1,053 research outputs found
Correlated Spectral and Temporal Variability in the High-Energy Emission from Blazars
Blazar flare data show energy-dependent lags and correlated variability
between optical/X-ray and GeV-TeV energies, and follow characteristic
trajectories when plotted in the spectral-index/flux plane. This behavior is
qualitatively explained if nonthermal electrons are injected over a finite time
interval in the comoving plasma frame and cool by radiative processes.
Numerical results are presented which show the importance of the effects of
synchrotron self-Compton cooling and plasmoid deceleration. The use of INTEGRAL
to advance our understanding of these systems is discussed.Comment: 8 pages, 5 figures, uses epsf.sty, rotate.sty Invited paper in "The
Extreme Universe," 3rd INTEGRAL Workshop, 14-18 September 1998, Taorimina,
Ital
Compton Scattering in Jets: A Mechanism for 0.4 and \lsim 0.2 Mev Line Production
We show that gamma ray line emission at 0.4 MeV and \lsim 0.2 MeV can
be produced by Compton scattering of beamed radiation in the jets of Galactic
black hole candidates. This mechanism has the novel feature of not invoking the
presence of -- pairs. To produce the two lines we employ a symmetric
double sided jet with bulk flow velocity of about 0.5 c and incident beam
radiation with a hard energy spectrum. We show that the two lines can be seen
at viewing angle cosines relative to the jet ranging from 0.2 to 0.6. This
comprises 40\% of the total solid angle. In addition, the line radiation is
approximately 10\% polarized. Depending on the bulk flow and viewing angle the
model can produce lines at other energies as well. In particular a broad
feature near 1 MeV can be seen by viewing the jet close to its axis. Our model
can also accommodate single line spectra if the beamed gamma ray emission or
the jets themselves are asymmetric.Comment: 10 pages, uuencoded compressed postscript with figures, NRL94-05-0
Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039
Recent HESS observations show that microquasars in high-mass systems are
sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray
emission is developed. Using the head-on approximation for the Compton cross
section and taking into account angular effects from the star's orbital motion,
we derive expressions to calculate the spectrum of gamma rays when nonthermal
jet electrons Compton-scatter photons of the stellar radiation field.
Calculations are presented for power-law distributions of nonthermal electrons
that are assumed to be isotropically distributed in the comoving jet frame, and
applied to -ray observations of LS 5039. We conclude that (1) the TeV
emission measured with HESS cannot result only from Compton-scattered stellar
radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a
combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data
associated with LS 5039 requires a very improbable leptonic model with a very
hard electron spectrum. Because the gamma rays would be variable in a leptonic
jet model, the data sets are unlikely to be representative of a simultaneously
measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily
to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic
modulation of the TeV emission from LS 5039 would favor a leptonic SSC or
cascade hadron origin of the emission in the inner jet, whereas stochastic
variability alone would support a more extended leptonic model. The puzzle of
the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.
On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts
A simple model for variability in relativistic plasma outflows is studied, in
which nonthermal electrons are continuously and uniformly injected in the
comoving frame over a time interval dt. The evolution of the electron
distribution is assumed to be dominated by synchrotron losses, and the energy-
and time-dependence of the synchrotron and synchrotron self-Compton (SSC)
fluxes are calculated for a power-law electron injection function with index s
= 2. The mean time of a flare or pulse measured at photon energy E with respect
to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for
synchrotron and SSC processes, respectively, until the time approaches the
limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is
the Doppler factor. This dependence is in accord with recent analyses of blazar
and GRB emissions, and suggests a method to discriminate between external
Compton and SSC models of high-energy gamma radiation from blazars and GRBs.
The qualititative behavior of the X-ray spectral index/flux relation observed
from BL Lac objects can be explained with this model. This demonstrates that
synchrotron losses are primarily responsible for the X-ray variability behavior
and strengthens a new test for beaming from correlated hard X-ray/TeV
observations.Comment: 10 pages, 2 figures, accepted for publication in Astrophysical
Journal Letters; uses aaspp4.sty, epsf.st
Implications of very rapid TeV variability in blazars
We discuss the implications of rapid (few-minute) variability in the TeV flux
of blazars, which has been observed recently with the HESS and MAGIC
telescopes. The variability timescales seen in PKS 2155-304 and Mrk 501 are
much shorter than inferred light-crossing times at the black hole horizon,
suggesting that the variability involves enhanced emission in a small region
within an outflowing jet. The enhancement could be triggered by dissipation in
part of the black hole's magnetosphere at the base of the outflow, or else by
instabilities in the jet itself. By considering the energetics of the observed
flares, along with the requirement that TeV photons escape without producing
pairs, we deduce that the bulk Lorentz factors in the jets must be >50. The
distance of the emission region from the central black hole is less
well-constrained. We discuss possible consequences for multi-wavelength
observations.Comment: 5 pages, no figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Letter
Is the High-Energy Emission from Centaurus A Compton-Scattered Jet Radiation?
We consider whether the hard X-ray and soft gamma-ray emission from Centaurus
A is beamed radiation from the active nucleus which is Compton-scattered into
our line-of-sight. We derive the spectrum and degree of polarization of
scattered radiation when incident beamed radiation is scattered from a cold
() electron cloud moving with bulk relativistic motion along the
jet axis, and calculate results for an unpolarized, highly-beamed incident
power-law photon source. We fit the OSSE data from Centaurus A with this model
and find that if the scatterers are not moving relativistically, then the angle
the jet makes with respect to our line-of-sight is . We
predict a high degree of polarization of the scattered radiation below
keV. Future measurements with X-ray and gamma-ray polarimeters could
be used to constrain or rule out such a scenario.Comment: 12 pages, Postscript file with 3 Figures, NRL 017-331-09
X-Ray Spectral Variability of Extreme BL Lac AGN H1426+428
Between 7 March 2002 and 15 June 2002, intensive X-ray observations were
carried out on the extreme BL Lac object H1426+428 with instruments on board
the Rossi X-ray Timing Explorer (RXTE). These instruments provide measurements
of H1426+428 in the crucial energy range that characterizes the first peak of
its spectral energy distribution. This peak, which is almost certainly due to
synchrotron emission, has previously been inferred to be in excess of 100 keV.
By taking frequent observations over a four-month campaign, which included
450 ksec of RXTE time, studies of flux and spectral variability on
multiple timescales were performed, along with studies of spectral hysteresis.
The 3-24 keV X-ray flux and spectra exhibited significant variability, implying
variability in the location of the first peak of the spectral energy
distribution. Hysteresis patterns were observed, and their characteristics have
been discussed within the context of emission models.Comment: accepted for publication in Astrophysical Journa
The obscured gamma-ray and UHECR universe
Auger results on clustering of > 60 EeV ultra-high energy cosmic ray (UHECR)
ions and the interpretation of the gamma-ray spectra of TeV blazars are
connected by effects from the extragalactic background light (EBL). The EBL
acts as an obscuring medium for gamma rays and a reprocessing medium for UHECR
ions and protons, causing the GZK cutoff. The study of the physics underlying
the coincidence between the GZK energy and the clustering energy of UHECR ions
favors a composition of > 60 EeV UHECRs in CNO group nucleons. This has
interesting implications for the sources of UHECRs. We also comment on the
Auger analysis.Comment: 11 pages, 10 figures, in the International Conference on Topics in
Astroparticle and Underground Physics (TAUP) 2007, Sendai, Japan, September
11-15, 200
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