2009 Fermi Symposium, Washington, D.C., Nov. 2-5 3
Table I VERITAS AGN Detections. The only non-blazar
object is the radio galaxy M 87. The blazars discovered
at VHE by VERITAS are marked with a dagger.
Object Class Redshift
M 87 FR I 0.004
Mkn 421 HBL 0.030
Mkn 501 HBL 0.034
1ES 2344+514 HBL 0.044
1ES 1959+650 HBL 0.047
W Comae† IBL 0.102
RGB J0710+591† HBL 0.125
H 1426+428 HBL 0.129
1ES 0806+524† HBL 0.138
1ES 0229+200 HBL 0.139
1ES 1218+304 HBL 0.182
RBS 0413† HBL 0.190
1ES 0502+675† HBL 0.341
3C 66A† IBL 0.444?
PKS 1424+240† IBL ?
VER J0521+211† ? ?
(∼ 5.5σ; 3% Crab ﬂux above 300 GeV; Γ VHE ∼ 2. 7)
during VERITAS observations from December 2008
to March 2009. The initial announcement of the VHE
discovery [19] led to its discovery above 1 GeV in the
Fermi-LAT data using a special analysis. RBS 0413,
a relatively distant HBL (z=0.19), was observed for
16 h good-quality live time in 2008-092. These data
resulted in the discovery of VHE gamma-rays ( >270γ,
∼ 6σ) at a ﬂux ( >200 GeV) of ∼ 2% of the Crab Neb-
ula ﬂux. The discovery [20] was announced simultane-
ously with the LAT MeV-GeV detection. The VHE
and other MWL observations, including Fermi-LAT
data, for each of these three sources will be the sub-
ject of a joint publication involving both the VERI-
TAS and LAT collaborations.
5.2. Discoveries Motivated by Fermi-LAT
The successful VHE discovery observations by
VERITAS of three blazars was motivated primarily
by results from the ﬁrst year of LAT data taking. In
particular, the VHE detections of PKS 1424+240 [21]
and 1ES 0502+675 [22] were the result of VERITAS
observations triggered by the inclusion of these objects
in the Fermi-LAT Bright AGN List [13]. The former
is only the third IBL known to emit VHE gamma-
rays, and the latter is the most distant BL Lac object
2RBS 0413 was observed further by VERITAS in Fall 2009.
(z = 0 . 341) detected in the VHE band. In addition,
VER J0521+211, likely associated with the radio-loud
AGN RGB J0521.8+2112, was detected by VERTAS
in∼ 4 h of observations in October 2009 [23]. These
observations were motivated by its identiﬁcation as a
>30 GeV γ-ray source in the public Fermi-LAT data.
Its VHE ﬂux is 5% of the Crab Nebula ﬂux, placing it
among the brightest VHE blazars detected in recent
years. VERITAS later observed even brighter VHE
ﬂaring from VER J0521+211 in November 2009 [24],
leading to deeper VHE observations.
6. Blazars Upper Limits
More than 50 VHE blazar candidates were observed
by VERITAS between September 2007 and June 2009.
The total exposure on the 49 non-detected candi-
dates is∼ 305 h live time (average of 6.2 h per can-
didate). Approximately 55% of the total exposure is
split amongst the 27 observed HBL. The remainder is
divided amongst the 8 IBL (26%), 5 LBL (6%), and 9
FSRQ (13%). There are no clear indications of signiﬁ-
cant VHEγ-ray emission from any of these 49 blazars
[25]. However, the observed signiﬁcance distribution is
clearly skewed towards positive values (see Figure 1).
A stacking analysis performed on the entire data sam-
ple shows an overall excess of 430γ-rays, correspond-
ing to a statistical signiﬁcance of 4.8 σ, observed from
the directions of the candidate blazars. The IBL and
HBL targets make up 96% of the observed excess. Ob-
servations 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 dif-
ferent event selection criteria (e.g.soft cutsoptimized
for sources with Γ VHE > 4). The distribution of VHE
ﬂux upper limits is shown in Figure 1. These 49 VHE
ﬂux upper limits are generally the most-constraining
ever reported for these objects.
7. Multi-wavelength Studies of VHE
Blazars
During the ﬁrst three seasons of VERITAS obser-