615 research outputs found
Relativistic Doppler-boosted emission in gamma-ray binaries
Gamma-ray binaries could be compact pulsar wind nebulae formed when a young
pulsar orbits a massive star. The pulsar wind is contained by the stellar wind
of the O or Be companion, creating a relativistic comet-like structure
accompanying the pulsar along its orbit. The X-ray and the very high energy
(>100 GeV, VHE) gamma-ray emissions from the binary LS 5039 are modulated on
the orbital period of the system. Maximum and minimum flux occur at the
conjunctions of the orbit, suggesting that the explanation is linked to the
orbital geometry. The VHE modulation has been proposed to be due to the
combined effect of Compton scattering and pair production on stellar photons,
both of which depend on orbital phase. The X-ray modulation could be due to
relativistic Doppler boosting in the comet tail where both the X-ray and VHE
photons would be emitted. Relativistic aberrations change the seed stellar
photon flux in the comoving frame so Doppler boosting affects synchrotron and
inverse Compton emission differently. The dependence with orbital phase of
relativistic Doppler-boosted (isotropic) synchrotron and (anisotropic) inverse
Compton emission is calculated, assuming that the flow is oriented radially
away from the star (LS 5039) or tangentially to the orbit (LS I +61 303, PSR
B1259-63). Doppler boosting of the synchrotron emission in LS 5039 produces a
lightcurve whose shape corresponds to the X-ray modulation. The observations
imply an outflow velocity of 0.15-0.33c consistent with the expected flow speed
at the pulsar wind termination shock. In LS I +61 303, the calculated Doppler
boosted emission peaks in phase with the observed VHE and X-ray maximum.
Doppler boosting might provide an explanation for the puzzling phasing of the
VHE peak in this system.Comment: 8 pages, 7 figures, accepted for publication in A&
Diffractive Interaction and Scaling Violation in pp->pi^0 Interaction and GeV Excess in Galactic Diffuse Gamma-Ray Spectrum of EGRET
We present here a new calculation of the gamma-ray spectrum from pp->pi^0 in
the Galactic ridge environment. The calculation includes the diffractive pp
interaction and incorporates the Feynman scaling violation for the first time.
Galactic diffuse gamma-rays come, predominantly, from pi^0->gamma gamma in the
sub-GeV to multi-GeV range. Hunter et al. found, however, an excess in the GeV
range ("GeV Excess") in the EGRET Galactic diffuse spectrum above the
prediction based on experimental pp->pi^0 cross-sections and the Feynman
scaling hypothesis. We show, in this work, that the diffractive process makes
the gamma-ray spectrum harder than the incident proton spectrum by ~0.05 in
power-law index, and, that the scaling violation produces 30-80% more pi^0 than
the scaling model for incident proton energies above 100GeV. Combination of the
two can explain about a half of the "GeV Excess" with the local cosmic proton
(power-law index ~2.7). The excess can be fully explained if the proton
spectral index in the Galactic ridge is a little harder (~0.2 in power-law
index) than the local spectrum. Given also in the paper is that the diffractive
process enhances e^+ over e^- and the scaling violation gives 50-100% higher
p-bar yield than without the violation, both in the multi-GeV range.Comment: 35 pages, 11 figures, to appear in Astrophysical Journa
The modulation of the gamma-ray emission from the binary LS 5039
Gamma-ray binaries, composed of a massive star and compact object, have been
established as a new class of sources of very high energy (VHE) photons. The
gamma-rays are produced by inverse Compton scattering of the stellar light by
VHE electrons accelerated in the vicinity of the compact object. The VHE
emission from LS 5039 displays an orbital modulation.
The inverse Compton spectrum depends on the angle between the incoming and
outgoing photon in the electron rest frame. Since the angle at which an
observer sees the star and electrons changes with the orbit, a phase dependence
of the spectrum is expected. The phase-dependent spectrum of LS 5039 is
calculated, assuming a continuous injection of electrons. The shape of the
electron distribution depends on the injected power-law and on the magnetic
field intensity.
Anisotropic scattering produces hard emission at inferior conjunction, when
attenuation due to pair production of the VHE gamma-rays on star light is
minimum. The computed lightcurve and spectra provide good fits to the HESS and
EGRET observations, except at phases of maximum attenuation where pair cascade
emission may be significant for HESS. Detailed predictions are made for a
modulation in the GLAST energy range. The magnetic field intensity at
periastron is 0.8+-0.2 G.
Anisotropic inverse Compton scattering plays a major role in LS 5039. The
derived magnetic field intensity, injection energy and slope suggest a
rotation-powered pulsar wind nebula. Gamma-ray binaries are promising sources
to study the environment of pulsars on small scales.Comment: 12 pages, 8 figures, accepted for publication in A&
Neutrino, Neutron, and Cosmic Ray Production in the External Shock Model of Gamma Ray Bursts
The hypothesis that ultra-high energy (>~ 10^19 eV) cosmic rays (UHECRs) are
accelerated by gamma-ray burst (GRB) blast waves is assumed to be correct.
