Nucleosynthesis in the gamma-ray burst accretion disks and associated outflows. (arXiv:1302.3067v1 [astro-ph.HE])

Nucleosynthesis in the gamma-ray burst accretion disks and associated outflows. (arXiv:1302.3067v1 [astro-ph.HE]):
We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks formed by the Type II collapsars and outflows launched from these disks.
We deal with accretion disks having relatively low accretion rates: 0.001 M_sun
s^{-1} <~ Mdot <~ 0.01 M_sun s^{-1} and hence they are predominantly advection
dominated. We report the synthesis of several unusual nuclei like 31P, 39K,
43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese and
copper in the disk. We also confirm that isotopes of iron, cobalt, nickel,
argon, calcium, sulphur and silicon get synthesized in the disk, as shown by
previous authors. Much of these heavy elements thus synthesized are ejected
from the disk and survive in the outflows. Indeed, emission lines of many of
these heavy elements have been observed in the X-ray afterglows of several
GRBs.

X-ray plateaus followed by sharp drops in GRBs 060413, 060522, 060607A and 080330: Further evidences for central engine afterglow from Gamma-ray Bursts. (arXiv:1301.3975v1 [astro-ph.HE])

X-ray plateaus followed by sharp drops in GRBs 060413, 060522, 060607A and 080330: Further evidences for central engine afterglow from Gamma-ray Bursts. (arXiv:1301.3975v1 [astro-ph.HE]):
The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are
characterized by plateaus that are followed by very sharp drops. An X-ray
plateau is interpretable within the framework of the external forward shock
model but the sharp drop is not. In this work we interpret these peculiar X-ray
afterglow data as the central engine afterglows from some magnetized central
engines, plausibly magnetars. In this model, the X-ray afterglows are powered
by the internal magnetic energy dissipation and the sudden drop is caused by
the collapse of the magnetar. Accordingly, the X-ray plateau photons should
have a high linear polarization, which can be tested by the future X-ray
polarimetry.

Early Thermal X-ray Emission from Long Gamma-ray Bursts and Their Circumstellar Environments. (arXiv:1301.2421v1 [astro-ph.HE])

Early Thermal X-ray Emission from Long Gamma-ray Bursts and Their Circumstellar Environments. (arXiv:1301.2421v1 [astro-ph.HE]):
We performed a series of hydrodynamical calculations of an ultra-relativistic
jet propagating through a massive star and the circumstellar matter to
investigate the interaction between the ejecta and the circumstellar matter. We
succeed in distinguishing two qualitatively different cases in which the ejecta
are shocked and adiabatically cool. To examine whether the cocoon expanding at
subrelativistic speeds emits any observable signal, we calculate expected
photospheric emission from the cocoon. It is found that the emission can
explain early thermal X-ray emission recently found in some long gamma-ray
bursts. The result implies that the difference of the circumstellar environment
of long gamma-ray bursts can be probed by observing their early thermal X-ray
emission.

Bright broad-band afterglows of gravitational wave bursts from binary neutron star mergers as a probe of millisecond magnetars. (arXiv:1301.0439v1 [astro-ph.HE])

Bright broad-band afterglows of gravitational wave bursts from binary neutron star mergers as a probe of millisecond magnetars. (arXiv:1301.0439v1 [astro-ph.HE]):
If double neutron star mergers leave behind a massive magnetar rather than a
black hole, a bright early afterglow can follow the gravitational wave burst
(GWB) even if there is no short gamma-ray burst (SGRB) - GWB association or
there is an association but the SGRB does not beam towards earth(zhang 2012).
Besides directly dissipating the proto-magnetar wind, we here suggest that the
magnetar wind could push the ejecta launched during the merger process, and
under certain conditions, would reach a relativistic speed. Such a
magnetar-powered ejecta, when interacting with the ambient medium, would
develop a bright broad-band afterglow due to synchrotron radiation. We study
this physical scenario in detail, and present the predicted X-ray, optical and
radio light curves for a range of magnetar and ejecta parameters. We show that
the X-ray and optical lightcurves usually peak around the magnetar spindown
time scale (10^3-10^5s), reaching brightness readily detectable by wide-field
X-ray and optical telescopes, and remain detectable for an extended period. The
radio afterglow peaks later, but is much brighter than the case without a
magnetar energy injection. Therefore, such bright broad-band afterglows, if
detected and combined with GWBs in the future, would be a probe of massive
millisecond magnetars and stiff equation-of-state for nuclear matter.

