ABSTRACT
Between the beginning of its full-scale scientific operations in 2007 and 2012, the VERITAS Cherenkov telescope array observed more than 130 blazars; of these, 26 were detected as very-high-energy (VHE; E > 100 GeV) γ-ray sources. In this work, we present the analysis results of a sample of 114 undetected objects. The observations constitute a total live-time of ∼570 hr. The sample includes several unidentified Fermi-Large Area Telescope (LAT) sources (located at high Galactic latitude) as well as all the sources from the second Fermi-LAT catalog that are contained within the field of view of the VERITAS observations. We have also performed optical spectroscopy measurements in order to estimate the redshift of some of these blazars that do not have spectroscopic distance estimates. We present new optical spectra from the Kast instrument on the Shane telescope at the Lick observatory for 18 blazars included in this work, which allowed for the successful measurement or constraint on the redshift of four of them. For each of the blazars included in our sample, we provide the flux upper limit in the VERITAS energy band. We also study the properties of the significance distributions and we present the result of a stacked analysis of the data set, which shows a 4σ excess.
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1. INTRODUCTION
The current generation of imaging atmospheric Cherenkov telescopes (IACTs), sensitive to very-high-energy (VHE; E > 100 GeV) γ-ray photons, has significantly increased our knowledge of blazars. They represent the class of objects that dominates the VHE extragalactic sky. Among the 66 extragalactic VHE sources currently detected,32 about 90% of them are blazars.
In the framework of the unified model of active galactic nuclei (AGNs), blazars are radio-loud AGNs, characterized by a pair of relativistic jets of plasma emitted along the polar axis of the super-massive black-hole powering the system, aligned along the line of sight of the observer (see Urry & Padovani 1995). The observational properties of blazars include a spectral energy distribution (SED) characterized by a non-thermal continuum from radio to γ-rays, extreme temporal variability, and strong polarization. These properties can be explained by considering that the emission from the jet, enhanced by relativistic effects, dominates the SED (Angel & Stockman 1980). Spectroscopic measurements in optical and UV reveal that two distinct sub-classes of blazars exist: BL Lac objects, characterized by a featureless optical/UV spectrum, and flat-spectrum radio-quasars (FSRQs), which show instead broad emission lines. The traditional division between the two classes of objects is an equivalent width of the emission lines equal to (see Stickel et al. 1991). The two sub-classes are also characterized by different luminosity and redshift distributions. The FSRQs are, on average, brighter and located at higher redshifts (see, e.g., Padovani 1992; Massaro et al. 2009). In the unified AGN model, this dichotomy is associated with a similar dichotomy seen in radio galaxies. FSRQs are considered the blazar version of FR II radio galaxies (Fanaroff & Riley 1974), while BL Lac objects correspond to FR I (Urry & Padovani 1995).
Two broad non-thermal components characterize blazar SEDs. The first, peaking between millimeter and X-rays, is attributed to synchrotron emission by a population of electrons/positrons in the blazar jet. The second, peaking in γ-rays, is associated, in leptonic models, with inverse-Compton scattering between the same leptons and their own synchrotron emission (synchtron-self-Compton model, SSC, Konigl 1981), or an external photon field, such as the emission from the dusty torus, the accretion disk or the broad-line region (see Sikora et al. 1994). Alternatively, in hadronic scenarios, the second SED component is attributed to synchrotron emission by protons, or by secondary particles produced in p–γ interactions (Mücke & Protheroe 2001). The position of the first peak is used to further classify BL Lac objects into low- and high-frequency-peaked BL Lac objects (LBL/HBL), depending on whether the peak frequency is located in infrared/optical or UV/X-rays, respectively. The transition between LBL and HBL is smooth, and a population of intermediate-frequency-peaked BL Lac objects exists as well (IBL, with 1014 Hz < νsyn < 1015 Hz, see Laurent-Muehleisen et al. 1998). While BL Lac objects present a variety of synchrotron peak frequencies, FSRQs are almost all characterized by a low-frequency peak. This classification can be seen as a more recent version of the older classification (see, e.g., Padovani & Giommi 1995b) of blazars into radio-selected objects (RBLs, which are more likely FSRQs and LBLs) and X-ray-selected objects (XBLs, which are more likely HBLs).33
The measurement of blazar spectral properties at VHE is important not only to characterize the blazar emission itself, but also to indirectly study the extragalactic background-light (EBL) in the infrared and visible bands due to the absorption that it causes on VHE photons via pair-production, (see Salamon & Stecker 1998). VHE observations can also be used to put limits on the strength of the intergalactic magnetic field according to the non-detection of the emission from the cascade triggered by the interaction of the pairs with the cosmic microwave background (see, e.g., Taylor et al. 2011).
Even though the detection and the measurement of the VHE spectrum of a blazar is of paramount importance for the comprehension of the physics of relativistic jets in AGNs and for cosmological studies, a non-detection in the VHE regime can also be extremely useful. It can constrain the source emission model, especially when the flux upper-limit is significantly lower than the extrapolation of the Fermi-Large Area Telescope (LAT; Atwood et al. 2009) measurement in high-energy γ-rays (HE; 100 MeV < E < 100 GeV) up to VHE, implying the presence of a spectral cut-off. It can also constrain the variability properties of the source at VHE if the blazar has been previously detected or is detected at a later time during a higher flux state. Additionally, for the upcoming Cherenkov Telescope Array (CTA, Actis et al. 2011), it will be useful to have the information from all past observations performed by the current generation of IACTs, in order to make predictions for expected outcomes (see, e.g., Sol et al. 2013).
All three major IACTs (H.E.S.S., MAGIC, and VERITAS) have published upper limits on blazar VHE emission, including more than 70 sources in their lists (Aharonian et al. 2005, 2008b; Aleksić et al. 2011a; Aliu et al. 2012; Abramowski et al. 2014). Past IACTs, such as CAT, HEGRA, and Whipple, have also presented the results from undetected blazars (Piron 2000; Aharonian et al. 2004; Falcone et al. 2004; Horan et al. 2004), though, in general, their limits have been superseded by the current instruments. At higher energies, upper limits on blazars have also been estimated using the air-shower technique with Milagro (Williams 2005).
The study of blazars is complicated by the uncertainty in their redshifts. In fact, the almost featureless spectra of BL Lac objects imply that a redshift estimate can be obtained only via absorption features from the host galaxy or clouds in the intergalactic medium, or via molecular emission lines (see Fumagalli et al. 2012), or via a less precise photometric estimate (comparing the luminosity of the host galaxy to samples of giant elliptical galaxies). If the blazar belongs to a group of galaxies, its redshift can also be estimated by studying the non-active companions (see Muriel et al. 2015). For VHE studies, the knowledge of the redshift of the blazar is very important, because the absorption by the EBL increases with the distance of the object. Currently, the farthest detected VHE blazar is the gravitationally lensed quasar S3 0218+357 (Mirzoyan 2014b) at z = 0.944, closely followed by the FSRQ PKS 1441+25 (Abeysekara et al. 2015; Ahnen et al. 2015) at z = 0.939, while the farthest, persistent (i.e., detected not only during episodic flaring activity) VHE blazar is PKS 1424+240 (, see Acciari et al. 2010; Furniss et al. 2013; Archambault et al. 2014). To improve the constraints on the distance of some VHE candidates, new redshift estimates obtained with the Kast spectrograph at the Lick Observatory (see Section 2 and the
In this paper, we present the results of the analysis of most of the non-detected blazars observed by VERITAS from 2007 (the beginning of full-scale scientific operations) to 2012 August (before the upgrade of the VERITAS array, see Kieda 2013). VERITAS upper limits on six VHE candidates were presented by Aliu et al. (2012). The sample also includes several unidentified Fermi-LAT objects (located at high Galactic latitude, and most likely associated with unidentified AGNs, see Ackermann et al. 2012a; Mirabal et al. 2012; Doert & Errando 2014).
The paper is organized as follows. In Section 2, we provide the details of the properties of the sample and the source selection. The details of the VERITAS data analysis and results are provided in Section 3. A stacked analysis of the data is presented in Section 4, studying the full data set as well as sub-data sets defined by redshifts and blazar classes. The conclusions are in Section 5.
2. THE SAMPLE AND NEW REDSHIFT ESTIMATES
Blazars targeted by VERITAS as VHE source candidates were selected according to a variety of criteria. Early source selections were based on blazar X-ray or radio catalogs, while more recent candidates have also been selected on the basis of their Fermi-LAT spectral characteristics (Abdo et al. 2009a, 2010a; Nolan et al. 2012) or on their association with clusters of HE γ-rays (see Archambault et al. 2013). The target list includes the following.
- 1.
- 2.
- 3.
- 4.All nearby () blazars detected by EGRET (Mukherjee 2001).
- 5.
- 6.
- 7.
- 8.Sources associated with clusters of HE γ-rays, but not included in Fermi-LAT catalogs, similar to VERJ0521+211 (Archambault et al. 2013).
In addition, several targets have been observed by VERITAS only as targets of opportunity (ToO), following flare alerts by multi-wavelength partners (see Errando 2011).
The sources included in our sample are listed in Table 1 (for sources without VHE detection, 82 targets) and Table 2 (for known VHE emitters, detected by other instruments, or by VERITAS after 2012 and during flaring activity, 11 targets). Sources are listed in order of increasing Right Ascension (R.A.). For every target, we indicate the name, the coordinates (R.A. and decl., in J2000), the catalog redshift z, the blazar class, the VERITAS dead-time corrected exposure (see Section 3), the average zenith angle of VERITAS observations, the dates of VERITAS observations (in MJD) and on which basis the source was selected as a VHE candidate. Names and coordinates are taken from the SIMBAD database.34 The references for the redshift estimates (and their uncertainties) and the blazar class are provided in the table notes. For every source, the archival SED from the ASDC SED Builder tool35 has been visually inspected. It is used to classify all BL Lac objects marked as HBLs. The total number of hours of VERITAS data analyzed is about 570, which represents about 60% of a single VERITAS yearly observing season, i.e., about one-tenth of all good quality VERITAS data taken from 2007 to 2012.
