337 research outputs found
Very Strong TeV Emission as Gamma-Ray Burst Afterglows
Gamma-ray bursts (GRBs) and following afterglows are considered to be
produced by dissipation of kinetic energy of a relativistic fireball and
radiation process is widely believed as synchrotron radiation or inverse
Compton scattering of electrons. We argue that the transfer of kinetic energy
of ejecta into electrons may be inefficient process and hence the total energy
released by a GRB event is much larger than that emitted in soft gamma-rays, by
a factor of \sim (m_p/m_e). We show that, in this case, very strong emission of
TeV gamma-rays is possible due to synchrotron radiation of protons accelerated
up to \sim 10^{21} eV, which are trapped in the magnetic field of afterglow
shock and radiate their energy on an observational time scale of \sim day. This
suggests a possibility that GRBs are most energetic in TeV range and such TeV
gamma-rays may be detectable from GRBs even at cosmological distances, i.e., z
\sim 1, by currently working ground-based telescopes. Furthermore, this model
gives a quantitative explanation for the famous long-duration GeV photons
detected from GRB940217. If TeV gamma-ray emission which is much more energetic
than GRB photons is detected, it provides a strong evidence for acceleration of
protons up to \sim 10^{21} eV.Comment: 10 pages, no figure. To appear in ApJ Letter
Relativistic Winds from Compact Gamma-Ray Sources: II. Pair Loading and Radiative Acceleration in Gamma-ray Bursts
We consider the effects of rapid pair creation by an intense pulse of
gamma-rays propagating ahead of a relativistic shock. Side-scattered photons
colliding with the main gamma-ray beam amplify the density of scattering
charges. The acceleration rate of the pair-loaded medium is calculated, and its
limiting bulk Lorentz factor related to the spectrum and compactness of the
photon source. One obtains, as a result, a definite prediction for the relative
inertia in baryons and pairs. The deceleration of a relativistic shock in the
moving medium, and the resulting synchrotron emissivity, are compared with
existing calculations for a static medium. The radiative efficiency is
increased dramatically by pair loading. When the initial ambient density
exceeds a critical value, the scattering depth traversed by the main gamma-ray
pulse rises above unity, and the pulse is broadened. These considerations place
significant constraints on burst progenitors: a pre-burst mass loss rate
exceeding 10^{-5} M_\odot per year is difficult to reconcile with individual
pulses narrower than 10 s, unless the radiative efficiency is low. An
anisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives a
large velocity shear that greatly increases the energy in the seed magnetic
field forward of the propagating shock.Comment: 19 pp., LaTeX (aaspp4.sty), revised 12/23/99, Ap. J. in press;
summary section added and several minor improvements in presentatio
Small-Angle Scattering of X-Rays from Extragalactic Sources by Dust in Intervening Galaxies
Gamma-ray bursts are now known to be a cosmological population of objects,
which are often accompanied by X-ray and optical afterglows. The total energy
emitted in the afterglow can be similar to the energy radiated in the gamma-ray
burst itself. If a galaxy containing a large column density of dust is near the
line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to
diffraction by the dust grains will give rise to an X-ray echo of the
afterglow. A measurement of the angular size of the echo at a certain time
after the afterglow is observed yields a combination of the angular diameter
distances to the scattering galaxy and the gamma-ray burst that can be used to
constrain cosmological models in the same way as a time delay in a
gravitational lens. The scattering galaxy will generally cause gravitational
lensing as well, and this should modify the shape of the X-ray echo from a
circular ring.
The main difficulty in detecting this phenomenon is the very low flux
expected for the echo. The flux can be increased when the gamma-ray burst is
highly magnified by gravitational lensing, or when the deflecting galaxy is at
low redshift. X-ray echos of continuous (but variable) sources, such as
quasars, may also be detectable with high-resolution instruments and would
allow similar measurements.Comment: To be published in Ap
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
The Evolution of Relativistic Binary Progenitor Systems
Relativistic binary pulsars, such as B1534+12 and B1913+16 are characterized
by having close orbits with a binary separation of ~ 3 R_\sun. The progenitor
of such a system is a neutron star, helium star binary. The helium star, with a
strong stellar wind, is able to spin up its compact companion via accretion.
