691 research outputs found
The Effect of Pair Cascades on the High-Energy Spectral Cutoff in Gamma-Ray Bursts
The highly luminous and variable prompt emission in Gamma-Ray Bursts (GRBs)
arises in an ultra-relativistic outflow. The exact underlying radiative
mechanism shaping its non-thermal spectrum is still uncertain, making it hard
to determine the outflow's bulk Lorentz factor . GRBs with spectral
cutoff due to pair production () at energies
MeV are extremely useful for inferring . We find that
when the emission region has a high enough compactness, then as it becomes
optically thick to scattering, Compton downscattering by non-relativistic
-pairs can shift the spectral cutoff energy well below the
self-annihilation threshold, . We treat this
effect numerically and show that obtained assuming
can under-predict its true value by as much as an order of magnitude.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letter
Hot Electromagnetic Outflows. III. Displaced Fireball in a Strong Magnetic Field
The evolution of a dilute electron-positron fireball is calculated in the
regime of strong magnetization and very high compactness (l ~10^3-10^8).
Heating is applied at a low effective temperature (< 25 keV), and the fireball
is allowed to expand, so that the formation of a black-body spectral
distribution is inhibited by pair annihilation. The diffusion equation for
Compton scattering is coupled to a single-temperature pair gas and an exact
(trans-relativistic) cyclo-synchrotron photon source. We find that the photon
spectrum develops a quasi-thermal peak, with a power-law slope below it that is
characteristic of gamma-ray bursts. The formation of a thermal high-frequency
spectrum is checked using the full kinetic equations at l ~ 10^3. These results
have several implications for the central engine of GRBs, and the mechanism of
energy transport. 1. Baryon rest mass carries less than ~ 10^{-5} of the energy
flux at jet breakout inside ~ 10^{12} cm from the engine, with most carried by
the magnetic field. 2. This degree of baryon purity points to the presence of
an event horizon in the engine, and neutrons play a negligible role in the
prompt emission mechanism. 3. X-ray flashes are emitted by outflows carrying
enough baryons that the photosphere is pair-depleted, which we show results in
faster thermalization. 4. The relation between observed peak frequency and
burst luminosity is bounded below by the observed Amati et al. relation if jet
Lorentz factor ~ 1/(opening angle) at breakout. 5. Stellar models are used to
demonstrate an inconsistency between the highest observed GRB energies, and a
hydrodynamic nozzle: magnetic collimation is required. 6. The magnetized pair
gas is dilute enough that high-frequency Alfven waves may become charge
starved. Finally, we suggest that limitations on magnetic reconnection from
plasma collisionality have been overestimated.Comment: 29 pages, 34 figures, submitted to the Ap
Pulse Structure of Hot Electromagnetic Outflows with Embedded Baryons
Gamma-ray bursts (GRBs) show a dramatic pulse structure that requires bulk
relativistic motion, but whose physical origin has remained murky. We focus on
a hot, magnetized jet that is emitted by a black hole and interacts with a
confining medium. Strongly relativistic expansion of the magnetic field, as
limited by a corrugation instability, may commence only after it forms a thin
shell. Then the observed burst duration is dominated by the curvature
delay, and null periods arise from angular inhomogeneities, not the duty cycle
of the engine. We associate the s timescale observed in the pulse width
distribution of long GRBs with the collapse of the central 2.5-3 of a
massive stellar core. A fraction of the baryons are shown to be embedded in the
magnetized outflow by the hyper-Eddington radiation flux; they strongly disturb
the magnetic field after the compactness drops below . The high-energy photons so created have a compressed pulse
structure. Delayed breakout of magnetic field from heavier baryon shells is
also a promising approach to X-ray flares. In the second part of the paper, we
calculate the imprint of an expanding, scattering photosphere on pulse
evolution. Two models for generating the high-energy spectral tail are
contrasted: i) pair breakdown due to reheating of an optically thin pair plasma
embedded in a thermal radiation field; and ii) continuous heating extending
from large to small scattering depth. The second model is strongly inconsistent
with the observed hard-to-soft evolution in GRB pulses. The first shows some
quantitative differences if the emission is purely spherical, but we show that
finite shell width, mild departures from spherical curvature, and latitudinal
Lorentz factor gradients have interesting effects.Comment: Submitted to the ApJ, 21 pages, 22 figure
Non-thermal Gamma-ray Emission from Delayed Pair Breakdown in a Magnetized and Photon-rich Outflow
We consider delayed, volumetric heating in a magnetized outflow that has
broken out of a confining medium and expanded to a high Lorentz factor () and low optical depth to scattering (). The energy flux at breakout is dominated by the magnetic
field, with a modest contribution from quasi-thermal gamma rays whose spectrum
was calculated in Paper I. We focus on the case of extreme baryon depletion in
the magnetized material, but allow for a separate baryonic component that is
entrained from a confining medium. Dissipation is driven by relativistic motion
between these two components, which develops once the photon compactness drops
below . We first calculate the acceleration of the
magnetized component following breakout, showing that embedded MHD turbulence
provides significant inertia, the neglect of which leads to unrealistically
high estimates of flow Lorentz factor. After re-heating begins, the pair and
photon distributions are evolved self-consistently using a one-zone kinetic
code that incorporates an exact treatment of Compton scattering, pair
production and annihilation, and Coulomb scattering. Heating leads to a surge
in pair creation, and the scattering depth saturates at
1-4. The plasma maintains a very low ratio of particle to magnetic pressure,
and can support strong anisotropy in the charged particle distribution, with
cooling dominated by Compton scattering. High-energy power-law spectra with
photon indices in the range observed in GRBs () are obtained
by varying the ratio of heat input to the seed energy in quasi-thermal photons.