Table 1. List of Sources Observed by VERITAS
Source Name | R.A. | Decl. | za | Typeb | Exposure | MJDs | Selectionc | |
---|---|---|---|---|---|---|---|---|
(h m s) | (° m s) | (hr) | (degree) | (−50000) | ||||
RBS 0042 | 00 18 27.8 | +29 47 32 | 0.100: (1) | HBL | 7.1 | 15 | 4731/32/33/40 | SHBL |
4741/42/46/73 | ||||||||
5089/90/91 | ||||||||
5131/43 | ||||||||
RBS 0082 | 00 35 14.7 | +15 15 04 | 1.28: (2) | HBL | 6.3 | 19 | 5100/01/02 | SHBL |
5119/20/29/30 | ||||||||
1ES 0037+405 | 00 40 13.8 | +40 50 05 | - | HBL | 36.0 | 14 | 4767/68/71/72 | ToO |
4773/89/91 | ||||||||
4800/02/22/29/46 | ||||||||
5156/57/58/59 | ||||||||
5457/70/71/72 | ||||||||
5475/76/77/78/79 | ||||||||
5495/96/97/98 | ||||||||
5500/01/23/26 | ||||||||
5546/54/56 | ||||||||
1RXS J0045.3+2127 | 00 45 19.2 | +21 27 43 | - | HBL | 1.2 | 22 | 5512 | N06, 1FGL |
RGB J0110+418 | 01 10 04.9 | +41 49 51 | 0.096 (3) | IBL (38) | 4.0 | 13 | 4832/33 | P00 |
5866/67/68/96 | ||||||||
1ES 0120+340 | 01 23 08.7 | +34 20 51 | 0.272 (4) | HBL | 5.9 | 16 | 4383/84/93/94 | SHBL, CG02 |
4414/37/5171 | ||||||||
5470/73/99 | ||||||||
5512/26 | ||||||||
QSO 0133+476 | 01 36 58.6 | +47 51 29 | 0.859 (5) | FSRQ (39) | 0.8 | 40 | 4508 | ToO |
B2 0200+30 | 02 03 45.6 | +30 41 30 | 0.761 (6) | - | 1.8 | 21 | 5569/70/88 | ToO |
CGRaBS J0211+1051 | 02 11 13.1 | +10 51 35 | 0.20: (7) | LBL (40) | 4.0 | 31 | 5588/89/90/91 | ToO |
5595/99 | ||||||||
RGB J0214+517 | 02 14 17.9 | +51 44 52 | 0.049 (8) | IBL (41) | 5.1 | 22 | 4773/89/90/91/94 | P00, CG02 |
4800/5156/81 | ||||||||
5201/04/5489 | ||||||||
RBS 0298 | 02 16 30.9 | +23 15 13 | 0.289 (9) | HBL | 3.1 | 14 | 4731/32/33/73 | SHBL |
RBS 0319 | 02 27 16.6 | +02 02 00 | 0.457 (10) | HBL | 0.3 | 31 | 4412 | SHBL |
AO 0235+16 | 02 38 38.9 | +16 36 59 | 0.94 (11) | LBL (41) | 4.3 | 21 | 4737/38/39/42/45 | 1FGL, ToO |
RGB J0250+172 | 02 50 38.0 | +17 12 08 | 0.243 (12) | IBL (38) | 5.1 | 22 | 5144/59/5472 | 1FGL |
5528/42/71/98 | ||||||||
2FGL J0312.8+2013 | 03 12 23.0 | +20 07 50 | - | - | 9.7 | 14 | 5209 | 2FGL |
5830/33/34/40 | ||||||||
5855/56/57/58 | ||||||||
5860/61/62 | ||||||||
RGB J0314+247 | 03 14 02.7 | +24 44 33 | 0.056 (3) | LBL (42) | 3.1 | 28 | 4441 | P00 |
4761/62/63/64/73 | ||||||||
RGB J0314+063 | 03 14 23.9 | +06 19 57 | - | HBL | 0.3 | 26 | 5868 | SHBL |
RGB J0321+236 | 03 22 00.0 | +23 36 11 | - | IBL (38) | 9.2 | 12 | 5501/02/03/04 | 1FGL |
5507/08/10/11 | ||||||||
5512/13/14 | ||||||||
B2 0321+33 | 03 24 41.2 | +34 10 46 | 0.061 (8) | 9.1 | 12 | 4409/12/16/37 | P00, F04 | |
FSRQ/ | 4438/39/40/47 | |||||||
NLS1 (43) | 4448/49/50/64 | |||||||
1FGL J0333.7+2919 | 03 33 49.2 | +29 16 32 | - | IBL (44) | 0.6 | 3 | 5571 | 1FGL |
1RXS J044127.8+150455 | 04 41 27.4 | +15 04 56 | 0.109 (13) | HBL | 10.1 | 21 | 4747/48/49/89/90 | SHBL |
4831/32/33 | ||||||||
4847/49/51/79 | ||||||||
4880/82/91 | ||||||||
2FGL J0423.3+5612 | 04 23 27.0 | +56 12 24 | - | - | 1.6 | 28 | 5855/5926 | 2FGL |
1FGL J0423.8+4148 | 04 23 56.1 | +41 50 03 | - | - | 1.0 | 15 | 5599 | 1FGL |
1ES 0446+449 | 04 50 07.3 | +45 03 12 | 0.203: (4) | IBL (38) | 7.1 | 22 | 4734/42/61/62 | S96 |
4763/64/65 | ||||||||
5209/13/34/35 | ||||||||
5838 | ||||||||
RGB J0505+612 | 05 05 58.7 | +61 13 36 | - | - | 9.2 | 32 | 5502/03/29/30 | 1FGL |
5535/36/40/41 | ||||||||
5543/44/57/58/72 | ||||||||
1FGL J0515.9+1528 | 05 15 47.3 | +15 27 17 | - | - | 3.9 | 18 | 5480/81/82 | 1FGL |
5558/59 | ||||||||
2FGL J0540.4+5822 | 05 40 26.0 | +58 22 54 | - | - | 1.3 | 30 | 5856/5926/27 | 2FGL |
RGB J0643+422 | 06 43 26.8 | +42 14 19 | 0.080 (14) | HBL | 1.2 | 23 | 4439/4790/4892 | B97 |
RGB J0656+426 | 06 56 10.7 | +42 37 02 | 0.061 (15) | IBL (37) | 9.4 | 16 | 4746/66/67/77 | P00 |
4778/79/90/4800 | ||||||||
1ES 0735+178 | 07 38 07.4 | +17 42 19 | 0.424 (16) | IBL (44) | 5.2 | 18 | 5531/32/58/59 | 1FGL |
5574/87/5602 | ||||||||
BZB J0809+3455 | 08 09 38.9 | +34 55 37 | 0.082 (8) | HBL | 1.6 | 13 | 5928/29/30 | 1FGL |
PKS 0829+046 | 08 31 48.9 | +04 29 39 | 0.174 (17) | IBL (37) | 2.4 | 30 | 4822/4921/5181 | M01 |
Mrk 1218 | 08 38 10.9 | +24 53 43 | 0.028 (18) | FSRQ/ | 5.9 | 13 | 4423/25/39/40 | F04 |
Sy1 (45) | 4448/49/50/52 | |||||||
OJ 287 | 08 54 48.9 | +20 06 31 | 0.306 (19) | LBL (46) | 10.2 | 19 | 4438/39/40/48 | CG02, M01 |
4449/50/52/65/66 | ||||||||
5233/35/37/66 | ToO | |||||||
5302 | ||||||||
B2 0912+29 | 09 15 52.4 | +29 33 24 | 0.36: (7) | HBL | 11.7 | 10 | 5571/72/74/75 | 1FGL |
5576/87/88/89/90 | ||||||||
5630/44/72/73 | ||||||||
1ES 0927+500 | 09 30 37.6 | +49 50 26 | 0.188 (4) | HBL | 11.7 | 23 | 4466/4770/71 | SHBL, ROXA |
4800/01/02/03/06 | ||||||||
4807/20/21/22 | ||||||||
5247/75 | ||||||||
RBS 0831 | 10 08 11.4 | +47 05 22 | 0.343 (20) | HBL | 1.6 | 18 | 5531/59/89 | SHBL, ROXA |
RGB J1012+424 | 10 12 44.3 | +42 29 57 | 0.36: (21) | IBL (37) | 1.7 | 15 | 5589/5931/86 | ROXA |
1ES 1028+511 | 10 31 18.5 | +50 53 36 | 0.36: (10) | HBL | 24.1 | 23 | 4412/13/15/4530 | SHBL, CG02 |
4828/29/30/31 | ROXA | |||||||
4832/59/83 | ||||||||
4905/06/11/21 | ||||||||
4922/23/27/28 | ||||||||
5292/93/95/98 | ||||||||
5301/03 | ||||||||
5919/23/27/31/45 | ||||||||
5946/58/70/79/82 | ||||||||
6000/02/09/27 | ||||||||
6035/38 | ||||||||
RGB J1037+571 | 10 37 44.3 | +57 11 56 | >0.62: (7) | IBL (37) | 3.7 | 26 | 5241/42/43/46 | 1FGL |
RGB J1053+494 | 10 53 44.1 | +49 29 56 | 0.140 (22) | HBL | 7.8 | 24 | 4879/81/82/88/91 | 1FGL |
4921/22 | ||||||||
5157/58/59 | ||||||||
RBS 0921 | 10 56 06.6 | +02 52 14 | 0.236 (23) | HBL | 2.7 | 30 | 4821/22/51 | SHBL |
RBS 0929 | 11 00 21.1 | +40 19 28 | - | HBL | 4.4 | 16 | 5333/5587/88/89 | SHBL, ROXA |
1ES 1106+244 | 11 09 16.2 | +24 11 20 | 0.482 (24) | HBL | 1.0 | 17 | 5981/82 | C07 |
RX J1117.1+2014 | 11 17 06.3 | +20 14 07 | 0.139 (15) | HBL | 9.1 | 16 | 4940/41/42/5538 | SHBL, CG02 |
5540/41/42/43/44 | 1FGL, ToO | |||||||
5543/63/64/65/71 | ||||||||
1ES 1118+424 | 11 20 48.1 | +42 12 12 | 0.230 (12) | HBL | 6.4 | 17 | 5212/32/74/75 | SHBL, S96 |
5276/89/90/91 | ||||||||
S4 1150+497 | 11 53 24.5 | +49 31 09 | 0.334 (25) | FSRQ (38) | 3.8 | 24 | 5701/02/03/04/05 | ROXA, ToO |
5706/07/08/09/10 | ||||||||
RGB J1231+287 | 12 31 43.6 | +28 47 50 | 1.03 (26) | HBL | 5.1 | 17 | 5239/66/91/98 | 1FGL |
5300/03 | ||||||||
1ES 1239+069 | 12 41 48.3 | +06 36 01 | 0.150 (4) | HBL | 1.9 | 26 | 4979/80 | S96 |
PG 1246+586 | 12 48 18.8 | +58 20 29 | >0.73: (10) | IBL (37) | 9.6 | 29 | 5595/ 5601/03/05 | 1FGL |
5621/24/25/29 | ||||||||
5630/31 | ||||||||
1ES 1255+244 | 12 57 31.9 | +24 12 40 | 0.141 (27) | HBL | 26.0 | 16 | 4531/34/68/80 | SHBL, S96 |
4581/82/83/84 | ||||||||
4585/86/87/91 | ||||||||
4907/11/23/50 | ||||||||
4970/79/80 | ||||||||
5207/08/36 | ||||||||
5591/5617 | ||||||||
5947/49/53/59 | ||||||||
5972/78/89 | ||||||||
6016/42/44/45 | ||||||||
6072/73/74 | ||||||||
BZB J1309+4305 | 13 09 25.5 | +43 05 06 | 0.691 (6) | HBL | 9.4 | 15 | 5594/96/98 | 1FGL |
5600/02/04/06 | ||||||||
5620/22/23/25 | ||||||||
1FGL J1323.1+2942 | 13 23 02.4 | +29 41 35 | - | FSRQ (47) | 8.4 | 14 | 4832/34/92/93 | 1FGL, ToO |
4909/14/79 | ||||||||
5596/97/99 | ||||||||
5602/05/48/49 | ||||||||
RX J1326.2+2933 | 13 26 15.0 | +29 33 31 | 0.431 (14) | HBL | 8.4 | 14 | Same as above | ROXA, C07 |
RGB J1341+399 | 13 41 05.2 | +39 59 46 | 0.169 (14) | HBL | 2.7 | 26 | 4938/78 | ROXA, N06 |
5972/6045 | ||||||||