Prompt Emission from Tidal Disruptions of White Dwarfs by Intermediate Mass Black Holes. (arXiv:1212.5267v1 [astro-ph.HE])

Prompt Emission from Tidal Disruptions of White Dwarfs by Intermediate Mass Black Holes. (arXiv:1212.5267v1 [astro-ph.HE]):
We present a qualitative picture of prompt emission from tidal disruptions of
white dwarfs (WD) by intermediate mass black holes (IMBH). The smaller size of
an IMBH compared to a supermassive black hole and a smaller tidal radius of a
WD disruption lead to a very fast event with high peak luminosity. Magnetic
field is generated in situ following the tidal disruption, which leads to
effective accretion. Since large-scale magnetic field is also produced,
geometrically thick super-Eddington inflow leads to a relativistic jet. The
dense jet possesses a photosphere, which emits quasi-thermal radiation in soft
X-rays. The source can be classified as a long low-luminosity gamma-ray burst
(ll-GRB). Tidal compression of a WD causes nuclear ignition, which is
observable as an accompanying supernova. We suggest that GRB060218 and SN2006aj
is such a pair of ll-GRB and supernova. We argue that in a flux-limited sample
the disruptions of WDs by IMBHs are more frequent then the disruptions of other
stars by IMBHs.

Pop III GRBs: an estimative of the event rate for future surveys. (arXiv:1209.0823v1 [astro-ph.CO])

Pop III GRBs: an estimative of the event rate for future surveys. (arXiv:1209.0823v1 [astro-ph.CO]):
We discuss the theoretical event rate of gamma-ray bursts (GRBs) from the
collapse of massive primordial stars. We construct a theoretical model to
calculate the rate and detectability of these GRBs taking into account all
important feedback and recent results from numerical simulations of pristine
gas. We expect to observe a maximum of N $\lesssim$ 0.2 GRBs per year
integrated over at z > 6 with \textit{Swift} and N $\lesssim$ 10 GRBs per year
integrated over at z > 6 with EXIST.

General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE])

General Relativistic Simulations of Accretion Induced Collapse of Neutron Stars to Black Holes. (arXiv:1209.0783v1 [astro-ph.HE]):
Neutron stars (NSs) in the astrophysical Universe are often surrounded by
accretion disks. Accretion of matter onto a NS may increase its mass above the
maximum value allowed by its equation of state, inducing its collapse to a
black hole (BH). Here we study this process for the first time, in 3D, and in
full general relativity. By considering three initial NS configurations, each
with and without a surrounding disk (of mass ~7% M_{NS}), we investigate the
effect of the accretion disk on the dynamics of the collapse and its imprint on
both the gravitational wave (GW) and electromagnetic (EM) signals that can be
emitted by these sources. We show in particular that, even if the GW signal is
similar for the accretion induced collapse (AIC) and the collapse of a NS in
vacuum (and detectable only for Galactic sources), the EM counterpart could
allow to discriminate between these two types of events. In fact, our
simulations show that, while the collapse of a NS in vacuum leaves no
appreciable baryonic matter outside the event horizon, an AIC is followed by a
phase of rapid accretion of the surviving disk onto the newly formed BH. The
post-collapse accretion rates, on the order of ~10^{-2} M_{sun} s^{-1}, make
these events tantalizing candidates as engines of short Gamma-Ray Bursts.

