## On the rate and on the gravitational wave emission of short and long GRBs. (arXiv:1602.03545v1 [astro-ph.HE])

### February 12th, 2016

Short and long-duration gamma-ray bursts (GRBs) are expected to originate in
binary systems. For short bursts, we have as possible progenitors neutron star
(NS) binaries (NS-NS), or of NS-black hole (BH) binaries (NS-BH). For long
bursts, the induced gravitational collapse (IGC) paradigm introduces a tight
binary system composed of a carbon-oxygen core (CO$_{\rm core}$) undergoing
supernova (SN) explosion that in turn triggers a hypercritical accretion
process onto a NS companion, or onto an already formed BH. These IGC events
lead lead either to NS-NS or to NS-BH binaries, the latter formed when the
accretion is sufficient to bring the NS to its critical mass. We use recent
estimates of the observed rates in X and gamma-rays of the above systems to
estimate the gravitational wave emission in all these binaries and their
expected detection rate by Advanced LIGO.

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## Collision-induced magnetic reconnection and a unified interpretation of polarization properties of GRBs and blazars. (arXiv:1602.03879v1 [astro-ph.HE])

### February 12th, 2016

The jet composition and energy dissipation mechanism of Gamma-ray bursts
(GRBs) and Blazars are fundamental questions which remain not fully understood.
One plausible model is to interpret the $\gamma$-ray emission of GRBs and
optical emission of blazars as synchrotron radiation of electrons accelerated
from the collision-induced magnetic dissipation regions in
Poynting-flux-dominated jets. The polarization observation is an important and
independent information to test this model. Based on our recent 3D relativistic
MHD simulations of collision-induced magnetic dissipation of magnetically
dominated blobs, here we perform calculations of the polarization properties of
the emission in the dissipation region and apply the results to model the
polarization observational data of GRB prompt emission and blazar optical
emission. We show that the same numerical model with different input parameters
can reproduce well the observational data of both GRBs and blazars, especially
the $90^{\circ}$ polarization angle (PA) change in GRB 100826A and the
$180^{\circ}$ PA swing in Blazar 3C279. This supports a unified model for GRB
and blazar jets, suggesting that collision-induced magnetic reconnection is a
common physical mechanism to power the relativistic jet emission from events
with very different black hole masses.

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## Constraints On Holographic Cosmological Models From Gamma Ray Bursts. (arXiv:1601.00183v3 [astro-ph.CO] UPDATED)

### February 12th, 2016

We use Gamma Ray Bursts (GRBs) data from Y. Wang (2008) to put additional
constraints on a set of cosmological dark energy models based on the
holographic principle. GRBs are among the most complex and energetic
astrophysical events known in the universe offering us the opportunity to
obtain information from the history of cosmic expansion up to about redshift of
$z\sim 6$. These astrophysical objects provide us a complementary observational
test to determine the nature of dark energy by complementing the information of
data from Supernovas (e.g. Union 2.1 compilation). We found that the $\Lambda CDM$ model gives the best fit to the observational data, although our
statistical analysis ($\Delta AIC$ and $\Delta BIC$) shows that the models
studied in this work (“Hubble Radius Scale” and “Ricci Scale Q”) have a
reasonable agreement with respect to the most successful, except for the “Ricci
Scale CPL” and “Future Event Horizon” models, which can be ruled out by the
present study. However, these results reflect the importance of GRBs
measurements to provide additional observational constraints to alternative
cosmological models, which are mandatory to clarify the way in which the
paradigm of dark energy or any alternative model is correct.

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## Photodisintegrated gamma rays and neutrinos from heavy nuclei in the gamma-ray burst jet of GRB 130427A. (arXiv:1512.02434v3 [astro-ph.HE] UPDATED)

### February 11th, 2016

Detection of ~ 0.1-70 GeV prompt gamma-ray emission from the exceptionally
bright gamma-ray burst (GRB) 130427A by the Fermi-Large Area Telescope provides
an opportunity to explore the physical processes of GeV gamma-ray emission from
the GRB jets. In this work we discuss interactions of Iron and Oxygen nuclei
with observed keV-MeV photons in the jet of GRB 130427A in order to explain an
additional, hard spectral component observed during 11.5-33 second after
trigger. The photodisintegration time scale for Iron nuclei is comparable to or
shorter than this duration. We find that gamma rays resulting from the Iron
nuclei disintegration can account for the hard power-law component of the
spectra in the 1-70 GeV range, before the gamma-gamma to electron-positron pair
production with low-energy photons severely attenuates emission of higher
energy photons. Electron antineutrinos from the secondary neutron decay, on the
other hand, can be emitted with energies up to 2 TeV. The flux of these
neutrinos is low and consistent with non-detection of GRB 130427A by the
IceCube Neutrino Observatory. The required total energy in the Iron nuclei for
this hadronic model for GeV emission is approximately 10 times the observed
total energy released in the prompt keV-MeV emission.