RGB J1351+112 | 13 51 20.8 | +11 14 53 | >0.619 (28) | HBL | 6.2 | 24 | 5210/21/39/40 | 1FGL |
5241/43/46 | ||||||||
5293/97/98 | ||||||||
RX J1353.4+5601 | 13 53 28.1 | +53 00 57 | 0.370 (14) | HBL | 2.5 | 26 | 5589/90/6002 | ROXA, N06 |
RBS 1350 | 14 06 59.2 | +16 42 06 | >0.623 (13) | HBL | 4.5 | 22 | 5269/97/98 | 1FGL |
5300/01 | ||||||||
RBS 1366 | 14 17 56.7 | +25 43 56 | 0.237 (12) | HBL | 10.0 | 17 | 4591/92/93/94 | SHBL, CG02 |
4596/99 | ROXA | |||||||
4611/12/13/15/16 | ||||||||
6046 | ||||||||
1ES 1421+582 | 14 22 38.9 | +58 01 56 | 0.683 (14) | HBL | 3.4 | 28 | 5324/25/26/28/30 | SHBL |
5333/35/50/51 | ||||||||
RGB J1439+395 | 14 39 17.5 | +39 32 43 | 0.344 (13) | HBL | 1.5 | 12 | 5620/6002 | SHBL, ROXA |
1RXS J144053.2+061013 | 14 40 52.9 | +06 10 16 | 0.396 (28) | IBL (48) | 2.5 | 27 | 5731/32/34/35/36 | 1FGL |
RBS 1452 | 15 01 01.8 | +22 38 06 | 0.235 (29) | IBL (49) | 4.1 | 21 | 5297/98/99 | 1FGL |
RGB J1532+302 | 15 32 02.3 | +30 16 29 | 0.065 (3) | HBL | 6.5 | 17 | 4939/40/67/68 | P00 |
4970/75/76/77 | ||||||||
RGB J1533+189 | 15 33 11.3 | +18 54 29 | 0.305 (13) | HBL | 2.9 | 20 | 5648/77 | SHBL, ROXA |
5705/06/20 | N06 | |||||||
1ES 1533+535 | 15 35 00.9 | +53 20 37 | 0.89: (10) | HBL | 1.0 | 23 | 4256/6002 | SHBL |
RGB J1610+671B | 16 10 04.1 | +67 10 26 | 0.067 (14) | HBL | 6.6 | 36 | 4908/38/5268 | P00 |
5292/93/94/95 | ||||||||
1ES 1627+402 | 16 29 01.3 | +40 08 00 | 0.272 (3) | HBL/ | 13.1 | 16 | 4229/35/36 | P02, F04 |
NLS1 (50), (51) | 4537/38/39/40/57 | |||||||
4559/60/61/62/63 | ||||||||
4564/65/69/83 | ||||||||
4954 | ||||||||
GB6 J1700+6830 | 17 00 09.3 | +68 30 07 | 0.301 (30) | FSRQ (30) | 0.8 | 37 | 4914/16 | 1FGL, ToO |
PKS 1717+177 | 17 19 13.0 | +17 45 06 | >0.58 (31) | LBL (52) | 5.1 | 18 | 4909/11/17/18 | 1FGL |
4920/21/22 | ||||||||
PKS 1725+045 | 17 28 25.0 | +04 27 05 | 0.2966 (32) | FSRQ (53) | 0.3 | 27 | 4412 | M01 |
PKS 1749+096 | 17 51 32.8 | +09 39 01 | 0.32 (33) | LBL (54) | 0.3 | 22 | 6072 | ToO |
RGB J1838+480 | 18 39 49.2 | +48 02 34 | 0.30: (21) | IBL (37) | 0.5 | 26 | 6090 | 2FGL |
RGB J1903+556 | 19 03 11.6 | +55 40 39 | >0.58: (7) | IBL (41) | 1.0 | 27 | 5099 | 1FGL |
1FGL J1926.8+6153 | 19 26 41.9 | +61 54 41 | - | - | 1.3 | 31 | 5706/07 | 1FGL |
PKS 2233-148 | 22 36 34.1 | -14 33 22 | >0.49: (34) | LBL (55) | 0.3 | 49 | 6100 | ToO |
3C 454.3 | 22 53 57.7 | +16 08 54 | 0.859 (35) | FSRQ (56) | 1.0 | 24 | 5504/31 | ToO |
RGB J2322+346 | 23 22 44.0 | +34 36 14 | 0.098 (3) | IBL (37) | 2.9 | 10 | 4731/36/39 | P00 |
4745/46/47 | ||||||||
1ES 2321+419 | 23 23 52.1 | +42 10 59 | >0.45 (36) | HBL | 4.2 | 21 | 4773/76 | S96 |
4802/03/30/31 | ||||||||
5091 | ||||||||
B3 2322+396 | 23 25 17.9 | +39 57 37 | >1.05 (32) | LBL (54) | 1.0 | 16 | 5118/30 | 1FGL |
1FGL J2329.2+3755 | 23 29 14.2 | +37 54 15 | - | - | 3.3 | 11 | 5470/71/72/75/76 | 1FGL |
5477/78/80 | ||||||||
1RXS J234332.5+343957 | 23 43 33.8 | +34 40 04 | 0.366 (13) | HBL | 1.5 | 17 | 5912 | SHBL |
Notes.
aUnconstrained redshifts are indicated with a hyphen (-). If the redshift value is uncertain, it is followed by a colon (:). Redshift references: (1) Fischer et al. (1998), (2) Rau et al. (2012), (3) Laurent-Muehleisen et al. (1998), (4) Perlman et al. (1996), (5) Lawrence et al. (1986), (6) Shaw et al. (2013), (7) Meisner & Romani (2010), (8) Marcha et al. (1996), (9) Böhringer et al. (2000), (10) Sbarufatti et al. (2005), (11) Cohen et al. (1987), (12) this work, (13) Piranomonte et al. (2007), (14) Bauer et al. (2000), (15) Lavaux & Hudson (2011), (16) Carswell et al. (1974), (17) Falomo (1991), (18) Osterbrock & Dahari (1983), (19) Stickel et al. (1989), (20) Plotkin et al. (2010), (21) Nilsson et al. (2003), (22) Stocke et al. (1991), (23) Cao et al. (1999), (24) Sbarufatti et al. (2009), (25) Burbidge et al. (1977), (26) White et al. (2000), (27) Padovani & Giommi (1995a), (28) Sandrinelli et al. (2013), (29) Jannuzi et al. (1993), (30) Henstock et al. (1997), (31) Shaw et al. (2009), (32) Eracleous & Halpern (2004), (33) Stickel et al. (1988), (34) Sbarufatti et al. (2006), (35) Smith et al. (1976), (36) Falomo & Kotilainen (1999). bBlazars of unknown type are indicated with a hyphen (-). Blazar type references: (37) Laurent-Muehleisen et al. (1999), (38) Giommi et al. (2012), (39) Chandra et al. (2012), (40) Ackermann et al. (2012b), (41) Nieppola et al. (2006), (42) Abdo et al. (2009b), (43) Massaro et al. (2012), (44) Rani et al. (2011), (45) Osterbrock & Dahari (1983), (46)Impey & Neugebauer (1988), (47) Cornwell et al. (1986), (48) Ajello et al. (2014), (49) Massaro et al. (2003), (50) Padovani et al. (2002), (51) Komossa et al. (2006), (52) Li et al. (2010), (53) Drinkwater et al. (1997), (54) Abdo et al. (2010b), (55) Lister et al. (2011), (56) Smith et al. (1976). cSource selection references: 1FGL, Abdo et al. (2010a); 2FGL, Nolan et al. (2012); B97, Brinkmann et al. (1997); CG02, Costamante & Ghisellini (2002); C07, Costamante (2007); F04, Falcone et al. (2004); M01, Mukherjee (2001); N06, Nieppola et al. (2006); ROXA, Turriziani et al. (2007); S96, Stecker et al. (1996); SHBL, Giommi et al. (2005); P00, Perlman (2000); P02, Padovani et al. (2002); ToO, Target of Opportunity.Table 2. List of Known VHE Sources Observed but Not Detected by VERITAS in 2007–2012
Source Name | R.A. | Decl. | za | Type | Exposure | MJDs | Selectionb | VHE detectionc | |
---|---|---|---|---|---|---|---|---|---|
(h m s) | (° m s) | (hr) | (degree) | (−50000) | |||||
1ES 0033+595 | 00 35 52.6 | +59 50 05 | 0.086: (1) | HBL | 22.6 | 31 | 4411/18/19/20 | CG02, P00 | 1 |
4421/38/40/48 | |||||||||
4464/66/76 | |||||||||
4734/36/37/74 | |||||||||
4775/76/77 | |||||||||
4803/04/06 | |||||||||
5866/67 | |||||||||
RGB J0152+017 | 01 52 33.5 | +01 46 40 | 0.080 (2) | HBL | 8.4 | 35 | 4421/22/37/38 | CG02 | 2 |
4439/40/47/48 | |||||||||
4449/50/64/65/78 | |||||||||
5537/5832/33/68 | |||||||||
RGB J0847+115 | 08 47 13.0 | +11 33 50 | 0.198 (3) | HBL | 12.1 | 25 | 4499/4505/07/08 | SHBL | 3 |
4522/23/24/25/26 | |||||||||
5303 | |||||||||
5502/03/31/59 | |||||||||
RX J1136.5+6737 | 11 36 30.1 | +67 37 04 | 0.134 (4) | HBL | 7.7 | 37 | 4860/61/91/92 | CG02, SHBL | 4 |
4918/21 | ROXA | ||||||||
5292/94/99 | |||||||||
5303 | |||||||||
PKS 1222+216 | 12 24 54.4 | +21 22 47 | 0.432 (5) | FSRQ | 25.8 | 16 | 4939/5182 | ToO | 5a, 5b |
5318/19/20/21 | |||||||||
5322/24/25/26 | |||||||||
5327/28/30/33 | |||||||||
5622/24/25/26 | |||||||||
5631/33/34 | |||||||||
3C 279 | 12 56 11.1 | -05 47 22 | 0.536 (6) | FSRQ | 8.3 | 40 | 5623 | ToO | 6a, 6b |
5707/08/09/10 | |||||||||
5711/15/17 | |||||||||
5923/24/25/26/27 | |||||||||
6016 | |||||||||
PKS 1510-089 | 15 12 52.2 | -09 06 22 | 0.361 (7) | FSRQ | 14.9 | 42 | 4909/11/48 | ToO | 7a, 7b |
5976/77/78/79 | |||||||||
5980/81/82/83 | |||||||||
5984/87 | |||||||||
RGB J1725+118 | 17 25 04.3 | +11 52 16 | >0.35: (8) | HBL | 10.0 | 23 | 4593/94 | CG02 | 8 |
4615/16/17/18 | |||||||||
4619/20/21/22 | |||||||||
0FGL J2001.0+4352 | 20 01 13.5 | +43 53 03 | 0.18: (9) | HBL | 4.9 | 25 | 5143/44/46/51/52 | 1FGL | 9 |
5326/52/57 | |||||||||
RGB J2243+203 | 22 43 54.7 | +20 21 04 | >0.39: (10) | IBL | 4.1 | 17 | 5094/98/99 | 1FGL | 10 |
5101/16/28/29 | |||||||||
B3 2247+381 | 22 50 06.6 | +38 25 58 | 0.118 (2) | HBL | 6.0 | 13 | 5092/93/95/96/97 | 1FGL | 11 |
5832/89 |
Notes.
