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Highlights of the VERITAS Blazar Program

The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona began full-scale operations in 2007, and is one of the world's most-sensitive detectors of astrophysical VHE (E>100 GeV) gamma rays. Approximately 50 blazars are known to emit VHE photons, and observations of blazars are a major focus of the VERITAS Collaboration. Nearly 2000 hours have been devoted to this program and ~130 blazars have already been observed with the array, in most cases with the deepest-ever VHE exposure. These observations have resulted in 21 detections, including 10 VHE discoveries. Recent highlights of the VERITAS blazar observation program, and the collaboration's long-term blazar observation strategy, are presented.

An upgrade of VERITAS was completed in 2012. The new energy threshold is expected to be ∼60 GeV with an overall sensitivity increase of ∼20%.
The VERITAS collaboration hopes to improve the understanding of VHE blazars and their related science by expanding the known population and by making precision measurements of their spectra and variability patterns. Contemporaneous multi-wavelength (MWL) observations are a key component of this program since blazars are highlyvariable at all wavelengths. Modeling of the resulting SEDs and searches for MWL correlations are used to provide insight into origin of the observed emission.
Since September 2007, VERITAS has observed 128 blazars, for a total of 1981 h (annual average of ∼400 h) during good weather conditions. Until 2010, ∼80% of the VERITAS blazar data were used for VHE discovery observations and follow-up studies of new sources. In 2010, the focus became deep studies of known VHE sources, and ∼60% of the data are now devoted to known sources. Target-of-Opportunity (ToO) observations are a key part of the blazar program and typically average 50 h per year. The 21 blazars detected by VERITAS are shown in Table 1; see [2,3] for more details. the Crab Nebula flux, and the flare exponentially decayed in τ = 13 ± 4 minutes. Despite being the brightest VHE flux ever observed from the object, the observed photon index (Γ = 3.6 ± 0.4) is consistent with that measured by MAGIC during a lower-flux (3% Crab Nebula) flare in 2006. More details on the VERITAS and other MWL observations of this exceptional flare can be found in these proceedings [5].
The comparatively distant (z = 0.287) HBL 1ES 0414+009 is a known VHE emitter and was observed with VERITAS for 56 h of quality-selected live time between 2008 and 2011. These data resulted in the significant detection of a source with flux ∼2% of the Crab Nebula flux [6]. The observed spectrum is soft Γ = 3.4 ± 0.5, largely due to the effects of EBL absorption. Although an HBL, its SED (see Figure 1) is not well fit by one-zone leptonic models; a lepto-hadronic model provides a better fit.
Since September 2007, ∼1260 h of VERITAS data were devoted to the discovery and follow-up observations of more than 100 blazars at VHE. The candidates observed are largely HBLs, but also include IBLs, LBLs, FSRQs, and Fermi-LAT sources likely associated with blazars. From 2007-09, the discovery program [8] focused on nearby (redshift, z < 0.3) X-ray-bright HBLs, IBLs, and FSRQs recommended in the literature and nearby EGRET-detected blazars. Some of the X-ray-bright targets were objects that met the selection criteria applied in the literature, but were first reported in later catalogs. Following the release of the first Fermi-LAT catalog of MeV-GeV-bright blazars in 2009 [9], the discovery program has focused on objects detected by Fermi-LAT [10].
These observations resulted in the VERITAS discovery of VHE γ-ray emission from 6 HBLs and the first 4 IBLs known to emit VHE photons [2]. The initial observations of 60% of these new VHE sources were motivated by the X-ray/EGRET-based selection criteria, and all these objects are detected by Fermi-LAT. In general, these new sources have soft VHE spectra (Γ avg = 3.5), and only three exhibit strong VHE flux variability. The VHE flux upper limits derived from the unsuccesful discovery observations are typically less than 2% of the Crab Nebula flux and often the strictest yet.
The successful VHE discovery observations [11] of RX J0648.7+1516 were motivated by the identification of a cluster of E > 10 GeV photons in the Fermi-LAT data. A total of 19 h of quality-selected live time was taken in March-April 2010. Significant VHE emission (Γ = 4.4 ± 0.8, ∼3% Crab Nebula flux) was detected at the position of RX J0648.7+1516, an unidentified radio, optical and X-ray emitter. Follow-up optical spectroscopy of this object yielded a continuum-dominated spectrum typical of BL Lac objects and weak absorption lines compatible with z = 0.179. The SED of this blazar indicates it is an HBL, but it is not well fit by an SSC model. RBS 0413 is a bright HBL, at z = 0.190, that was initially targeted based on its inclusion in the Sedentary Survey [12]. It was observed for ∼26 h of quality-selected live time between 2008 and 2010 [7]. The data yielded the detection of a soft-spectrum (Γ = 3.2 ± 0.7) source with ∼1% Crab Nebula flux. Its SED, shown in Figure 1, is again not well described by a SSC model. The modeling favors the inclusion of an additional external-Compton (EC) component, and can also be fit by a lepto-hadronic scenario.

THE LONG-TERM BLAZAR OBSERVATION STRATEGY
In 2010, VERITAS began a systematic campaign to acquire deep exposures on 14 VHE blazars. Much of these data are acquired via regular monitoring exposures, and data taking is intensified during any flaring behavior. Regular simultaneous MWL observations are also organized. The 14 selected blazars are indicated in Table 1, and include: • 6 comparatively distant, hard-VHE-spectrum HBLs for EBL and IGMF studies. • 4 bright HBLs to easily enable MWL studies with high VHE statistics. These may also have the highest likelihood for exhibiting greater than Crab Nebula flux flares. • 4 non-HBLs to provide insights into the blazar sequence, to discover their low VHE states, and to study future VHE flares. The total targeted exposure on these 14 VHE blazars is ∼220 h per year. A discovery program will also continue, with a goal of ∼100 h per year. It will focus on Fermi-LAT selected objects but will have a larger emphasis on higher-risk / higher reward targets that may require deep exposures. An active ToO program on both known VHE sources and VHE discovery candidates will also be maintained. The triggers will be based on optical, X-ray, MeV-GeV gamma-ray and TeV gamma-ray flares. The VERITAS collaboration hopes to increase the amount of blazar ToO data to ∼100 h per year.