1,310 research outputs found
Possible evolutionary transition from rapidly rotating neutron stars to strange stars due to spin-down
We present a scenario of formation of strange stars due to spin-down of {\it
rapidly rotating} neutron stars left after supernova explosions . By assuming a
process where the total baryon mass is conserved but the angular momentum is
lost due to emission of gravitational waves and/or the magnetic braking, we
find that the transition from rapidly rotating neutron stars to slowly rotating
strange stars is possible; a large amount of energy could
be released. The liberated energy might become a new energy source for a
delayed explosion of supernova. Furthermore, our scenario suggests that the
supernova associated with gamma-ray bursts could become candidates for targets
in the future observation of gravitational waves.Comment: 11 pages, 3 figures, Received November 5, 200
A Fourier-Based Algorithm for Modelling Aberrations in HETE-2's Imaging System
The High-Energy Transient Explorer (HETE-2), launched in October 2000, is a
satellite experiment dedicated to the study of gamma-ray bursts in a very wide
energy range from soft X-ray to gamma-ray wavelengths. The intermediate X-ray
range (2-30keV) is covered by the Wide-field X-ray Monitor WXM, a coded
aperture imager. In this article, an algorithm for reconstructing the positions
of gamma-ray bursts is described, which is capable of correcting systematic
aberrations to approximately 1 arcmin throughout the field of view.
Functionality and performance of this algorithm have been validated using data
from Monte Carlo simulations as well as from astrometric observations of the
X-ray source Scorpius X-1.Comment: 14 pages, 9 figures, 2 tables; Nucl.Instr.Meth., in pres
Delayed soft X-ray emission lines in the afterglow of GRB 030227
Strong, delayed X-ray line emission is detected in the afterglow of GRB
030227, appearing near the end of the XMM-Newton observation, nearly twenty
hours after the burst. The observed flux in the lines, not simply the
equivalent width, sharply increases from an undetectable level (<1.7e-14
erg/cm^2/s, 3 sigma) to 4.1e-14 erg/cm^2/s in the final 9.7 ks. The line
emission alone has nearly twice as many detected photons as any previous
detection of X-ray lines. The lines correspond well to hydrogen and/or
helium-like emission from Mg, Si, S, Ar and Ca at a redshift z=1.39. There is
no evidence for Fe, Co or Ni--the ultimate iron abundance must be less than a
tenth that of the lighter metals. If the supernova and GRB events are nearly
simultaneous there must be continuing, sporadic power output after the GRB of a
luminosity >~5e46 erg/s, exceeding all but the most powerful quasars.Comment: Submitted to ApJL. 14 pages, 3 figures with AASLaTe
Measurement and control of a mechanical oscillator at its thermal decoherence rate
In real-time quantum feedback protocols, the record of a continuous
measurement is used to stabilize a desired quantum state. Recent years have
seen highly successful applications in a variety of well-isolated
micro-systems, including microwave photons and superconducting qubits. By
contrast, the ability to stabilize the quantum state of a tangibly massive
object, such as a nanomechanical oscillator, remains a difficult challenge: The
main obstacle is environmental decoherence, which places stringent requirements
on the timescale in which the state must be measured. Here we describe a
position sensor that is capable of resolving the zero-point motion of a
solid-state, nanomechanical oscillator in the timescale of its thermal
decoherence, a critical requirement for preparing its ground state using
feedback. The sensor is based on cavity optomechanical coupling, and realizes a
measurement of the oscillator's displacement with an imprecision 40 dB below
that at the standard quantum limit, while maintaining an
imprecision-back-action product within a factor of 5 of the Heisenberg
uncertainty limit. Using the measurement as an error signal and radiation
pressure as an actuator, we demonstrate active feedback cooling (cold-damping)
of the 4.3 MHz oscillator from a cryogenic bath temperature of 4.4 K to an
effective value of 1.10.1 mK, corresponding to a mean phonon number of
5.30.6 (i.e., a ground state probability of 16%). Our results set a new
benchmark for the performance of a linear position sensor, and signal the
emergence of engineered mechanical oscillators as practical subjects for
measurement-based quantum control.Comment: 24 pages, 10 figures; typos corrected in main text and figure
Swift and Fermi observations of X-ray flares: the case of Late Internal Shock
Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a
unique broadband view of their afterglow emission, spanning more than ten
decades in energy. We present the sample of X-ray flares observed by both Swift
and Fermi during the first three years of Fermi operations. While bright in the
X-ray band, X-ray flares are often undetected at lower (optical), and higher
(MeV to GeV) energies. We show that this disfavors synchrotron self-Compton
processes as origin of the observed X-ray emission. We compare the broadband
properties of X-ray flares with the standard late internal shock model, and
find that, in this scenario, X-ray flares can be produced by a late-time
relativistic (Gamma>50) outflow at radii R~10^13-10^14 cm. This conclusion
holds only if the variability timescale is significantly shorter than the
observed flare duration, and implies that X-ray flares can directly probe the
activity of the GRB central engine.Comment: 13 pages, 4 figures, accepted for publication in Ap
Prompt and delayed emission properties of Gamma-Ray Bursts observed with BeppoSAX
We investigated the spectral evolution in the 2--700 keV energy band of
Gamma-Ray Bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and
localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite
before May 1998. Most of them have been followed-up with the Narrow Field
Instruments aboard the same satellite. In the light of these results we discuss
open issues on the GRB phenomenon. We find that the optically thin synchrotron
shock model (SSM) provides an acceptable representation of most of the
time-resolved GRB spectra extending down to 2 keV, except in the initial phases
of several bursts and during the whole duration of the quite strong GRB970111,
where a low-energy photon depletion with respect to the thin SSM spectrum is
observed. We find that the X-ray afterglow starts at about 50% of the GRB
duration, and that its fluence, as computed from the WFC light curve, is
consistent with the decay law found from the afterglow NFI observations. We
also investigate the hydrodynamical evolution of the GRB in our sample and
their associated afterglow, when it was detected. We find that the photon index
of the latest spectrum of the GRB prompt emission is correlated with the index
of the afterglow fading law, when available, as expected on the basis of an
external shock of a relativistic fireball.Comment: 35 pages, 1 LaTeX file, 20 postscript figures, 1 postscript table,
accepted for pubblication in Astrophysical Journal Supplement Series.
Corrected error bars in Fig.2/GRB980425/panel B and GRB980425 fluence in Tab.
The X-ray afterglow of the Gamma-ray burst of May 8, 1997: spectral variability and possible evidence of an iron line
We report the possible detection (99.3% of statistical significance) of
redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst
GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of
the putative host galaxy determined from optical spectroscopy. The line
disappeared about 1 day after the burst. We have also analyzed the spectral
variability during the outburst event that characterizes the X-ray afterglow of
this GRB. The spectrum gets harder during the flare, turning to steep when the
flux decreases. The variability, intensity and width of the line indicate that
the emitting region should have a mass approximately greater than 0.5 solar
masses (assuming the iron abundance similar to its solar value), a size of
about 3 times 10^15 cm, be distributed anisotropically, and be moving with
sub-relativistic speed. In contrast to the fairly clean environment expected in
the merging of two neutron stars, the observed line properties would imply that
the site of the burst is embedded in a large mass of material, consistent with
pre-explosion ejecta of a very massive star. This material could be related
with the outburst observed in the afterglow 1 day after the GRB and with the
spectral variations measured during this phase.Comment: To appear in The Astrophysical Journal Letters, AASTEX LateX, 2
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