aIf the redshift value is uncertain it is followed by a colon (:). Redshift references: (1) private communication from Perlman, see Falomo & Kotilainen (1999), (2) Laurent-Muehleisen et al. (1998), (3) Cao et al. (1999), (4) Bade et al. (1994), (5) Burbidge & Kinman (1966), (6) Marziani et al. (1996), (7) Thompson et al. (1990), (8) Landoni et al. (2014), (9) Aleksić et al. (2014a), (10) Meisner & Romani (2010). bSelection references: see Table 1. cVHE detection references (see also for the blazar subclass classification): (1) Aleksić et al. (2015a), (2) Aharonian et al. (2008a), (3) Mirzoyan (2014c), (4) Mirzoyan (2014a), (5a) Aleksić et al. (2011b), (5b) Holder (2014a), (6a) Albert et al. (2008), (6b) Aleksić et al. (2014c), (7a) Abramowski et al. (2013a), (7b) Aleksić et al. (2014b), (8) Cortina (2013), (9) Aleksić et al. (2014a), (10) Holder (2014b), (11) Aleksić et al. (2012).Download table as: ASCIITypeset image
The field of view (FOV) of the VERITAS telescope array is about 35 and for every observation there is a chance that, in addition to the targeted blazar, other γ-ray sources are contained within the FOV. For every target included in our sample, we checked if other known γ-ray sources (included in the two-year Fermi-LAT catalog, 2FGL, see Nolan et al. 2012) were present in the FOV. Twenty-one 2FGL sources were indirectly observed by VERITAS through proximity to the blazar of interest, and are listed in Table 3. We also indicate the counterpart name (from the 2FGL catalog), the coordinates (R.A. and decl.) of the counterpart, the redshift, and the blazar sub-class (if known). The majority of these additional 2FGL sources are AGNs without classification.
Table 3. List of 2FGL Sources in the VERITAS Field of View of Sources Listed in Tables 1 and 2
Source Name | Counterpart | R.A.a | Decl.a | zb | Typec | in FOV ofd |
---|---|---|---|---|---|---|
(h m s) | (° m s) | |||||
2FGL J0047.9+2232 | BWE 0045+2218 | 00 48 02.5 | +22 34 53 | 1.161 (1) | FSRQ (12) | RGB J0045+214 |
2FGL J0148.6+0127 | PMN J0148+0129 | 01 48 33.8 | +01 29 01 | 0.940 (2) | - | RGB J0152+017 |
2FGL J0158.4+0107 | - | 01 58 25.4 | +01 07 31 | - | - | RGB J0152+017 |
2FGL J0205.4+3211 | 1Jy 0202+319 | 02 05 04.9 | +32 12 30 | 1.466 (3) | FSRQ (13) | B2 0200+30 |
2FGL J0212.1+5318 | - | 02 12 09.4 | +53 18 19 | - | - | RGB J0214+517 |
2FGL J0213.1+2245 | 1RXS J021252.2+224510 | 02 12 52.8 | +22 44 52 | 0.459 (2) | HBL (14) | RBS 0298 |
2FGL J0326.1+2226 | TXS 0322+222 | 03 25 36.8 | +22 24 00 | 2.06 (4) | FSRQ (15) | RGB J0321+236 |
2FGL J0440.4+1433 | TXS 0437+145 | 04 40 21.1 | +14 37 57 | - | - | 1RXS J044127.8+150455 |
2FGL J0856.3+2058 | TXS 0853+211 | 08 56 39.7 | +20 57 43 | >0.388 (5) | - | OJ 287 |
2FGL J0929.5+5009 | QSO J0929+5013 | 09 29 15.4 | +50 13 36 | 0.370 (6) | IBL (16) | 1ES 0927+500 |
2FGL J1058.4+0133 | 4C 01.28 | 10 58 29.6 | +01 33 58 | 0.888 (7) | FSRQ (13) | RBS 0921 |
2FGL J1059.0+0222 | PMN J1059+0225 | 10 59 06.0 | +02 25 12 | - | - | RBS 0921 |
2FGL J1141.0+6803 | 1RXS J114118.3+680433 | 11 41 18.0 | +68 04 33 | - | - | RX J1136.5+6737 |
2FGL J1239.5+0728 | PKS 1236+077 | 12 38 24.6 | +07 30 17 | 0.400 (8) | FSRQ (17) | 1ES 1239+069 |
2FGL J1245.1+5708 | GB6 J1245+5710 | 12 45 10.0 | +57 09 54 | >0.521 (5) | LBL (18) | PG 1246+586 |
2FGL J1303.1+2435 | VIPS J13030+2433 | 13 03 03.2 | +24 33 56 | 0.993 (9) | LBL (19) | 1ES 1255+244 |
2FGL J1359.4+5541 | VIPS J13590+5544 | 13 59 05.7 | +55 44 29 | 1.014 (1) | FSRQ (12) | RX J1353.4+5601 |
2FGL J1722.7+1013 | TXS 1720+102 | 17 22 44.6 | +10 13 36 | 0.732 (10) | FSRQ (20) | RGB J1725+118 |
2FGL J1727.9+1220 | PKS 1725+123 | 17 28 07.1 | +12 15 39 | 0.583 (11) | FSRQ (20) | RGB J1725+118 |
2FGL J1927.5+6117 | S4 1926+611 | 19 27 30.4 | +61 17 33 | - | LBL (21) | 1FGL J1926.8+6153 |
2FGL J1959.9+4212 | 1RXS J195956.1+421339 | 19 59 56.1 | +42 13 39 | - | - | 0FGL J2001.0+4352 |
Notes.
aCoordinates are provided for the counterpart. If the Fermi-LAT source is not associated with any lower-energy blazar, coordinates from the 2FGL catalog are given instead. bUnconstrained redshifts are indicated with a hyphen (−). Redshift references: (1) Shaw et al. (2012), (2) Shaw et al. (2013), (3) Kraus & Gearhart (1975), (4) Halpern et al. (1986), (5) Plotkin et al. (2010), (6) Healey et al. (2008), (7) Hewitt & Burbidge (1993), (8) White et al. (1988), (9) Glikman et al. (2007), (10) Afanas'Ev et al. (2005), (11) Sowards-Emmerd et al. (2005). cBlazars of unknown type are indicated with a hyphen (−). Blazar type references: (12) Shaw et al. (2012), (13) Kraus & Gearhart (1975), (14) Ajello et al. (2014), (15) Ghisellini et al. (2011), (16) Laurent-Muehleisen et al. (1999), (17) Hewitt & Burbidge (1993), (18) Plotkin et al. (2010), (19) Glikman et al. (2007), (20) Afanas'Ev et al. (2005), (21) Maselli et al. (2010). dSee Tables 1 and 2 for information on the exposure and the zenith angle of the observations.Download table as: ASCIITypeset image
2.1. New Redshift Measurements
In order to measure the distance of some γ-ray blazars, we observed 18 of the VERITAS targets using the dual-arm Kast spectrograph at the Cassegrain focus of the Shane telescope at the Lick Observatory. For all the observations presented here, the instrument was configured with the 600/4310 grism on the blue arm, and the 600/7500 grating on the red arm, the D55 dichroic, and a slit. The dichroic crossover creates an instrumental gap located at ∼5500 Å and affects approximately 200 Å of the spectrum. In the spectroscopic Figures 2.1–2.6 (
- 1.RGB J0250+172. Observations of the source were obtained on 2010 August 15 (UT) and resulted in the detection of galactic features at z = 0.243. We detected Ca ii (H, equivalent width (EW) K, ), G-band (), and Mg i () absorption. In the literature, Bauer et al. (2000) quotes a redshift of z = 1.10 for RGB J0250+172; however, there is no information on spectroscopic lines provided within the reference. Nilsson et al. (2003) present optical images of BL Lac objects, including RGB J0250+172. They find that the object is clearly resolved and state that z = 1.10 is too high because it results in a host galaxy that is exceedingly bright (). Based on fits to the observed light profile, a redshift estimate of z = 0.25 is provided, which is similar to the value measured within this work.
- 2.1ES 1118+424.Observations of the source were obtained on 2013 February 14 (UT) and resulted in the detection of galactic features at z = 0.230. We detected Ca ii (H, EW = 11500 ± 1500 mÅ; K, EW = 13000 ± 1700 mÅ), G-band (EW = 3790 ± 862 mÅ), Ca i (EW = 1208 ± 199 mÅ), and Mg i (EW = 2896 ± 254 mÅ) absorption lines. In the literature, the redshift for 1ES 1118+424 is quoted as z = 0.124 from a private communication (see Falomo & Kotilainen 1999). However, Falomo & Kotilainen (1999) derive a lower limit of based on images taken using the Nordic Optical Telescope, where the source is unresolved. They simulate an elliptical host galaxy with and an effective radius of 10 kpc to determine the lowest redshift at which it would not be resolved. These galactic parameters are what they find from other BL Lac objects in their study, and they note that assuming a less luminous and smaller host galaxy would result in a lower redshift estimate.
- 3.RBS 1366 (=1E 1415+25.9).Observations of the sources obtained on 2014 May 30 (UT) resulted in the detection of galactic features at z = 0.237. We detected Ca ii (H, EW = 1300 ± 130 mÅ; K, EW = 1570 ± 140 mÅ), G-band (EW = 210 ± 160 mÅ), Ca i (EW = 710 ± 64 mÅ), and Mg i (EW = 1900 ± 100 mÅ) absorption.Halpern et al. (1986) also measure a redshift of z = 0.237 based on Ca ii, G-band, Fe i, Mg i, and Na absorption. This source displays the significant variability associated with BL Lac objects. The spectrum taken in 2013 has a lower overall flux than the spectrum taken in 2014 (see Table 7), indicating that the source might have been in different flux states.