We contrast our results with those for continuous heating across an expanding
photosphere, and show that the latter model produces soft-hard evolution that
is inconsistent with observations of GRBs.Comment: Submitted to the ApJ, 27 pages, 19 figure
Magnetic Reconnection Instabilities in Soft-Gamma Repeaters
We examine an external trigger mechanism that gives rise to the intense soft
gamma-ray repeater (SGR) giant flares. Out of the three giant flares, two
showcased the existence of a precursor, which we show to have had initiated the
main flare. We develop a reconnection model based on the hypothesis that
shearing motion of the footpoints causes the materialization of a Sweet-Parker
current layer in the magnetosphere. The thinning of this macroscopic layer due
to the development of an embedded super-hot turbulent current layer switches on
the impulsive Hall reconnection, which powers the giant flare. We show that the
thinning time is on the order of the pre-flare quiescent time.Comment: 4 pages, 2 figures, submission for the Proceedings of the Thirteenth
Marcel Grossman Meeting on General Relativity edited by Kjell Rosquist,
Robert T Jantzen, Remo Ruffini World Scientific, Singapore, 201
Statistical ages and the cooling rate of X-ray dim isolated neutron stars
The cooling theory of neutron stars is corroborated by its comparison with
observations of thermally emitting isolated neutron stars and accreting neutron
stars in binary systems. An important ingredient for such an analysis is the
age of the object, which, typically, is obtained from the spin-down history.
This age is highly uncertain if the object's magnetic field varies appreciably
over time. Other age estimators, such as supernova remnant ages and kinematic
ages, only apply to few handful of neutron stars. We conduct a population
synthesis study of the nearby isolated thermal emitters and obtain their ages
statistically from the observed luminosity function of these objects. We argue
that a more sensitive blind scan of the galactic disk with the upcoming space
telescopes can help to constrain the ages to higher accuracy.Comment: 9 pages, 2 figures, 2 tables. Figures and tables updated. Added
discussion on errors. Accepted for publication in MNRA
Lessons from the short GRB170817A - the First Gravitational Wave Detection of a Binary Neutron Star Merger
The first, long awaited, detection of a gravitational wave (GW) signal from
the merger of a binary neutron-star (NS-NS) system was finally achieved
(GW170817), and was also accompanied by an electromagnetic counterpart --
the short-duration GRB 170817A. It occurred in the nearby (Mpc)
elliptical galaxy NGC4993, and showed optical, IR and UV emission from half
a day up to weeks after the event, as well as late time X-ray (at days) and radio (at days) emission. There was a delay of
s between the GW merger chirp signal and the
prompt-GRB emission onset, and an upper limit of
was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal.
