2,642 research outputs found
Giant X-ray Bump in GRB 121027A: Evidence for Fall-back Disk Accretion
A particularly interesting discovery in observations of GRB 121027A is that
of a giant X-ray bump detected by the Swift/X-Ray Telescope. The X-ray
afterglow re-brightens sharply at about 1000 s after the trigger by more than
two orders of magnitude in less than 200 s. This X-ray bump lasts for more than
10 ks. It is quite different from typical X-ray flares. In this Letter we
propose a fall-back accretion model to interpret this X-ray bump within the
context of the collapse of a massive star for a long-duration gamma-ray burst.
The required fall-back radius of about 3.5e10 cm and mass of about 0.9-2.6
solar masses imply that a significant part of the helium envelope should
survive through the mass loss during the last stage of the massive progenitor
of GRB 121027A.Comment: 5 pages, 3 figures, 2013, ApJL, 767:L3
Jet Luminosity of Gamma-ray Bursts: Blandford-Znajek Mechanism v.s. Neutrino Annihilation Process
A neutrino-dominated accretion flow (NDAF) around a rotating stellar-mass
black hole (BH) is one of the plausible candidates for the central engine of
gamma-ray bursts (GRBs). Two mechanisms, i.e., Blandford-Znajek (BZ) mechanism
and neutrino annihilation process, are generally considered to power GRBs.
Using the analytic solutions from Xue et al. (2013) and ignoring the effects of
the magnetic field configuration, we estimate the BZ and neutrino annihilation
luminosities as the functions of the disk masses and BH spin parameters to
contrast the observational jet luminosities of GRBs. The results show that,
although the neutrino annihilation processes could account for most of GRBs,
the BZ mechanism is more effective, especially for long-duration GRBs.
Actually, if the energy of afterglows and flares of GRBs is included, the
distinction between these two mechanisms is more significant. Furthermore,
massive disk mass and high BH spin are beneficial to power high luminosities of
GRBs. Finally, we discuss possible physical mechanisms to enhance the disk mass
or the neutrino emission rate of NDAFs and relevant difference between these
two mechanisms.Comment: 20 pages, 3 tables, 2 figures, accepted for publication in ApJ
Physical origin of multi-wavelength emission of GRB 100418A and implications for its progenitor
GRB 100418A is a long burst at z=0.624 without detection of any associated
supernova (SN). Its lightcurves in both the prompt and afterglow phases are
similar to GRB 060614, a nearby long GRB without an associated SN. We analyze
the observational data of this event and discuss the possible origins of its
multi-wavelength emission. We show that its joint lightcurve at 1 keV derived
from Swift BAT and XRT observations is composed of two distinguished
components. The first component, whose spectrum is extremely soft (\Gamma =
4.32), ends with a steep decay segment, indicating the internal origin of this
component. The second component is a slowly-rising, broad bump which peaks at
~10^5 seconds post the BAT trigger. Assuming that the late bump is due to onset
of the afterglow, we derive the initial Lorentz factor (Gamma_0) of the GRB
fireball and find that it significantly deviates from the relation between the
Gamma_0 and Eiso of typical GRBs. We also check whether it follows the same
anti-correlation between X-ray luminosity and the break time observed in the
shallow decay phase of many typical GRBs, which is usually regarded as a signal
of late energy injection from the GRB central engine. However, we find that it
does not obey this correlation. We propose that the late bump could be
contributed by a two-component jet. We fit the second component with an
off-axis jet model for a constant medium density and find the late bump can be
represented by the model. The derived jet half-opening angle is 0.30 rad and
the viewing angle is 0.315 rad. The medium density is 0.05 cm^-3, possibly
suggesting that it may be from a merger of compact stars. The similarity
between GRBs 060614 and 100418A may indicate that the two GRBs are from the
same population and the late bump observed in the two GRBs may be a signal of a
two-component jet powered by the GRB central engine.Comment: 8 pages, 3 figures, accepted for publication in Research in Astron.
Astrophy
Can black-hole neutrino-cooled disks power short gamma-ray bursts?
Stellar-mass black holes (BHs) surrounded by neutrino-dominated accretion
flows (NDAFs) are the plausible candidates to power gamma-ray bursts (GRBs) via
neutrinos emission and their annihilation. The progenitors of short-duration
GRBs (SGRBs) are generally considered to be compact binaries mergers. According
to the simulation results, the disk mass of the NDAF has been limited after
merger events. We can estimate such disk mass by using the current SGRB
observational data and fireball model. The results show that the disk mass of a
certain SGRB mainly depends on its output energy, jet opening angle, and
central BH characteristics. Even for the extreme BH parameters, some SGRBs
require massive disks, which approach or exceed the limits in simulations. We
suggest that there may exist alternative magnetohydrodynamic processes or some
mechanisms increasing the neutrino emission to produce SGRBs with the
reasonable BH parameters and disk mass.Comment: 17 pages, 1 table, 2 figures, accepted for publication in Ap
Is There a Relation between Duration and E_iso in Gamma-Ray Bursts?
The system of accretion disk and black hole is usually considered as the
central engine of Gamma-ray Bursts (GRBs). It is usually thought that the disk
in the central engine of GRBs is the advection-dominated accretion disk, which
is developed from a massive (mass M_disk) torus at radius r_disk. We find a
positive correlation between the isotropic gamma-ray energy E_iso and duration
(so-called T_90) for GRBs. We interpret this correlation within the
advection-dominated accretion disk model, associating E_iso and T_90 with M_
disk and viscous timescale respectively.Comment: 2 pages, 3 figures. Feeding compact objects: Accretion on all scales.