- 4.1ES 2321+419.Observations of the source obtained on 2014 October 28 (UT) resulted in the detection of absorption features at z = 0.267. We detected Ca ii (H, EW = 260 ± 47 mÅ; K, EW = 180 ± 52 mÅ) and Mg ii (2796 Å, EW = 740 ± 83 mÅ; 2803 Å, EW = 510 ± 67 mÅ) absorption. Because the Ca ii absorption is narrow, and there are no other galactic features, only a lower limit can be set on the redshift of the source. Additionally, there is potentially Mg ii absorption at a higher redshift, z = 0.346. In the literature, Falomo & Kotilainen (1999) derive a lower limit for this source of using the same technique as for 1ES 1118+424. While our value is not inconsistent, it is considerably lower than that placed based on assumptions about the host galaxy.
3. VERITAS OBSERVATIONS AND DATA ANALYSIS
The VERITAS (Very Energetic Radiation Imaging Telescope Array System) telescope array is composed of four IACTs of 12 m diameter each, located at the Fred Lawrence Whipple Observatory, on the slopes of Mount Hopkins, in southern Arizona (31° N, 110° W). Each telescope has a segmented mirror that focuses light onto a camera composed of 499 photomultipliers located at the focal plane. The instrument FOV is 35. For further details on the VERITAS instrument, see Holder et al. (2006) and Holder (2011).
The telescopes measure the faint Cherenkov light induced by the electromagnetic showers triggered by the interaction of the γ-ray photons with the Earth atmosphere. Similar cascades triggered by cosmic-rays are also detected by VERITAS, and can be rejected by applying specific cuts on the shape of the Cherenkov image (Hillas 1985).
The results presented in this paper have been obtained using a set of γ-hadron separation cuts specifically optimized for the detection of soft spectrum sources (differential spectrum parametrized by a power-law function with ). The spectral index assumed is in line with the typical value of Γ observed for VHE blazars (see Şentürk et al. 2013).
All the observations presented in this paper were made using the "wobble" observing strategy (Fomin et al. 1994). Here, the telescopes point 05 away from the target, alternatively in each of the four cardinal directions, to enable background estimation from the same FOV. This procedure ensures a similar acceptance for both the source (ON) and the background (OFF) regions. Regions overlapping bright stars are excluded from background estimates. The ratio of the ON over the OFF region size defines the background normalization parameter α. The dead time of the telescope array is explicitly calculated and is approximately 10% for the observations described here. The exposure values provided in Table 1 are all corrected for dead time, and represent the effective live-time of VERITAS observations.
The VERITAS observations here have an average length of 20 minutes (referred to as a run), before switching targets or wobble directions. For quality assurance, all the runs with a length lower than 10 minutes were excluded, often being associated with technical problems, resulting in the early termination of the run. Additionally, all observations characterized by non-optimal weather conditions or malfunctioning hardware were excluded from the run selection. On certain occasions, one of the VERITAS telescopes can be non-operational due to technical problems; all the runs analyzed in this paper have at least three telescopes in operation. Runs with all four telescopes in operation represent the bulk (92%) of the data.
In the standard configuration, VERITAS observations are not performed under bright-moonlight conditions (moon illumination of full moon). Since 2012, the VERITAS collaboration has started a new observing program in order to extend the duty cycle of the observatory and perform observations also under bright moonlight (Archambault et al. 2015). None of the data presented in this paper were taken under bright-moonlight conditions. Observations performed under moderate moonlight (moon illumination ) are included and analyzed in the same manner as dark-time observations, with appropriate instrument response functions to account for the increased night-sky background.
The significance at the source location is computed using Equation (17) in Li & Ma (1983). The upper limit on the VHE flux is estimated according to Rolke et al. (2005) at the confidence level. It is first calculated assuming three different values of the spectral index (, 3.5 and 4.5) in order to estimate the decorrelation energy Edec (the energy at which the upper limit estimate depends the least on the spectral index). The upper limit is then recomputed at the reference energy Edec assuming a spectral index . The threshold of the analysis (which depends mainly on the zenith angle of the observations) is also calculated. For every source, we verified that, not only the overall significance is lower than five standard deviations (σ), but that no flares have been detected, i.e., that none of the sources was detected at more than during any single run.
For sources that are detected by Fermi-LAT (76% of the sample), the flux is extrapolated into the VERITAS energy band, taking into account the absorption from the EBL using the model by Franceschini et al. (2008), which is in agreement with the most recent observational constraints (Abramowski et al. 2013b). The extrapolated flux is then compared to the VERITAS upper limit. If the VERITAS measurement is lower than the extrapolation, it means that an additional cut-off should be present in the γ-ray component between the Fermi-LAT and the VERITAS energy bands.
The results of the analysis are reported in Tables 4 and 5 (for known VHE sources) and 6 (for 2FGL sources in the VERITAS FOV). For every target, we provide the significance, the number of ON and OFF counts, the value of the α parameter (ratio of the ON over OFF region size), the threshold of the analysis Eth, the decorrelation energy Edec, the differential flux upper limit at Edec, the integral flux upper limit (above Eth, provided in Crab units, following Hillas et al. 1998)37 , the ratio between the VERITAS differential upper limit and the extrapolation of the Fermi-LAT detection (, evaluated at Edec). Note that the values of the decorrelation and threshold energies are provided with three decimal values to ease any extrapolation to other energy bands, but they are known only to the second decimal value.
Table 4. Analysis Results and Flux Upper Limits for the Non-detected AGNs Observed by VERITAS
Source Name | σ | ON | OFF | α | UL | UL | |||
---|---|---|---|---|---|---|---|---|---|
(TeV) | (TeV) | (10−12 ) | (% C.U.) | ||||||
RBS 0042 | 0.02 | 1239 | 6455 | 0.192 | 0.182 | 0.345 | 8.7 | 2.2 | 0.2 (z = 0.1) |
RBS 0082 | 0.16 | 1680 | 9062 | 0.185 | 0.166 | 0.264 | 16.9 | 1.8 | 45.0 |
1ES 0037+405 | 1.50 | 7515 | 64132 | 0.115 | 0.166 | 0.322 | 29.3 | 6.3 | - |
1RXS J0045.3+2127 | 2.04 | 224 | 1116 | 0.172 | 0.166 | 0.297 | 54.6 | 8.9 | 3.0/14.8 (z = 0.1/0.5) |
RGB J0110+418 | −0.02 | 801 | 4810 | 0.167 | 0.182 | 0.299 | 22.4 | 3.4 | - |
1ES 0120+340 | 1.47 | 1174 | 5215 | 0.214 | 0.166 | 0.283 | 24.6 | 3.4 | 1.4 |
QSO 0133+476 | 1.24 | 114 | 496 | 0.202 | 0.417 | 0.728 | 11.8 | 17.4 | 1.9e4 |
B2 0200+30 | 1.38 | 495 | 2775 | 0.167 | 0.151 | 0.273 | 51.9 | 6.9 | 59.6 |
CGRaBS J0211+1051 | 1.01 | 977 | 5659 | 0.167 | 0.200 | 0.318 | 20.8 | 3.6 | 3.4 |
RGB J0214+517 | 0.33 | 1113 | 5877 | 0.187 | 0.182 | 0.336 | 15.9 | 3.7 | - |
RBS 0298 | 1.76 | 606 | 3245 | 0.173 | 0.240 | 0.435 | 21.5 | 9.2 | - |
RBS 0319 | −0.52 | 61 | 393 | 0.167 | 0.219 | 0.351 | 38.4 | 8.6 | 44.9 |
AO 0235+16 | 0.63 | 704 | 4116 | 0.167 | 0.182 | 0.311 | 18.7 | 3.2 | 9.6 |
RGB J0250+172 | −0.06 | 1274 | 5637 | 0.226 | 0.166 | 0.316 | 13.5 | 2.7 | 1.4 |
2FGL J0312.8+2013 | −0.36 | 2124 | 9046 | 0.238 | 0.166 | 0.257 | 10.0 | 1.0 | 0.5/0.6 (z = 0.1/0.5) |
RGB J0314+247 | 1.07 | 691 | 3967 | 0.167 | 0.240 | 0.460 | 16.4 | 8.5 | - |
RGB J0314+063 | 1.18 | 76 | 326 | 0.200 | 0.182 | 0.294 | 76.7 | 11.0 | - |
RGB J0321+236 | 1.24 | 3065 | 17948 | 0.167 | 0.138 | 0.233 | 31.2 | 2.6 | 2.0/2.9 (z = 0.1/0.5) |
B2 0321+33 | −0.03 | 2190 | 8223 | 0.267 | 0.166 | 0.272 | 9.7 | 1.2 | 15.9 |
1FGL J0333.7+2919 | 0.37 | 158 | 606 | 0.223 | 0.138 | 0.226 | 101.0 | 7.6 | 2.7/7.4 (z = 0.1/0.5) |
1RXS J044127.8+150455 | 1.83 | 2351 | 10636 | 0.212 | 0.182 | 0.338 | 17.5 | 4.1 | - |
2FGL J0423.3+5612 | 0.67 | 283 | 1522 | 0.178 | 0.240 | 0.457 | 16.4 | 8.3 | 1.9/32.6 (z = 0.1/0.5) |
