249 research outputs found
Linkage between Accretion Disks and Blazars
The magnetic field in an accretion disk is estimated assuming that all of the
angular momentum within prescribed accretion disk radii is removed by a jet.
The magnetic field estimated at the base of the jet is extrapolated to the
blazar emission region using a model for a relativistic axisymmetric jet
combined with some simplifying assumptions based on the relativistic nature of
the flow. The extrapolated magnetic field is compared with estimates based upon
the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN
421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-
Compton models are inconsistent with the magnetic fields extrapolated in this
way. However, in two cases inverse Compton models in which a substantial part
of the soft photon field is generated locally agree well, mainly because these
models imply magnetic field strengths which are closer to being consistent with
Poynting flux dominated jets. This comparison is based on estimating the mass
accretion rate from the jet energy flux. Further comparisons along these lines
will be facilitated by independent estimates of the mass accretion rate in
blazars and by more detailed models for jet propagation near the black hole.Comment: Submiteed to the Astrophysics & Space Science special issue on the
5th Stromlo Symposiu
The cyclo-synchrotron process and particle heating through the absorption of photons
We propose a new approximation for the cyclo-synchrotron emissivity of a
single electron. In the second part of this work, we discuss a simple
application for our approximation, and investigate the heating of electrons
through the self-absorption process. Finally, we investigate the self-absorbed
part of the spectrum produced by a power-law population of electrons. In
comparison to earlier approximations, our formula provides a few significant
advantages. Integration of the emissivity over the whole frequency range,
starting from the proper minimal emitting frequency, gives the correct cooling
rate for any energy particle. Further, the spectrum of the emission is well
approximated over the whole frequency range, even for relatively low particle
energies (beta << 0.1), where most of the power is emitted in the first
harmonic. In order to test our continuous approximation, we compare it with a
recently derived approximation of the first ten harmonics. Finally, our formula
connects relatively smooth to the synchrotron emission at beta=0.9. We show
that the self-absorption is a very efficient heating mechanism for low energy
particles, independent of the shape of the particle distribution responsible
for the self-absorbed synchrotron emission. We find that the energy gains for
low energy particles are always higher than energy losses by cyclo-synchrotron
emission. We show also that the spectral index of the self-absorbed part of the
spectrum at very low frequencies differs significantly from the well known
standard relation I(nu) ~ nu^(5/2).Comment: 9 pages, 4 figures, accepted for publication in A&
X-ray variability patterns in blazars
We study the expected variability patterns of blazars within the two-zone
acceleration model putting special emphasis on flare shapes and spectral lags.
We solve semi-analytically the kinetic equations which describe the particle
evolution in the acceleration and radiation zone. We then perturb the solutions
by introducing Lorentzian variations in its key parameters and examine the
flaring behavior of the system. We apply the above to the X-ray observations of
blazar 1ES 1218+304 which exhibited a hard lag behavior during a flaring
episode and discuss possibilities of producing it within the context of our
model. The steady-state radio to X-rays emission of 1ES 1218+304 can be
reproduced with parameters which lie well within the ones generally accepted
from blazar modeling. Additionally, we find that the best way to explain its
flaring behavior is by varying the rate of particles injected in the
acceleration zone.Comment: accepted by A&
On the correlation between the X-ray and gamma-ray emission in TeV blazars
The observations of TeV blazars published recently show an unexpected
quadratic or even cubic correlation between the X-ray and gamma-ray emission. A
standard model of the synchrotron self-Compton emission of a compact source
inside a jet is not able to explain such a correlation. Therefore, we propose
an alternative scenario where the emission of at least two independent compact
components is observed at the same time.
We compare two different models. The first model assumes the injection of
relativistic particles into a downstream region of a shock wave inside a jet
that creates the emitting source. The model precisely describes the evolution
of the particle energy spectrum inside the source and takes into account a
light-crossing time effect for the produced radiation. The second model assumes
an intrinsically constant emission of a homogeneous source that travels inside
the jet along a curved trajectory, where the activity is produced simply by
different values of the source's Doppler factor. To verify the two models we
use recentlu published observations of Mrk 421.
