890 research outputs found
Thermal emission from bare quark matter surfaces of hot strange stars
We consider the thermal emission of photons and electron-positron pairs from
the bare quark surface of a hot strange star. The radiation of high-energy (>
20 MeV) equilibrium photons prevails at the surface temperature T_S > 5 x
10^{10} K, while below this temperature, 8 x 10^8 < T_S < 5 x 10^{10} K,
emission of electron-positron pairs created by the Coulomb barrier at the quark
surface dominates. The thermal luminosity of a hot strange star in both photons
and pairs is estimated.Comment: 10 pages, 2 figures, ApJLetters, in pres
Afterglow Light Curve Modulated by a Highly Magnetized Millisecond Pulsar
We investigate consequences of a continuously energy-injecting central engine
of gamma-ray burst (GRB) afterglow emission, assuming that a highly magnetized
pulsar is left beaming in the core of a GRB progenitor. Beaming and continuous
energy-injection are natural consequences of the pulsar origin of GRB
afterglows. Whereas previous studies have considered continuous
energy-injection from a new-born pulsar to interpret the deviation of afterglow
light curves of GRBs from those with the simple power law behavior, a beaming
effect, which is one of the most important aspects of pulsar emissions, is
ignored in earlier investigations. We explicitly include the beaming effect and
consider a change of the beaming with time due to a dynamical evolution of a
new-born pulsar. We show that the magnitude of the afterglow from this fireball
indeed first decreases with time, subsequently rises, and declines again. One
of the most peculiar optical afterglows light curve of GRB 970508 can be
accounted for by continuous energy injection with beaming due to a highly
magnetized new-born pulsar. We discuss implications on such observational
evidence for a pulsar.Comment: 4 pages, 1 table, submitted to Astronomy and Astrophysics (Letters
Linearly polarized X-ray flares following short gamma-ray bursts
Soft X-ray flares were detected to follow the short-duration gamma-ray burst
GRB 050724. The temporal properties of the flares suggest that they are likely
due to the late time activity of the central engine. We argue that if short
GRBs are generated through compact star mergers, as is supported by the recent
observations, the jet powering the late X-ray flares must be launched via
magnetic processes rather than via neutrino-antineutrino annihilations. As a
result, the X-ray flares following short GRBs are expected to be linearly
polarized. The argument may also apply to the X-ray flares following long GRBs.
Future observations with the upcoming X-ray polarimeters will test this
prediction.Comment: 4 pages (no figure), accepted for publication in ApJL, typos
correcte
Gamma-ray burst early afterglows: reverse shock emission from an arbitrarily magnetized ejecta
Evidence suggests that the gamma-ray burst (GRB) ejecta is likely magnetized,
although the degree of magnetization of the ejecta is unknown. We derive a
rigorous analytical solution for the relativistic 90 degree shocks under the
ideal MHD condition, and use them to study the reverse shock emission
properties of an arbitrarily magnetized ejecta. Contrary to the previous
belief, we find that strong relativistic shocks still exist in the high-sigma
limit. Assuming a constant density of the circumburst medium, we study the
shell-medium interaction in detail and categorize various critical radii for
shell evolution. With typical GRB parameters, a reverse shock exists when sigma
is less than a few tens or a few hundreds. The shell evolution can be still
categorized into the thick and thin shell regimes, but the separation between
the two regime now depends on sigma and the thick shell regime greatly shrinks
at high-sigma. The early optical afterglow lightcurves are calculated for GRBs
with a wide range of the sigma values. We find that the reverse shock emission
level increases steadily with sigma initially, but starts to decline when sigma
becomes larger than unity. In the high-sigma regime the reverse shock peak is
usually broadened due to the separation of the shock crossing radius and the
deceleration radius in the thin shell regime. The early afterglow data and
tight upper limits of known GRBs could be understood within the theoretical
framework developed in this paper, with the inferred sigma value varying in a
wide range.Comment: Updated to match the version to appear in Ap
GaN evaporation and enhanced diffusion of Ar during high-temperature ion implantation
GaN films were implanted with 150 keV Ar+ at temperatures up to 1100 °C to a dose of 3×1015 cm-2. Concentration profiles of Ar were measured by secondary ion mass spectroscopy and depth distributions of ion-induced damage were estimated from Rutherford backscattering/channeling spectra. No redistribution of Ar atoms was detected up to 700 °C. At 1000 °C a deep penetrating diffusion tail and a shift of the Ar peak to the surface were observed. At temperatures higher than 800 °C shift of the damage peak to the surface was also observed. We attributed the shift of the Ar peak and the damage peaks to evaporation of thin layer of GaN during high-temperature implantation and estimated its temperature dependence
Environment and Energy Injection Effects in GRB Afterglows
In a recent paper (Dai & Lu 1999), we have proposed a simple model in which
the steepening in the light curve of the R-band afterglow of the gamma-ray
burst (GRB) 990123 is caused by the adiabatic shock which has evolved from an
ultrarelativistic phase to a nonrelativistic phase in a dense medium. We find
that such a model is quite consistent with observations if the medium density
is about . Here we discuss this model in more
details. In particular, we investigate the effects of synchrotron self
absorption and energy injection. A shock in a dense medium becomes
nonrelativistic rapidly after a short relativistic phase. The afterglow from
the shock at the nonrelativistic stage decays more rapidly than at the
relativistic stage. Since some models for GRB energy sources predict that a
strongly magnetic millisecond pulsar may be born during the formation of GRB,
we discuss the effect of such a pulsar on the evolution of the nonrelativistic
shock through magnetic dipole radiation. We find that after the energy which
the shock obtains from the pulsar is much more than the initial energy of the
shock, the afterglow decay will flatten significantly. When the pulsar energy
input effect disappears, the decay will steepen again. These features are in
excellent agreement with the afterglows of GRB 980519, GRB 990510 and GRB
980326. Furthermore, our model fits very well all the observational data of GRB
980519 including the last two detections.Comment: 21 pages, LaTeX, accepted for publication in ApJ, one paragraph adde
The Compact Central Object in the Supernova Remnant G266.2-1.2
We observed the compact central object CXOU J085201.4--461753 in the
supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector
in timing mode. The spectrum of this object can be described by a blackbody
model with the temperature kT=404 eV and radius of the emitting region R=0.28
km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the
source spectrum. The spectrum shows a marginally significant feature at 1.68
keV. Search for periodicity yields two candidate periods, about 301 ms and 33
ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions
are 13% and 9%, respectively. We find no evidence for long-term variability of
the source flux, nor do we find extended emission around the central object. We
suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the
central source of the supernova remnant Cas A. It could be either a neutron
star with a low or regular magnetic field, slowly accreting from a fossil disk,
or, more likely, an isolated neutron star with a superstrong magnetic field. In
either case, a conservative upper limit on surface temperature of a 10 km
radius neutron star is about 90 eV, which suggests accelerated cooling for a
reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur
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