1FGL J0423.8+4148 | −0.22 | 240 | 1462 | 0.167 | 0.166 | 0.274 | 46.4 | 5.7 | 0.6/2.2 (z = 0.1/0.5) |
1ES 0446+449 | −1.49 | 1482 | 10866 | 0.142 | 0.219 | 0.363 | 4.7 | 1.2 | - |
RGB J0505+612 | −1.50 | 2167 | 9896 | 0.227 | 0.219 | 0.377 | 4.1 | 1.2 | 0.6/5.7 (z = 0.1/0.5) |
1FGL J0515.9+1528 | −0.54 | 1149 | 6734 | 0.173 | 0.151 | 0.283 | 16.7 | 2.5 | 0.9/3.8 (z = 0.1/0.5) |
2FGL J0540.4+5822 | 0.43 | 226 | 1315 | 0.167 | 0.240 | 0.459 | 16.5 | 8.5 | 2.8/49.4 (z = 0.1/0.5) |
RGB J0643+422 | 0.46 | 240 | 1282 | 0.181 | 0.200 | 0.369 | 32.7 | 9.5 | - |
RGB J0656+426 | 1.16 | 1960 | 9607 | 0.198 | 0.200 | 0.322 | 20.2 | 3.6 | - |
1ES 0735+178 | −1.20 | 1259 | 6865 | 0.190 | 0.166 | 0.260 | 9.0 | 0.9 | 0.5 |
BZB J0809+3455 | −0.24 | 252 | 1537 | 0.167 | 0.151 | 0.251 | 39.8 | 4.0 | - |
PKS 0829+046 | −0.77 | 465 | 2465 | 0.196 | 0.240 | 0.379 | 10.1 | 2.7 | 0.6 |
Mrk 1218 | 2.44 | 1589 | 8994 | 0.165 | 0.166 | 0.280 | 43.1 | 5.7 | - |
OJ 287 | 0.97 | 2197 | 12966 | 0.166 | 0.182 | 0.296 | 17.4 | 2.6 | 3.1 |
B2 0912+29 | 3.49 | 3466 | 19492 | 0.167 | 0.138 | 0.228 | 45.9 | 3.6 | 1.6 |
1ES 0927+500 | −0.18 | 2378 | 11404 | 0.209 | 0.182 | 0.346 | 11.0 | 2.8 | - |
RBS 0831 | −0.31 | 394 | 2403 | 0.167 | 0.166 | 0.297 | 29.5 | 4.8 | - |
RGB J1012+424 | 0.18 | 270 | 1324 | 0.167 | 0.219 | 0.316 | 43.5 | 6.7 | 22.2 |
1ES 1028+511 | 1.16 | 4610 | 27154 | 0.167 | 0.182 | 0.305 | 12.4 | 2.0 | 1.4 |
RGB J1037+571 | −1.53 | 790 | 3798 | 0.221 | 0.200 | 0.331 | 5.7 | 1.1 | 2.5 (z = 0.6) |
RGB J1053+494 | −0.76 | 1397 | 8567 | 0.167 | 0.200 | 0.386 | 6.4 | 2.2 | 0.5 |
RBS 0921 | 0.84 | 633 | 3534 | 0.173 | 0.240 | 0.354 | 20.1 | 4.2 | - |
RBS 0929 | −0.62 | 923 | 4678 | 0.202 | 0.166 | 0.319 | 11.3 | 2.4 | 0.5/2.7 (z = 0.1/0.5) |
1ES 1106+244 | −1.65 | 200 | 1151 | 0.197 | 0.151 | 0.257 | 14.3 | 1.5 | 3.3 |
RX J1117.1+2014 | 0.16 | 2545 | 12950 | 0.190 | 0.151 | 0.281 | 12.5 | 1.8 | 0.18 |
1ES 1118+424 | 0.39 | 1685 | 9703 | 0.172 | 0.151 | 0.267 | 22.4 | 2.8 | 0.39 |
S4 1150+497 | −0.53 | 749 | 4589 | 0.167 | 0.182 | 0.315 | 12.9 | 2.3 | 70.0 |
RGB J1231+287 | 0.74 | 1258 | 6664 | 0.185 | 0.138 | 0.243 | 36.6 | 3.5 | 12.2 |
1ES 1239+069 | −0.86 | 224 | 1429 | 0.167 | 0.240 | 0.369 | 8.7 | 2.1 | - |
PG 1246+586 | 0.23 | 2123 | 12670 | 0.167 | 0.200 | 0.363 | 9.5 | 2.4 | 14.5 (z = 0.73) |
1ES 1255+244 | 2.24 | 5127 | 29732 | 0.167 | 0.166 | 0.315 | 12.4 | 2.5 | - |
BZB J1309+4305 | 0.54 | 2359 | 14020 | 0.167 | 0.151 | 0.298 | 17.8 | 3.2 | 7.9 |
1FGL J1323.1+2942 | 0.24 | 1781 | 14622 | 0.121 | 0.151 | 0.243 | 18.0 | 1.6 | 1.2/3.7 (z = 0.1/0.5) |
RX J1326.2+2933 | 1.36 | 1771 | 14150 | 0.127 | 0.166 | 0.256 | 17.8 | 1.7 | - |
RGB J1341+399 | 0.00 | 381 | 2286 | 0.167 | 0.200 | 0.405 | 12.7 | 5.1 | - |
RGB J1351+112 | 1.44 | 1715 | 8994 | 0.184 | 0.151 | 0.248 | 30.0 | 2.8 | 2.9 (z = 0.62) |
RX J1353.4+5601 | 0.65 | 569 | 3163 | 0.175 | 0.200 | 0.339 | 24.8 | 5.3 | - |
RBS 1350 | 0.55 | 1387 | 8190 | 0.167 | 0.151 | 0.248 | 27.9 | 2.6 | - |
RBS 1366 | 1.89 | 1789 | 9843 | 0.173 | 0.200 | 0.327 | 17.1 | 3.3 | 7.9 |
1ES 1421+582 | 0.17 | 674 | 4016 | 0.167 | 0.219 | 0.378 | 13.2 | 3.8 | - |
RGB J1439+395 | 0.80 | 404 | 2321 | 0.167 | 0.151 | 0.246 | 62.4 | 5.7 | 5.0 |
1RXS J144053.2+061013 | −0.09 | 424 | 2556 | 0.167 | 0.200 | 0.343 | 13.8 | 3.1 | 16.7 |
RBS 1452 | 0.03 | 1232 | 7474 | 0.165 | 0.151 | 0.254 | 25.9 | 2.7 | 0.5 |
RGB J1532+302 | −0.56 | 812 | 3648 | 0.227 | 0.166 | 0.348 | 10.7 | 3.0 | - |
RGB J1533+189 | −1.44 | 653 | 4460 | 0.167 | 0.151 | 0.293 | 8.5 | 1.5 | - |
1ES 1533+535 | 0.54 | 191 | 973 | 0.188 | 0.182 | 0.324 | 41.7 | 8.9 | - |
RGB J1610+671B | −1.75 | 1318 | 6510 | 0.214 | 0.263 | 0.516 | 1.6 | 1.1 | - |
1ES 1627+402 | −0.33 | 2191 | 13244 | 0.167 | 0.182 | 0.360 | 7.3 | 2.1 | - |
GB6 J1700+6830 | −1.98 | 81 | 610 | 0.167 | 0.316 | 0.528 | 3.5 | 2.2 | 161 |
PKS 1717+177 | 0.42 | 934 | 4343 | 0.212 | 0.182 | 0.290 | 19.3 | 2.6 | 3.1(z = 0.58) |
PKS 1725+045 | −0.58 | 41 | 271 | 0.167 | 0.200 | 0.329 | 37.5 | 7.3 | 127 |
PKS 1749+096 | −1.00 | 64 | 438 | 0.167 | 0.166 | 0.257 | 43.6 | 4.3 | 1.6 |
RGB J1838+480 | 0.27 | 87 | 329 | 0.167 | 0.200 | 0.346 | 52.3 | 12.1 | 4.2 |
RGB J1903+556 | 0.19 | 164 | 968 | 0.167 | 0.240 | 0.398 | 22.4 | 7.0 | 22.2 (z = 0.58) |
1FGL J1926.8+6153 | 0.62 | 231 | 1326 | 0.167 | 0.240 | 0.408 | 21.6 | 7.4 | 1.1/12.5 (z = 0.1/0.5) |
PKS 2233-148 | 0.06 | 32 | 190 | 0.167 | 0.501 | 0.829 | 7.8 | 15.0 | 1.4e3 (z = 0.49) |
3C 454.3 | −1.21 | 220 | 981 | 0.25 | 0.138 | 0.250 | 23.0 | 2.5 | 0.6 |
RGB J2322+346 | −0.42 | 518 | 2926 | 0.181 | 0.182 | 0.296 | 19.2 | 2.8 | 1.4 |
1ES 2321+419 | 2.05 | 992 | 3686 | 0.250 | 0.219 | 0.414 | 23.7 | 9.4 | 15.9 (z = 0.5) |
B3 2322+396 | −0.63 | 131 | 705 | 0.197 | 0.166 | 0.256 | 49.2 | 4.8 | 80 (z = 1.05) |
1FGL J2329.2+3755 | −0.13 | 847 | 5106 | 0.167 | 0.166 | 0.254 | 27.3 | 2.6 | 1.0/3.4 (z = 0.1/0.5) |
1RXS J234332.5+343957 | 0.80 | 341 | 1952 | 0.167 | 0.151 | 0.250 | 53.5 | 5.2 | 3.2 |
Table 5. Results and Upper Limits for the Known VHE Sources
Source Name | σ | ON | OFF | α | UL | UL | |||
---|---|---|---|---|---|---|---|---|---|
(TeV) | (TeV) | (10−12 cm s TeV) | (% C.U.) | ||||||
1ES 0033+595 | 3.23 | 4560 | 20321 | 0.214 | 0.288 | 0.490 | 10.0 | 5.4 | 0.8 (z = 0.086) |
RGB J0152+017 | 1.83 | 1720 | 8693 | 0.188 | 0.240 | 0.376 | 14.1 | 3.6 | 1.2 |
RGB J0847+115 | −0.03 | 3391 | 19889 | 0.171 | 0.182 | 0.338 | 8.63 | 2.0 | 0.3 |
RX J1136.5+6737 | 1.10 | 1324 | 7271 | 0.177 | 0.288 | 0.519 | 7.86 | 5.2 | 1.3 |
PKS 1222+216 | 3.40 | 7482 | 39770 | 0.180 | 0.182 | 0.257 | 24.1 | 2.2 | 0.2 |
3C 279 | 0.10 | 1479 | 8849 | 0.167 | 0.263 | 0.430 | 5.5 | 2.1 | 2.6 |
PKS 1510-089 | 2.00 | 2691 | 13698 | 0.167 | 0.263 | 0.496 | 4.7 | 2.9 | 1.1 |
RGB J1725+118 | 2.29 | 1979 | 10833 | 0.079 | 0.200 | 0.316 | 18.7 | 3.2 | 0.6/2.6 (z = 0.1/0.5) |
0FGL J2001.0+4352 | 0.76 | 1102 | 6449 | 0.167 | 0.200 | 0.384 | 15.5 | 5.2 | 0.3 (z = 0.2) |
RGB J2243+203 | −0.12 | 1111 | 6458 | 0.173 | 0.166 | 0.263 | 19.3 | 2.1 | 0.3 (z = 0.39) |
B3 2247+381 | −0.93 | 1447 | 8042 | 0.185 | 0.166 | 0.315 | 8.8 | 1.8 | 0.5 |
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Table 6. Results and Upper Limits for the 2FGL Sources in the VERITAS Field of View
Source Name | σ | ON | OFF | α | UL | UL | |||
---|---|---|---|---|---|---|---|---|---|
(TeV) | (TeV) | (10−12 cm) | (% C.U.) | ||||||
2FGL J0047.9+2232 | 0.32 | 123 | 2058 | 0.0770 | 0.200 | 0.314 | 70.5 | 11.6 | 1.6e4 |
2FGL J0148.6+0127 | 1.15 | 1239 | 17272 | 0.063 | 0.240 | 0.412 | 25.1 | 8.9 | 6.7e3 |
2FGL J0158.4+0107 | −1.62 | 582 | 13194 | 0.0625 | 0.240 | 0.435 | 16.6 | 7.1 | 150/2.2e3 (z = 0.1/0.5) |
2FGL J0205.4+3211 | 1.76 | 232 | 3743 | 0.0524 | 0.166 | 0.290 | 102 | 15.4 | 5.4e5 |
2FGL J0212.1+5318 | −1.22 | 389 | 8089 | 0.0512 | 0.200 | 0.377 | 37.8 | 11.9 | 1.0/9.1 (z = 0.1/0.5) |
2FGL J0213.1+2245 | 0.57 | 501 | 7460 | 0.0655 | 0.182 | 0.295 | 63.3 | 9.2 | 29.3 |
2FGL J0326.1+2226 | 0.22 | 1342 | 25891 | 0.0515 | 0.151 | 0.249 | 92.9 | 9.0 | 2.5e4 |
2FGL J0440.4+1433 | 0.20 | 1028 | 19458 | 0.0525 | 0.182 | 0.340 | 50.3 | 12.1 | 60/416 (z = 0.1/0.5) |
2FGL J0856.3+2058 | 0.19 | 1686 | 21961 | 0.0764 | 0.182 | 0.297 | 37.1 | 5.6 | 53 (z = 0.5) |
2FGL J0929.5+5009 | 3.28 | 1712 | 17578 | 0.0884 | 0.200 | 0.349 | 40.2 | 9.6 | 28 |
2FGL J1058.4+0133 | 1.19 | 319 | 5882 | 0.0506 | 0.240 | 0.373 | 51.7 | 12.9 | 985 |
2FGL J1059.0+0222 | −0.37 | 533 | 7097 | 0.0764 | 0.240 | 0.359 | 25.8 | 5.7 | 30/240 (z = 0.1/0.5) |
2FGL J1141.0+6803 | 0.17 | 1195 | 12124 | 0.0981 | 0.288 | 0.525 | 7.5 | 5.1 | 0.7/18.9 (z = 0.1/0.5) |
2FGL J1239.5+0728 | 1.41 | 198 | 2766 | 0.0644 | 0.240 | 0.368 | 55.1 | 13.1 | 67 |
2FGL J1245.1+5708 | 2.34 | 1418 | 24171 | 0.0545 | 0.219 | 0.369 | 40.7 | 10.8 | 276 (z = 0.52) |
2FGL J1303.1+2435 | 0.33 | 3021 | 57951 | 0.0518 | 0.166 | 0.322 | 36.2 | 7.8 | 117 |
2FGL J1359.4+5541 | 1.74 | 507 | 5493 | 0.0850 | 0.219 | 0.347 | 50.5 | 10.8 | 1.4e5 |
2FGL J1722.7+1013 | 1.73 | 693 | 14010 | 0.0462 | 0.200 | 0.332 | 72.2 | 14.5 | 433 |
2FGL J1727.9+1220 | −0.56 | 1816 | 23431 | 0.0789 | 0.200 | 0.315 | 15.3 | 2.5 | 41 |
2FGL J1927.5+6117 | −1.94 | 127 | 1964 | 0.077 | 0.240 | 0.412 | 12.1 | 4.3 | 5.1/62 (z = 0.1/0.5) |
2FGL J1959.9+4212 | 1.82 | 458 | 9118 | 0.037 | 0.219 | 0.347 | 64.4 | 13.8 | 24/176 (z = 0.1/0.5) |
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All the results presented in this paper have been cross-checked using a separate analysis, which provided consistent results for the single upper limit values, significance distributions (Section 3.2) and stacked analysis (Section 4).
