OPTICAL SPECTROSCOPIC OBSERVATIONS OF GAMMA-RAY BLAZAR CANDIDATES. VI. FURTHER OBSERVATIONS FROM TNG, WHT, OAN, SOAR, AND MAGELLAN TELESCOPES

Unidentified γ-ray sources (UGSs) have been known to constitute a large fraction of the γ-ray sources since the observations of the Energetic Gamma Ray Experiment Telescope (EGRET) survey on board the Compton Gamma-Ray Observatory (Mukherjee et al. 1997; Hartman et al. 1999). For this reason "resolving" the γ-ray sky, i.e., searching for the counterparts of the UGSs, was listed as one of the four key scientific objectives of the Fermi mission launched in 2008 (Atwood et al. 2009).

Given the large position uncertainty of Fermi sources (of the order of 01) compared to the density of potential optical counterparts, there is no simple procedure to assign them a potential counterpart, and a multifrequency approach is thus necessary, in particular to decrease the number of UGSs. Consequently, a large number of follow-up observations have been carried out since the launch of Fermi to decrease the number of UGSs, mostly focused on searching for blazars, the most numerous class of active galaxies detected in the γ-rays, and pulsars (see, e.g., Ackermann et al. 2012; Nolan et al. 2012; Abdo et al. 2015). Both low and high radio frequency observations (such as the All-sky Survey of Flat-Spectrum Radio Sources CRATES [Healey et al. 2007] and the All-sky Survey of Gamma-ray Blazar Candidates CGRaBS [Healey et al. 2008]) have been successfully used in recent years to select compact radio sources as blazar candidates (see, e.g., Petrov et al. 2013; Schinzel et al. 2015). The discovery of the peculiar infrared (IR) colors of the known γ-ray blazar population (Massaro et al. 2011; D'Abrusco et al. 2012) led to the development of several methods to search for sources with similar proprieties within the UGS sample (Massaro et al. 2012; D'Abrusco et al. 2013; Massaro et al. 2013a, 2013b). In addition, a dedicated X-ray survey of all the UGSs is currently being carried out by the Swift satellite¹⁵ (see Stroh & Falcone 2013 and also Paggi et al. 2013; Takeuchi et al. 2013; Acero et al. 2015), aiming at the determination of the X-ray counterpart of the Fermi sources. Finally, statistical analyses have been also performed trying to characterize the nature of the γ-ray sources by their γ-ray spectral properties and temporal behaviors (see, e.g., Ackermann et al. 2012; Doert & Errando 2014).

In addition, 20% of the sources above 100 MeV in the Fermi-LAT Third Source Catalog, (3FGL; Acero et al. 2015) are listed as blazar candidates of uncertain type (BCUs). They present flat radio spectra and/or X-ray counterparts and have a multifrequency behavior similar to blazars, as defined according to the 3FGL and the Third Catalog of Active Galactic Nuclei Detected by the Fermi-LAT (Ackermann et al. 2015).

Definitive confirmation of the blazar-like nature of candidates selected as potential counterparts of UGSs comes from optical spectroscopy (see Massaro et al. 2014 for more details). In 2012 we started an optical spectroscopic campaign to follow up and confirm the nature of the blazar-like sources selected on the basis of their IR colors or their low-frequency (i.e., below ∼1 GHz) flat radio spectrum (Massaro et al. 2013b, 2013c) to confirm their nature. Using the results achieved to date, the fraction of UGSs listed in the First Fermi-LAT source catalog (1FGL; Abdo et al. 2010) and the Second Fermi-LAT source catalog (2FGL; Nolan et al. 2012) is decreased by ∼10%–20%.

Sources observed during our campaign were classified as BL Lac objects, labeled as BZBs according to the nomenclature of the Roma-BZCAT catalog (Massaro et al. 2009), whenever the optical spectrum is featureless or shows only optical emission/absorption features with equivalent widths smaller than 5 Å (Stickel et al. 1991), or as blazars of quasar type (i.e., BZQs) if they have a typical quasar-like optical spectrum with broad emission lines. The members of this last class are also known as flat-spectrum radio quasars (FSRQs) since they present the same optical spectra as quasars and a flat radio spectra. We also considered sources classified according to the latest release of the Roma-BZCAT (Massaro et al. 2015a); in this catalog, BL Lacs exhibiting optical spectra of a typical elliptical galaxy (nonthermal continuum with a low Ca H&K break contrast) are classified as BZGs that stands for BL Lac of galaxy type (Marchã et al. 1996; Landt et al. 2002).

