90 research outputs found
Simulations of GRB Jets in a Stratified External Medium: Dynamics, Afterglow Lightcurves, Jet Breaks and Radio Calorimetry
The dynamics of GRB jets during the afterglow phase is most reliably and
accurately modelled using hydrodynamic simulations. All published simulations,
however, have considered only a uniform external medium, while a stratified
external medium is expected around long duration GRB progenitors. Here we
present simulations of the dynamics of GRB jets and the resulting afterglow
emission for both uniform and stratified external media with for k = 0, 1, 2. The simulations are performed in 2D using the special
relativistic version of the Mezcal code. The dynamics for stratified external
media are broadly similar to those derived for expansion into a uniform
external medium. The jet half-opening angle start increasing logarithmically
with time once the Lorentz factor drops below 1/theta_0. For larger k values
the lateral expansion is faster at early times and slower at late times with
the jet expansion becoming Newtonian and slowly approaching spherical symmetry
over progressively longer timescales. We find that contrary to analytic
expectations, there is a reasonably sharp jet break in the lightcurve for k = 2
although the shape of the break is affected more by the viewing angle than by
the slope of the external density profile. Steeper density profiles are found
to produce more gradual jet breaks while larger viewing angles cause smoother
and later appearing jet breaks. The counter-jet becomes visible as it becomes
sub-relativistic, and for k=0 this results in a clear bump-like feature in the
light curve. However, for larger k values the jet decelerates more gradually,
causing only a mild flattening in the radio light curve that might be hard to
discern when k=2. Late time radio calorimetry is likely to consistently
over-estimate the true energy by up to a factor of a few for k=2, and either
over-predict or under-predict it by a smaller factor for k = 0,1.Comment: 10 pages, 13 figures, submitted to Ap
Diversity Of Short Gamma-Ray Burst Afterglows From Compact Binary Mergers Hosting Pulsars
Short gamma-ray bursts (sGRBs) are widely believed to result from the mergers
of compact binaries. This model predicts an afterglow that bears the
characteristic signatures of a constant, low density medium, including a smooth
prompt-afterglow transition, and a simple temporal evolution. However, these
expectations are in conflict with observations for a non-negligible fraction of
sGRB afterglows. In particular, the onset of the afterglow phase for some of
these events appears to be delayed and, in addition, a few of them exhibit
late- time rapid fading in their lightcurves. We show that these peculiar
observations can be explained independently of ongoing central engine activity
if some sGRB progenitors are compact binaries hosting at least one pulsar. The
Poynting flux emanating from the pulsar companion can excavate a bow-shock
cavity surround- ing the binary. If this cavity is larger than the shock
deceleration length scale in the undisturbed interstellar medium, then the
onset of the afterglow will be delayed. Should the deceleration occur entirely
within the swept-up thin shell, a rapid fade in the lightcurve will ensue. We
identify two types of pulsar that can achieve the conditions necessary for
altering the afterglow: low field, long lived pulsars, and high field pulsars.
We find that a sizable fraction (~20-50%) of low field pulsars are likely to
reside in neutron star binaries based on observations, while their high field
counterparts are not. Hydrodynamical calculations motivated by this model are
shown to be in good agreement with observations of sGRB afterglow lightcurves.Comment: Accepted to ApjL. Direct comparison to observed X-Ray afterglows now
included. 5 Figure
What determines the structure of short gamma-ray burst jets?
The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray
burst arising from a neutron star merger, has challenged our understanding of
the angular structure of relativistic jets. Studies of the jet propagation
usually assume that the jet is ejected from the central engine with a top-hat
structure and its final structure, which determines the observed light curve
and spectra, is primarily regulated by the interaction with the nearby
environment. However, jets are expected to be produced with a structure that is
more complex than a simple top-hat, as shown by global accretion simulations.
