MULTIWAVELENGTH OBSERVATIONS OF THE VERY HIGH ENERGY BLAZAR 1ES 2344+514

The majority of blazars detected at very high energy (VHE, E > 100 GeV) γ-rays are high-frequency-peaked BL Lac objects (HBLs), with currently ∼30 HBLs from a total of ∼40 VHE blazars.²⁵ The short timescale variability seen in the broadband spectral energy distributions (SEDs) of blazars is explained by highly relativistic plasma jets oriented close to the line of sight (Blandford & Königl 1979). HBLs are blazars exhibiting synchrotron radiation peaking typically at UV to X-ray energies and a second SED component peaking at GeV to TeV energies. Leptonic models (Coppi 1992; Böttcher & Chang 2002; Krawczynski et al. 2004) describe the high-energy (HE) peak as inverse Compton upscattering of low-energy photons by electrons, while hadronic models attribute the emission to proton-induced cascades, synchrotron radiation by protons, pion production in dense clumps of jet plasma, or curvature radiation (Mannheim 1993; Aharonian 2000; Mücke & Protheroe 2001). Detailed multiwavelength studies from optical to γ-ray energies of the temporal and spectral variability of HBLs promise to constrain the physical parameters of the underlying particle distributions, particularly the Doppler factor and magnetic field strength (Tavecchio et al. 1998).

The HBL 1ES 2344+514 was first detected in the Einstein Slew Survey at X-ray energies (0.2–4 keV) and has a redshift of z = 0.044 (Elvis et al. 1992; Perlman et al. 1996). VHE γ-ray emission from 1ES 2344+514 was discovered by the Whipple 10 m telescope in energies >350 GeV at a 6.0σ detection level during a 1 day flare on 1995 December 20 (Catanese et al. 1998). X-ray observations of 1ES 2344+514 with BeppoSAX revealed variability in the 2–10 keV flux by a factor of ∼2 during a 7 day period in 1996 December (Giommi et al. 2000). 1ES 2344+514 was later observed by Swift in 2005 during April, May, and December at a lower flux than in 1996 December (Tramacere et al. 2007). The X-ray spectrum is reasonably well described by an absorbed power law, with photon indices ranging from Γ ≈ 1.8 to 2.3. Marginal evidence for optical variability in 1ES 2344+514 is seen from the sparse data set of two observations in 1998 (Nilsson et al. 1999; Falomo & Kotilainen 1999), six nights in 2000 (Xie et al. 2002), and one observation on 2007 January 12 (Gupta et al. 2008). In the radio band, Very Large Array observations reveal an unusual morphology at arcsec scales for a blazar of two compact high brightness emission regions connected via a diffuse halo, more reminiscent of a radio galaxy (Rector et al. 2003; Giroletti et al. 2004). High-resolution Very Long Baseline Array observations show a well-collimated jet extending ∼10 pc from the core, which then bends 25° and broadens into a cone of ∼35° opening angle. In HE (0.1 > E < 300 GeV) γ-rays, 1ES 2344+514 is associated with the source 1FGL J2347.1+5142 from the Fermi LAT First Source Catalog (Abdo et al. 2010), detected at a significance of 10.6σ and showing a hard HE γ-ray spectrum of 1.57 ± 0.17 with no indication of HE γ-ray flux variability during the period of 2008 August to 2009 July.

Evidence for long-term VHE γ-ray flux variability in 1ES 2344+514 is seen in observations between 1995 and 2005 by the Whipple 10 m telescope, HEGRA, and MAGIC. The Whipple 10 m γ-ray photon spectrum on the flaring night of 1995 December 20 is best fit by a power law over the energy range 0.8–9 TeV, with a photon index Γ = 2.54 ± 0.17_stat ± 0.07_syst (Schroedter et al. 2005). The measured flux F(>1 TeV) = (3.3 ± 0.7) × 10⁻¹¹) photons cm⁻² s⁻¹ for the flare night is a factor of 1.7 ± 0.3 higher than the Crab Nebula flux. Excluding 1995 December 20, the time-averaged Whipple 10 m data between 1995 and 1997 yielded a marginal 4σ detection at a significantly lower flux level, corresponding to ∼10% of the Crab Nebula flux (Catanese et al. 1998). Further VHE γ-ray observations of 1ES 2344+514 with HEGRA in 1997, 1998, and 2002 resulted in a 4.4σ detection in a low flux level state of (3.3 ± 1.0)% of the Crab Nebula flux (Aharonian et al. 2004). Observations in 1995 with TACTIC above 1.5 TeV in 2004 and 2005 yielded a weakly constraining flux upper limit near the Whipple 10 m detection flux level (Godambe et al. 2007). In 2005 August to 2006 January, the MAGIC telescope measured a time-averaged photon spectrum between 0.14 and 5.4 TeV at a low flux of 5% of the Crab Nebula flux characterized by a power law with Γ = 2.95 ± 0.12_stat ± 0.20_syst (Albert et al. 2007). This article presents the first multiwavelength campaign on 1ES 2344+514, with contemporaneous UV, X-ray, and VHE γ-ray observations over a several month long period.

