arXiv subjects

Fabio De Colle

Publications and source records attributed to Fabio De Colle.

At least 55 records · Page 3Linked to original sources

arXiv2018

Off-axis afterglow light curves and images from 2D hydrodynamic simulations of double-sided GRB jets in a stratified external medium

Gamma-ray burst (GRB) jets are narrow, and thus typically point away from us. They are initially ultra-relativistic, causing their prompt $γ$-ray and early afterglow emission to be beamed away from us. However, as the jet gradually decelerates its beaming cone widens and eventually reaches our line of sight and the afterglow emission may be detected. Such orphan afterglows were not clearly detected so far. Nevertheless, they should be detected in upcoming optical or radio surveys, and it would be challenging to clearly distinguish between them and other types of transients. Therefore, we perform detailed, realistic calculations of the expected afterglow emission from GRB jets viewed at different angles from the jet's symmetry axis. The dynamics are calculated using 2D relativistic hydrodynamics simulations of jets propagating into different power-law external density profiles, $ρ_{\rm ext}\propto{}R^{-k}$ for $k=0,\,1,\,1.5,\,2$, ranging from a uniform ISM-like medium ($k=0$) to a stratified steady stellar-wind like profile ($k=2$). We calculate radio, optical and X-ray lightcurves, and the evolution of the radio afterglow image size, shape and flux centroid. This may help identify misaligned relativistic jets, whether initially ultra-relativistic and producing a GRB for observers within their beam, or (possibly intrinsically more common) moderately relativistic, in either (i) nearby supernovae Ib/c (some of which are associated with long duration GRBs), or (ii) in binary neutron star mergers, which may produce short duration GRBs, and may also be detected in gravitational waves (e.g. GW$\,$170827/GRB$\,$170817A with a weak prompt $γ$-ray emission may harbor an off-axis jet).

astro-ph.HE↗

arXiv2018

Radio emission from the cocoon of a GRB jet: implications for relativistic supernovae and off-axis GRB emission

Relativistic supernovae constitute a sub-class of type Ic supernovae (SNe). Their non-thermal, radio emission differs notably from that of regular type Ic supernovae as they have a fast expansion speed (with velocities $\sim$ 0.6-0.8 c) which can not be explained by a "standard", spherical SN explosion but advocates for a quickly evolving, mildly relativistic ejecta associated with the SN. In this paper, we compute the synchrotron radiation emitted by the cocoon of a long gamma-ray burst jet (GRB). We show that the energy and velocity of the expanding cocoon, and the radio non-thermal light curves and spectra are consistent with those observed in relativistic SNe. Thus, the radio emission from this events is not coming from the SN shock front, but from the mildly relativistic cocoon produced by the passage of a GRB jet through the progenitor star. We also show that the cocoon radio emission dominates the GRB emission at early times for GRBs seen off-axis, and the flux can be larger at late times compared with on-axis GRBs if the cocoon energy is at least comparable with respect to the GRB energy.

astro-ph.HE↗

arXiv2018

Thermal and non-thermal emission from the cocoon of a gamma-ray burst jet

We present hydrodynamic simulations of the hot cocoon produced when a relativistic jet passes through the gamma-ray burst (GRB) progenitor star and its environment, and we compute the lightcurve and spectrum of the radiation emitted by the cocoon. The radiation from the cocoon has a nearly thermal spectrum with a peak in the X-ray band, and it lasts for a few minutes in the observer frame; the cocoon radiation starts at roughly the same time as when $γ$-rays from a burst trigger detectors aboard GRB satellites. The isotropic cocoon luminosity ($\sim 10^{47}$ erg s$^{-1}$) is of the same order of magnitude as the X-ray luminosity of a typical long-GRB afterglow during the plateau phase. This radiation should be identifiable in the Swift data because of its nearly thermal spectrum which is distinct from the somewhat brighter power-law component. The detection of this thermal component would provide information regarding the size and density stratification of the GRB progenitor star. Photons from the cocoon are also inverse-Compton (IC) scattered by electrons in the relativistic jet. We present the IC lightcurve and spectrum, by post-processing the results of the numerical simulations. The IC spectrum lies in 10 keV--MeV band for typical GRB parameters. The detection of this IC component would provide an independent measurement of GRB jet Lorentz factor and it would also help to determine the jet magnetisation parameter.

astro-ph.HE↗

arXiv2017

Off-Axis Emission of Short GRB Jets from Double Neutron Star Mergers and GRB 170817A

