arXiv subjects

Om S. Salafia

Publications and source records attributed to Om S. Salafia.

7 recordsLinked to original sources

arXiv2022

Accretion-to-jet energy conversion efficiency in GW170817

Based on previously published multi-wavelength modelling of the GRB 170817A jet afterglow, that includes information from the VLBI centroid motion, we construct the posterior probability density distribution on the total energy in the bipolar jets launched by the GW170817 merger remnant. By applying a new numerical-relativity-informed fitting formula for the accretion disk mass, we construct the posterior probability density distribution of the GW170817 remnant disk mass. By combining the two, we estimate the accretion-to-jet energy conversion efficiency in this system, carefully accounting for uncertainties. The accretion-to-jet energy conversion efficiency in GW170817 is $η\sim 10^{-3}$ with an uncertainty of slightly less than two orders of magnitude. This low efficiency is in good agreement with expectations from the $ν\barν$ mechanism, which therefore cannot be excluded by this measurement alone. Such an efficiency also agrees with that anticipated for the Blandford-Znajek mechanism, provided that the magnetic field in the disk right after the merger is predominantly toroidal (which is expected as a result of the merger dynamics).

astro-ph.HE↗

arXiv2020

High latitude emission from structured jet of Gamma-Ray Bursts observed off-axis

The X-ray emission of gamma-ray bursts (GRBs) is often characterized by an initial steep decay, followed by a nearly constant emission phase (so called "plateau") which can extend up to thousands of seconds. While the steep decay is usually interpreted as the tail of the prompt gamma-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a "structured jet" whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand over this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth and (c) the lightcurve dependence on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE lightcurve is smoothly decaying with no clear distinction between the steep and flat phase, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall lightcurve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first Swift-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.

astro-ph.HE↗

arXiv2018

Interpreting GRB170817A as a giant flare from a jet-less double neutron-star merger

We show that the delay between GRB170817A and GW170817 is incompatible with de-beamed emission from an off-axis relativistic jet. The prompt emission and the subsequent radio and X-ray observations can instead be interpreted within a giant-flare-like scenario, being the result of a relativistic outflow driven by the ultra-strong magnetic field produced by magnetohydrodynamic amplification during the merger of the progenitor double neutron-star binary. Within such picture, the data indicate that the outflow must be endowed with a steep velocity profile, with a relatively fast tail extending to Gamma~8. Since the conditions for the launch of such an outflow are quite general, and the presence of a velocity profile is a natural expectation of the acceleration process, most neutron star binary mergers should feature this quasi-isotropic, hard X-ray emission component, that can be a powerful guide to the discovery of additional kilonovae associated to relatively nearby gravitational wave events.

astro-ph.HE↗

arXiv2017

Jet-driven and jet-less fireballs from compact binary mergers

During a compact binary merger involving at least one neutron star, a small fraction of the gravitational energy could be liberated in such a way to accelerate a small fraction (~ 10^-6) of the neutron star mass in an isotropic or quasi-isotropic way. In presence of certain conditions, a pair-loaded fireball can form, which undergoes accelerated expansion reaching relativistic velocities. As in the standard fireball scenario, internal energy is partly transformed into kinetic energy. At the photospheric radius, the internal radiation can escape, giving rise to a pulse that lasts for a time equal to the delay time since the merger. The subsequent interaction with the interstellar medium can then convert part of the remaining kinetic energy back into radiation in a weak isotropic afterglow at all wavelengths. This scenario does not require the presence of a jet: the associated isotropic prompt and afterglow emission should be visible for all NS-NS and BH-NS mergers within 90 Mpc, independent of their inclination. The prompt emission is similar to that expected from an off-axis jet, either structured or much slower than usually assumed (Gamma ~ 10), or from the jet cocoon. The predicted afterglow emission properties can discriminate among these scenarios.

astro-ph.HE↗

arXiv2017

Where and when: optimal scheduling of the electromagnetic follow-up of gravitational-wave events based on counterpart lightcurve models