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## Beaming of particles and synchrotron radiation in relativistic magnetic reconnection. (arXiv:1601.07349v2 [astro-ph.HE] UPDATED)

### February 11th, 2016

Relativistic reconnection has been invoked as a mechanism for particle
acceleration in numerous astrophysical systems. According to idealised
analytical models reconnection produces a bulk relativistic outflow emerging
from the reconnection sites (X-points). The resulting radiation is therefore
highly beamed. Using two-dimensional particle-in-cell (PIC) simulations, we
investigate particle and radiation beaming, finding a very different picture.
Instead of having a relativistic average bulk motion with isotropic electron
velocity distribution in its rest frame, we find that the bulk motion of
particles in X-points is similar to their Lorentz factor gamma, and the
particles are beamed within about 5/gamma. On the way from the X-point to the
magnetic islands, particles turn in the magnetic field, forming a fan confined
to the current sheet. Once they reach the islands they isotropise after
completing a full Larmor gyration and their radiation is not strongly beamed
anymore. The radiation pattern at a given frequency depends on where the
corresponding emitting electrons radiate their energy. Lower energy particles
that cool slowly spend most of their time in the islands, and their radiation
is not highly beamed. Only particles that quickly cool at the edge of the
emerging from these fast cooling particles is above the burn-off limit (about
100 MeV in the overall rest frame of the reconnecting plasma.) This has
significant implications for models of GRBs and AGNs that invoke beaming in
that frame at much lower energies.

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## Machine Learning Model of the Swift/BAT Trigger Algorithm for Long GRB Population Studies. (arXiv:1509.01228v2 [astro-ph.HE] UPDATED)

### February 10th, 2016

To draw inferences about gamma-ray burst (GRB) source populations based on
Swift observations, it is essential to understand the detection efficiency of
the Swift burst alert telescope (BAT). This study considers the problem of
modeling the Swift/BAT triggering algorithm for long GRBs, a computationally
expensive procedure, and models it using machine learning algorithms. A large
sample of simulated GRBs from Lien 2014 is used to train various models: random
forests, boosted decision trees (with AdaBoost), support vector machines, and
artificial neural networks. The best models have accuracies of $\gtrsim97\%$
($\lesssim 3\%$ error), which is a significant improvement on a cut in GRB flux
which has an accuracy of $89.6\%$ ($10.4\%$ error). These models are then used
to measure the detection efficiency of Swift as a function of redshift $z$,
which is used to perform Bayesian parameter estimation on the GRB rate
distribution. We find a local GRB rate density of $n_0 \sim 0.48^{+0.41}_{-0.23} \ {\rm Gpc}^{-3} {\rm yr}^{-1}$ with power-law indices of
$n_1 \sim 1.7^{+0.6}_{-0.5}$ and $n_2 \sim -5.9^{+5.7}_{-0.1}$ for GRBs above
and below a break point of $z_1 \sim 6.8^{+2.8}_{-3.2}$. This methodology is
able to improve upon earlier studies by more accurately modeling Swift
detection and using this for fully Bayesian model fitting. The code used in
this is analysis is publicly available online
(https://github.com/PBGraff/SwiftGRB_PEanalysis).

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## Analysis of the observed and intrinsic durations of gamma-ray bursts with known redshift. (arXiv:1602.02363v1 [astro-ph.HE])

### February 9th, 2016

The duration distribution of 408 GRBs with measured both duration $T_{90}$
and redshift $z$ is examined. Mixtures of a number of distributions (standard
normal, skew-normal, sinh-arcsinh, and alpha-skew-normal) are fitted to the
observed and intrinsic durations using the maximum log-likelihood method. The
best fit is chosen via the Akaike information critetion. The aim of this work
is to assess the presence of the presumed intermediate GRB class, and to
provide a phenomenological model more appropriate than the common mixture of
standard Gaussians. While $\log T^{obs}_{90}$ are well described by a truly
trimodal fit, after moving to the rest frame the statistically most significant
fit is unimodal. To trace the source of this discrepancy, 334 GRBs observed
only by $Swift$/BAT are examined in the same way. In the observer frame, this
results in a number of statistically plausible descriptions, being uni- and
bimodal, and with the number of components ranging from one to three. After
moving to the rest frame, no unambiguous conclusions may be put forward. It is
concluded that the size of the sample is not big enough to infer reliably GRB
properties based on a univariate statistical reasoning only.

