1,267 research outputs found
Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst
The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is
analyzed in terms of different variations of the homogeneous leptonic jet model
for the production of high-energy radiation from blazars. We find that a
two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and
an external Compton component, is required in order to yield an acceptable fit
to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other
BL Lac objects, the broad emission line region plays an important role for the
high-energy emission. Several alternative blazar jet models are briefly
discussed. In the appendix, we describe the formalism in which the process of
Comptonization of reprocessed accretion disk photons is treated in the
previously developed blazar jet simulation code which we use.Comment: Now accepted for publication in The Astronomical Journal.
Significantly extended discussion w.r.t. original version. 3 Figures included
using epsf.sty, rotate.st
Transformation Properties of External Radiation Fields, Energy-Loss Rates and Scattered Spectra, and a Model for Blazar Variability
We treat transformation properties of external radiation fields in the proper
frame of a plasma moving with constant speed. The specific spectral energy
densities of external isotropic and accretion-disk radiation fields are derived
in the comoving frame of relativistic outflows, such as those thought to be
found near black-hole jet and gamma-ray burst sources. Nonthermal electrons and
positrons Compton-scatter this radiation field, and high-energy protons and
ions interact with this field through photomeson and photopair production. We
revisit the problem of the Compton-scattered spectrum associated with an
external accretion-disk radiation field, and clarify a past treatment by the
authors. Simple expressions for energy-loss rates and Thomson-scattered spectra
are given for ambient soft photon fields consisting either of a surrounding
external isotropic monochromatic radiation field, or of an azimuthally
symmetric, geometrically thin accretion-disk radiation field. A model for
blazar emission is presented that displays a characteristic spectral and
variability behavior due to the presence of a direct accretion-disk component.
The disk component and distinct flaring behavior can be bright enough to be
detected from flat spectrum radio quasars with {\it GLAST}. Spectral states of
blazars are characterized by the relative importance of the accretion-disk and
scattered radiation fields and, in the extended jet, by the accretion disk,
inner jet, and cosmic microwave background radiation fields.Comment: 43 pages, 12 figures, ApJ, in press; includes improvements in
response to referee report, added references, section of detectability with
GLAS
X-ray spectral features from GRBs: Predictions of progenitor models
We investigate the potentially observable prompt or delayed X-ray spectral
features from the currently popular gamma-ray burst (GRB) models. During the
evolution of many GRB progenitors, a disk around the central GRB source is
produced. Shock heating as the GRB ejecta collide with the disk may produce
observable X-ray features. We first summarize predictions deduced from previous
calculations which invoke photoionization and relativistic blast waves. We then
calculate the quasi-thermal X-ray line features produced assuming the ejecta
are nonrelativistic (which is more likely for the disk interactions of many GRB
models). In the framework of the Hypernova/Collapsar model, delayed (a few days
- several months after the GRB) bursts of line-dominated, thermal X-ray
emission may be expected. The He-merger scenario predicts similar X-ray
emission line bursts <~ a few days after the GRB. These X-ray signatures should
be observable with Chandra and XMM-Newton out to at least z ~ 1. Weak emission
line features <~ a few days after the GRB may also result from the supranova
GRB scenario. In all three cases, significant X-ray absorption features, in
particular during the prompt GRB phase, are expected. No significant X-ray
spectral features might result from compact-object binary mergers.Comment: 20 pages, including 8 figures and 3 tables. Uses epsf.sty,
rotate.sty. Final version, accepted for publication in to ApJ. Revised
analytical estimate of maximum emission line luminosity. Numerical results
and conclusions unchange
Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April
The well studied blazar Markarian 421 (Mrk 421, =0.031) was the subject of
an intensive multi-wavelength campaign when it flared in 2013 April. The
recorded X-ray and very high energy (VHE, E100 GeV) -ray fluxes are
the highest ever measured from this object. At the peak of the activity, it was
monitored by the hard X-ray focusing telescope {\it Nuclear Spectroscopic
Telescope Array} ({\it NuSTAR}) and {\it Swift} X-Ray Telescope (XRT). In this
work, we present a detailed variability analysis of {\it NuSTAR} and {\it
Swift}-XRT observations of Mrk 421 during this flaring episode. We obtained the
shortest flux doubling time of 14.015.03 minutes, which is the shortest
hard X-ray (379 keV) variability ever recorded from Mrk 421 and is on the
order of the light crossing time of the black hole's event horizon. A pattern
of extremely fast variability events superposed on slowly varying flares is
found in most of the {\it NuSTAR} observations. We suggest that these peculiar
variability patterns may be explained by magnetic energy dissipation and
reconnection in a fast moving compact emission region within the jet. Based on
the fast variability, we derive a lower limit on the magnetic field strength of
~G, where is the
Doppler factor in units of 10, and is the characteristic X-ray
synchrotron frequency in units of ~Hz.Comment: 23 pages, 5 figures, 2 tables, to appear in the Astrophysical Journa
The Energy Dependence of the Aperiodic Variability for Cygnus X-1, GX 339-4, GRS 1758-258, & 1E 1740.7-2942
Using the data from the Rossi X-ray Timing Explorer (RXTE), we report the
different energy dependence of the variability of the four persistent hard
X-ray sources in the low-hard state: Cygnus X-1, GX 339-4, GRS 1758-258 and 1E
1740.7-2942. Cygnus X-1 is found to have a flatter power density spectrum (PDS)
shape at higher energies. The other three sources have energy independent PDS
shapes. The energy dependence of the overall variability (the integrated rms
amplitude) varies from source to source and from observation to observation.