Implications of this assumption are then derived for the external shock model
of gamma-ray bursts. The evolving synchrotron radiation spectrum in GRB blast
waves provides target photons for the photomeson production of neutrinos and
neutrons. Decay characteristics and radiative efficiencies of the neutral
particles that escape from the blast wave are calculated. The diffuse
high-energy GRB neutrino background and the distribution of high-energy GRB
neutrino events are calculated for specific parameter sets, and a scaling
relation for the photomeson production efficiency in surroundings with
different densities is derived. GRBs provide an intense flux of high-energy
neutrons, with neutron-production efficiencies exceeding ~ 1% of the total
energy release. The radiative characteristics of the neutron beta-decay
electrons from the GRB "neutron bomb" are solved in a special case. Galaxies
with GRB activity should be surrounded by radiation halos of ~ 100 kpc extent
from the outflowing neutrons, consisting of a nonthermal optical/X-ray
synchrotron component and a high-energy gamma-ray component from
Compton-scattered microwave background radiation. The luminosity of sources of
GRBs and relativistic outflows in L* galaxies such as the Milky Way is at the
level of ~10^40+-1 ergs/s. This is sufficient to account for UHECR generation
by GRBs. We briefly speculate on the possibility that hadronic cosmic rays
originate from the subset of supernovae that collapse to form relativistic
outflows and GRBs. (abridged)Comment: 53 pages, 8 figures, ApJ, in press, 574, July 20, 2002. Substantial
revision, previous Appendix expanded to ApJ, 556, 479; cosmic ray origin
speculations to Heidelberg (astro-ph/001054) and Hamburg ICRC
(astro-ph/0202254) proceeding
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)
We examine the radio properties of EGRET-detected blazars observed as part of
the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly
an order of magnitude below the MOJAVE survey and most other samples that have
been used to study the properties of EGRET blazars. At lower flux levels, radio
flux density does not directly correlate with gamma-ray flux density. We do
find that the EGRET-detected blazars tend to have higher brightness
temperatures, greater core fractions, and possibly larger than average jet
opening angles. A weak correlation is also found with jet length and with
polarization. All of the well-established trends can be explained by
systematically larger Doppler factors in the gamma-ray loud blazars, consistent
with the measurements of higher apparent velocities found in monitoring
programs carried out at radio frequencies above 10 GHz.Comment: 20 pages, 7 figures, accepted to Ap
The new model of fitting the spectral energy distributions of Mkn 421 and Mkn 501
The spectral energy distribution (SED) of TeV blazars has a double-humped
shape that is usually interpreted as Synchrotron Self Compton (SSC) model. The
one zone SSC model is used broadly but cannot fit the high energy tail of SED
very well. It need bulk Lorentz factor which is conflict with the observation.
Furthermore one zone SSC model can not explain the entire spectrum. In the
paper, we propose a new model that the high energy emission is produced by the
accelerated protons in the blob with a small size and high magnetic field, the
low energy radiation comes from the electrons in the expanded blob. Because the
high and low energy photons are not produced at the same time, the requirement
of large Doppler factor from pair production is relaxed. We present the fitting
results of the SEDs for Mkn 501 during April 1997 and Mkn 421 during March 2001
respectively.Comment: 5 pages, 1 figures, 1table. accepted for publication in Sciences in
China --
GRB 070311: a direct link between the prompt emission and the afterglow
We present prompt gamma-ray, early NIR/optical, late optical and X-ray
observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to
gain clues on the mechanisms responsible for the prompt gamma-ray pulse as well
as for the early and late multi-band afterglow of GRB 070311. We fitted with
empirical functions the gamma-ray and optical light curves and scaled the
result to the late time X-rays. The H-band light curve taken by REM shows two
pulses peaking 80 and 140 s after the peak of the gamma-ray burst and possibly
accompanied by a faint gamma-ray tail. Remarkably, the late optical and X-ray
afterglow underwent a major rebrightening between 3x10^4 and 2x10^5 s after the
burst with an X-ray fluence comparable with that of the prompt emission
extrapolated in the same band. Notably, the time profile of the late
rebrightening can be described as the combination of a time-rescaled version of
the prompt gamma-ray pulse and an underlying power law. This result supports a
common origin for both prompt and late X-ray/optical afterglow rebrightening of
GRB 070311 within the external shock scenario. The main fireball would be
responsible for the prompt emission, while a second shell would produce the
rebrightening when impacting the leading blastwave in a refreshed shock
(abridged).Comment: 14 pages, 11 figures, 4 tables, accepted to A&
The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary Report
Original paper can be found at: http://proceedings.aip.org/proceedings/ Copyright American Institute of Physics DOI: 10.1063/1.2943545otherPeer reviewe
A Viewing Angle - Kinetic Luminosity Unification Scheme For BL Lacertae Objects
We propose a unified classification for BL Lac objects (BLs), focusing on the
synchrotron peak frequency of the spectral energy distribution. The unification
scheme is based on the angle Theta that describes the orientation of the
relativistic jet and on the electron kinetic luminosity Lambda of the jet. We
assume that Lambda scales with the size of the jet r in a self-similar fashion
(Lambda propto r^2), as supported by observational data. The jets are
self-similar in geometry and have the same pressure and median magnetic field
at the inlet, independent of size. The self-similarity is broken for the
highest energy electrons, which radiate mainly at high frequencies, since for
large sources they suffer more severe radiative energy losses over a given
fraction of the jet length. We calculate the optically thin synchrotron
spectrum using an accelerating inner jet model based on simple relativistic gas
dynamics and show that it can fit the observed infrared to X-ray spectrum of
PKS 2155--304. We couple the accelerating jet model to the unification scheme
and compare the results to complete samples of
BLs. The negative apparent evolution of X-ray selected BLs is explained as a
result of positive evolution of the jet electron kinetic luminosity
. We review observational arguments in favor of the existence of
scaled-down accretion disks and broad emission-line regions in BLs. The
proposed unification scheme can explain the lack of observed broad emission
lines in X-ray selected BLs, as well as the existence of those lines
preferentially in luminous radio-selected BLs. Finally, we review observational
arguments that suggest the extension of this unification scheme to all blazars.Comment: 32 pages, 8 figures, to be published in the ApJ (Oct 20, 1998
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