3.1. Notes on Individual Sources
Among the blazars targeted by VERITAS between 2007 and 2012, 11 of them were later identified as VHE emitters. They are listed in Table 2. The VERITAS upper limits are useful in these cases to constrain the properties of the VHE emission during low-flux states, as well as the variability properties of the source. The discussion of these blazar observations, in order of R.A., follows.
- 1.1ES 0033+595 (HBL, ).VHE emission from 1ES 0033+595 was discovered by MAGIC (Aleksić et al. 2015a). The flux, measured from 24 hours of observations taken from 2009 August to October, is Crab above 290 GeV.38 The observed spectral index during the MAGIC observations is 3.8 ± 0.7. The VERITAS upper limit (5.4% Crab above 290 GeV) is fully consistent with the MAGIC measurement. The VHE variability of the source has been demonstrated by more recent VERITAS observations, which detected a bright VHE flare (with integral flux higher than 10% Crab) from the source during 2013 September, with simultaneous X-ray and ultraviolet coverage by Swift (Benbow 2015). A paper presenting the results of this multi-wavelength campaign is currently in preparation.
- 2.RGB J0152+017 (HBL, z = 0.08).This source has been known as a VHE emitter since 2008 (Aharonian et al. 2008a), when it was detected by H.E.S.S. at a flux of Crab above 240 GeV, during 15 hr of observation. VERITAS observed the blazar in three different seasons: 2007–2008 (covering the H.E.S.S. period), 2010–2011, and 2011–2012. The VERITAS upper limit ( Crab above 240 GeV) is fully consistent with the H.E.S.S. detection.
- 3.RGB J0847+115 (HBL, z = 0.198).VHE emission from this blazar was announced by MAGIC in 2014, at a flux corresponding to Crab above 200 GeV (Mirzoyan 2014c). No spectral information is currently available, but the MAGIC collaboration reported a preliminary classification of the source as an extreme-HBL, with synchrotron peak frequency in hard-X-rays, and inverse-Compton peak frequency at TeV energies. Evidence of VHE and optical variability was also claimed by MAGIC. The VERITAS upper limit ( of the Crab Nebula flux above 180 GeV) is marginally consistent with the preliminary flux estimate by MAGIC, and could be related to variability of the VHE emission.
- 4.RX J1136.5+6737 (HBL, z = 0.134).The MAGIC collaboration recently reported the detection of this source at a flux of Crab above 200 GeV, in 20 hours of observations between 2014 January and April (Mirzoyan 2014a). No spectral information is available at the present time. The VERITAS upper limit ( Crab above 290 GeV) is fully consistent with the preliminary flux estimate by MAGIC.
- 5.PKS 1222+216 (FSRQ, z = 0.432).VHE emission from this FSRQ was detected by MAGIC in 2010 at a flux of the order of the Crab Nebula flux (Aleksić et al. 2011b). The detection of this VHE flare is of paramount importance for blazar physics: the rapid variability, and the fact that VHE photons can escape the bright photon field present in FSRQs was used to put constraints on the location of the γ-ray emitting region in blazars. The VERITAS non-detection constrains the low-flux state at a level of Crab above 180 GeV. During 2014 May, PKS 1222+216 underwent another γ-ray flare, and VERITAS detected VHE emission at a flux of 3% Crab (Holder 2014a). A paper describing the VERITAS detection in 2014 is currently in preparation.
- 6.3C 279 (FSRQ, z = 0.536).This quasar is the first of its class detected as a VHE emitter (Albert et al. 2008). VERITAS observations during 2011 were triggered by flaring activity observed at lower wavelengths (optical, X-rays, and HE γ-rays). The same flare triggered observations with the MAGIC telescopes (Aleksić et al. 2014c), which resulted as well in no VHE detection (flux upper limit equal to Crab above 260 GeV). The VERITAS flux upper limit ( Crab above 260 GeV) is similar to the one measured with the MAGIC telescopes.
- 7.PKS 1510-089 (FSRQ, z = 0.361).Two VHE flares from this quasar have been reported so far: the first during 2009 March–April, seen by H.E.S.S. (around 0.6% of the Crab Nebula flux above 260 GeV, see Abramowski et al. 2013a), the second during 2012 February, seen by MAGIC (around 1% of the Crab Nebula flux above 260 GeV, see Aleksić et al. 2014b). The VERITAS observations presented in this paper are quasi-simultaneous with both flares (see Table 2 for details). For the 2009 flare, VERITAS observations were taken a few days before the VHE flare seen by H.E.S.S. For the 2012 flare, VERITAS observations were taken every night from February 19 to 27, covering the Fermi-LAT flare. The VERITAS upper limit is 2.9% of the Crab Nebula flux above 260 GeV.
- 8.RGB J1725+118 (HBL, ).The discovery of VHE emission from this blazar was recently reported by MAGIC (Cortina 2013) at a flux of Crab above 140 GeV during observations in 2013 May triggered by an elevated optical state. No spectral information is available at the present time. The VERITAS upper limit ( Crab above 200 GeV) is consistent with the MAGIC measurement.
- 9.0FGL J2001.0+4352 (HBL, ).Early results from Fermi-LAT indicated that this source was a good candidate for IACTs, in particular, due to its hard GeV spectrum (Abdo et al. 2009a). The MAGIC collaboration detected the source at a flux of Crab during a single night (1.4 hr on 2010 July 16, see Aleksić et al. 2014a). The VERITAS upper limit clearly indicates that this blazar is variable at VHE, and that its baseline flux is below Crab above 200 GeV.
- 10.RGB J2243+203 (HBL, ).VHE emission from this source was detected by VERITAS during 2014 December, following a trigger from a high Fermi-LAT flux (Holder 2014b; Abeysekara 2015). Preliminary analysis indicates that the flux from the flaring blazar was at Crab above 180 GeV. The upper limits computed from 2009 observations indicate that the VHE emission from this source is variable, being significantly lower (2.1% Crab above 170 GeV) than the 2014 detection.
- 11.B3 2247+381 (HBL, z = 0.119).This source was detected by MAGIC in 14 hours of observations from 2010 September to October, at a flux of Crab above 170 GeV (Aleksić et al. 2012). The MAGIC observations were triggered by a high optical state, and there is evidence of variability in the simultaneous X-ray light curve. VERITAS observations do not cover the MAGIC detection, nor the high optical flux state measured by the Tuorla observatory. The non-detection by VERITAS (flux upper limit equal to Crab above 170 GeV) constrains the low-state flux of this blazar to be lower than the MAGIC detection, suggesting that it may have been related to a VHE high-flux state.
In addition to these known VHE emitters, we discuss a few other targets with noteworthy histories.
- 1.1ES 0037+405 (HBL, z unknown).VERITAS observations of this target were taken as a self-triggered ToO. During observations of the Andromeda galaxy (M31, see Bird 2015), a 4σ hotspot coincident with this blazar was observed in the reconstructed sky-map. However, further observations did not confirm the hotspot, and the cumulative significance is 1.5σ in 36 hr.
- 2.OJ 287 (LBL, z = 0.306).This blazar is one of the most studied objects of its kind due to a clear periodicity in its optical lightcurve, with a period of about 12 years. The VERITAS observations presented in this work cover the last active phase in Fall 2007 (from 2007 December 4 to 2008 January 1), with additional observations during 2010. The VHE upper limits are comparable to the ones measured with the MAGIC telescopes and presented by Seta et al. (2009).
- 3.1FGL J1323.1+2942 (FSRQ, z unknown) and RX J1326.2+2933 (HBL, z = 0.431).Although the angular distance between the two sources is only , they are not the same blazar. VERITAS can resolve the two objects, and they have been targeted by VERITAS independently (see last column of Table 1). Since they are well within the VERITAS FOV, the exposures on these two objects have been merged into a single data set.
- 4.B2 0912+29 (HBL, z = 0.36).This blazar shows the highest significance in our data set ( in 11.7 hr). This excess was confirmed at the same significance level by the cross-check analysis chain. Further observations were taken during the 2013 and 2014 observing seasons, but the initial excess did not increase. While VHE blazars are known to be variable, and one could interpret the lack of a detection in 2013–2014 as due to variability, we also note that the probability of a excess reduces to only when 103 trials (the sources from Tables 1 and 3) are taken into account, and it is thus not enough to make any claim.