In this sixth paper of the series we present 30 additional optical spectra collected during observational gaps in different programs or in service mode. The data were collected at Telescopio Nazionale Galileo (TNG) in La Palma (Spain), William Herschel Telescope (WHT), Observatorio Astronómico Nacional (OAN) in San Pedro Mártir (Mexico) for the Northern Hemisphere, and at the Southern Astrophysical Research Telescope (SOAR) and at the Magellan Telescopes at the Carnegie Observatories for the Southern Hemisphere. Results from our exploratory program were presented in Paggi et al. (2014); while those results for observations carried out in 2013 with SOAR and KPNO, together with additional OAN spectra, are discussed in Landoni et al. (2015), Massaro et al. (2015b, 2015c), and Ricci et al. (2015) respectively, and a release of ∼25 γ-ray BCUs has been recently presented in Álvarez-Crespo et al. (2016).

The paper is organized as follows: Section 2 describes the sample analyzed. Section 3 is dedicated to the data reduction procedures, while Section 4 details the results of our spectral analysis. Finally, the summary and conclusions are presented in Section 5. We use cgs units unless otherwise stated. Spectral indices, α, are defined by flux density ${S}_{\nu }\propto {\nu }^{-\alpha }$, and flat spectra are considered for sources with $\alpha \lt 0.5$.

The aim of our campaign is to perform spectroscopic observations of a large number of γ-ray blazar candidates selected on the basis of their characteristic IR colors (Massaro et al. 2011). The observing strategy, successfully applied in recent years (see, e.g., Álvarez-Crespo et al. 2016; Ricci et al. 2015), is to request small subsamples of our main list from different telescopes to minimize the impact on their individual schedules. More details on the observing strategy and a summary of the observations performed on the γ-ray blazar candidates found for the 2FGL catalog will be presented in a forthcoming paper (R. D'Abrusco et al. 2016, in preparation).

Our present sample lists 30 sources; a large fraction of them (i.e., 23 out of 30) lie within the positional uncertainty region of the corresponding Fermi source listed in the 3FGL, while four of the sources that do not appear in the 3FGL are included in the 2FGL (Nolan et al. 2012), one is listed in the first catalog release 1FGL (Abdo et al. 2010), and the remaining two are reported in the First Fermi-LAT Catalog of Sources above 10 GeV (1FHL; Ackermann et al. 2013). All their pointed counterparts lie within the Fermi position uncertainty at 95% level of confidence.

The sample selection was mainly driven by source visibility during the nights obtained at each telescope. We chose our targets considering the optimal conditions of visibility and airmass lower than 1.5. Our sample of 30 sources was selected as follows:

• 1.  
  Nine targets belong to the WISE Blazar-like Radio-Loud Source (WIBRaLS; D'Abrusco et al. 2014). This is a catalog of radio-loud candidate γ-ray-emitting blazars with WISE mid-infrared colors similar to the colors of confirmed γ-ray blazars. Details of the sources are reported in Table 1.
• 2.  
  Seven objects are included in Massaro et al. (2015b), which reports refined associations of the 1FGL and 2FGL catalogs based on multifrequency properties.
• 3.  
  Twelve targets are selected because they are radio sources lying within the positional uncertainty region of UGSs. This is because all the Fermi blazars associated up to date have a radio counterpart, as highlighted by the radio–gamma-ray connection (Abdo et al. 2010; Ghirlanda et al. 2010; Mahony et al. 2010). To search for potential blazar-like objects, these radio sources are the primary candidates for optical spectroscopic observations.

