The VERITAS Blazar Key Science ProjectWystan Benbow1 for the VERITAS collaboration2

1 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA; wbenbow@cfa.harvard.edu2 See R.A. Ong et al. (these proceedings) or http//veritas.sao.edu/conferences/authors?icrc2009

Introduction:The VERITAS array of four 12-m diameter atmospheric-Cherenkov telescopes in southern Arizona is used to study very high energy (VHE; E>100 GeV) γ-ray emission from astrophysical objects. One of the VERITAS collaborationʼs Key Science Projects (KSP) is the study of blazars. These active galactic nuclei are the most numerous class of identified VHE sources, with ~25 known to emit VHE photons. More than 60 blazars have been observed with the VERITAS array since 2007, in most cases with the deepest-ever VHE exposure. These observations have resulted in the detection of VHE γ-rays from 11 blazars, including 5 for the first time at these energies. Here the blazar KSP is summarized and flux upper limits from 49 blazars observed (6.2 h average exposure) in a discovery program are presented.

The Key Science Project:VERITAS observes for ~750 h and ~250 h each year during periods of astronomical darkness and partial moonlight, respectively. The moonlight observations are almost exclusively used for a blazar discovery program, and a large fraction of the dark time is used for the blazar KSP, which consists of:•A VHE blazar discovery program (~200 h / yr): Each year ~10 targets are selected to receive ~10 h of observations each (200 min. guaranteed) during astronomical darkness. These data are supplemented by discovery observations during periods of partial moonlight. Since 2007, VERITAS has discovered VHE emission from 5 blazars (Please see J. Perkinsʼ talk in OG 2.3).•A target-of-opportunity (ToO) observation program (~40 h / yr): VERITAS blazar observations can be triggered by either a VERITAS blazar discovery, a flaring alert (>2 Crab) from the VHE blazar monitoring program of the Whipple 10-m telescope, or a lower-energy bazar flaring alert (optical, X-ray or Fermi-LAT). Should the guaranteed allocation be exhausted, further time can be requested from a pool of directorʼs discretionary time. Please see the Perkins & Grube OG 2.3 talks.•Multi-wavelength (MWL) studies of VHE blazars (~30 h / yr + ToO): Each year one blazar receives a deep VERITAS exposure in a pre-planned campaign of extensive, simultaneous MWL (X-ray, optical, radio) measurements. ToO observation proposals for similar measurements are also submitted each year to lower-energy observatories (e.g. Swift & RXTE). These proposals are triggered by a VERITAS discovery or a Whipple 10-m flaring alert. Please see J. Grubeʼs OG 2.3 talk.•Distant VHE blazar studies to constrain the extragalactic background light (EBL): Here distant targets are given a higher priority in the blazar discovery program as well as for the MWL observations of known VHE blazars with hard VHE spectra. Please see A. Imranʼs OG 2.3 talk.

HBL t[h]

<θ>[º]

Eth[GeV]

F (>Eth)[10-12 cm-2 s-1]

F(>Eth)[Crab %] z Notes

RBS 0042* 4.9 11 170 < 9.5 < 3.1 0.100 SS1ES 0033+595 20.2 31 240 < 4.5 < 2.4 0.086 CG, P

RGB J0110+418 2.1 15 170 < 14 < 4.7 0.096 P1ES 0120+340 2.8 13 170 < 14 < 4.7 0.272 CG, SS

RGB J0214+517 3.0 22 200 < 9.8 < 4.0 0.049 CG, PRBS 0298* 3.0 14 170 < 6.1 < 2.0 0.289 SSRBS 0319* 0.3 31 240 < 8.5 < 4.5 0.457 SSRBS 0413 8.0 18 170 < 6.5 < 2.1 0.190 SS

1ES 0414+009 8.6 32 240 < 4.5 < 2.4 0.287 CG, SS1RXS J044127.8+150455* 10.1 21 180 < 3.2 < 1.1 0.109 SS

1ES 0647+250 15.8 18 170 < 3.8 < 1.2 0.203 CGRGB J0656+426 9.4 16 170 < 6.1 < 2.0 0.059 PRGB J0847+115 7.1 24 200 < 3.0 < 1.2 0.199 SS1ES 0927+500 10.4 22 190 < 7.0 < 2.7 0.187 R, SS1ES 1028+511 10.9 25 200 < 6.3 < 2.6 0.360 CG, R, SS

