2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1
Submillimeter Variability and the Gamma-ray Connection inFermi
Blazars
A. Strom
Univ. of Arizona, AZ 85721, USA
A. Siemiginowska, M. Gurwell, B. Kelly
CfA, MA 02138, USA
We present multi-epoch observations from the Submillimeter Array (SMA) for a sample of 171 bright blazars,
43 of which were detected by Fermi during the ﬁrst three months of observations. We explore the correlation
between their gamma-ray properties and submillimeter observations of their parsec-scale jets, with a special
emphasis on spectral index in both bands and the variability of the synchrotron component. Subclass is de-
termined using a combination of Fermi designation and the Candidate Gamma-Ray Blazar Survey (CGRaBS),
resulting in 35 BL Lac objects and 136 ﬂat-spectrum radio quasars (FSRQs) in our total sample. We calculate
submillimeter energy spectral indices using contemporaneous observations in the 1 mm and 850 micron bands
during the months August–October 2008. The submillimeter light curves are modeled as ﬁrst-order continuous
autoregressive processes, from which we derive characteristic timescales. Our blazar sample exhibits no diﬀer-
ences in submillimeter variability amplitude or characteristic timescale as a function of subclass or luminosity.
All of the the light curves are consistent with being produced by a single process that accounts for both low
and high states, and there is additional evidence that objects may be transitioning between blazar class during
ﬂaring epochs.
1. INTRODUCTION
The timescales on which high-amplitude ﬂaring
events occur in blazars indicate that much of the en-
ergy is being produced deep within the jet on small,
sub-parsec scales [1, 2]. Understanding if/how emis-
sion diﬀers between blazar subclasses (i.e., BL Lacs
objects and ﬂat-spectrum radio quasars (FSRQs))
may oﬀer important insight into the similarity be-
tween blazars and, furthermore, can provide con-
straints on the formation and acceleration of the jets
themselves.
For the synchrotron component of blazar spectra,
the low-frequency spectral break due to synchrotron
self-absorption moves to higher frequencies as one
measures closer to the base of the jet [2]. This of-
ten places the peak of the spectrum in the millime-
ter and submillimeter bands, where the emission is
optically-thin and originates on parsec and sub-parsec
scales [3], allowing direct observation of the most com-
pact regions near the central engine. The high en-
ergy γ-ray emission originates as a Compton process,
typically a combination of synchrotron-self-Compton
(SSC) and external-radiation-Compton (ERC). De-
pending on the source properties, the synchrotron
photons or external photons are upscattered by the
same population of electrons that emit the millimeter
and submillimeter spectra. Therefore the submillime-
ter and γ-ray emission are closely linked and give the
full information about the source emission.
A systematic study of the submillimeter properties
of the entire sample ofFermi blazars has yet to be con-
ducted and is one of the primary goals of our work. We
present here preliminary analysis of the submillimeter
properties of Fermi blazars detected by the Submil-
limeter Array1 (SMA) at 1mm and 850 µm, including
an investigation of variable behavior and the deter-
mination of submillimeter energy spectral indices. In
addition, we consider the connection to the observed
γ-ray indices and luminosities.
2. SMA BLAZARS
The Submillimeter Array [4] consists of eight 6 m
antennas located near the summit of Mauna Kea. The
SMA is used in a variety of baseline conﬁgurations
and typically operates in the 1mm and 850 µm win-
dows, achieving spatial resolution as ﬁne as 0.25” at
850µm. The sources used as phase calibrators for the
array are compiled in a database known as the SMA
Calibrator List2 [5]. Essentially a collection of bright
objects (stronger than 750 mJy at 230 GHz and 1 Jy