In addition to the above selection, we also pointed at serendipitous objects based on opportunity. The reasons for the selection of these targets are described in the following list:

• 1.  
  3FGL J0721.5–0221 was selected for being an active galaxy of uncertain type (AGU) in the 2FGL.
• 2.  
  The bright flat-spectrum radio source, PMN J1038–5311, was proposed as the counterpart of the LAT-detected flaring source 3FGL J1038.9–5311 in 2012 March by Ciprini et al. (2012). The first appearance of the γ-ray source in a Fermi-LAT catalog is in the 3FGL, associated with the PMN source.
• 3.  
  3FGL J1221.5–0632 is unassociated in all existing Fermi-LAT catalogs. We observed the optical source, USNO B1.0 0835–0232187, coincident with the brightest X-ray source found within the 1FGL J1221.4–0635 and 2FGL J1221.4–0633 error circles (XRT Src#1 in Takeuchi et al. 2013). Interestingly, Takeuchi et al. (2013) found no radio emission from the optical/X-ray source in the FIRST survey data.

It is worth noting that the γ-ray class of BCUs in the 3FGL corresponds to the old class AGU in both the 1FGL and 2FGL catalogs. Gamma-ray information and classification are shown in Table 1, while in Table 2 we report the log of the observations together with results of our analysis and the multifrequency notes collected for each source. We note that since all but one of our sources have an IR counterpart in the WISE catalog (Wright et al. 2010; Cutri et al. 2012), we decide to use the WISE name to label/identity them in both tables. For 2FGL J1803.6+2523c, the WISE counterpart is contaminated by a nearby star, so instead we report the name of its counterpart in the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006).

Here we describe the telescopes and the instruments used to carry out our spectroscopic campaign and provide the basic details of the data reduction procedures adopted.

3.1. TNG

3.2. William Herschel Telescope

3.3. Observatorio Astronómico Nacional San Pedro Mártir

3.4. Southern Astrophysical Research Telescope

3.5. Magellan Telescopes at the Carnegie Observatories

3.6. Data Reduction Procedures

We have explored several catalogs, surveys, and databases such as the NASA Extragalactic Database (NED) and SIMBAD Astronomical Database searching for the multifrequency information of the sources listed in our sample. All the multifrequency notes collected in our analysis are listed in Table 2, and surveys and catalogs are listed in Table 3. There is also a note when the spectral energy distribution (SED) of a source is presented in Takeuchi et al. (2013) and when the radio counterpart has a flat radio spectrum (marked as "rf"). For the XMM-Newton, Chandra, and Swift catalogs we decided to use the same symbol to indicate whether the blazar candidate has an X-ray counterpart because these X-ray observatories performed only pointed observations that could have been done serendipitously in the field of the Fermi source. In the following we provide the description of the results divided in subsections according to the classification in the Fermi catalogs. The spectra of the individual targets, together with their finding charts, are shown in the figure set of Figure 1.

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Tarball of figure set images

4.1. Unidentified γ-ray Sources

4.2. Blazar Candidates of Uncertain Type

4.3. Reobservations of Fermi BL Lac Objects

Our main goal was to confirm the nature of the counterparts for the UGSs and BCUs selected on the basis of their low radio frequency spectra and their peculiar IR colors (Massaro et al. 2012b, 2013d).

In addition, we reobserved two sources already classified as BL Lacs at uncertain redshift in the literature because of the optimal conditions at the moment of the observation for these sources. We aim to find a variation of the continuum emission that enables us to estimate their redshifts.

The total number of sources presented is 30, and the results can be summarized as follows:

• 1.  
  In the UGS subsample all the sources have a blazar nature. Five of them are BL Lacs whose redshifts could not be determined owing to their featureless continuum, while three others (even though they have a BL Lac nature) have features in their optical spectra that led to the possibility of determining their redshifts. These sources are WISE J003020.44–164713.1, which was detected at z = 0.237, WISE J004348.66–111607.2 at z = 0.264, and WISE J170409.58+123421.7 at z = 0.45. The source WISE J154824.38+145702.8 is a BZG, dominated by absorption from the host galaxy, and we were able to detect absorption lines in the optical spectrum leading to a redshift measurement of z = 0.23. There is a QSO, WISE J122127.20–062847.8, at z = 0.44. Since there is no radio emission for this optical source, it is possible that this QSO is not the real counterpart.
• 2.  
  We analyzed the spectra of 18 sources classified as BCUs. We found that six of them are BL Lacs, and their featureless spectra made it impossible to measure their redshifts. Even though the source WISE J010345.74+132345.3 was found to be a BL Lac dominated by nonthermal emission, some spectral features enabled us to determine its redshift of z = 0.49. The same situation was found for the source WISE J203649.49–332830.7, a BL Lac dominated by nonthermal emission at z = 0.237. The emissions of the sources WISE J091714.61–034314.2 at z = 0.308 and WISE J104204.30–055816.5 at z = 0.39 were both dominated by the host galaxy rather than by the jet continuum. Finally, because of the radio and optical data available, eight of them were classified as FSRQs, and we measured their redshifts: WISE J033223.25–111950.6 was determined to be at z = 0.2074, WISE J050818.99–193555.7 at z = 1.88, WISE J061822.65–242637.7 at z = 0.2995, WISE J070001.49+170921.9 at z = 1.08, WISE J103840.66–531142.9 at z = 1.45, WISE J133120.35–132605.7 at z = 0.25, WISE 162444.79+110959.3 at z = 2.1, and 2MASS J18031240+2521185 at z = 0.77.
• 3.  
  We reobserved the spectra of two sources previously characterized as BL Lacs and confirmed this classification for both of them. The spectra of the source WISE J010509.19+392815.1 observed by Shaw et al. (2013) showed emission dominated by the host galaxy rather than by nonthermal emission. In our observations, instead, the spectrum is dominated by continuum emission arising from the jet; it might be a transition object.

