## Towards an understanding of GRB prompt emission mechanism: I. The origin of spectral lags. (arXiv:1511.08807v1 [astro-ph.HE])

### December 1st, 2015

Z. Lucas Uhm, Bing Zhang (UNLV)

Despite decades of investigations, the physical mechanism that powers the
bright prompt $\gamma$-ray emission from gamma-ray bursts (GRBs) is still not
identified. One important observational clue that remains not properly
interpreted so far is the existence of time lags of broad light curve pulses in
different energy bands, named “spectral lags”. Here we show that the
traditional “kinematic” view invoking the high-latitude emission “curvature
effect” of a relativistic jet cannot account for spectral lags. Rather, the
observed spectral lags demand the sweep of a spectral peak across the observing
energy band in a specific manner. The duration of the broad pulses and inferred
typical Lorentz factor of GRBs require that the emission region is in an
optically thin emission region far from the GRB central engine. We construct a
simple physical model invoking synchrotron radiation from a rapidly expanding
outflow. We show that the observed spectral lags appear naturally in our model
light-curves given that (1) the gamma-ray photon spectrum is curved (as
observed), (2) the magnetic field strength in the emitting region decreases
with radius as the region expands in space, and (3) the emission region itself
undergoes rapid bulk acceleration as the prompt $\gamma$-rays are produced.
These requirements are consistent with a Poynting-flux-dominated jet abruptly
dissipating magnetic energy at a large distance from the engine.

Posted in All Recent GRB Papers | Comments Off on Towards an understanding of GRB prompt emission mechanism: I. The origin of spectral lags. (arXiv:1511.08807v1 [astro-ph.HE])

## Analysis of the observed and intrinsic durations of $Swift$/BAT gamma-ray bursts. (arXiv:1511.08925v1 [astro-ph.HE])

### December 1st, 2015

The duration distribution of 947 GRBs observed by $Swift$/BAT, as well as its
subsample of 347 events with measured redshift, allowing to examine the
durations in both the observer and rest frames, are examined. Using a maximum
log-likelihood method, mixtures of two and three standard Gaussians are fitted
to each sample, and the adequate model is chosen based on the value of the
difference in the log-likelihoods, Akaike information criterion and Bayesian
information criterion. It is found that a two-Gaussian is a better description
than a three-Gaussian, and that the presumed intermediate-duration class is
unlikely to be present in the $Swift$ duration data.

Posted in All Recent GRB Papers | Comments Off on Analysis of the observed and intrinsic durations of $Swift$/BAT gamma-ray bursts. (arXiv:1511.08925v1 [astro-ph.HE])

## High Precision Analyses of Lyman alpha Damping Wing of Gamma-Ray Bursts in the Reionization Era: On the Controversial Results from GRB 130606A at z = 5.91. (arXiv:1508.05067v3 [astro-ph.CO] UPDATED)

### November 26th, 2015

Tomonori Totani (UTokyo), Kentaro Aoki (NAOJ), Takashi Hattori (NAOJ), Nobuyuki Kawai (TITech)

The unprecedentedly bright afterglow of Swift GRB 130606A at z = 5.91 gave us
a unique opportunity to probe the reionization era by high precision analyses
of the redward damping wing of Ly alpha absorption, but the reported
constraints on the neutral hydrogen fraction (f_HI) in intergalactic medium
(IGM) derived from spectra taken by different telescopes are in contradiction.
Here we examine the origin of this discrepancy by analyzing the spectrum taken
by VLT with our own analysis code previously used to fit the Subaru spectrum.
Though the VLT team reported no evidence for IGM HI using the VLT spectrum, we
confirmed our previous result of preferring non-zero IGM HI (the best-fit f_HI
~ 0.06, when IGM HI extends to the GRB redshift). The fit residuals of the VLT
spectrum by the model without IGM HI show the same systematic trend as the
Subaru spectrum. We consider that the likely origin of the discrepancy between
the two teams is the difference of the wavelength ranges adopted in the
fittings; our wavelength range is wider than that of the VLT team, and also we
avoided the shortest wavelength range of deep Ly alpha absorption (lambda_obs <
8426 A), because this region is dominated by HI in the host galaxy and the
systematic uncertainty about host HI velocity distribution is large. We also
study the sensitivity of these results to the adopted Ly alpha cross section
formulae, ranging from the classical Lorentzian function to the most recent one
taking into account fully quantum mechanical scattering. It is found that the
preference for non-zero IGM HI is robust against the choice of the cross
section formulae, but it is quantitatively not negligible and hence one should
be careful in future analyses.