Our simulations show that simultaneous radiation of at least two independent
sources, where the first source dominates the emission in the X-ray range and
the second source radiates strongly in the gamma-ray range, can explain the
observed correlations. However, the injection model provides inadequate results
because it gives different values for the correlation of the rise and decay of
a flare. This problem is negligible in the scenario that uses the Doppler
boosting effect. Therefore, this approach yields much better results.Comment: 6 pages, 2 figures, accepted for publication in A&
Dissipation of jet bulk kinetic energy in powerful blazars
We investigate the dissipation of the bulk kinetic energy of a relativistic
jet at different distances from the central power--house and analyse in detail
how the dissipated energy is radiated away. We assume that the location of the
dissipation region is a function of the bulk Lorentz factor Gamma of the jet,
being closer to the center for smaller Gamma. This assumption is naturally
fulfilled in the internal shock scenario. The dissipated energy is partially
used to accelerate electrons and to amplify the magnetic field. This process
creates a source inside the jet (blob). Such blobs may efficiently produce
synchrotron and inverse Compton emission. We find that even if the blobs or
shells responsible for the blazar activity carry the same energy (in bulk
kinetic form), the fact that they move at different Gamma can produce dramatic
variations in different bands, even if the bolometric luminosity is instead
very similar. This is due to the relative importance of the synchrotron,
self-Compton and external Compton radiation processes, which greatly changes by
changing Gamma and the compactness of the source, even if the total radiated
energy is constant. We then find that the jet can produce most of its radiative
output at small distances from the putative black-hole and its accretion disk,
if this implies a low level of emitted MeV-GeV flux. Our findings, which we
apply for illustrative purposes to the blazar 3C 454.3, will be easily testable
by the coming gamma-ray satellite, such as AGILE and GLAST.Comment: accepted for publication in A&
X-ray Time Lags in TeV Blazars
We use Monte Carlo/Fokker-Planck simulations to study the X-ray time lags.
Our results show that soft lags will be observed as long as the decay of the
flare is dominated by radiative cooling, even when acceleration and cooling
timescales are similar. Hard lags can be produced in presence of a competitive
achromatic particle energy loss mechanism if the acceleration process operates
on a timescale such that particles are slowly moved towards higher energy while
the flare evolves. In this type of scenario, the {\gamma} -ray/X-ray quadratic
relation is also reproduced.Comment: 4 pages, 6 figures, Proceeding of `Multiwavelength Variability of
Blazars', Guangzhou, Chin
Near Infrared Intraday Variability of Mrk 421
We report results from our monitoring of the BL Lac object Mrk 421 in the
near-IR band. The observations, aimed at studying the intraday variability
(IDV) of the object, were carried out systematically over an extended (and
near-continuous) period of eight nights from the 1.2m Mount Abu Infrared
Telescope, India. There are limited studies for Mrk 421 in the band for
such an extended period. The observation epoch for this study (25 February - 5
March 2003) was chosen to significantly overlap other concurrent studies of Mrk
421 in the X-ray/-ray regions being conducted using the Rossi X-ray
timing explorer (RXTE) and the solar tower atmospheric Cherenkov effect
experiment (STACEE). Hence these results could be useful for a multi-wavelength
analysis of the variability behavior of Mrk 421. We find that Mrk 421 was quite
active during the observed period and showed significant IDV and short term
variability. A maximum variation of 0.89 magnitudes is seen over the entirety
of the observed period. Flaring activity, with typical brightness variations of
, are also seen on several occasions. The extent of the variability
observed by us is compared with the results of other similar studies of Mrk 421
in the band.Comment: accepted for publication in Astronomy & Astrophysics (6 pages, 2
figures
Extragalactic jets on subpc and large scales
Jets can be probed in their innermost regions (d~0.1 pc) through the study of
the relativistically-boosted emission of blazars. On the other extreme of
spatial scales, the study of structure and dynamics of extragalactic
relativistic jets received renewed impulse after the discovery, made by
Chandra, of bright X-ray emission from regions at distances larger than
hundreds of kpc from the central engine. At both scales it is thus possible to
infer some of the basic parameters of the flow (speed, density, magnetic field
intensity, power). After a brief review of the available observational
evidence, I discuss how the comparison between the physical quantities
independently derived at the two scales can be used to shed light on the global
dynamics of the jet, from the innermost regions to the hundreds of kpc scale.Comment: Proceedings of the 5th Stromlo Symposium: Disks, Winds, and Jets -
from Planets to Quasars. Accepted, to be published in Astrophysics & Space
Scienc
Searches for very high energy gamma rays from blazars with CANGAROO-III telescope in 2005-2009
We have searched for very high energy (VHE) gamma rays from four blazars
using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the
results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C
279, performed from 2005 to 2009, applying a new analysis to suppress the
effects of the position dependence of Cherenkov images in the field of view. No
significant VHE gamma ray emission was detected from any of the four blazars.
The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT
archival data. Non-simultaneous wide range (radio to VHE gamma-ray bands)
spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV
gamma-ray spectra, and archival data are discussed using a one-zone synchrotron
self-Compton (SSC) model in combination with a external Compton (EC) radiation.
The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model,
and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC
model. We find a consistency with the blazar sequence in terms of strength of
magnetic field and component size.Comment: 11 pages, 8 figures, Accepted for publication in Astroparticle
Physic
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