We thank the anonymous referee for useful comments that led to improvements in the paper. We are grateful to Dr. Mendez Alvarez, Dr. Dominguez, Dr. Karjalainen, Dr. Riddick, Dr. Skillen at WHT, and Dr. Boschin at TNG for their help in scheduling, preparing, and performing the observations. This investigation is supported by NASA grants NNX12AO97G, NNX13AP20G, and NNX14AJ61G. H.A.S. acknowledges partial support from NASA/JPL grant RSA 1369566. The work by G.T. is supported by the ASI/INAF contract I/005/12/0. V.C. and V.P.-A. are supported by the CONACyT research grant 151494 (Mexico). Work by C.C.C. at NRL is supported in part by NASA DPR S-15633-Y. J.S. is supported by a Packard Foundation Fellowship.

Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) åda República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). Part of this work is based on archival data, software, or online services provided by the ASI Science Data Center. This research has made use of data obtained from the high-energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA's Goddard Space Flight Center; the SIMBAD database operated at CDS, Strasbourg, France; and the NASA/IPAC Extragalactic Database (NED), operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Part of this work is based on the NVSS (NRAO VLA Sky Survey): The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation and on the VLA low-frequency Sky Survey (VLSS). The Molonglo Observatory site manager, Duncan Campbell-Wilson, and the staff, Jeff Webb, Michael White, and John Barry, are responsible for the smooth operation of Molonglo Observatory Synthesis Telescope (MOST) and the day-to-day observing program of SUMSS. The SUMSS survey is dedicated to Michael Large, whose expertise and vision made the project possible. The MOST is operated by the School of Physics with the support of the Australian Research Council and the Science Foundation for Physics within the University of Sydney. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/). Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III Web site is http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration, including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University. The WENSS project was a collaboration between the Netherlands Foundation for Research in Astronomy and the Leiden Observatory. We acknowledge the WENSS team, which consisted of Ger de Bruyn, Yuan Tang, Roeland Rengelink, George Miley, Huub Rottgering, Malcolm Bremer, Martin Bremer, Wim Brouw, Ernst Raimond, and David Fullagar, for the extensive work aimed at producing the WENSS catalog, and TOPCAT¹⁶ (Taylor 2005) for the preparation and manipulation of the tabular data and the images. The Aladin Java applet¹⁷ was used to create the finding charts reported in this paper (Bonnarel et al. 2000). It can be started from the CDS (Strasbourg—France), from the CFA (Harvard—USA), from the ADAC (Tokyo—Japan), from the IUCAA (Pune—India), from the UKADC (Cambridge—UK), or from the CADC (Victoria—Canada).

The Molonglo Observatory site manager, Duncan Campbell-Wilson, and the staff, Jeff Webb, Michael White and John Barry, are responsible for the smooth operation of Molonglo Observatory Synthesis Telescope (MOST) and the day-to-day observing programme of SUMSS. The SUMSS survey is dedicated to Michael Large whose expertise and vision made the project possible. The MOST is operated by the School of Physics with the support of the Australian Research Council and the Science Foundation for Physics within the University of Sydney.