VERITAS is an array of four 12 m diameter imaging atmospheric Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in Southern Arizona (Weekes et al. 2002). Each VERITAS camera contains 499 pixels (015 diameter) and has a field of view of 35. VERITAS is sensitive over an energy range of ∼100 GeV to ∼30 TeV with an energy resolution of 15%–20% and an angular resolution (68% containment) of less than 014 per event (Acciari et al. 2008). VERITAS can detect VHE γ-ray fluxes of 5% and 1% of the Crab Nebula flux at the 5σ level in <2.5 and <50 hr, respectively.

The VERITAS observations of 1ES 2344+514 presented here were taken on 37 nights between 2007 October 4 and 2008 January 11. After applying quality-selection criteria, the total exposure is 18.1 hr live time. Data-quality selection requires clear atmospheric conditions, based on infrared sky temperature measurements, and normal hardware operation. During each night, the quality-selected data ranged from 0.3 to 2.7 hr live time. The zenith angle of observations ranges from 19° to 48°, with a mean of 28°. All data were taken during moon-less periods in wobble mode with pointings of 05 from the blazar alternating from north, south, east, and west directions to enable simultaneous background estimation (Aharonian et al. 2001).

Data reduction followed the methods described in Acciari et al. (2008). Signals in each event are first calibrated (Holder et al. 2006), and the images are parameterized (Hillas 1985). The γ-ray direction and air shower impact parameter on the ground are then reconstructed using stereoscopic techniques (Hofmann et al. 1999; Krawczynski et al. 2006). The background of cosmic rays is rejected with a very high efficiency using event-by-event cuts on the arrival direction (θ²), mean scaled width and length, integrated charge (size), and location of the image centroids in the camera (distance). The cuts applied here are optimized a priori for a source strength of 10% of the Crab Nebula flux and a similar photon index to the Crab Nebula. The energy of each event is reconstructed using lookup tables from Monte Carlo simulations of γ-rays (Acciari et al. 2008). Results from two independent VERITAS analysis packages (Daniel et al. 2007) yield results consistent with those presented here.

A total of 1275 on-source and 4494 off-source events were measured, with an on–off normalization of α = 0.143. This corresponds to a total excess of 632 events from the direction of 1ES 2344+514. The statistical significance of this excess is 20.2 standard deviations (σ). Figure 1 shows the θ² distribution of the total data set, which is the squared angular distance between the reconstructed event direction and the nominal source position. The shape of the excess is consistent with a simulated point source for VERITAS.

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The X-ray satellite mission RXTE (Bradt et al. 1993) observed 1ES 2344+514 between 2007 October 7 and 2008 January 11 (ObsID 93132). Data are presented from Proportional Counter Unit 2 (PCU 2) of the Proportional Counter Array (PCA) instrument (Jahoda et al. 1996) since for nearly all observations the other PCUs were not in operation. The 52 nightly PCA observations were taken in snapshots ranging from 0.11 to 1.00 hr live time and are listed in Table 1. Data reduction is performed with the HEAsoft 6.5 package. Only the top layer (X1L and X1R) signal is used. The data are filtered following the standard criteria advised by the NASA Guest Observer Facility. Background data are parameterized with the pcabackest tool using the most recent model for faint sources. The photon spectrum of each observation is extracted using the saextrct tool. Response matrices are generated using pcarsp with the latest calibration files.