3.2. Significance Distributions
In Figure 1, we present the distribution of the significances for all the sources presented in our work. Given that the blazar population at VHE is not homogeneous (see the Introduction), and depends on both the blazar sub-class (which is correlated with the energy of the high-energy SED peak) and the blazar redshift (which implies a different level of EBL absorption), significance distributions are produced as a function of these two parameters. For the redshift division (left plot of Figure 1), redshifts lower or higher than 0.6 were considered, along with unknown redshift. Concerning the division of blazar sub-classes (right plot of Figure 1), the sources were categorized as HBLs, IBLs/LBLs/FSRQs, and blazars of unknown type. The Gaussian distribution expected from a sample with average and is overlaid on the significance distribution. A fit of the histogram with a Gaussian function instead yields and .
4. STACKED ANALYSIS
Motivated by the skew in the significance distribution and in order to study if there is any evidence of emission from a population of blazars below the VERITAS sensitivity level, a stacked analysis of the data set is performed. For every source the γ-ray excess (ON—α OFF) and its uncertainty (the excess divided by the significance) are calculated. We then compute the sum of the excesses, and its uncertainty (the square root of the sum of the squared uncertainties), whose ratio provides the significance of the stacked excess. Sources known as VHE emitters are excluded from the stacked analysis, which only includes sources listed in Tables 1 and 3.
The stacked analysis indicates that there is evidence of VHE emission at a level of , corresponding to an excess of 1990 γ-rays. The same study is then performed for sub-samples of the overall data set. The majority of the excess () comes from nearby () HBLs. On the other hand, the stacked analysis including only non-HBL sources located at an unknown distance or results in a stacked significance of . However, because nearby HBLs are considered, the most likely VHE candidates, there are more of them and they often have deeper exposures. So this study has more sensitivity to the nearby HBLs. Indeed, the VERITAS exposure on HBLs located at is about 196 hr. By assuming that the stacked excess comes from a constant signal from all sources in 570 hr, one would expect a excess in 196 hr. The excess from the HBLs is thus compatible with this expectation, and it is not possible to claim that the stacked excess is dominated by a particular blazar population.
The MAGIC collaboration has also reported evidence for VHE emission from a stacked sample of IBL/HBL sources (Aleksić et al. 2011a), detecting a signal at a significance level of from an exposure of 394 hr. The following sources included in the present work are also part of the MAGIC sample: 1ES 0120+340, 1RXS J044127.8+150455, 1ES 0927+500, 1ES 1028+511, RX J1117.1+2014, RX 1136.5+6737, and RBS 1366. The four sources with the highest significance in the MAGIC publication (1ES 0033+595, 1ES 1011+496, B2 1215+30, and 1ES 1741+196) were, notably, later confirmed as VHE emitters, either during flaring activity, or by increasing the integration time.
5. CONCLUSIONS
The results from the analysis of the observations of non-detected blazars targeted by VERITAS from 2007 to 2012 have been presented. In addition, γ-ray sources from the 2FGL catalog that were within the FOV of these VERITAS observations were included in this study. For all the 114 sources included in this data set, we provided the VERITAS upper limit at VHE. Given that the redshift estimate of blazars is particularly important for VHE extragalactic astronomy, due to the γ-ray absorption on the EBL, we also presented the results from optical spectroscopy of 18 of these targets, determining the redshift for three of them, and providing a lower limit for the redshift of one of the sources.
We have presented the results from a stacked analysis of the data set, showing that there is some evidence of signal with a significance level of .
In the near future, the sensitivity of VHE astronomy will be significantly increased thanks to the Cherenkov Telescope Array (CTA), which will be capable of detecting sources with VHE fluxes of the order of 0.001 Crab units, about a factor of 10 better than current IACTs (Actis et al. 2011). Among the scientific goals of CTA, an important endeavor will be to increase the number of known VHE blazars in order to perform population studies. Among the VERITAS targets presented in this work, the sources with the highest significance could be considered as primary candidates for observations with CTA, which may be able to detect many of them on the basis of the extrapolation of their Fermi-LAT spectra to higher energies. The non-detection of a number of later detected VHE blazars emphasizes the variable nature of these sources, highlighting the importance of monitoring observations in order to increase the likelihood of catching the sources at detectable VHE states.
This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and by NSERC in Canada. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The VERITAS Collaboration is grateful to Trevor Weekes for his seminal contributions and leadership in the field of VHE gamma-ray astrophysics, which made this study possible. M.F. acknowledges support by the Science and Technology Facilities Council (grant number ST/L00075X/1).
APPENDIX: NEW REDSHIFT ESTIMATES
In this appendix, we present optical spectra of 18 blazars taken at the Lick Observatory in an attempt to spectroscopically measure their redshift. The sources selected for spectroscopy were selected independently from the sources selected in the main text, so the overlap is not complete. The spectra we show were taken between 2010 August and 2014 October. During the observations at Lick we often observed the same source more than once. Duplicate observations are noted in Table 7, but we only show one spectrum per source in the figures. Four of these spectra (shown in Figure 2) have host galaxy features that allow an accurate redshift determination, and are discussed in the main text.
Table 7. Blazars Observed at the Lick Observatory using the Shane 3 m Kast Spectrograph
Target Namea | Obs. Date | Obs. Date | Exposure | Signal to Noiseb | Standard Star | z | Figurec |
---|---|---|---|---|---|---|---|
(UT) | (MJD—50000) | (s) | |||||
RBS 0082 | 2010 Aug 13 | 5421 | 3600 | 50, 77 | BD+28 4211 | ⋯ | 2.2 |
1ES 0033+595 | 2012 Aug 22 | 6161 | 3600 | 20, 88 | BD+28 4211 | ⋯ | ⋯ |
1ES 0033+595 | 2013 Dec 4 | 6630 | 3600 | 2.8, 34 | G191B2B | ⋯ | 2.2 |
1RXS J0045.3+2127 | 2012 Aug 22 | 6161 | 1800 | 56, 106 | BD+28 4211 | ⋯ | ⋯ |
1RXS J0045.3+2127 | 2014 Oct 28 | 6958 | 1800 | 57, 121 | Feige 110 | ⋯ | 2.2 |
RGB J0250+172 | 2010 Aug 15 | 5423 | 5400 | 21, 44 | BD+28 4211 | 0.243 | 2.1 |
1ES 0446+449 | 2013 Feb 14 | 6337 | 3600 | n/a, 65 | HZ2 | ⋯ | 2.3 |
RGB J0505+612 | 2013 Feb 14 | 6337 | 900 | 0.4, 4.3 | HZ2 | ⋯ | 2.3 |
2FGL J0540.4+5822 | 2014 Oct 28 | 6958 | 3600 | 14, 46 | G19B2B | ⋯ | 2.3 |
B2 0912+29 | 2013 Apr 7 | 6389 | 3600 | 57, 179 | Feige 34 | ⋯ | ⋯ |
B2 0912+29 | 2013 Jan 4 | 6299 | 3600 | 81, 144 | Feige 34 | ⋯ | 2.4 |
B2 0912+29 | 2013 Dec 4 | 6630 | 3600 | 48, 102 | G191B2B | ⋯ | ⋯ |
RBS 0929 | 2013 Apr 7 | 6389 | 3600 | 13, 33 | Feige 34 | ⋯ | 2.4 |
RGB J1037+571 | 2013 Feb 14 | 6337 | 3600 | 54, 128 | Feige 34 | 2.4 | |
1ES 1118+424 | 2013 Feb 14 | 6337 | 3600 | 1.5, 22 | Feige 34 | 0.230 | 2.1 |
PG 1246+586 | 2013 Apr 7 | 6389 | 3600 | 80, 229 | GD 153 | ⋯ | ⋯ |
PG 1246+586 | 2014 May 29 | 6806 | 3600 | 86, 175 | HZ44 | ⋯ | ⋯ |
PG 1246+586 | 2014 May 30 | 6807 | 3600 | 80, 203 | HZ44 | ⋯ | 2.5 |
RBS 1366 | 2013 Apr 7 | 6389 | 900 | 7, 34 | BD+33 2642 | ⋯ | ⋯ |
RBS 1366 | 2014 May 30 | 6807 | 3600 | 26, 72 | BD+33 2642 | 0.237 | 2.1 |
1RXS J144053.2+061013 | 2013 Jan 4 | 6299 | 3800 | 13, 53 | BD+33 2642 | ⋯ | 2.5 |
RGB J1725+118 | 2013 Jun 12 | 6455 | 3600 | 48, 129 | BD+33 2642 | ⋯ | ⋯ |
RGB J1725+118 | 2014 May 29 | 6806 | 3600 | 81, 189 | BD+33 2642 | ⋯ | ⋯ |
RGB J1725+118 | 2014 May 30 | 6807 | 3600 | 111, 205 | BD+33 2642 | ⋯ | 2.5 |
RGB J1903+556 | 2013 Jun 13 | 6456 | 1800 | 18, 46 | BD+28 4211 | 2.6 | |
BZB J2243+2021 | 2010 Aug 13 | 5421 | 3600 | 92, 167 | BD+28 4211 | ⋯ | 2.6 |
1ES 2321+419 | 2012 Aug 22 | 6161 | 3000 | 20, 33 | BD+28 4211 | ⋯ | ⋯ |
1ES 2321+419 | 2014 Oct 28 | 6958 | 3600 | 52, 121 | Feige 110 | 2.1 |
Notes.
aSee Table 1 in main text for coordinates. bThis is the average signal-to-noise per pixel. The first number is for the blue side CCD, between 3500 and 5400 A. The second number is for the red side CCD, between 5700 and 6800 Angstroms. cSeveral targets have spectra from multiple nights. Only one spectrum per target is shown; this column indicates the corresponding figure, if applicable.Download table as: ASCIITypeset image
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Standard image High-resolution imageFootnotes
- 32
For a recent review, see, e.g., Şentürk et al. (2013); for an updated list of known VHE sources see http://tevcat.uchicago.edu.
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The classification of BL Lac objects as LBL/IBL/HBL is sometimes replaced by LSP/ISP/HSP (low/intermediate/high-synchrotron-peaked blazars, see Abdo et al. 2010b), making an explicit reference to the synchrotron origin of the first component of the SED. For BL Lac objects, the two triplets of acronyms can be considered as synonyms.
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The best-fit of the VHE emission from the Crab Nebula as measured with the Whipple 10 m telescope and presented in Hillas et al. (1998) is a power-law function with index and normalization at 1 TeV. The integral upper limits are computed from the differential ones and provided here as a reference. They can easily be recomputed for different values of Eth, or for other definitions of the Crab unit. For example, using as a reference the MAGIC spectrum of the Crab Nebula (Aleksić et al. 2015b), the Crab unit above 200 GeV is 74% of the Whipple Crab unit above the same threshold.
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In order to ease the comparison between the results from different instruments, the integral fluxes provided in this section have been recomputed above the VERITAS threshold, when the spectral information is available, and are expressed in Crab units as defined in Hillas et al. (1998).