Proceedings IAU Symp. No. 290, 201
Central Engine-Powered Bright X-ray Flares in Short Gamma-Ray Bursts: A Hint of Black Hole-Neutron Star Merger?
Short gamma-ray bursts may originate from the merger of double neutron stars
(NS) or that of a black hole (BH) and an NS. We propose that the bright X-ray
flare related to the central engine reactivity may hint a BH-NS merger, since
such a merger can provide more fall-back materials and therefore a more massive
accretion disk than the NS-NS merger. Based on the observed 49 short bursts
with Swift/X-ray Telescope follow-up observations, we find that three bursts
have bright X-ray flares, among which three flares from two bursts are probably
related to the central engine reactivity. We argue that these two bursts may
originate from the BH-NS merger rather than the NS-NS merger. Our suggested
link between the central engine-powered bright X-ray flare and the BH-NS merger
event can be checked by the future gravitational wave detections from advanced
LIGO and Virgo.Comment: 15 pages, 6 figures, accepted for publication in Ap
Evolutions of stellar-mass black hole hyperaccretion systems in the center of gamma-ray bursts
A neutrino-dominated accretion disk around a stellar-mass black hole (BH) can
power a gamma-ray burst (GRB) via annihilation of neutrinos launched from the
disk. For the BH hyperaccretion system, high accretion rate should trigger the
violent evolution of the BH's characteristics, which further leads to the
evolution of the neutrino annihilation luminosity. In this paper, we consider
the evolution of the accretion system to analyze the mean time-dependent
neutrino annihilation luminosity with the different mean accretion rates and
initial BH parameters. By time-integrating the luminosity, the total neutrino
annihilation energy with the reasonable initial disk mass can satisfy the most
of short-duration GRBs and about half of long-duration GRBs. Moreover, the
extreme Kerr BH should exist in the cental engines of some high-luminosity
GRBs. GRBs with higher energy have to request the alternative
magnetohydrodynamics processes in the centers, such as the Blandford-Znajek jet
from the accretion system or the millisecond magnetar.Comment: 20 pages, 6 figure
Photosphere emission in the X-Ray Flares of Swift Gamma-Ray Bursts and Implications for the Fireball Properties
X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft
X-ray range and the spectral coverage is limited. In this paper, we present an
analysis of 32 GRB X-ray flares that are simultaneously observed by both BAT
and XRT on board the Swift mission, so a joint spectral analysis with a wider
spectral coverage is possible. Our results show that the joint spectra of 19
flares are fitted with the absorbed single power-law or the Band function
models. More interestingly, the joint spectra of the other 13 X-ray flares are
fitted with the absorbed single power-law model plus a black body (BB)
component. Phenomenally, the observed spectra of these 13 flares are analogous
to several GRBs with a thermal component, but only with a much lower
temperature of keV. Assuming that the thermal emission is the
photosphere emission of the GRB fireball, we derive the fireball properties of
the 13 flares that have redshift measurements, such as the bulk Lorentz factor
of the outflow. The derived range from
to and a relation of to the thermal emission luminosity
is found. It is consistent with the relation that are
derived for the prompt gamma-ray emission. We discuss the physical implications
of these results within the content of jet composition and radiation mechanism
of GRBs and X-ray flares.Comment: 31 pages, 8 figures, and 4 tables. Accepted for publication in Ap
Spectroscopic Identification and Chemical Distribution of HII Regions in the Galactic Anti-center Area from LAMOST
We spectroscopically identify 101 Galactic HII regions using spectra from the
Large Sky Area Multi- Object Fiber Spectroscopic Telescope (LAMOST) survey,
cross-matched with an HII region catalog derived from the all-sky Wide-Field
Infrared Survey Explorer(WISE) data. Among all HII regions in our sample, 47
sources are newly confirmed. Spatially, most of our identified HII regions are
located in the anti-center area of the Galaxy. For each of the HII regions, we
accurately extract and measure the nebular emission lines of the spectra, and
estimate the oxygen abundances using the strong-line method. We focus on the
abundance distribution of HII regions in the Galactic anti-center area.
Accordingly, we derive the oxygen abundance gradient with a slope of -0.036
+/-0.004 dex/kpc, covering a range of RG from 8.1 to 19.3 kpc. In particular,
we also fit the outer disk objects with a slope of -0.039 +/- 0.012 dex /kpc,
which indicates that there is no flattening of the radial oxygen gradient in
the outer Galactic disk.Comment: accepted by PASP, 13 pages, 5 figure
X-ray light curve in GRB 170714A: evidence for quark star?
Two plateaus and one following bump in the X-ray light curve of GRB 170714A
have been detected by the \textit{Swift}/X-Ray Telescope, which could be very
meaningful for the central engine of gamma-ray bursts (GRBs), implying that the
origin of this burst might be different from that of other ultra-long GRBs. We
propose that merging two neutron stars into a hyper-massive quark star (QS) and
then collapsing into a black hole (BH), with a delay time around ~s,
could be responsible for those X-ray components. The hyper-massive QS is
initially in a fluid state, being turbulent and differentially rotating, but
would be solidified and release its latent heat injected into the GRB fireball
(lasting about ~s during the liquid-solid phase transition). Magnetic
field as high as ~G could be created by dynamo action of the
newborn liquid QS, and a magnetar-like central engine (after solidification)
supplies significant energy for the second plateau. More energy could be
released during a fall-back accretion after the post-merger QS collapses to a
BH, and the X-ray bump forms. This post-merger QS model might be tested by
future observations, with either advanced gravitational wave detectors (e.g.,
advanced LIGO and VIRGO) or X-ray/optical telescopes.Comment: 6 pages, 1 figure, 1 table, accepted for publication in Ap
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