The electromagnetic (EM) follow-up of a gravitational wave (GW) event requires to scan a wide sky region, defined by the so called "skymap", for the detection and identification of a transient counterpart. We propose a novel method that exploits information encoded in the GW signal to construct a "detectability map", which represents the time-dependent ("when") probability to detect the transient at each position of the skymap ("where"). Focusing on the case of a neutron star binary inspiral, we model the associated short gamma-ray burst afterglow and macronova emission, using the probability distributions of binary parameters (sky position, distance, orbit inclination, mass ratio) extracted from the GW signal as inputs. The resulting family of possible lightcurves is the basis to construct the detectability map. As a practical example, we apply the method to a simulated GW signal produced by a neutron star merger at 75 Mpc whose localization uncertainty is very large (about 1500 square degrees). We construct observing strategies based on the detectability maps for optical, infrared and radio facilities, taking VST, VISTA and MeerKAT as prototypes. Assuming limiting fluxes of r ~ 24.5, J ~ 22.4 (AB magnitudes) and 500 uJy @ 1.4 GHz for ~ 1000 s of exposure each, the afterglow and macronova emissions are successfully detected with a minimum observing time of 7, 15 and 5 hours respectively.

astro-ph.HE↗

arXiv2016

Light curves and spectra from off-axis gamma-ray bursts

If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles $θ_{\rm view} \gtrsim θ_{\rm jet} + Γ^{-1}$, the pulse broadening significantly smears out the light curve variability. This is largely independent of geometry and emission processes. To explore a specific case, we set up a simple model of a single pulse under the assumption that the pulse rise and decay are dominated by the shell curvature effect. We show that such a pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a different hardness-intensity correlation with respect to the same pulse seen on-axis. For each of these effects, we provide an intuitive physical explanation. We then show how a synthetic light curve made by a superposition of pulses changes with increasing viewing angle. We find that a highly variable light curve, (as seen on-axis) becomes smooth and apparently single-pulsed (when seen off-axis) because of pulse overlap. To test the relevance of this fact, we estimate the fraction of off-axis gamma-ray bursts detectable by \textit{Swift} as a function of redshift, finding that a sizable fraction (between 10\% and 80\%) of nearby ($z<0.1$) bursts are observed with $θ_{\rm view} \gtrsim θ_{\rm jet} + Γ^{-1}$. Based on these results, we argue that low luminosity gamma-ray bursts are consistent with being ordinary bursts seen off-axis.

astro-ph.HE↗

arXiv2016

An unexpected new explanation of seasonality in suicide attempts: Grey's Anatomy broadcasting

Seasonality is one of the oldest and most elucidation-resistant issues in suicide epidemiological research. Despite winter depression (also known as Seasonal Affective Disorder, SAD) is known and treated since many years, worldwide cross-sectional data from 28 countries show a lower frequency of suicide attempts around the equinoxes and a higher frequency in spring (both in Northern and Southern Hemisphere). This peak is not compatible with the SAD explanation. However, in recent years epidemiological research has yielded new results, which provide new perspectives on the matter. In fact, the discovery of a new pathology called Post-Series Depression (PSD) could provide an explanation of the suicide attempts pattern. The aim of this study is to analyse weekly data in order to compare them with the TV series broadcasting. Since medical observations in our sample are distributed over many years, in order to compare them as best as we can with the television programming, Grey's Anatomy series was chosen. This medical drama has been in the top 10 of most viewed TV series since 12 years and it is broadcast all over the world, so that it can be considered a universal and homogeneous phenomenon. A full season of the series is split into two separate units with a hiatus around the end of the calendar year, and it runs from September through May. Data analysis was made in order to prove the correlation between PSD and the increase of suicide attempts. Surprisingly, the data analysis shows that the increase of rate of suicide attempts does not coincide with the breaks in Grey's Anatomy scheduling, but with the series broadcasting. This therefore suggests that it is the series itself to increase the viewer's depression.

physics.pop-ph↗