The short-duration ($\lesssim2\;$s) GRB 170817A in the nearby ($D=40\;$Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived ($\sim1\;$s) hypermassive NS, which is also supported by the $Δt\approx1.74\;$s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9$\;$days) and radio (16.4$\;$days) emission, together with the low isotropic equivalent $γ$-ray energy output ($E_{\rmγ,iso}\approx5\times10^{46}\;$erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle $θ_{\rm obs}\approx16^\circ-26^\circ$, GRB jet energy $E\sim10^{48.5}-10^{49.5}~{\rm erg}$, and external density $n\sim10^{-5}-10^{-1}~{\rm cm}^{-3}$ for a $ξ_e\sim 0.1$ non-thermal electron acceleration efficiency.

astro-ph.HE↗

arXiv2017

Dynamics of jets during the Common Envelope phase

Common envelope (CE) is an important phase in the evolution of many binary systems. Giant star / compact object interaction in binaries plays an important role in high-energy phenomena as well as in the evolution of their environment. Material accreted onto the compact object may form a disk and power a jet. We study analytically and through numerical simulations the interaction between the jet and the CE. We determine the conditions under which accreting material quenches the jet or allows it to propagate successfully, in which case even the envelope may be ejected. Close to the stellar core of the companion the compact object accretes at a larger rate. A jet launched from this region needs a larger accretion-to-ejection efficiency to successfully propagate through the CE compared to a jet launched far from the stellar core, and is strongly deflected by the orbital motion. The energy deposited by the jet may be larger than the binding energy of the envelope. The jet can, thus, play a fundamental role in the CE evolution. We find that the energy dissipation of the jet into the CE may stop accretion onto the disk. We expect the jet to be intermittent, unless the energy deposited is large enough to lead to the unbinding of the outer layers of the CE. Given that the energy and duration of the jet are similar to those of ultra-long GRBs, we suggest this as a new channel to produce these events.

astro-ph.HE↗

arXiv2016

Transverse velocity shifts in protostellar jets: rotation or velocity asymmetries?

Observations of several protostellar jets show systematic differences in radial velocity transverse to the jet propagation direction, which have been interpreted as evidence of rotation in the jets. In this paper we discuss the origin of these velocity shifts, and show that they could be originated by rotation in the flow, or by side to side asymmetries in the shock velocity, which could be due to asymmetries in the jet ejection velocity/density or in the ambient medium. For typical poloidal jet velocities (~ 100-200 km/s), an asymmetry >~ 10% can produce velocity shifts comparable to those observed. We also present three dimensional numerical simulations of rotating, precessing and asymmetric jets, and show that, even though for a given jet there is a clear degeneracy between these effects, a statistical analysis of jets with different inclination angles can help to distinguish between the alternative origins of transverse velocity shifts. Our analysis indicate that side to side velocities asymmetries could represent an important contribution to transverse velocity shifts, being the most important contributor for large jet inclination angles (with respect the the plane of the sky), and can not be neglected when interpreting the observations.

astro-ph.SR↗

arXiv2016

The Properties of Short gamma-ray burst Jets Triggered by neutron star mergers

The most popular model for short gamma-ray bursts (sGRBs) involves the coalescence of binary neutron stars. Because the progenitor is actually hidden from view, we must consider under which circumstances such merging systems are capable of producing a successful sGRB. Soon after coalescence, winds are launched from the merger remnant. In this paper, we use realistic wind profiles derived from global merger simulations in order to investigate the interaction of sGRB jets with these winds using numerical simulations. We analyze the conditions for which these axisymmetric winds permit relativistic jets to breakout and produce a sGRB. We find that jets with luminosities comparable to those observed in sGRBs are only successful when their half-opening angles are below ~20°. This jet collimation mechanism leads to a simple physical interpretation of the luminosities and opening angles inferred for sGRBs. If wide, low luminosity jets are observed, they might be indicative of a different progenitor avenue such as the merger of a neutron star with a black hole. We also use the observed durations of sGRB to place constraints on the lifetime of the wind phase, which is determined by the time it takes the jet to breakout. In all cases we find that the derived limits argue against completely stable remnants for binary neutron star mergers that produce sGRBs.

astro-ph.HE↗

arXiv2015

A Study of Radio Polarization in Protostellar Jets

Synchrotron radiation is commonly observed in connection with shocks of different velocities, ranging from relativistic shocks associated with active galactic nuclei, gamma-ray bursts or microquasars to weakly- or non-relativistic flows as those observed in supernova remnants. Recent observations of synchrotron emission in protostellar jets are important not only because they extend the range over which the acceleration process works, but also because they allow to determine the jet and/or interstellar magnetic field structure, thus giving insights on the jet ejection and collimation mechanisms. In this paper, we compute for the first time polarized (synchrotron) and non polarized (thermal X-ray) synthetic emission maps from axisymmetrical simulations of magnetized protostellar jets. We consider models with different jet velocities and variability, as well as toroidal or helical magnetic field. Our simulations show that variable, low-density jets with velocities $\sim$ 1000 km s$^{-1}$ and $\sim$ 10 times lighter than the environment can produce internal knots with significant synchrotron emission, and thermal X-rays in the shocked region of the leading bow shock moving in a dense medium. While models with a purely toroidal magnetic field show a very large degree of polarization, models with helical magnetic field show lower values and a decrease of the degree of polarization, in agreement with observations of protostellar jets.