Swift (Gehrels et al. 2004) Target of Opportunity observations of 1ES 2344+514 from 2007 October 27 to 2008 January 1 were triggered by the VERITAS detection. The Swift data set consists of eight snapshot observations of 10–45 minute duration each, as listed in Table 1. All Swift X-ray Telescope (XRT) data (Burrows et al. 2005) are reduced using the HEAsoft 6.5 package. Event files are calibrated and cleaned following the standard filtering criteria using the xrtpipeline task and applying the most recent Swift XRT calibration files. All data were taken in photon counting (PC) mode, with grades 0–12 selected over the energy range 0.3–10 keV. Due to photon pile-up in the core of the point-spread function (PSF) at count rates >0.5 counts s⁻¹ (PC mode), the source events are extracted from an annular region with inner radius ranging from 2 to 6 pixels and an outer radius of 30 pixels (47.2 arcsec). Background counts are extracted from a 40 pixel radius circle in a source-free region. Ancillary response files are generated using the xrtmkarf task, with corrections applied for the PSF losses and CCD defects. The latest response matrix from the XRT calibration files is applied. To ensure valid χ² minimization statistics during spectral fitting, the extracted XRT energy spectra are rebinned to contain a minimum of 30 counts in each bin.

Swift Ultraviolet and Optical Telescope (UVOT) observations were taken in the photometric bands of UVW1, UVM2, and UVW2 (Poole et al. 2008). The uvotsource tool is used to extract counts, correct for coincidence losses, apply background subtraction, and calculate the source flux. The standard 5 arcsec radius source aperture is used, with a 20 arcsec background region. The source fluxes are dereddened using the interstellar extinction curve in Fitzpatrick (1999). In the UV filters only a low level of host galaxy flux is evident (Tramacere et al. 2007), so no corrections are needed. The uncertainty in PSF variations with time is fixed for this analysis at 15%. Observations were taken with just one UV filter during each pointing, and only UVM2 data were taken on multiple nights. Within the conservative errors, no variability is seen between the four UVM2 filter observations.

The VERITAS differential photon spectrum over the energy range of 390 GeV to 8.3 TeV for the time-averaged data excluding a strong flaring night on 2007 December 7 is shown in Figure 2. A power-law model fit to the data is best described by the form dN/dE = F_○ · (E/1 TeV)^−Γ yielding a χ²/dof of 8.39/4 for a flux normalization constant of F_○ = (3.38 ± 0.23_stat ± 0.70_syst) × 10⁻¹² photons cm⁻² s⁻¹ TeV⁻¹ and a photon index of Γ = 2.78 ± 0.09_stat ± 0.15_syst. A fit to a power law with an exponential cutoff model, which contains one additional degree of freedom, does not yield a significantly better fit. On 2007 December 7 the weather conditions were affected by moving cloud coverage, so the observations do not pass the standard data-quality criteria. However, online analysis showed that the source was in the most active state yet observed. We therefore include these data and increase the systematic uncertainties in flux and spectral shape. Conservative systematic uncertainties in the flux of 30% and photon index of 0.2 are determined, based on the results of observations of the Crab Nebula taken in similarly variable conditions. The VHE γ-ray spectrum from the high-flux night is well described by a power law (χ²/dof of 1.27/3) with a slightly harder photon index Γ = 2.43 ± 0.22_stat ± 0.2_syst and a higher flux normalization of F_○ = (17.3 ± 1.92_stat ± 5.19_syst) × 10⁻¹² photons cm⁻² s⁻¹ TeV⁻¹.

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The best-fit results presented here are tested with a dedicated Monte Carlo study, which explores the uncertainty ranges for the two measured spectra. In the Monte Carlo study photons are selected from a power-law distribution matching the measured distribution convolved with the effective area as a function of energy. The selected photon energies are then smeared with a Gaussian corresponding to the energy resolution of ∼15% at these energies. The number of photons selected is matched to the observed number of photons with a Poisson scatter. Five million synthetic photon spectra are created using similar binning to the original spectra, and the best-fit power-law index and normalization parameters are extracted for each synthetic spectrum providing dense coverage of the parameter space. The Monte Carlo mean value and one-dimensional standard deviation of the power-law parameters are consistent with the best-fit parameters of the measured spectra. For the time-averaged spectrum excluding the flaring night of 2007 December 7, the Monte Carlo study yields a mean value and 1σ error for the flux normalization of F_{○, MC} = (3.35 ± 0.30) × 10⁻¹² photons cm⁻² s⁻¹ TeV⁻¹ and for the photon index of Γ_MC = 2.76 ± 0.15. For the high flux 2007 December 7 spectrum, the Monte Carlo study yields F_{○, MC} = (17.3 ± 2.67) × 10⁻¹² ph cm⁻² s⁻¹ TeV⁻¹ and Γ_MC = 2.40 ± 0.28.