1E~1740.7-2942, for example, has a variability generally increasing with energy
while GX 339-4 has a decreasing variability. A general trend is found in the
four sources that the integrated rms amplitude anti-correlates with the X-ray
flux. We compare these distinct energy dependent behaviors with several
emission models. None of the models can fully explain all the features that we
have found.Comment: 18 pages, 6 figures. Accepted for publication in Ap
Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell
We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted
for publication in ApJ. Extended introduction and discussio
Photon-Photon Absorption of Very High Energy Gamma-Rays from Microquasars: Application to LS 5039
Very high energy (VHE) gamma-rays have recently been detected from the
Galactic black-hole candidate and microquasar LS 5039. A plausible site for the
production of these VHE gamma-rays is the region close to the mildly
relativistic outflow. However, at distances comparable to the binary
separation, the intense photon field of the stellar companion will lead to
substantial gamma-gamma absorption of VHE gamma-rays. If the system is viewed
at a substantial inclination (i > 0), this absorption feature will be modulated
on the orbital period of the binary as a result of a phase-dependent
stellar-radiation intensity and pair-production threshold. We apply our results
to LS 5039 and find that (1) gamma-gamma absorption effects will be substantial
if the photon production site is located at a distance from the central compact
object of the order of the binary separation (~ 2.5e12 cm) or less; (2) the
gamma-gamma absorption depth will be largest at a few hundred GeV, leading to a
characteristic absorption trough; (3) the gamma-gamma absorption feature will
be strongly modulated on the orbital period of the binary, characterized by a
spectral hardening accompanying periodic dips of the VHE gamma-ray flux; and
(4) gamma rays can escape virtually unabsorbed, even from within ~ 10^{12} cm,
when the star is located behind the production site as seen by the observer.Comment: Submitted to ApJ Letters. AASTeX, 12 ms pages, including 4 eps
figure
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks
Sari, Piran, and Narayan have derived analytic formulas to model the spectra
from gamma-ray burst blast waves that are energized by sweeping up material
from the surrounding medium. We extend these expressions to apply to general
radiative regimes and to include the effects of synchrotron self-absorption.
Electron energy losses due to the synchrotron self-Compton process are also
treated in a very approximate way. The calculated spectra are compared with
detailed numerical simulation results. We find that the spectral and temporal
breaks from the detailed numerical simulation are much smoother than the
analytic formulas imply, and that the discrepancies between the analytic and
numerical results are greatest near the breaks and endpoints of the synchrotron
spectra. The expressions are most accurate (within a factor of ~ 3) in the
optical/X-ray regime during the afterglow phase, and are more accurate when
epsilon_e, the fraction of swept-up particle energy that is transferred to the
electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude
accuracy in the self-absorbed radio/infrared regime, and give poor fits to the
self-Compton spectra due to complications from Klein-Nishina effects and
photon-photon opacity.Comment: 16 pages, 7 figures, ApJ, in press, 537, July 1, 2000. Minor changes
in response to referee report, corrected figure
Broadband Spectral Analysis of PKS 0528+134: A Report on Six Years of EGRET Observations
The multiwavelength spectra of PKS 0528+134 during six years of observations
by EGRET have been analyzed using synchrotron self-Compton (SSC) and external
radiation Compton (ERC) models. We find that a two-component model, in which
the target photons are produced externally to the gamma-ray emitting region,
but also including an SSC component, is required to suitably reproduce the
spectral energy distributions of the source. Our analysis indicates that there
is a trend in the observed properties of PKS 0528+134, as the source goes from
a gamma-ray low state to a flaring state. We observe that during the higher
gamma-ray states, the bulk Lorentz factor of the jet increases and the ERC
component dominates the high-energy emission. Our model calculations indicate
the trend that the energies of the electrons giving rise to the synchrotron
peak decreases, and the power-ratio of the gamma-ray and low energy spectral
components increases, as the source goes from a low to a high gamma-ray state.Comment: 36 pages, 13 figures, final version accepted for publication in ApJ;
includes minor modification
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