arXiv subjects

Brian J. Morsony

Publications and source records attributed to Brian J. Morsony.

At least 19 recordsLinked to original sources

arXiv2018

Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multi-wavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma-rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubts on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multi-wavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30 degrees away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.

astro-ph.HE↗

arXiv2018

Suppression of AGN-Driven Turbulence by Magnetic Fields in a Magnetohydrodynamic Model of the Intracluster Medium

We investigate the role of AGN feedback in turbulent heating of galaxy clusters. Specifically, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work which neglects magnetic fields has shown that this process is inefficient, with less than 1% of the injected energy ending up in turbulence. This inefficiency is primarily due to the fact that the bubbles are shredded apart by hydrodynamic instabilities before they can excite sufficiently strong g-modes. Using a plane-parallel model of the ICM and 3D ideal MHD simulations, we examine the role of a large-scale magnetic field which is able to drape around these rising bubbles, preserving them from hydrodynamic instabilities. We find that, while magnetic draping appears better able to preserve AGN-driven bubbles, the driving of g-modes and the resulting production of turbulence is still inefficient. The magnetic tension force prevents g-modes from transitioning into the nonlinear regime, suppressing turbulence in our model ICM. Our work highlights the ways in which ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high $β$ plasma kinetic effects. These results suggest the hypothesis that other mechanisms of heating the ICM plasma such as sound waves or cosmic rays may be responsible for observed feedback in galaxy clusters.

astro-ph.HE↗

arXiv2017

Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts

We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15<theta<45 degrees and gives origin to X-ray transients with a duration of a few to ~10 seconds, delayed by a few seconds from the time of the merger. We also discuss the distance at which such transients can be detected, finding that it depends sensitively on the assumptions that are made about the radiation spectrum. Purely thermal cocoon transients are detectable only out to a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out to several tens of Mpc.

astro-ph.HE↗

arXiv2017

Off-axis emission of short gamma-ray bursts and the detectability of electromagnetic counterparts of gravitational wave detected binary mergers

We present calculations of the wide angle emission of short-duration gamma-ray bursts from compact binary merger progenitors. Such events are expected to be localized by their gravitational wave emission, fairly irrespective of the orientation of the angular momentum vector of the system, along which the gamma-ray burst outflow is expected to propagate. We show that both the prompt and afterglow emission are dim and challenging to detect for observers lying outside of the cone within which the relativistic outflow is propagating. If the jet initially propagates through a baryon contaminated region surrounding the merger site, however, a hot cocoon forms around it. The cocoon subsequently expands quasi-isotropically producing its own prompt emission and external shock powered afterglow. We show that the cocoon prompt emission is detectable by Swift BAT and Fermi GBM, We also show that the cocoon afterglow peaks a few hours to a few days after the burst and is detectable for up to a few weeks at all wavelengths. The timing and brightness of the transient are however uncertain due to their dependence on unknown quantities such as the density of the ambient medium surrounding the merger site, the cocoon energy, and the cocoon Lorentz factor. For a significant fraction of the gravitationally-detected neutron-star-binary mergers, the cocoon afterglow could possibly be the only identifiable electromagnetic counterpart, at least at radio and X-ray frequencies.

astro-ph.HE↗

arXiv2016

Modeling the afterglow of the possible Fermi-GBM event associated with GW150914

We model the possible afterglow of the Fermi GBM event associated with LIGO detection GW150914, under the assumption that the gamma-ray are produced by a short GRB-like relativistic outflow. We model GW150914-GBM as both a weak, on-axis short GRB and normal short GRB seen far off axis. Given the large uncertainty in the position of GW150914, we determine that the best chance of finding the afterglow is with ASKAP or possibly the MWA, with the flux from an off-axis short GRB reaching 0.2 - 4 mJy (0.12 - 16 mJy) at 150 MHz (863.5 MHz) by 1 - 12 months after the initial event. At low frequencies, the source would evolve from a hard to soft spectrum over several months. The radio afterglow would be detectable for several months to years after it peaks, meaning the afterglow may still be detectable and increasing in brightness NOW (mid-July 2016). With a localization from the MWA or ASKAP, the afterglow would be detectable at higher radio frequencies with the ATCA and in X-rays with Chandra or XMM.

