886 research outputs found
Properties of the Intergalactic Magnetic Field Constrained by Gamma-ray Observations of Gamma-Ray Bursts
The magnetic field in intergalactic space gives important information about
magnetogenesis in the early universe. The properties of this field can be
probed by searching for radiation of secondary ee pairs created by TeV
photons, that produce GeV range radiation by Compton-scattering cosmic
microwave background (CMB) photons. The arrival times of the GeV "echo" photons
depend strongly on the magnetic field strength and coherence length. A Monte
Carlo code that accurately treats pair creation is developed to simulate the
spectrum and time-dependence of the echo radiation. The extrapolation of the
spectrum of powerful gamma-ray bursts (GRBs) like GRB 130427A to TeV energies
is used to demonstrate how the IGMF can be constrained if it falls in the
- G range for 1 Mpc coherence length.Comment: 8 pages, 6 figure
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
On the Redshift Distribution of Gamma Ray Bursts in the Swift Era
A simple physical model for long-duration gamma ray bursts (GRBs) is used to
fit the redshift (z) and the jet opening-angle distributions measured with
earlier GRB missions and with Swift. The effect of different sensitivities for
GRB triggering is sufficient to explain the difference in the z distributions
of the pre-Swift and Swift samples, with mean redshifts of ~1.5 and
~2.7, respectively. Assuming that the emission properties of GRBs do not
change with time, we find that the data can only be fitted if the comoving
rate-density of GRB sources exhibits positive evolution to z >~ 3-5. The mean
intrinsic beaming factor of GRBs is found to range from ~34-42, with the Swift
average opening half-angle ~10 degree, compared to the pre-Swift
average of ~7 degree. Within the uniform jet model, the GRB
luminosity function is proportional to L^{-3.25}_*, as inferred from our best
fit to the opening angle distribution. Because of the unlikely detection of
several GRBs with z <~ 0.25, our analysis indicates that low redshift GRBs
represent a different population of GRBs than those detected at higher
redshifts. Neglecting possible metallicity effects on GRB host galaxies, we
find that ~1 GRB occurs every 600,000 yrs in a local L_* spiral galaxy like the
Milky Way. The fraction of high-redshift GRBs is estimated at 8-12% and 2.5-6%
at z >= 5 and z >= 7, respectively, assuming continued positive evolution of
the GRB rate density to high redshifts.Comment: Accepted for publication in ApJ. The paper contains 29 pages and 24
figure
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.
Gamma Ray Burst Neutrinos Probing Quantum Gravity
Very high energy, short wavelength, neutrinos may interact with the
space-time foam predicted by theories of quantum gravity. They would propagate
like light through a crystal lattice and be delayed, with the delay depending
on the energy. This will appear to the observer as a violation of Lorenz
invariance. Back of the envelope calculations imply that observations of
neutrinos produced by gamma ray bursts may reach Planck-scale sensitivity. We
revisit the problem considering two essential complications: the imprecise
timing of the neutrinos associated with their poorly understood production
mechanism in the source and the indirect nature of their energy measurement
made by high energy neutrino telescopes.Comment: 14 pages, 2 figure
Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for GLAST
A study of the statistics of cosmological black-hole jet sources is applied
to EGRET blazar data, and predictions are made for GLAST. Black-hole jet
sources are modeled as collimated relativistic plasma outflows with radiation
beamed along the jet axis due to strong Doppler boosting. The comoving rate
density of blazar flares is assumed to follow a blazar formation rate (BFR),
modeled by analytic functions based on astronomical observations and fits to
EGRET data. The redshift and size distributions of gamma-ray blazars observed
with EGRET, separated into BL Lac object (BL) and flat spectrum radio quasar
(FSRQ) distributions, are fit with monoparametric functions for the
distributions of the jet Lorentz factor \Gamma, comoving directional power
l'_e, and spectral slope. A BFR factor ~10 x greater at z ~ 1 than at present
is found to fit the FSRQ data. A smaller comoving rate density and greater
luminosity of BL flares at early times compared to the present epoch fits the
BL data. Based on the EGRET observations, ~1000 blazars consisting of ~800
FSRQs and FR2 radio galaxies and ~200 BL Lacs and FR1 radio galaxies will be
detected with GLAST during the first year of the mission. Additional AGN
classes, such as hard-spectrum BL Lacs that were mostly missed with EGRET,
could add more GLAST sources. The FSRQ and BL contributions to the EGRET
gamma-ray background at 1 GeV are estimated at the level of ~10 - 15% and ~2 -
4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered
in the optimal case, and a GLAST analysis method for blazar detection is
outlined.Comment: 17 pages, 9 figures, ApJ, in press, v.660, May 1, 2007 (minor changes
from previous version
On the equipartition of thermal and non-thermal energy in clusters of galaxies
Clusters of galaxies are revealing themselves as powerful sources of non
thermal radiation in a wide range of wavelengths. In order to account for these
multifrequency observations equipartition of cosmic rays (CRs) with the thermal
gas in clusters of galaxies is often invoked. This condition might suggest a
dynamical role played by cosmic rays in the virialization of these large scale
structures and is now testable through gamma ray observations. We show here, in
the specific case of the Coma and Virgo clusters, for which upper limits on the
gamma ray emission exist, that equipartition implies gamma ray fluxes that are
close or even in excess of the EGRET limit, depending on the adopted model of
CR injection. We use this bound to limit the validity of the equipartition
condition. We also show that, contrary to what claimed in previous
calculations, the equipartition assumption implies gamma ray fluxes in the TeV
range which can be detectable even by currently operating gamma ray
observatories if the injection cosmic ray spectrum is flatter than .Comment: 20 pages + 2 figures. To appear in the Astrophysical Journa
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
- âŠ