arXiv:1001.0770v1  [astro-ph.HE]  5 Jan 2010
2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1
VERITAS Observations of Blazars
W. Benbow for the VERIT AS Collaboration
Harvard-Smithsonian Center for Astrophysics, F .L. Whippl e Observatory , PO Box 6369, Amado, AZ 85645,
USA
The VERITAS array of four 12-m diameter imaging atmospheric -Cherenkov telescopes in southern Arizona is
used to study very high energy (VHE; E >100 GeV) γ-ray emission from astrophysical objects. VERITAS is
currently the most sensitive VHE γ-ray observatory in the world and one of the VERITAS collabor ation’s Key
Science Projects (KSP) is the study of blazars. These active galactic nuclei (AGN) are the most numerous class
of identiﬁed VHE sources, with ∼ 30 known to emit VHE photons. More than 70 AGN, almost all of wh ich
are 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 detectio n of VHE γ-rays from 16 AGN (15 blazars), including
8 for the ﬁrst time at these energies. The VERITAS blazar KSP i s summarized in this proceeding and selected
results are presented.
1. Introduction
Active galactic nuclei are the most numerous class
of identiﬁed VHE γ-ray sources. These objects emit
non-thermal radiation across ∼ 20 orders of magnitude
in energy and rank among the most powerful particle
accelerators in the universe. A small fraction of AGN
possess strong collimated outﬂows (jets) powered by
accretion onto a supermassive black hole (SMBH).
VHEγ-ray emission can be generated in these jets,
likely in a compact region very near the SMBH event
horizon. Blazars, a class of AGN with jets pointed
along the line-of-sight to the observer, are of par-
ticular interest in the VHE regime. Approximately
30 blazars, primarily high-frequency-peaked BL Lacs
(HBL), are identiﬁed as sources of VHEγ-rays, and
some are spectacularly variable on time scales com-
parable to the light crossing time of their SMBH (∼ 2
min; [1]). VHE blazar studies probe the environment
very near the central SMBH and address a wide range
of physical phenomena, including the accretion and
jet-formation processes. These studies also have cos-
mological implications, as VHE blazar data can be
used to strongly constrain primordial radiation ﬁelds
(see the extragalactic background light (EBL) con-
straints from, e.g., [2, 3]).
VHE blazars have double-humped spectral energy
distributions (SEDs), with one peak at UV/X-ray en-
ergies and another at GeV/TeV energies. The ori-
gin of the lower-energy peak is commonly explained
as synchrotron emission from the relativistic electrons
in the blazar jets. The origin of the higher-energy
peak is controversial, but is widely believed to be the
result of inverse-Compton scattering of seed photons
oﬀ the same relativistic electrons. The origin of the
seed photons in these leptonic scenarios could be the
synchrotron photons themselves, or photons from an
external source. Hadronic scenarios are also plausible
explanations for the VHE emission, but generally are
not favored.
Contemporaneous multi-wavelength (MWL) obser-
vations of VHE blazars, can measure both SED peaks
and are crucial for extracting information from the
observations of VHE blazars. They are used to con-
strain the size, magnetic ﬁeld and Doppler factor of
the emission region, as well as to determine the origin
(leptonic or hadronic) of the VHEγ-rays. In leptonic
scenarios, such MWL observations are used to mea-
sure the spectrum of high-energy electrons producing
the emission, as well as to elucidate the nature of the
seed photons. Additionally, an accurate measure of
the cosmological EBL density requires accurate mod-
eling of the blazar’s intrinsic VHE emission that can
only be performed with contemporaneous MWL ob-
servations.
2. VERITAS
VERITAS, a stereoscopic array of four 12-m
atmospheric-Cherenkov telescopes located in Arizona,
is used to study VHEγ-rays from a variety of astro-