astro-ph.HE↗

arXiv2016

Three-dimensional simulations of long duration gamma-ray burst jets: time scales from variable engines

Gamma-ray burst light curves are characterized by marked variability, each showing unique properties. The origin of such variability, at least for a fraction of long GRBs, may be the result of an unsteady central engine. It is thus important to study the effects that an episodic central engine has on the jet propagation and, eventually, on the prompt emission within the collapsar scenario. Thus, in this study we follow the interaction of pulsed outflows with their progenitor stars with hydrodynamic numerical simulations in both two and three dimensions. We show that the propagation of unsteady jets is affected by the interaction with the progenitor material well after the break-out time, especially for jets with long quiescent times, comparable to or larger than a second. We also show that this interaction can lead to an asymmetric behavior in which pulse durations and quiescent periods are systematically different. After the pulsed jets drill through the progenitor and the interstellar medium, we find that, on average, the quiescent epochs last longer than the pulses (even in simulations with symmetrical active and quiescent engine times). This could explain the asymmetry detected in the light curves of long-quiescent times duration gamma-ray bursts.

astro-ph.HE↗

arXiv2015

Numerical Simulations of Gamma-Ray Burst Explosions

Gamma-ray bursts are a complex, non-linear system that evolves very rapidly through stages of vastly different conditions. They evolve from scales of few hundred kilometers where they are very dense and hot to cold and tenuous on scales of parsecs. As such, our understanding of such a phenomenon can truly increase by combining theoretical and numerical studies adopting different numerical techniques to face different problems and deal with diverse conditions. In this review, we will describe the tremendous advancement in our comprehension of the bursts phenomenology through numerical modeling. Though we will discuss studies mainly based on jet dynamics across the progenitor star and the interstellar medium, we will also touch upon other problems such as the jet launching, its acceleration, and the radiation mechanisms. Finally, we will describe how combining numerical results with observations from Swift and other instruments resulted in true understanding of the bursts phenomenon and the challenges still lying ahead.

astro-ph.HE↗

arXiv2014

Modeling Increased Metal Production in Galaxy Clusters with Pair-Instability Supernovae

Galaxy clusters contain much more metal per star, typically 3 times as much, than is produced in normal galaxies. We set out to determine what changes are needed to the stellar mass function and supernovae rates to account for this excess metal. In particular, we vary the Type Ia supernovae rate, IMF slope, upper and lower mass cutoffs, and the merger rate of massive stars. We then use existing simulation results for metal production from AGB stars, Type Ia SNe and core-collapse SNe to calculate the total amount of each element produced per solar mass of star formation. For models with very massive stars, we also include metal production from pair-instability supernovae (PISNe).We find that including PISNe makes it much easier to increase the amount of metal produced per stellar mass. Therefore a separate population of high-mass stars is not needed to produce the high amounts of metal found in galaxy clusters. We also find that including at least some PISNe increases the abundance of intermediate-mass elements relative to both oxygen and iron, consistent with observations of ICM abundances.

astro-ph.CO↗

arXiv2013

Three-dimensional AMR simulations of long-duration Gamma-Ray Burst jets inside massive progenitor stars

We present the results of special relativistic, adaptive mesh refinement, 3D simulations of gamma-ray burst jets expanding inside a realistic stellar progenitor. Our simulations confirm that relativistic jets can propagate and break out of the progenitor star while remaining relativistic. This result is independent of the resolution, even though the amount of turbulence and variability observed in the simulations is greater at higher resolutions. We find that the propagation of the jet head inside the progenitor star is slightly faster in 3D simulations compared to 2D ones at the same resolution. This behavior seems to be due to the fact that the jet head in 3D simulations can wobble around the jet axis, finding the spot of least resistance to proceed. Most of the jet properties, such as density, pressure, and Lorentz factor, are only marginally affected by the dimensionality of the simulations and therefore results from 2D simulations can be considered reliable.

