arXiv subjects

Brian Morsony

Publications and source records attributed to Brian Morsony.

7 recordsLinked to original sources

arXiv2021

Two steps forward and one step sideways: the propagation of relativistic jets in realistic binary neutron star merger ejecta

The association of GRB170817A with GW170817 has confirmed the long-standing hypothesis that binary neutron star (BNS) mergers are the progenitors of at least some short gamma-ray bursts (SGRBs). This connection has ushered in an era in which broadband observations of SGRBs, together with measurements of the time delay between the gravitational waves and the electromagnetic radiation, allow to probe the properties of the emitting outflow and its engine to an unprecedented detail. Since the structure of the radiating outflow is molded by the interaction of a relativistic jet with the binary ejecta, it is of paramount importance to study the system in a realistic setting. Here we present a three-dimensional hydrodynamic simulation of a relativistic jet propagating in the ejecta of a BNS merger, which were computed with a general relativistic magnetohydrodynamic simulation. We find that the jet's centroid oscillates around the axis of the system, due to inhomogeneities encountered in the propagation. These oscillations allow the jet to find the path of least resistance and travel faster than an identical jet in smooth ejecta. In our setup the breakout time is ~0.6 sec, comparable to the expected central engine duration in SGRBs and possibly a non-negligible fraction of the total delay between the gravitational and gamma-ray signals. Our simulation also shows that energy is carried in roughly equal amounts by the jet and by the cocoon, and that about 20 per cent of the injected energy is transferred to the ejecta via mechanical work.

astro-ph.HE↗

arXiv2017

G2 and Sgr A*: A Cosmic Fizzle At The Galactic Center

We carry out a series of simulations of G2-type clouds interacting with the black hole at the galactic center, to determine why no large changes in the luminosity of Sgr A* were seen, and to determine the nature of G2. We measure the accretion rate from the gas cloud onto Sgr A* for a range of simulation parameters, such as cloud structure, background structure, background density, grid resolution, and accretion radius. For a broad range of parameters, the amount of cloud material accreted is small relative to the amount of background material accreted. The total accretion rate is not significantly effected for at least 30 years after periapsis. We find that reproducing observations of G2 likely requires two components for the object: an extended, cold gas cloud responsible for the Br-gamma emission, and a compact core or dusty stellar object dominating the bolometric luminosity. In simulations, the bolometric and X-ray luminosity have a peak lasting from about 1 year before to 1 year after periapsis, a feature not detected in observations. Our simulated Br-gamma emission is largely consistent with observations leading up to periapsis, with a slight increase in luminosity and a large increase in the FWHM of the line velocity. All emission from a gaseous component of G2 should fade rapidly after periapsis and be undetectable after 1 year, due to shock heating and expansion of the cloud. Any remaining emission should be from the compact component of G2.

astro-ph.HE↗

arXiv2014

Photospheric emission from long duration gamma-ray bursts powered by variable engines

We present the results of a set of numerical simulations of long-duration gamma-ray burst jets aimed at studying the effect of a variable engine on the peak frequency of the photospheric emission. Our simulations follow the propagation of the jet inside the progenitor star, its break-out, and the subsequent expansion in the environment out to the photospheric radius. A constant and two step-function models are considered for the engine luminosity. We show that our synthetic light-curves follow a luminosity-peak frequency correlation analogous to the Golenetskii correlation found in long-duration gamma-ray burst observations. Within the parameter space explored, it appears that the central engine luminosity profile does not have a significant effect on the location of a gamma-ray burst in the Luminosity-peak frequency plane, bursts from different central engines being indistinguishable from each other.

astro-ph.HE↗

arXiv2013

On the observed duration distribution of gamma-ray bursts from collapsars

The duration of the prompt emission of long gamma-ray bursts is generally considered to be fairly similar to the duration of the activity of the engine in the center of the progenitor star. Here, we investigate the relation between the duration of the engine activity and that of the observed light curve, using inputs from both numerical simulations and observations. We find that the observed burst duration is a good proxy for the engine duration after the time necessary for the jet to break out the star's surface is subtracted. However, the observed duration is a function of the viewing angle and can be significantly shorter than the duration of the engine activity. We also show that the observed, redshift-corrected burst duration evolves only moderately with redshift for both observations and synthetic light curves. We conclude that the broad distribution of the observed duration of long BATSE gamma-ray bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being due to viewing and redshift effects. Our results do not rule out the existence of engines with very long duration. However, we find that they are constrained to be a small minority of the BATSE detected bursts.

astro-ph.HE↗

arXiv2011

Jet trails and Mach cones: The interaction of microquasars with the ISM

A sub-set of microquasars exhibit high peculiar velocity with respect to the local standard of rest due to the kicks they receive when being born in supernovae. The interaction between the radio plasma released by microquasar jets from such high-velocity binaries with the ISM must lead to the production of trails and bow shocks similar to what is observed in narrow-angle tailed radio galaxies and pulsar wind nebulae. We present a set of numerical simulations of this interaction that illuminate the long term dynamical evolution and the observational properties of these microquasar bow shock nebulae and trails. We find that this interaction always produces a structure that consists of a bow shock, a trailing neck, and an expanding bubble. Using our simulations to model emission, we predict that the shock surrounding the bubble and the neck should be visible in Hα emission, the interior of the bubble should be visible in synchrotron radio emission, and only the bow shock is likely to be detectable in X-ray emission. We construct an analytic model for the evolution of the neck and bubble shape and compare this model with observations of X-ray binary SAX J1712.6-3739.

astro-ph.HE↗

arXiv2008

Jitter radiation in gamma-ray bursts and their afterglows: emission and self-absorption

Relativistic electrons moving into a highly tangled magnetic field emit jitter radiation. We present a detailed computation of the jitter radiation spectrum, including self-absorption, for electrons inside Weibel-like shock generated magnetic fields. We apply our results to the case of the prompt and afterglow emission of gamma-ray bursts. We show that jitter emission can reproduce most of the observed features with some important differences with respect to standard synchrotron, especially in the frequency range between the self-absorption and the peak frequency. We discuss the similarities and differences between jitter and synchrotron and discuss experiments that can disentangle the two mechanisms.

astro-ph↗

arXiv2006

Long Gamma-Ray Bursts as standard candles

As soon as it was realized that long GRBs lie at cosmological distances, attempts have been made to use them as cosmological probes. Besides their use as lighthouses, a task that presents mainly the technological challenge of a rapid deep high resolution follow-up, researchers attempted to find the Holy Grail: a way to create a standard candle from GRB observables. We discuss here the attempts and the discovery of the Ghirlanda correlation, to date the best method to standardize the GRB candle. Together with discussing the promises of this method, we will underline the open issues, the required calibrations and how to understand them and keep them under control. Even though GRB cosmology is a field in its infancy, ongoing work and studies will clarify soon if and how GRBs will be able to keep up to the promises.

astro-ph↗