27 research outputs found
Blazar Variability: A Study of Non-stationarity and the Flux-RMS Relation
We analyze X-ray light curves of the blazars Mrk 421, PKS 2155-304, and 3C
273 using observations by the Soft X-ray Telescope on board AstroSat and
archival XMM-Newton data. We use light curves of length 30-90 ks each from 3-4
epochs for all three blazars. We apply the autoregressive integrated moving
average (ARIMA) model which indicates the variability is consistent with short
memory processes for most of the epochs. We show that the power spectral
density (PSD) of the X-ray variability of the individual blazars are consistent
within uncertainties across the epochs. This implies that the construction of
broadband PSD using light curves from different epochs is accurate. However,
using certain properties of the variance of the light curves and its segments,
we show that the blazars exhibit hints of non-stationarity beyond that due to
their characteristic red noise nature in some of those observations. We find a
linear relationship between the root-mean-squared amplitude of variability at
shorter timescales and the mean flux level at longer timescales for light
curves of Mrk 421 across epochs separated by decades as well as light curves
spanning 5 days and 10 yr. The presence of flux-rms relation over very
different timescales may imply that, similar to the X-ray binaries and Seyfert
galaxies, longer and shorter timescale variability are connected in blazars.Comment: 12 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Magnetic Field Amplification and Blazar Flares
Recent multiwavelength observations of PKS 0208-512 by SMARTS, Fermi, and
Swift revealed that gamma-ray and optical light curves of this flat spectrum
radio quasars are highly correlated, but with an exception of one large optical
flare having no corresponding gamma-ray activity or even detection. On the
other hand, recent advances in SNRs observations and plasma simulations both
reveal that magnetic field downstream of astrophysical shocks can be largely
amplified beyond simple shock compression. These amplifications, along with
their associated particle acceleration, might contribute to blazar flares,
including the peculiar flare of PKS 0208-512. Using our time dependent
multizone blazar emission code, we evaluate several scenarios that may
represent such phenomena. This code combines Monte Carlo method that tracks the
radiative processes including inverse Compton scattering, and Fokker-Planck
equation that follows the cooling and acceleration of particles. It is a
comprehensive time dependent code that fully takes into account the light
travel time effects. In this study, both the changes of the magnetic field and
acceleration efficiency are explored as the cause of blazar flares. Under these
assumption, synchrotron self-Compton and external Compton scenarios produce
distinct features that favor the external Compton scenario. The optical flares
with/without gamma-ray counterparts can be explained by different allocations
of energy between the magnetization and particle acceleration, which in turn
can be affected by the relative orientation between the magnetic field and the
shock flow. We compare the details of the observations and simulation, and
highlight what implications this study has on our understanding of relativistic
jets.Comment: 6 pages. To appear in the proceeding of 'The Innermost Regions of
Relativistic Jets and Their Magnetic Fields', Granada, Spai
Locating the GeV Emission Region in the Jets of Blazars from Months-Timescale Multi-Wavelength Outbursts
It is well-known that the -ray emission in blazars originate in the
relativistic jet pointed at the observers. However, it is not clear whether the
exact location of the GeV emission is less than a pc from the central engine,
such that it may receive sufficient amount of photons from the broad line
region (BLR) or farther out at 1-100 pc range. The former assumption has been
successfully used to model the spectral energy distribution of many blazars.
However, simultaneous detection of TeV -rays along with GeV outbursts
in some cases indicate that the emission region must be outside the BLR. In
addition, GeV outbursts have sometimes been observed to be simultaneous with
the passing of a disturbance through the so called "VLBI core," which is
located tens of pc away from the central engine. Hence, the exact location of
-ray emission remains ambiguous. Here we present a method we have
developed to constrain the location of the emission region. We identify
simultaneous months-timescale GeV and optical outbursts in the light curves
spanning over 8 years of a sample of eleven blazars. Using theoretical jet
emission models we show that the energy ratio of simultaneous optical and GeV
outbursts is strongly dependent on the location of the emission region.
Comparing the energy dissipation of the observed multi-wavelength outbursts and
that of the simulated flares in our theoretical model we find that most of the
above outbursts originate beyond the BLR at approximately a few pc from the
central engine