Posted in All Recent GRB Papers | Comments Off on High Precision Analyses of Lyman alpha Damping Wing of Gamma-Ray Bursts in the Reionization Era: On the Controversial Results from GRB 130606A at z = 5.91. (arXiv:1508.05067v3 [astro-ph.CO] UPDATED)

## Ultra High Energy Neutrinos from Gamma-Ray Burst Afterglows Using the Swift-UVOT Data. (arXiv:1511.07010v1 [astro-ph.HE])

### November 24th, 2015

We consider a sample of 107 Gamma Ray Bursts (GRBs) for which early UV
emission was measured by Swift, and extrapolate the photon intensity to lower
energies. Protons accelerated in the GRB jet may interact with such photons to
produce charged pions and subsequently ultra high energy neutrinos
$\varepsilon_\nu\geq 10^{16}$ eV. We use simple energy conversion efficiency
arguments to predict the maximal neutrino flux expected from each GRB. We
estimate the neutrino detection rate at large area radio based neutrino
detectors and conclude that the early afterglow neutrino emission is too weak
to be detected even by next generation neutrino observatories.

Posted in All Recent GRB Papers | Comments Off on Ultra High Energy Neutrinos from Gamma-Ray Burst Afterglows Using the Swift-UVOT Data. (arXiv:1511.07010v1 [astro-ph.HE])

## The energy budget of GRBs based on updated prompt \& afterglow observations. (arXiv:1504.01056v2 [astro-ph.HE] UPDATED)

### November 24th, 2015

We compare the isotropic equivalent 15-2000 keV gamma-ray energy, E_gamma,
emitted by a sample of 91 swift Gamma-Ray Bursts (GRBs) with known redshifts,
with the isotropic equivalent fireball energy, E_fb, as estimated within the
fireball model framework from X-ray afterglow observations of these bursts. The
uncertainty in E_gamma, which spans the range of ~10^51 erg to ~10^53.5 erg, is
approximately 25% on average, due mainly to the extrapolation from the BAT
detector band to the 15-2000 keV band. The uncertainty in E_fb is approximately
a factor of 2, due mainly to the X-ray measurements' scatter. We find E_gamma
and E_fb to be tightly correlated. The average(std) of {\eta}^11hr_gamma is
approximately log_10(E_gamma/(3{\epsilon} _eE^11hr_fb)) are -0.34(0.60), and
the upper limit on the intrinsic spread of {\eta}_gamma is approximately 0.5
({\epsilon}_e is the fraction of shocked plasma energy carried by electrons and
E^x hr_fb is inferred from the X-ray flux at x hours). We also find that E_fb
inferred from X-ray observations at 3 and 11 hours are similar, with an
average(std) of log_10(E^3hr_fb/E^11hr_fb) of 0.04(0.28). The small variance of
{\eta}_gamma implies that burst-to-burst variations in {\epsilon}_e and in the
efficiency of fireball energy conversion to gamma-rays are small, and suggests
that both are of order unity. The small variance of {\eta}_gamma and the
similarity of E^3hr_fb and E^11hr_fb further imply that {\epsilon}_e does not
vary significantly with shock Lorentz factor, and that for most bursts the
modification of fireball energy during the afterglow phase, by processes such
as radiative losses or extended duration energy injection, are not significant.
Finally, our results imply that if fireballs are indeed jets, then the jet
opening angle satisfies {\theta}>0.1 for most cases. [abridged]

Posted in All Recent GRB Papers | Comments Off on The energy budget of GRBs based on updated prompt \& afterglow observations. (arXiv:1504.01056v2 [astro-ph.HE] UPDATED)

## X-ray-powered macronovae. (arXiv:1508.05093v2 [astro-ph.HE] UPDATED)

### November 24th, 2015

A macronova (or kilonova) was observed as an infrared excess several days
after short gamma-ray burst, GRB 130603B. Although the $r$-process
radioactivity is widely discussed as an energy source, it requires huge mass of
ejecta from a neutron star (NS) binary merger. We propose a new model that the
X-ray excess gives rise to the simultaneously observed infrared excess via
thermal re-emission and explore what constraints this would place on the mass
and velocity of the ejecta. This X-ray-powered model explains both the X-ray
and infrared excesses with a single energy source by the central engine like a
black hole, and allows for broader parameter region, in particular smaller
ejecta mass $\sim10^{-3}-10^{-2}M_{\odot}$ with iron mixed as suggested by
general relativistic simulations for typical NS-NS mergers, than the previous
models. We also discuss the other macronova candidates in GRB 060614 and GRB
080503, and implications for the search of electromagnetic counterparts to
gravitational waves.