The neutron star's magnetic field is then lowered to observed values of about
10^{10} Gauss. As the pulsar lifetime is inversely proportional to its magnetic
field, the possibility of observing such a system is, thus, enhanced by this
type of evolution. We will show that a nascent (Crab-like) pulsar in such a
system can, through accretion-braking torques (i.e. the "propeller effect") and
wind-induced spin-up rates, reach equilibrium periods that are close to
observed values. Such processes occur within the relatively short helium star
lifetimes. Additionally, we find that the final outcome of such evolutionary
scenarios depends strongly on initial parameters, particularly the initial
binary separation and helium star mass. It is, indeed, determined that the
majority of such systems end up in the pulsar "graveyard", and only a small
fraction are strongly recycled. This fact might help to reconcile theoretically
expected birth rates with limited observations of relativistic binary pulsars.Comment: 24 pages, 10 Postscript figures, Submitted to The Astrophysical
Journa
Jet break time -- flux density relationship and constraints on physical parameters of GRB afterglows
We derive a relation between the flux density at the light-curve
break of a gamma-ray burst (GRB) afterglow and the break time . The
break is due to the transition from the spherical-like to jet-like evolution of
the afterglow, when the Lorentz factor of the jet equals the inverse of the
initial half-opening angle, i.e., . We show that this
relation indeed behaves as among GRBs for the
slow-cooling case, where is the power-law index of electron distribution. A
statistical analysis of the optical jet breaks of nine GRBs gives , which is consistent with the shock acceleration theory. The value of
derived in this way is different from the observed temporal index
() of the late-time light curve after ,
which suffers several uncertainties from the unclear hydrodynamics of the
sideways expansion and exhibits a large dispersion. Our results not only
confirm that the remnants of GRBs are standard candles, but also provide the
first evidence that the physical parameters of relativistic shocks are
universal, with the favored values and .Comment: 11 pages including 3 color figure
Radio Supernova SN 1998bw and Its Relation to GRB 980425
SN 1998bw is an unusual Type Ic supernova that may be associated with the
-ray burst GRB 980425. We use a synchrotron self-absorption model for
its radio emission to deduce that the synchrotron-emitting gas is expanding
into a circumstellar medium of approximately density profile, at a
speed comparable to the speed of light. We assume that the efficiencies of
production of relativistic electrons and magnetic field are constant through
the evolution. The circumstellar density is consistent with that expected
around the massive star core thought to be the progenitor of SN 1998bw. The
explosion energy in material moving with velocity is ergs, with some preference for the high values. The rise in
the radio light curves observed at days 20-40 is inferred to be the result of a
rise in the energy of the blast wave by a factor . Interaction with a
jump in the ambient density is not consistent with the observed evolution. We
infer that the boost in energy is from a shell of matter from the explosion
that catches up with the decelerating shock front. Both the high explosion
energy and the nature of the energy input to the blast wave are difficult to
reconcile with energy input from the shock-accelerated high velocity ejecta
from a supernova. The implication is that there is irregular energy input from
a central engine, which is the type of model invoked for normal -ray
bursts. The link between SN 1998bw and GRB 980425 is thus strengthened.Comment: 21 pages, 9 figures, revised version to appear in Ap
Implications of the PSR 1257+12 Planetary System for Isolated Millisecond Pulsars
The first extrasolar planets were discovered in 1992 around the millisecond
pulsar PSR 1257+12. We show that recent developments in the study of accretion
onto magnetized stars, plus the existence of the innermost, moon-sized planet
in the PSR 1257+12 system, suggest that the pulsar was born with approximately
its current rotation frequency and magnetic moment. If so, this has important
implications for the formation and evolution of neutron star magnetic fields as
well as for the formation of planets around pulsars. In particular, it suggests
that some and perhaps all isolated millisecond pulsars may have been born with
high spin rates and low magnetic fields instead of having been recycled by
accretion.Comment: 17 pages including one figure, uses aaspp4, accepted by Ap
Issues in the Blandford-Znajek Process for GRB Inner Engine
Several issues regarding the Blandford-Znajek process are discussed to
demonstrate that it can be an effective mechanism for powering the gamma ray
bursts. Using a simple circuit analysis it is argued that the disk power
increases the effective power of the black hole-accretion disk system, although
a part of disk power can be dissipated into black hole entropy. Within the
framework of the force-free magnetosphere with the strong magnetic field, the
magnetically dominated MHD flow is found to support the Blandford-Znajek
process and it is demonstrated that the possible magnetic repulsion by the
rotating black hole will not affect the efficiency substantially.Comment: 10 pages, 1 figure, 3 references added, more discussions on the
magnetic field on the black hole, accepted for publication in Ap
SCUBA observations of the host galaxies of four dark gamma-ray bursts
We present the results of a search for submillimetre-luminous host galaxies
of optically dark gamma-ray bursts (GRBs) using the Submillimetre Common-User
Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We made
photometry measurements of the 850-micron flux at the location of four `dark
bursts', which are those with no detected optical afterglow despite rapid deep
searches, and which may therefore be within galaxies containing substantial
amounts of dust. We were unable to detect any individual source significantly.
Our results are consistent with predictions for the host galaxy population as a
whole, rather than for a subset of dusty hosts. This indicates that optically
dark GRBs are not especially associated with very submillimetre-luminous
galaxies and so cannot be used as reliable indicators of dust-enshrouded
massive star-formation activity. Further observations are required to establish
the relationship between the wider GRB host galaxy population and SCUBA
galaxies.Comment: 6 pages. Accepted for publication in MNRA
- âŠ