Black Hole-Neutron Star Mergers: Disk Mass Predictions. (arXiv:1207.6304v1 [astro-ph.HE])

Black Hole-Neutron Star Mergers: Disk Mass Predictions. (arXiv:1207.6304v1 [astro-ph.HE]):
Determining the final result of black hole-neutron star mergers, and in
particular the amount of matter remaining outside the black hole at late times,
has been one of the main motivations behind the numerical simulation of these
systems. Black hole-neutron star binaries are amongst the most likely
progenitors of short gamma-ray bursts --- as long as they result in the
formation of massive (at least ~0.1 solar mass) accretion disks around the
black hole. Whether this actually happens strongly depends on the physical
characteristics of the system, and in particular on the mass ratio, the spin of
the black hole, and the radius of the neutron star. We present here a simple
two-parameter model, fitted to existing numerical results, for the
determination of the mass remaining outside the black hole a few milliseconds
after a black hole-neutron star merger. This model predicts the remnant mass
within a few percents of the mass of the neutron star, at least for remnant
masses up to 20% of the neutron star mass. Results across the range of
parameters deemed to be the most likely astrophysically are presented here. We
find that, for 10 solar mass black holes, massive disks are only possible for
fairly large neutron stars (R>12km), or quasi-extremal black hole spins
(a/M>0.9). We also use our model to discuss how the equation of state of the
neutron star affects the final remnant, and the strong influence that this can
have on the rate of short gamma-ray bursts produced by black hole-neutron star
mergers.

The mystery of the missing GRB redshifts. (arXiv:1206.5558v1 [astro-ph.CO])

The mystery of the missing GRB redshifts. (arXiv:1206.5558v1 [astro-ph.CO]):
It is clear that optical selection effects have distorted the "true" GRB
redshift distribution to its presently observed biased distribution. We
constrain a statistically optimal model that implies GRB host galaxy dust
extinction could account for up to 40% of missing optical afterglows and
redshifts in $z = 0-3$, but the bias is negligible at very high-$z$. The
limiting sensitivity of the telescopes, and the time to acquire
spectroscopic/photometric redshifts, are significant sources of bias for the
very high-$z$ sample. We caution on constraining star formation rate and
luminosity evolution using the GRB redshift distribution without accounting for
these selection effects.

Long Duration X-Ray Flash and X-Ray Rich Gamma Ray Burst from Low Mass Population III Star. (arXiv:1207.2835v1 [astro-ph.HE])

Long Duration X-Ray Flash and X-Ray Rich Gamma Ray Burst from Low Mass Population III Star. (arXiv:1207.2835v1 [astro-ph.HE]):
Recent numerical simulations suggest that Population III (Pop III) stars are
born with masses not larger than $\sim 100M_\odot$ but typically $\sim
40M_{\odot}$. We investigate whether such a low mass Pop III star can raise a
Gamma Ray Burst (GRB) by considering the propagation of a jet, which is
launched from the black hole, in the stellar envelope. It is generally believed
that a super giant star is not an appropriate progenitor of a GRB, since the
large envelope prevents the successful jet breakout. Especially for Pop III
stars, the mass loss is not expected and the large hydrogen envelope is kept
due to the low opacity envelope. We find, however, that those Pop III stars who
end as blue super giants are compact enough for jets to break out the stellar
envelopes successfully. We evaluate observational characters of Pop III GRBs
and predict that Pop III GRBs have the duration of $\sim 10^5$ sec in the
observer frame and the peak luminosity of $\sim 5 \times 10^{50}{\rm erg/sec}$.
Moreover, assuming that the $E_p-L_p$ correlation (or the $E_p-E_{\gamma, \rm
iso}$ correlation) holds for Pop III GRBs, we find that the spectrum peak
energy falls $\sim$ a few keV (or $\sim 100$ keV) in the observer frame. We
discuss the detectability of Pop III GRBs by future satellite missions such as
EXIST and Lobster. If the $E_p-E_{\gamma, \rm iso}$ correlation holds for Pop
III GRBs, we find that EXIST is more appropriate for GRB detections and that
EXIST can detect Pop III GRBs at $z \lesssim 9$. We observe such Pop III GRBs
at $z \sim 9$ as long duration X-ray rich GRBs by EXIST. On the other hand, if
the $E_p-L_p$ correlation holds, we find that Lobster is more appropriate for
detecting GRBs and that Lobster can detect very high z Pop III GRBs up to $z
\sim 19$. We observe Pop III GRBs as long duration X-ray flashes by Lobster.