astro-ph.HE↗

arXiv2013

Simulations of bent-double radio sources in galaxy groups

Bent-double radio sources have been used as a probe to measure the density of intergalactic gas in galaxy groups. We carry out a series of high-resolution, 3D simulations of AGN jets moving through an external medium with a constant density in order to develop a general formula for the radius of curvature of the jets, and to determine how accurately the density of the intra-group medium (IGM) can be measured. Our simulations produce curved jets ending in bright radio lobes with an extended trail of low surface brightness radio emission. The radius of curvature of the jets varies with time by only about 25%. The radio trail seen in our simulations is typically not detected in known sources, but may be detectable in lower resolution radio observations. The length of this tail can be used to determine the age of the AGN. We also use our simulation data to derive a formula for the kinetic luminosity of observed jets in terms of the radius of curvature and jet pressure. In characterizing how well observations can measure the IGM density, we find that the limited resolution of typical radio observations leads to a systematic under-estimate of the density of about 50%. The unknown angles between the observer and the direction of jet propagation and direction of AGN motion through the IGM leads to an uncertainty of about 50% in estimates of the IGM density. Previous conclusions drawn using these sources, indicating that galaxy groups contain significant reservoirs of baryons in their IGM, are still valid when considering this level of uncertainty. In addition, we model the X-ray emission expected from bent-double radio sources. We find that known sources in reasonably dense environments should be detectable in ~100 ks Chandra observations. X-ray observations of these sources would place constraints on the IGM density and AGN velocity that are complementary to radio observations.

astro-ph.CO↗

arXiv2012

Unifying the Zoo of Jet-Driven Stellar Explosions

We present a set of numerical simulations of stellar explosions induced by relativistic jets emanating from a central engine sitting at the center of compact, dying stars. We explore a wide range of durations of the central engine activity, two candidate stellar progenitors, and two possible values of the total energy release. We find that even if the jets are narrowly collimated, their interaction with the star unbinds the stellar material, producing a stellar explosion. We also find that the outcome of the explosion can be very different depending on the duration of the engine activity. Only the longest-lasting engines result in successful gamma-ray bursts. Engines that power jets only for a short time result in relativistic supernova explosions, akin to observed engine-driven SNe such as SN2009bb. Engines with intermediate durations produce weak gamma-ray bursts, with properties similar to nearby bursts such as GRB 980425. Finally, we find that the engines with the shortest durations, if they exist in nature, produce stellar explosions that lack sizable amounts of relativistic ejecta and are therefore dynamically indistinguishable from ordinary core-collapse supernovae.

astro-ph.HE↗

arXiv2011

High-efficiency photospheric emission of long-duration gamma-ray burst jets: the effect of the viewing angle

We present the results of a numerical investigation of the spectra and light curves of the emission from the photospheres of long-duration gamma-ray burst jets. We confirm that the photospheric emission has high efficiency and we show that the efficiency increases slightly with the off-axis angle. We show that the peak frequency of the observed spectrum is proportional to the square root of the photosphere's luminosity, in agreement with the Amati relation. However, a quantitative comparison reveals that the thermal peak frequency is too small for the corresponding total luminosity. As a consequence, the radiation must be out of thermal equilibrium with the baryons in order to reproduce the observations. Finally, we show that the spectrum integrated over the emitting surface is virtually indistinguishable from a Planck law, and therefore an additional mechanism has to be identified to explain the non-thermal behavior of the observed spectra at both high and low frequencies.

astro-ph.HE↗

arXiv2010

The origin and propagation of variability in the outflows of long duration gamma-ray bursts

We present the results of hydrodynamical simulations of gamma-ray burst jets propagating through their stellar progenitor material and subsequently through the surrounding circumstellar medium. We consider both jets that are injected with constant properties in the center of the star and jets injected with a variable luminosity. We show that the variability properties of the jet outside the star are a combination of the variability injected by the engine and the variability caused by the jet propagation through the star. Comparing power spectra for the two cases shows that the variability injected by the engine is preserved even if the jet is heavily shocked inside the star. Such shocking produces additional variability at long time scales, of order several seconds. Our findings suggest that the broad pulses of several seconds duration typically observed in gamma-ray bursts are due to the interaction of the jet with the progenitor, while the short-timescale variability, characterized by fluctuations on time scales of milliseconds, has to be injected at the base of the jet. Studying the properties of the fast variability in GRBs may therefore provide clues to the nature of the inner engine and the mechanisms of energy extraction from it.

astro-ph.HE↗

arXiv2010

Swimming against the current: Simulations of central AGN evolution in dynamic galaxy clusters