In this letter we examine some of the implications of these groundbreaking
observations. The delay sets an upper limit on the prompt-GRB
emission radius, ,
for a jet with sharp edges at an angle . GRB
170817A's relatively low isotropic equivalent -ray energy-output may
suggest a viewing angle slightly outside the jet's sharp edge,
, but its peak
photon energy and afterglow emission suggest instead that the jet
does not have sharp edges and the prompt emission was dominated by less
energetic material along our line of sight, at . Finally, we consider the type of remnant that is produced by the
NS-NS merger and find that a relatively long-lived (s) massive NS is
strongly disfavored, while a hyper-massive NS of lifetime s appears to
be somewhat favored over the direct formation of a black hole.Comment: 5 pages, 3 figures, minor changes; accepted to ApJ
2D Relativistic MHD Simulations of the Kruskal-Schwarzschild Instability in a Relativistic Striped Wind
We study the linear and non-linear development of the Kruskal-Schwarzchild
Instability in a relativisitically expanding striped wind. This instability is
the generalization of Rayleigh-Taylor instability in the presence of a magnetic
field. It has been suggested to produce a self-sustained acceleration mechanism
in strongly magnetized outflows found in active galactic nuclei, gamma-ray
bursts, and micro-quasars. The instability leads to magnetic reconnection, but
in contrast with steady-state Sweet-Parker reconnection, the dissipation rate
is not limited by the current layer's small aspect ratio. We performed
two-dimensional (2D) relativistic magneto-hydrodynamic (RMHD) simulations
featuring two cold and highly magnetized () plasma
layers with an anti-parallel magnetic field separated by a thin layer of
relativistically hot plasma with a local effective gravity induced by the
outflow's acceleration. Our simulations show how the heavier relativistically
hot plasma in the reconnecting layer drips out and allows oppositely oriented
magnetic field lines to reconnect. The instability's growth rate in the linear
regime matches the predictions of linear stability analysis. We find turbulence
rather than an ordered bulk flow near the reconnection region, with turbulent
velocities up to c, largely independent of model parameters. However,
the magnetic energy dissipation rate is found to be much slower, corresponding
to an effective ordered bulk velocity inflow into the reconnection region
, of . This occurs due to the slow evacuation of hot plasma from the
current layer, largely because of the Kelvin-Helmholtz instability experienced
by the dripping plasma. 3D RMHD simulations are needed to further investigate
the non-linear regime.Comment: 12 pages, 12 figures, Accepted for Publication in MNRA
When Did the Remnant of GW170817 Collapse to a Black Hole?
The main hard pulse of prompt gamma-ray emission in GRB170817A had a
duration of and its onset was delayed with respect to the
gravitational-wave chirp signal by .
Detailed follow-up of the subsequent broadband kilonova emission revealed a
two-component ejecta -- a lanthanide-poor ejecta with mass that powered the early but rapidly fading blue
emission and a lanthanide-rich ejecta with mass that powered the longer lasting redder emission. Both the prompt
gamma-ray onset delay and the existence of the blue ejecta with modest electron
fraction, , can be explained if the collapse to a
black hole was delayed by the formation of a hypermassive neutron star (HMNS).
Here, we determine the survival time of the merger remnant by combining two
different constraints, namely, the time needed to produce the requisite
blue-ejecta mass and that necessary for the relativistic jet to bore its way
out of the expanding ejecta. In this way, we determine that the remnant of
GW170817 must have collapsed to a black hole after . We also discuss how future detections and
the delays between the gravitational and electromagnetic emissions can be used
to constrain the properties of the merged object.Comment: 20 pages, 7 figures. Incorporated comments from the referee. Accepted
for publication in Ap
Linear polarization in gamma-ray burst prompt emission
Despite being hard to measure, GRB prompt -ray emission polarization
is a valuable probe of the dominant emission mechanism and the outflow's
composition and angular structure. During the prompt emission the outflow is
ultra-relativistic with Lorentz factors . We describe in detail the
linear polarization properties of various emission mechanisms: synchrotron
radiation from different magnetic field structures (ordered: toroidal or radial , and random: normal to the radial direction
), Compton drag, and photospheric emission. We calculate the
polarization for different GRB jet angular structures (e.g. top-hat, Gaussian,
power-law) and viewing angles . Synchrotron with
can produce large polarizations, up to , for a
top-hat jet but only for lines of sight just outside the jet's sharp edge. The
same also holds for Compton drag, albeit with a slightly higher overall .
Moreover, we demonstrate how -variations during the GRB or smoother jet
edges would significantly reduce . We construct a semi-analytic model for
non-dissipative photospheric emission from structured jets. Such emission can
produce up to with reasonably high fluences, but this
requires steep gradients in . A polarization of
can robustly be produced only by synchrotron
emission from a transverse magnetic field ordered on angles
around our line of sight (like a global toroidal field). Therefore, such a
model would be strongly favored even by a single secure measurement within this
range. We find that such a model would also be favored if is
measured in most GRBs within a large enough sample, by deriving the
polarization distribution for our different emission and jet models.Comment: 31 pages, 18 figures. Minor changes to the text; Accepted for
publication in MNRA
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