astro-ph.SR↗

arXiv2014

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.

astro-ph.HE↗

arXiv2014

A Stellar Wind Origin for the G2 Cloud: Three-Dimensional Numerical Simulations

We present 3D, adaptive mesh refinement simulations of G2, a cloud of gas moving in a highly eccentric orbit towards the galactic center. We assume that G2 originates from a stellar wind interacting with the environment of the Sgr A* black hole. The stellar wind forms a cometary bubble which becomes increasingly elongated as the star approaches periastron. A few months after periastron passage, streams of material begin to accrete on the central black hole with accretion rates $\dot{M} \sim 10^{-8}$ M$_\odot$ yr$^{-1}$. Predicted Br$γ$ emission maps and position-velocity diagrams show an elongated emission resembling recent observations of G2. A large increase in luminosity is predicted by the emission coming from the shocked wind region during periastron passage. The observations, showing a constant Br$γ$ luminosity, remain puzzling, and are explained here assuming that the emission is dominated by the free-wind region. The observed Br$γ$ luminosity ($\sim 8 \times 10^{30}$ erg s$^{-1}$) is reproduced by a model with a $v_w=50$ km s$^{-1}$ wind velocity and a $10^{-7}$ M$_\odot$ yr$^{-1}$ mass loss rate if the emission comes from the shocked wind. A faster and less dense wind reproduces the Br$γ$ luminosity if the emission comes from the inner, free wind region. The extended cometary wind bubble, largely destroyed by the tidal interaction with the black hole, reforms a few years after periastron passage. As a result, the Br$γ$ emission is more compact after periastron passage.

astro-ph.GA↗

arXiv2014

Necessary Conditions for Short Gamma-Ray Burst Production in Binary Neutron Star Mergers

The central engine of short gamma-ray bursts (sGRBs) is hidden from direct view, operating at a scale much smaller than that probed by the emitted radiation. Thus we must infer its origin not only with respect to the formation of the {\it trigger} - the actual astrophysical configuration that is capable of powering a sGRB - but also from the consequences that follow from the various evolutionary pathways that may be involved in producing it. Considering binary neutron star mergers we critically evaluate, analytically and through numerical simulations, whether the neutrino-driven wind produced by the newly formed hyper-massive neutron star can allow the collimated relativistic outflow that follows its collapse to actually produce a sGRB or not. Upon comparison with the observed sGRB duration distribution, we find that collapse cannot be significantly delayed (<=100 ms) before the outflow is choked, thus limiting the possibility that long-lived hyper-massive remnants can account for these events. In the case of successful breakthrough of the jet through the neutrino-driven wind, the energy stored in the cocoon could contribute to the precursor and extended emission observed in sGRBs.

astro-ph.HE↗

arXiv2013

Understanding the Unusual X-Ray Emission Properties of the Massive, Close Binary WR 20a: A High Energy Window into the Stellar Wind Initiation Region

The problem of explaining the X-ray emission properties of the massive, close binary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR 20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solar masses orbiting with a period of only 3.7 days. Although Chandra observations were taken during the secondary optical eclipse, the X-ray light curve shows no signs of a flux decrement. In fact, WR 20a appears slightly more X-ray luminous and softer during the optical eclipse, opposite to what has been observed in other binary systems. To aid in our interpretation of the data, we compare with the results of hydrodynamical simulations using the adaptive mesh refinement code Mezcal that includes radiative cooling and a radiative acceleration force term. It is shown that the X-ray emission can be successfully explained in models where the wind-wind collision interface in this system occurs while the outflowing material is still being accelerated. Consequently, WR 20a serves as a critical test-case for how radiatively-driven stellar winds initiate and interact. Our models not only procure a robust description of current Chandra data, which cover the orbital phases between 0.3 to 0.6, but provide detailed predictions over the entire orbit.