RBS 0921* 2.1 30 220 < 6.7 < 3.2 0.236 SSRX J1117.1+2014* 1.8 14 160 < 13 < 3.8 0.139 CG, SS, Optical ToORX J1136.5+6737 4.4 36 300 < 7.7 < 5.7 0.134 CG, R, SS

1ES 1239+069 1.9 26 210 < 4.6 < 2.0 0.150 SDS1ES 1255+244 15.1 15 170 < 5.0 < 1.6 0.141 SDS, SS

RGB J1341+399 0.9 32 250 < 10 < 5.8 0.163RGB J1417+257 8.8 18 170 < 3.6 < 1.2 0.237 CG, R, SSRGB J1532+302 6.0 18 170 < 3.8 < 1.2 0.064 P

RGB J1610+671B 1.2 37 300 < 16 < 11 0.067 P1ES 1727+502 2.9 22 190 < 5.2 < 2.0 0.055 P, SDS1ES 1741+196 2.3 13 160 < 11 < 3.2 0.084 CG

RGB J2322+346 2.3 7 160 < 10 < 3.1 0.098 P

IBL1ES 0446+449** 5.6 24 200 < 5.5 < 2.3 0.203 SDS

RGB J0643+422*** 1.2 23 200 < 16 < 6.5 0.080OJ 287*** 6.5 18 170 < 7.1 < 2.3 0.306 CG, EGRET

RGB J1053+494 5.6 22 190 < 6.3 < 2.4 0.140 Fermi1ES 1215+303 28.7 12 170 < 3.5 < 1.1 0.13? CG, Fermi

RX J1326.2+2933*** 3.6 17 180 < 2.1 < 0.7 0.431 Co, R1ES 1440+122 18.9 22 190 < 4.2 < 1.6 0.162 CG, R, SDS

RGB J1725+118 9.9 23 190 < 4.1 < 1.6 0.018 CG, P

LBLAO 0235+16 4.3 19 170 < 2.6 < 0.9 0.94 Fermi ToO

RGB J0314+247 3.0 28 210 < 4.9 < 2.1 0.054 PPKS 0829+046 2.1 30 220 < 8.0 < 3.7 0.174 EGRETPKS 1717+177 5.1 18 170 < 4.3 < 1.4 0.137 Fermi1ES 2321+419 3.3 19 180 < 9.5 < 3.4 0.059 SDS

FSRQQSO 0133+476 0.8 41 360 < 12 < 12 0.859 Optical ToO1H 0323+342 8.0 24 170 < 5.4 < 1.7 0.061 HFSRQ, P

Mkn 1218 5.9 13 170 < 3.4 < 1.1 0.028 HFSRQPKS 1222+21 2.7 16 170 < 7.0 < 2.3 0.432 Fermi ToO

3C 273 9.2 33 250 < 5.4 < 3.1 0.158 EGRET, FermiPKS 1510-08 3.4 42 360 < 2.5 < 2.4 0.360 Fermi ToO

1ES 1627+402 9.8 15 170 < 3.8 < 1.2 0.272 HFSRQ, PaGB6 J1700+6830 0.8 37 300 < 9.1 < 6.7 0.301 Fermi ToO

PKS 1725+045 0.3 27 210 < 8.9 < 3.9 0.296 EGRET

The Blazar Discovery Program:The ~50 blazars observed in the VERITAS discovery program are largely high-frequency-peaked BL Lac objects (HBL). However, the program also includes IBLs (intermediate-peaked) and LBLs (low-peaked), as well as flat spectrum radio quasars (FSRQs), in an attempt to increase the types of blazars known to emit VHE γ-rays. The observed targets are drawn from a ”target list” containing objects visible to the telescopes at reasonable zenith angles (−8º < δ < 72º), without a previously published VHE limit below 1.5% Crab, and with a measured redshift z < 0.3. To further the study of the EBL a few objects having a large (z > 0.3) are also included in the target list. Although many of these candidates were already observed by VHE instruments, the pre-existing flux limits are generally well above (factor of ~10) the flux sensitivity of VERITAS. The target list includes: •All nearby (z < 0.3) HBL & IBL recommended as potential VHE emitters in [1, 2, 3].•The X-ray brightest HBL (z<0.3) in the recent Sedentary [4] and ROXA [5] surveys.•Four distant (z > 0.3) BL Lac objects recommended by [1, 6].•All nearby (z < 0.3) blazars detected by EGRET [7].•All nearby (z < 0.3) blazars detected by the Fermi-LAT [8].•Several FSRQ recommended as potential VHE emitters in [2, 9].