Previous VHE γ-ray observations of 1ES 2344+514 with the MAGIC telescope between 2005 August and 2006 January measured a constant flux at a slightly lower level than the time-averaged (flare excluded) flux measured here, and a consistent photon index of Γ = 2.95 ± 0.12_stat. ± 0.2_syst. (Albert et al. 2007). The high-flux VHE γ-ray photon spectrum measured by the Whipple 10 m on 1995 December 20 with a photon index of Γ = 2.54 ± 0.17_stat ± 0.07_syst (Schroedter et al. 2005) is marginally harder than the low flux spectrum and agrees with the high-flux spectrum measured here. The VHE observed γ-ray spectral shape and intensity are modified by absorption from pair production interactions with the infrared component of the extragalactic background light (EBL; Gould & Schréder 1967). Using the redshift z = 0.044 for 1ES 2344+514, we calculate a de-absorbed spectrum based on the EBL model of Franceschini et al. (2008), yielding intrinsic photon indices for the VERITAS low and high-flux states of Γ_{int, low} ≈ 2.5 and Γ_{int, high} ≈ 2.1, respectively.

X-ray spectral analysis of the Swift XRT and RXTE PCA data is performed with XSPEC 12.4. An absorbed power-law model, including the phabs model for the photoelectric absorption, is fit to each spectrum. First, a joint fit of the eight Swift XRTs over the energy range 0.4–10 keV using a tied column density N_H and each spectrum having a varying photon index and normalization is compared to a joint fit with a fixed Galactic column density N_{H, Gal} of 1.50 × 10²¹ cm⁻² (Kalberla et al. 2005). The joint fit with tied N_H yields a best-fit N_H of (2.06 ± 0.13) × 10²¹ cm⁻², with a reduced χ² of 1.03 for 203 degrees of freedom (dof) and chance probability for larger chi-squared statistics (null hypothesis) of 37%. The alternative joint fit with the fixed Galactic column density N_{H, Gal} yields a higher reduced χ² of 1.13 for 204 dof and null hypothesis of 9.2%. The best-fit N_H is then used in the absorbed power-law model for the RXTE PCA spectra, which are mostly unaffected by absorption due to the 3–20 keV energy range, and for all Swift XRT results presented here. Figure 3 shows Swift XRT (0.4–10 keV) and RXTE PCA (3–20 keV) spectra from example moderate-flux and high-flux observations. The ratio in each energy bin of the data to the absorbed power-law model is shown for the example spectra, with the full list of reduced χ² values per dof for each observation listed in Table 1. The model describes well the spectra, yielding an average chance probability for larger chi-squared statistics of 37% and 51% for the Swift XRT and RXTE PCA data, respectively. The measured Swift XRT 0.4–10 keV photon indices range from Γ = 2.33 ± 0.05 to Γ = 1.97 ± 0.05 for observations differing in 2–10 keV integrated flux by a factor of 6.54 ± 0.52. These results are consistent with the BeppoSAX 0.5–10 keV photon indices of Γ ≈ 1.8–2.3 at similar flux levels for an absorbed power-law model with fixed Galactic column density (Giommi et al. 2000). Detailed results from the nightly RXTE PCA and Swift XRT spectra focusing on the connection between the significant X-ray spectral and flux variability are presented in the following section.

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The nightly VHE γ-ray and X-ray light curve of 1ES 2344+514 from VERITAS, RXTE PCA, and Swift XRT is shown in Figure 4. A strong VHE γ-ray flare is seen on 2007 December 7 (54441.12 MJD) at an integrated flux F(>300 GeV) = (6.8 ± 0.6) × 10⁻¹¹ cm⁻² s⁻¹, corresponding to 48% of the Crab Nebula flux. The measured increase in flux of a factor of 1.9 ± 0.5 between the previous night and the flare night shows the first clear evidence of ∼day timescale VHE γ-ray variability from 1ES 2344+514 since the Whipple 10 m discovery of VHE γ-ray emission in 1995 (Catanese et al. 1998). Excluding the 2007 December 7 flaring event, the average F(E>300 GeV) is (1.1 ± 0.1) × 10⁻¹¹ photons cm⁻² s⁻¹, corresponding to 7.6% of the Crab Nebula flux. A fit to a constant flux excluding the flaring night is rejected with a chance probability of 7.2 × 10⁻⁸, indicating significant low-level variability. A measure of the integrated level of flux variability is the fractional root-mean-square variability amplitude F_var (Vaughan et al. 2003). For the full VERITAS data set of nightly exposures a high level of variability F_var = (75 ± 10)% is implied. Excluding the flare night, a still significant F_var = (34 ± 16)% is determined. Searches for short-term flux variations within each of the nightly observations showed no significant variability.