astro-ph.HE↗

arXiv2013

The Morphology and Dynamics of Jet-Driven Supernova Remnants: the Case of W49B

The circumstellar medium (CSM) of a massive star is modified by its winds before a supernova (SN) explosion occurs, and thus the evolution of the resulting supernova remnant (SNR) is influenced by both the geometry of the explosion as well as the complex structure of the CSM. Motivated by recent work suggesting the SNR W49B was a jet-driven SN expanding in a complex CSM, we explore how the dynamics and the metal distributions in a jet-driven explosion are modified by the interaction with the surrounding environment. In particular, we perform hydrodynamical calculations to study the dynamics and explosive nucleosynthesis of a jet-driven SN triggered by the collapse of a 25 solar masses Wolf-Rayet star and its subsequent interaction with the CSM up to several hundred years following the explosion. We find that although the CSM has small-scale effects on the structure of the SNR, the overall morphology and abundance patterns are reflective of the initial asymmetry of the SN explosion. Thus, we predict that jet-driven SNRs, such as W49B, should be identifiable based on morphology and abundance patterns at ages up to several hundred years, even if they expand into a complex CSM environment.

astro-ph.HE↗

arXiv2013

Conditions For Successful Helium Detonations In Astrophysical Environments

Several models for type Ia-like supernovae events rely on the production of a self-sustained detonation powered by nuclear reactions. In the absence of hydrogen, the fuel that powers these detonations typically consists of either pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that systematically determine the conditions required to initiate detonations in C/O material exist, but until now no analogous investigation of He matter has been conducted. We perform one-dimensional reactive hydrodynamical simulations at a variety of initial density and temperature combinations and find critical length scales for the initiation of He detonations that range between 1 - $10^{10}$ cm. A simple estimate of the length scales over which the total consumption of fuel will occur for steady-state detonations is provided by the Chapman-Jouguet (CJ) formalism. Our initiation lengths are consistently smaller than the corresponding CJ length scales by a factor of $\Sim 100$, providing opportunities for thermonuclear explosions in a wider range of low-mass white dwarfs (WDs) than previously thought possible. We find that virialized WDs with as little mass as 0.24 $M_\odot$ can be detonated, and that even less massive WDs can be detonated if a sizable fraction of their mass is raised to a higher adiabat. That the initiation length is exceeded by the CJ length implies that certain systems may not reach nuclear statistical equilibrium within the time it takes a detonation to traverse the object. In support of this hypothesis, we demonstrate that incomplete burning will occur in the majority of He WD detonations and that $^{40}$Ca, $^{44}$Ti, or $^{48}$Cr, rather than $^{56}$Ni, is the predominant burning product for many of these events. We anticipate that ...

astro-ph.HE↗

arXiv2012

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 $10^6 M_\odot$ 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]

astro-ph.HE↗

arXiv2011

Gamma-Ray Burst Dynamics and Afterglow Radiation from Adaptive Mesh Refinement, Special Relativistic Hydrodynamic Simulations

We report on the development of Mezcal-SRHD, a new adaptive mesh refinement, special relativistic hydrodynamics (SRHD) code, developed with the aim of studying the highly relativistic flows in Gamma-Ray Burst sources. The SRHD equations are solved using finite volume conservative solvers. The correct implementation of the algorithms is verified by one-dimensional (1D) shock tube and multidimensional tests. The code is then applied to study the propagation of 1D spherical impulsive blast waves expanding in a stratified medium with $ρ\propto r^{-k}$, bridging between the relativistic and Newtonian phases, as well as to a two-dimensional (2D) cylindrically symmetric impulsive jet propagating in a constant density medium. It is shown that the deceleration to non-relativistic speeds in one-dimension occurs on scales significantly larger than the Sedov length. This transition is further delayed with respect to the Sedov length as the degree of stratification of the ambient medium is increased. This result, together with the scaling of position, Lorentz factor and the shock velocity as a function of time and shock radius, is explained here using a simple analytical model based on energy conservation. The method used for calculating the afterglow radiation by post-processing the results of the simulations is described in detail. The light curves computed using the results of 1D numerical simulations during the relativistic stage correctly reproduce those calculated assuming the self-similar Blandford-McKee solution for the evolution of the flow. The jet dynamics from our 2D simulations and the resulting afterglow lightcurves, including the jet break, are in good agreement with those presented in previous works. Finally, we show how the details of the dynamics critically depend on properly resolving the structure of the relativistic flow.

astro-ph.HE↗

arXiv2011

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 $ρ\propto r^{-k}$ 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.

astro-ph.HE↗

arXiv2011

A relativistic jetted outburst from a massive black hole fed by a tidally disrupted star

While gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, the vast majority of MBHs are considered dormant. Occasionally, a star passing too near a MBH is torn apart by gravitational forces, leading to a bright panchromatic tidal disruption flare (TDF). While the high-energy transient Swift J164449.3+573451 ("Sw 1644+57") initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that the observations (Levan et al. 2011) suggest a sudden accretion event onto a central MBH of mass ~10^6-10^7 solar masses. We find evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 with a smaller-scale blazar. The phenomenologically novel Sw 1644+57 thus connects the study of TDFs and active galaxies, opening a new vista on disk-jet interactions in BHs and magnetic field generation and transport in accretion systems.

astro-ph.HE↗