We present a series of three-dimensional hydrodynamical simulations of central AGN driven jets in a dynamic, cosmologically evolved galaxy cluster. Extending previous work, we study jet powers ranging from L_jet = 10^44 erg/s to L_jet = 10^46 erg/s and in duration from 30 Myr to 200 Myr. We find that large-scale motions of cluster gas disrupt the AGN jets, causing energy to be distributed throughout the centre of the cluster, rather than confined to a narrow angle around the jet axis. Disruption of the jet also leads to the appearance of multiple disconnected X-ray bubbles from a long-duration AGN with a constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. We find that the "sphere of influence" of the AGN, the radial scale within which the cluster is strongly affected by the jet, scales as R ~ L_jet^(1/3). Increasing the duration of AGN activity does not increase the radius affected by the AGN significantly, but does change the magnitude of the AGN's effects. How an AGN delivers energy to a cluster will determine where that energy is deposited: a high luminosity is needed to heat material outside the core of the cluster, while a low-luminosity, long-duration AGN is more efficient at heating the inner few tens of kpc.

astro-ph.HE↗

arXiv2010

Short-duration gamma-ray bursts from off-axis collapsars

We present 2D high-resolution hydrodynamic simulations of the relativistic outflows of long-duration gamma-ray burst progenitors. We analyze the properties of the outflows at wide off-axis angles, produced by the expansion of the hot cocoon that surrounds the jet inside the progenitor star. We find that the cocoon emission at wide angles may have properties similar to those of the subclass of short-duration gamma-ray bursts with persistent X-ray emission. We compute the predicted duration distribution, redshift distribution, and afterglow brightness and we find that they are all in agreement with the observed properties of short GRBs with persistent emission. We suggest that a SN component, the properties of the host galaxies, and late afterglow observations can be used as a crucial test to verify this model.

astro-ph.HE↗

arXiv2009

Very high efficiency photospheric emission in long duration gamma-ray bursts

We numerically analyze the evolution of a long-duration gamma-ray burst jet as it leaves the progenitor star and propagates to the photospheric radius, where radiation can be released. We find that the interaction of the relativistic material with the progenitor star has influences well beyond the stellar surface. Tangential collimation shocks are observed throughout the jet evolution, out to about 100 stellar radii, which is the whole range of our simulation. We find that the jet is internally hot at the photospheric radius and we compute the photospheric emission. The photosphere is a very efficient radiator, capable of converting more than half of the total energy of the jet into radiation. We show that bright photospheres are a common feature of jets born inside massive progenitor stars and that this effect can explain the high radiative efficiency observed in long-duration bursts.

astro-ph.HE↗

arXiv2008

Jitter radiation images, spectra, and light curves from a relativistic spherical blastwave

We consider radiation emitted by the jitter mechanism in a Blandford-McKee self-similar blastwave. We assume the magnetic field configuration throughout the whole blastwave meets the condition for the emission of jitter radiation and we compute the ensuing images, light curves and spectra. The calculations are performed for both a uniform and a wind environment. We compare our jitter results to synchrotron results. We show that jitter radiation produces slightly different spectra than synchrotron, in particular between the self-absorption and the peak frequency, where the jitter spectrum is flat, while the synchrotron spectrum grows as ν^{1/3}. The spectral difference is reflected in the early decay slope of the light curves. We conclude that jitter and synchrotron afterglows can be distinguished from each other with good quality observations. However, it is unlikely that the difference can explain the peculiar behavior of several recent observations, such as flat X-ray slopes and uncorrelated optical and X-ray behavior.

astro-ph↗

arXiv2008

X-ray flares and their relation to the inner engine activity of GRBs

Flares overlaid on the smooth power-law decay of Swift X-ray afterglows are rather common, appearing in roughly half the observed light curves. They are a manifestation of the late time activity of the inner engine, since their temporal evolution is too fast to be linked to activity taking place in the external shock blastwave. In this paper we show that the energy emitted in the form of flares decreases with time as a power-law. We discuss several possibilities in which the flares can be powered and the source of the observed variability. We show that late time accretion from a disk can provide the necessary energy input in both classes of short duration and long duration gamma-ray bursts.

astro-ph↗