Data Set & Analysis MethodsMore than 50 VHE blazar candidates were observed by VERITAS between September 2007 and June 2009. Excluding the 2 HBL and 3 LBL discovered as VHE emitters by VERITAS [10, 11, 12, 13, 14], a total of 49 candidates have some exposure surviving data-quality selection. The total exposure on all these candidates is ~305 h live time, yielding an average exposure of ~6.2 h per candidate. Approximately 165 h (55%) of the total exposure is split amongst the 27 observed HBL. The remainder is divided amongst the 8 IBL (~80 h; 26%), 5 LBL (~20 h, 6%), and 9 FSRQ (~40 h, 13%). The calibration and analysis of the VERITAS data is performed with the standard analysis package [15, 16] with standard event selection criteria a-priori optimized using VERITAS Crab Nebula data for the detection of a weak (3% Crab flux) source. The presented results are verified by an independent calibration and analysis chain, and have undergone numerous checks for systematic effects in the data.

Figure (above): The distribution of the significance (standard deviations, σ), of the excess observed from each of the 49 blazars. The curve is a Gaussian distribution, with mean zero and standard deviation one, normalized to the number of observed blazars.

There are no clear indications of significant VHE γ-ray emission from any of the 49 blazars. However, the observed significance distribution is clearly skewed towards positive values. A stacking analysis performed on the entire data sample shows an overall excess of 430 γ-rays, corresponding to a statistical significance of 4.8σ, observed from the directions of the candidate blazars. The IBL & HBL targets make up 96% of the observed excess. Observations of these objects also comprise ~80% of the total exposure. An identical stacked analysis of all the extragalactic non-blazar targets observed, but not clearly detected (>5σ), by VERITAS does not show a significant excess (~120 h exposure). The stacked excess persists using alternate methods for estimating the background at each blazar location, and with different event selection criteria (e.g. “soft” cuts optimized for sources with Γ > 4).

Observational Results

This research is supported by grants from the US Department of Energy, the US National Science Foundation, and the

Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland, and by STFC in the UK. We acknowledge the excellent work of the technical support staff at the FLWO

and the collaborating institutions in the construction and operation of the instrument.

ConclusionsThe first two years of the VERITAS blazar KSP have been highly successful. Highlights include the detection of more than a dozen VHE blazars, including 5 discoveries, with the observations almost always having contemporaneous MWL data. All but a handful of the blazars on the initial VERITAS target list have been observed. The 49 flux upper limits presented here generally are the most-constraining ever for each of the blazars. The excess seen in the stacked blazar analysis presented here suggests that the direction of the VERITAS discovery program is well justified. Although some of the targets on the current VERITAS target list require initial or follow-up observations, a new list of candidates will be observed by VERITAS in future seasons. These targets will likely be drawn from very hard spectrum blazars detected by Fermi-LAT, perhaps only above 1 GeV, and will likely have a greater focus on high-risk/high-reward objects at larger redshifts (0.3 < z < 0.7). In addition, the number of VHE blazars studied in pre-planned MWL campaigns will increase as data from the Fermi-LAT will be publicly available. In particular, future MWL observations of 1ES 0229+200 (z = 0.139, Γ = 2.5) are tentatively planned given the potential EBL implications [17].

Upper Limits from the ObservationsFigure (right): The distribution of the integral flux upper limits, in Crab units, from each of the 49 observed blazars.

Table: The good-quality live time (t), mean zenith angle (θ), and energy threshold (Eth) of the VERITAS data for each of the 49 blazars observed, but not detected (>5σ) in the VERITAS blazar discovery program. Upper limits (99% confidence level [18]) on the integral flux (F>Eth) above Eth are calculated assuming a photon index Γ = 3.0 and are shown in both absolute units and percentage of the flux observed from the Crab Nebula above the same threshold. The systematic error on the integral flux limits is ~20%. The redshift (z) is from NED. The BL Lac classification (HBL, IBL, LBL) is taken from [19], unless otherwise noted (* from [4], ** from [20], *** from the BZCAT SED [21]. The notes indicate the following: VHE recommendation from Co [6], CG [1] , P [2], Pa [9], SDS [3]; SS = Sedentary survey [4]; R = ROXA survey [5]; Fermi = Fermi-LAT detection [8]; E = EGRET detection [7]; ToO = ToO trigger (optical or Fermi-LAT).

Time Weighted Average = ~2% Crab

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