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Figure 4 (lower panels) shows the 2–10 keV flux and photon index Γ measured over 3–20 keV from RXTE PCA and 0.4–10 keV from Swift XRT data. The X-ray flux is shown to be highly variable throughout the campaign, with F_var = (51 ± 1)%. In 2007 December, large amplitude flaring is evident with flux doubling timescales of ∼1 day. A 2–10 keV flux of (6.3 ± 0.3) × 10⁻¹¹ erg cm⁻² s⁻¹ is seen from the Swift XRT data on 2007 December 8, representing the highest X-ray flux ever measured for 1ES 2344+514. Analysis of all subsequent X-ray data of 1ES 2344+514, which currently consists solely of Swift XRT observations, shows a 2–10 keV flux consistent with the lowest flux measurements presented here. Detailed results from the more recent Swift XRT data set are beyond the scope of this publication, but automated count rate light curves are publicly available.²⁶

X-ray spectral variability between flaring nights is investigated by plotting the X-ray photon index Γ versus the 2–10 keV flux, shown in Figure 5. A logarithmic correlation of decreasing photon index with increasing flux is suggested from a power-law fit to the RXTE PCA and Swift XRT data with a χ²/dof probability of 0.14 and 0.70 compared to 7.9 × 10⁻²⁰ and 3.3 × 10⁻⁴ for a constant photon index, respectively. Due to the lack of significant curvature in the X-ray spectrum, the peak energy E_p for a νF(ν) SED representation is largely unconstrained. A comparison of the photon indices from the Swift XRT and RXTE PCA spectra restricted to the overlapping energy range of 3–10 keV is limited by the low number of energy bins above 3 keV, so a study of the systematically higher RXTE PCA (3–20 keV) photon indices compared with the Swift XRT (0.4–10 keV) photon indices at similar flux levels is inconclusive. Furthermore, the lack of purely simultaneous RXTE PCA and Swift XRT data does not allow for a detailed study of joint fits from 0.4–20 keV. Even during the bright flaring events in 2007 December, the RXTE PCA spectra from 3 to 20 keV show no sign of curvature, with measured photon indices of ∼1.9. For these high-flux states the implied peak energy E_p ⩽ 10 keV agrees with the estimated peak energies derived from BeppoSAX observations in 1998 (Giommi et al. 2000).

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Figure 6 shows the VERITAS flux F(>300 GeV) versus RXTE PCA and Swift XRT 2–10 keV fluxes for nights with observations in both energy bands. A Pearson coefficient of r = 0.60 ± 0.11 is calculated for the VHE γ-ray to X-ray flux points, suggestive of correlated variability. The best-fit linear model yields a slope of 0.56 ± 0.08 with a fit probability of 0.14. For the highest measured X-ray fluxes in 2007 December the weather conditions at the VERITAS site were poor, which excludes these nightly fluxes from Figure 6. Due to the sparse data sampling, a search for lags between the VHE γ-ray and X-ray emission is not investigated here.