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## On the nature of short and long GRBs and their occurrence rate. (arXiv:1602.02732v1 [astro-ph.HE])

### February 9th, 2016

There is mounting evidence for the binary nature of the progenitors of
gamma-ray bursts (GRBs). For a long GRB (L-GRB), the induced gravitational
collapse (IGC) paradigm proposes as its progenitor an “in-state”, namely a
tight binary system composed of a carbon-oxygen core (CO$_{\rm core}$)
undergoing a supernova (SN) explosion which triggers hypercritical accretion
onto a neutron star (NS) companion. For a short GRB (S-GRB), a NS-NS merger is
traditionally adopted as the progenitor. We divide L-GRBs and S-GRBs into two
sub-classes, depending whether or not a black hole (BH) is formed in the merger
or in the hypercritical accretion process exceeding the NS critical mass. For
long bursts, when no BH is formed we have the sub-class of X-ray flashes
(XRFs), with isotropic energy $E_{iso}\lesssim10^{52}$ erg and rest-frame
spectral peak energy $E_{p,i}\lesssim200$~keV. When a BH is formed we have the
sub-class of authentic L-GRBs, also referred to as binary-driven hypernovae
(BdHNe), with $E_{iso}\gtrsim10^{52}$ erg and $E_{p,i}\gtrsim200$ keV. In
analogy, short bursts are similarly divided into two sub-classes. When no BH is
formed, short gamma-ray flashes (S-GRFs) occur, with $E_{iso}\lesssim10^{52}$
erg and $E_{p,i}\lesssim2$ MeV. When a BH is formed, the authentic S-GRBs
occur, with $E_{iso}\gtrsim10^{52}$ erg and $E_{p,i}\gtrsim2$ MeV. We give
examples and observational signatures of these four sub-classes and their rate
of occurrence. From their respective rates it is possible that “in-states” of
S-GRFs and S-GRBs originate from “out-states” of XRFs. We indicate possible
outcomes of three additional progenitor systems: white dwarf-NS, BH-CO$_{\rm core}$ and BH-NS. These systems have hybrid features between S-GRBs and L-GRBs.
In the case of S-GRBs and BdHNe evidence is given of the coincidence of the
onset of the high energy GeV emission with the birth of a Kerr-Newman BH.

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## Cosmic explosions, life in the Universe and the Cosmological Constant. (arXiv:1508.01034v2 [astro-ph.CO] UPDATED)

### February 5th, 2016

Galactic Gamma-Ray Bursts (GRBs) are copious sources of gamma-rays that can
pose a threat to complex life. Using recent determinations of their rate and
the probability of GRBs causing massive extinction, we explore what type of
universes are most likely to harbour advanced forms of life. For this purpose
we use cosmological N-body simulations to determine at what time and for what
value of the cosmological constant ($\Lambda$) the chances of life being
unaffected by cosmic explosions are maximised. We find that $\Lambda-$dominated
universes favour the survival of life against GRBs. Within a $\Lambda$CDM
cosmology, the parameters that govern the likelihood of life survival to GRBs
are dictated by the value of $\Lambda$ and the age of the Universe. We find
that we seem to live in a favorable point in this parameter phase space which
minimises the exposure to cosmic explosions, yet maximises the number of main
sequence (hydrogen-burning) stars around which advanced life forms can exist.

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## Long-Duration Gamma-Ray Burst Host Galaxies in Emission and Absorption. (arXiv:1602.00770v1 [astro-ph.HE])

### February 3rd, 2016

The galaxy population hosting long-duration GRBs provides a means to
constrain the progenitor and an opportunity to use these violent explosions to
characterize the nature of the high-redshift universe. Studies of GRB host
galaxies in emission reveal a population of star-forming galaxies with great
diversity, spanning a wide range of masses, metallicities, and redshifts.
However, as a population GRB hosts are significantly less massive and poorer in
metals than the hosts of other core-collapse transients, suggesting that GRB
production is only efficient at metallicities significantly below Solar. GRBs
may also prefer compact galaxies, and dense and/or central regions of galaxies,
more than other types of core-collapse explosion. Meanwhile, studies of hosts
in absorption against the luminous GRB optical afterglow provide a unique means
of unveiling properties of the ISM in even the faintest and most distant
galaxies; these observations are helping to constrain the chemical evolution of
galaxies and the properties of interstellar dust out to very high redshifts.
New ground- and space-based instrumentation, and the accumulation of larger and
more carefully-selected samples, are continually enhancing our view of the GRB
host population.

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