We present numerical simulations of short GRBs launched with a wide range of
initial structures, durations and luminosities. We follow the jet interaction
with the merger remnant wind and compute its final structure at distances
~cm from the central engine. We show that the final jet
structure, as well as the resulting afterglow emission, depend strongly on the
initial structure of the jet, its luminosity and duration. While the initial
structure at the jet is preserved for long-lasting SGRBs, it is strongly
modified for jets barely making their way through the wind. This illustrates
the importance of combining the results of global simulations with propagation
studies in order to better predict the expected afterglow signatures from
neutron star mergers. Structured jets provide a reasonable description of the
GRB 170817A afterglow emission with an off-axis angle .Comment: 9 pages, 7 Figures; Added discussion; Accepted for publication in
MNRA
The dynamics, appearance and demographics of relativistic jets triggered by tidal disruption of stars in quiescent supermassive black holes
We examine the consequences of a model in which relativistic jets can be
triggered in quiescent massive black holes when a geometrically thick and hot
accretion disk forms as a result of the tidal disruption of a star. To estimate
the power, thrust and lifetime of the jet, we use the mass accretion history
onto the black hole as calculated by detailed hydrodynamic simulations of the
tidal disruption of stars. We go on to determine the states of the interstellar
medium in various types of quiescent galactic nuclei, and describe how this
external matter can affect jets propagating through it. We use this
information, together with a two-dimensional hydrodynamic model of the
structure of the relativistic flow, to study the dynamics of the jet, the
propagation of which is regulated by the density stratification of the
environment and by its injection history. The breaking of symmetry involved in
transitioning from one to two dimensions is crucial and leads to qualitatively
new phenomena. Many of the observed properties of the Swift 1644+57/GRB 110328A
event can be understood as resulting from accretion onto and jets driven by a
central mass black hole following the disruption of sun-like
star. With the inclusion of a stochastic contribution to the luminosity due to
variations in the feeding rate driven by instabilities near the tidal radius,
we find that our model can explain the X-ray light curve without invoking a
rarely-occurring deep encounter. In conjunction with the number density of
black holes in the local universe, we hypothesize that the conditions required
to produce the Swift event are not anomalous, but are in fact representative of
the jet-driven flare population arising from tidal disruptions.
[abridged]Comment: 16 pages, 16 figures, submitted to Ap
The Fate of the Merger Remnant in GW170817 and its Imprint on the Jet Structure
The first neutron star binary merger detected in gravitational waves,
GW170817 and the subsequent detection of its emission across the
electromagnetic spectrum showed that these systems are viable progenitors of
short -ray bursts (sGRB). The afterglow signal of GW170817 has been
found to be consistent with a structured GRB jet seen off-axis, requiring
significant amounts of relativistic material at large angles. This trait can be
attributed to the interaction of the relativistic jet with the external wind
medium. Here we perform numerical simulations of relativistic jets interacting
with realistic wind environments in order to explore how the properties of the
wind and central engine affect the structure of successful jets. We find that
the angular energy distribution of the jet depends primarily on the ratio
between the lifetime of the jet and the time it takes the merger remnant to
collapse. We make use of these simulations to constrain the time it took for
the merger remnant in GW170817 to collapse into a black hole based on the
angular structure of the jet as inferred from afterglow observations.
We conclude that the lifetime of the merger remnant in GW170817 was s, which, after collapse, triggered the formation of the jet.Comment: 13 figures, 15 pages, accepted in Ap
Hydrodynamical Simulations to Determine the Feeding Rate of Black Holes by the Tidal Disruption of Stars: The Importance of the Impact Parameter and Stellar Structure
The disruption of stars by supermassive black holes has been linked to more
than a dozen flares in the cores of galaxies out to redshift .
Modeling these flares properly requires a prediction of the rate of mass return
to the black hole after a disruption. Through hydrodynamical simulation, we
show that aside from the full disruption of a solar mass star at the exact
limit where the star is destroyed, the common assumptions used to estimate
, the rate of mass return to the black hole, are largely invalid.
While the analytical approximation to tidal disruption predicts that the
least-centrally concentrated stars and the deepest encounters should have more
quickly-peaked flares, we find that the most-centrally concentrated stars have
the quickest-peaking flares, and the trend between the time of peak and the
impact parameter for deeply-penetrating encounters reverses beyond the critical
distance at which the star is completely destroyed. We also show that the
most-centrally concentrated stars produced a characteristic drop in
shortly after peak when a star is only partially disrupted, with
the power law index being as extreme as -4 in the months immediately
following the peak of a flare. Additionally, we find that asymptotes to
for both low- and high-mass stars for approximately half of all
stellar disruptions. Both of these results are significantly steeper than the
typically assumed . As these precipitous decay rates are only seen
for events in which a stellar core survives the disruption, they can be used to
determine if an observed tidal disruption flare produced a surviving remnant.
These results should be taken into consideration when flares arising from tidal
disruptions are modeled.
[abridged]Comment: 16 pages, 13 figures (2 new figures in revised version). Published in
ApJ. Latest version incorporates erratum that fixes issue with fitting
formulae not including enough significant digit
Private equity and venture capital in Italy
This paper examines private equity and venture capital in Italy. The first part looks at the main features of the Italian market and its recent evolution; the second part considers the results of a survey of firms and intermediaries designed to gather information regarding contract features and the characteristics of investee firms and investing intermediaries. Finally, the paper discusses the main obstacles to the development of the sector using information from the survey of intermediaries.private equity, venture capital
- …