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The broadband SED of HBLs is often modeled within a synchrotron self-Compton (SSC) framework. Simple one-zone SSC models typically predict correlated X-ray and VHE γ-ray variability from the underlying electron distribution. Figure 7 shows a one-zone SSC model calculation (Krawczynski et al. 2004) to the SED of 1ES 2344+514 for illustration purposes in two flux states. A high-flux SED is constructed from the VERITAS photon spectrum in 2007 December 7 and Swift UVOT and XRT data on 2007 December 8. The time-averaged (flare excluded) VERITAS data are combined with Swift and RXTE PCA spectra on 2008 January 1 when the object was at a moderate X-ray flux state with respect to the full campaign. Shown also in Figure 7 are the non-contemporaneous MAGIC data from 2005 in a low-flux state and the flaring Whipple 10 m data from 1995 December 20. Also shown for reference is the Fermi LAT spectrum from 2008 August to 2009 July in a non-variable low-flux state. Each model curve for the broadband data contemporaneous with the VERITAS data represents the intrinsic source radiation, and all VHE γ-ray spectra are corrected for absorption by the EBL (Franceschini et al. 2008). The SSC model input parameters for the low (high, when different) states are a Doppler factor δ = 13 (20), magnetic field = 0.09 G (0.03 G), emission radius = 10¹⁶ cm, electron density = 0.05 ergs cm⁻³, and a broken power-law electron spectrum with low energies between 10⁸ eV and 10^11.3 eV (10^11.4 eV) with spectral index n₁ = 2.5 (2.3), and above the cutoff energy with index n₁ = 3.2 and highest energy of 10¹² eV.

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The light crossing time τ = R/(c × δ) ≈ 5 hr for the high-flux state with a Doppler factor δ = 20 is consistent with the ∼hour timescale X-ray variability seen from 1ES 2344+514 with BeppoSAX on 1996 December 7 (Giommi et al. 2000). Further hourly X-ray variability in 1ES 2344+514 could not be confirmed in this work due to the short (<hour) exposures with Swift and RXTE and moderate event statistics. In addition to the low- and high-flux state modeling from data in 2007 October to 2008 January, the same SSC model was applied to non-contemporaneous low- and high-flux states derived from BeppoSAX, MAGIC, and Whipple 10 m spectra (Albert et al. 2007). A reasonable agreement between model parameters, such as Doppler factors ∼10–20 and a magnetic field ∼0.03–0.3 G, is evident for both the low- and high-flux states with the archival 1ES 2344+514 data, and compared with one-zone SSC modeling of similar moderate-flux level flares in the HBLs 1ES 1959+650 (Tagliaferri et al. 2008), 1ES 0806+524 (Acciari et al. 2009), and 1ES 1101-213 (Aharonian et al. 2007). A detailed study of the SSC model parameter space (e.g., Tavecchio et al. 1998) is not pursued here due to sparseness in the SED coverage and degeneracy between model parameters. The simple one-zone SSC modeling presented here adequately describes the data, however, further optical, X-ray, and VHE γ-ray observations of 1ES 2344+514 simultaneous with Fermi LAT promise to offer stronger constraints to emission models.

In this paper, a ∼4 month long VHE γ-ray and X-ray observing campaign of 1ES 2344+514 revealed significant flux variability on ∼1 day timescales. In particular, flux doubling between nights is evident in both the X-ray and VHE γ-ray bands in 2007 December, with the highest ever X-ray flux measured from 1ES 2344+514 on 2007 December 8. Evidence of correlated nightly X-ray flux and VHE γ-ray flux variability is shown, as generally found in the well-studied HBLs Mrk 501 (Krawczynski et al. 2000), Mrk 421 (Fossati et al. 2008), and 1ES 1959+650 (Krawczynski et al. 2004), however, so-called orphan flares of increased flux in one energy band and not the other have also been observed (e.g., (Błażejowski et al. 2005; Krawczynski et al. 2004)). For these campaigns sparse temporal sampling and insufficient VHE sensitivity to short timescale variability limited any statements on a linear or quadratic relationship between VHE and X-ray flux expected under various SSC scenarios during rising or decaying flares (Fossati et al. 2008). The 1ES 2344+514 data presented here represent the first contemporaneous multiwavelength campaign of the object from UV to VHE γ-rays, but are similarly restricted by sparse temporal sampling and broadband coverage in the SEDs. Applying a simple one-zone SSC model reasonably describes the data, and is consistent with past modeling of the object and other HBLs. Further optical, X-ray, and VHE γ-ray observations of 1ES 2344+514 simultaneous with Fermi LAT in the HE γ-ray band promise to offer stronger constraints to emission models and high sensitivity broadband measurements of high-flux state flaring events.

This research is supported by grants from the U.S. Department of Energy, the U.S. National Science Foundation, and the Smithsonian Institution, by NSERC in Canada, by STFC in the UK and by Science Foundation Ireland. Special acknowledgment to the Swift and RXTE teams for the support of these observations.