83 research outputs found
Modeling the variability of the BL Lacertae object PKS 2155-304
The bright X-ray selected BL Lacertae object PKS 2155-304 has been the target
of two intense multiwavelength campaigns, in November 1991 and in May 1994.
Although the spectral energy distributions at both epochs were quite similar,
the source exhibited two very distinct variability patterns that cannot be
easily reconciled with homogeneous, one-zone jet models. During the first epoch
the variability was almost achromatic in amplitude, with a time lag between
X-rays and UV of h, while during the second epoch the variability
amplitude increased as a function of wavelength, with the EUV flare peaking
day after the X-ray flare. We model the source using a
time-dependent inhomogeneous accelerating jet model. e reproduce the general
characteristics of the different variability signatures by assuming that plasma
disturbances with different physical properties propagate downstream in an
underlying jet characterized by the same set of physical parameters at both
epochs. A time delay of 1 day between the hardening of the UV
spectral index and the UV flux, present at both epochs, is modeled with
stochastic fluctuations in the particle acceleration manifested through small
variations of the maximum energy of the injected electrons. We predict that
similar time delays will be present in future observations, even in the absence
of strong variability events. We stress the importance of observations at
neighboring frequencies as a diagnostic tool for the structure of the quiescent
jet in blazars, especially in the seemingly dull case when strong variability
is absent.Comment: 8 pages, 5 figures. Accepted in ApJ Letter
High Energy Variability Of Synchrotron-Self Compton Emitting Sources: Why One Zone Models Do Not Work And How We Can Fix It
With the anticipated launch of GLAST, the existing X-ray telescopes, and the
enhanced capabilities of the new generation of TeV telescopes, developing tools
for modeling the variability of high energy sources such as blazars is becoming
a high priority. We point out the serious, innate problems one zone
synchrotron-self Compton models have in simulating high energy variability. We
then present the first steps toward a multi zone model where non-local, time
delayed Synchrotron-self Compton electron energy losses are taken into account.
By introducing only one additional parameter, the length of the system, our
code can simulate variability properly at Compton dominated stages, a situation
typical of flaring systems. As a first application, we were able to reproduce
variability similar to that observed in the case of the puzzling `orphan' TeV
flares that are not accompanied by a corresponding X-ray flare.Comment: to appear in the 1st GLAST symposium proceeding
Constraints on the Intergalactic Magnetic Field from Gamma-Ray Observations of Blazars
Gamma rays from distant blazars interact with the extragalactic background
light, creating electron-positron pairs, and reducing the gamma-ray flux
measured by ground-based atmospheric Cherenkov gamma-ray telescopes. These
pairs can Compton-scatter the cosmic microwave background, creating a gamma-ray
signature observable by the Fermi Large Area Telesope (LAT). The signature is
also dependent on the intergalactic magnetic field (IGMF), since it can deflect
the pairs from our line of sight, reducing the gamma-ray emission. We present
preliminary constraints on the IGMF using Fermi-LAT and Cherenkov telescope
observations, ruling out both very large and very small values of the IGMF
strength.Comment: 7 pages, 4 figures. 2012 Fermi Symposium proceedings - eConf C121028
(fixed minor typo in title
Witnessing the gradual slow-down of powerful extragalactic jets: The X-ray -- optical -- radio connection
A puzzling feature of the {\it Chandra}--detected quasar jets is that their
X-ray emission decreases faster along the jet than their radio emission,
resulting to an outward increasing radio to X-ray ratio. In some sources this
behavior is so extreme that the radio emission peak is located clearly
downstream of that of the X-rays. This is a rather unanticipated behavior given
that the inverse
Compton nature of the X-rays and the synchrotron radio emission are
attributed to roughly the same electrons of the jet's non-thermal electron
distribution. In this note we show that this morphological behavior can result
from the gradual deceleration of a relativistic flow and that the offsets in
peak emission at different wavelengths carry the imprint of this deceleration.
This notion is consistent with another recent finding, namely that the jets
feeding the terminal hot spots of powerful radio galaxies and quasars are still
relativistic with Lorentz factors . The picture of the
kinematics of powerful jets emerging from these considerations is that they
remain relativistic as they gradually decelerate from Kpc scales to the hot
spots, where, in a final collision with the intergalactic medium, they
slow-down rapidly to the subrelativistic velocities of the hot spot advance
speed.Comment: Submitted in ApJ Letters on Jan. 14, 200
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