Posted in All Recent GRB Papers | Comments Off on X-ray-powered macronovae. (arXiv:1508.05093v2 [astro-ph.HE] UPDATED)

## Constraining the Lorentz invariance violation from the continuous spectra of short gamma-ray bursts. (arXiv:1506.08495v2 [astro-ph.HE] CROSS LISTED)

### November 18th, 2015

In quantum gravity, a foamy structure of space-time leads to Lorentz
invariance violation (LIV). As the most energetic astrophysical processes in
the Universe, gamma-ray bursts (GRBs) provide an effective way to probe quantum
gravity effects. We use continuous spectra of 20 short GRBs detected by the
Swift satellite to give a conservative lower limit of quantum gravity energy
scale $M_\textrm{QG}$. Due to the LIV effect, photons with different energy
have different velocities. This will lead to the delayed arrival of high energy
photons relative to the low energy ones. Based on the fact that the LIV-induced
time delay can't be longer than the duration of a GRB, we present the most
conservative estimation of the quantum gravity energy scales from 20 short
GRBs. The most strict constraint, $M_\textrm{QG}>5.05\times10^{14}$ GeV, is
from GRB 140622A.

Posted in All Recent GRB Papers | Comments Off on Constraining the Lorentz invariance violation from the continuous spectra of short gamma-ray bursts. (arXiv:1506.08495v2 [astro-ph.HE] CROSS LISTED)

## Machine Learning Search for Gamma-Ray Burst Afterglows in Optical Surveys. (arXiv:1511.04555v1 [astro-ph.IM])

### November 17th, 2015

Thanks to the advances in robotic telescopes, the time domain astronomy leads
to a large number of transient events detected in images every night. Data
mining and machine learning tools used for object classification are presented.
The goal is to automatically classify transient events for both further
follow-up by a larger telescope and for statistical studies of transient
events. A special attention is given to the identification of gamma-ray burst
afterglows. Machine learning techniques is used to identify GROND gamma-ray
burst afterglow among the astrophysical objects present in the SDSS archival
images based on the $g'-r'$, $r'-i'$ and $i'-z'$ colour indices. The
performance of the support vector machine, random forest and neural network
algorithms is compared. A joint meta-classifier, built on top of the individual
classifiers, can identify GRB afterglows with the overall accuracy of $\gtrsim 90\%$.

Posted in All Recent GRB Papers | Comments Off on Machine Learning Search for Gamma-Ray Burst Afterglows in Optical Surveys. (arXiv:1511.04555v1 [astro-ph.IM])

## Constraining the Lorentz invariance violation from the continuous spectra of short gamma-ray bursts. (arXiv:1506.08495v2 [astro-ph.HE] UPDATED)

### November 17th, 2015

In quantum gravity, a foamy structure of space-time leads to Lorentz
invariance violation (LIV). As the most energetic astrophysical processes in
the Universe, gamma-ray bursts (GRBs) provide an effective way to probe quantum
gravity effects. We use continuous spectra of 20 short GRBs detected by the
Swift satellite to give a conservative lower limit of quantum gravity energy
scale $M_\textrm{QG}$. Due to the LIV effect, photons with different energy
have different velocities. This will lead to the delayed arrival of high energy
photons relative to the low energy ones. Based on the fact that the LIV-induced
time delay can't be longer than the duration of a GRB, we present the most
conservative estimation of the quantum gravity energy scales from 20 short
GRBs. The most strict constraint, $M_\textrm{QG}>5.05\times10^{14}$ GeV, is
from GRB 140622A.

Posted in All Recent GRB Papers | Comments Off on Constraining the Lorentz invariance violation from the continuous spectra of short gamma-ray bursts. (arXiv:1506.08495v2 [astro-ph.HE] UPDATED)

## A Deep Search for Prompt Radio Emission from the Short GRB 150424A With The Murchison Widefield Array. (arXiv:1511.03656v1 [astro-ph.HE])

### November 13th, 2015

We present a search for prompt radio emission associated with the
short-duration gamma-ray burst (GRB) 150424A using the Murchison Widefield
Array (MWA) at frequencies from 80-133 MHz. Our observations span delays of 23
s-30 min after the GRB, corresponding to dispersion measures of 100-7700
pc/cm^3. We see no excess flux in images with timescales of 4 s, 2 min, or 30
min, and set a 3 sigma flux density limit of 3.0 Jy at 132 MHz on the shortest
timescales: some of the most stringent limits to date on prompt radio emission
from any type of GRB. We use these limits to constrain a number of proposed
models for coherent emission from short-duration GRBs, although we show that
our limits are not particularly constraining for fast radio bursts because of
reduced sensitivity for this pointing. Finally, we discuss the prospects for
using the MWA to search for prompt radio emission from gravitational wave
transients and find that while the flux density and luminosity limits are
likely to be very constraining, the latency of the gravitational wave alert may
limit the robustness of any conclusions.

Posted in All Recent GRB Papers | Comments Off on A Deep Search for Prompt Radio Emission from the Short GRB 150424A With The Murchison Widefield Array. (arXiv:1511.03656v1 [astro-ph.HE])