arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 127 records · Page 7Linked to original sources

arXiv2016

The Observable Signatures of GRB Cocoons

As a long GRB jet propagates within the surrounding stellar atmosphere it creates a cocoon composed of an outer Newtonian shocked stellar material and an inner (possibly relativistic) shocked jet material. The jet deposits $10^{51}-10^{52}$ erg into this cocoon. This energy is comparable to the GRB's energy and to the energy of the accompanying supernova, yet its signature has been largely neglected so far. A fraction of the cocoon energy is released during its expansion following the breakout from the star and later as it interacts with the surrounding matter. We explore here the possible signatures of the cocoon emission and outline a framework to calculate them from the conditions of the cocoon at the time of the jet breakout. We show that the cocoon signature depends strongly on the level of mixing between the shocked jet and shocked stellar material that fills it, which is currently unknown. We find that if there is no mixing at all then the $γ$-ray emission from the cocoon is so bright that it should have been already detected, and the lack of such detections indicates that mixing at some level must take place. We calculate also the expected signal for partial and full mixing. While the typical signals are weaker than GRBs' afterglows, the latter are highly beamed while the former have wide angles. We predict that future optical, UV and X-ray transient searches, like LSST, ZTF, ULTRASAT, ISS-Lobster and others will most likely detect such signals, providing a wealth of information on the progenitors and jets of GRBs. While we focus on long GRBs, we note that analogous (but weaker) cocoons may arise in short GRBs as well. Their signatures might be the most promising electromagnetic counterparts for gravitational waves merger's signals.

astro-ph.HE↗

arXiv2016

Radio Counterparts of Compact Binary Mergers detectable in Gravitational Waves: A Simulation for an Optimized Survey

Mergers of binary neutron stars and black hole-neutron star binaries produce gravitational-wave (GW) emission and outflows with significant kinetic energies. These outflows result in radio emissions through synchrotron radiation. We explore the detectability of these synchrotron generated radio signals by follow-up observations of GW merger events lacking a detection of electromagnetic counterparts in other wavelengths. We model radio light curves arising from (i) sub-relativistic merger ejecta and (ii) ultra-relativistic jets. The former produces radio remnants on timescales of a few years and the latter produces $γ$-ray bursts in the direction of the jet and orphan-radio afterglows extending over wider angles on timescales of weeks. Based on the derived light curves, we suggest an optimized survey at $1.4$ GHz with five epochs separated by a logarithmic time interval. We estimate the detectability of the radio counterparts of simulated GW-merger events to be detected by advanced LIGO and Virgo by current and future radio facilities. The detectable distances for these GW merger events could be as high as 1 Gpc. $20$--$60\%$ of the long-lasting radio remnants will be detectable in the case of the moderate kinetic energy of $3\cdot 10^{50}$ erg and a circum-merger density of $0.1 {\rm cm^{-3}}$ or larger, while $5$--$20\%$ of the orphan radio afterglows with kinetic energy of $10^{48}$ erg will be detectable. The detection likelihood increases if one focuses on the well-localizable GW events. We discuss the background noise due to radio fluxes of host galaxies and false positives arising from extragalactic radio transients and variable Active Galactic Nuclei and we show that the quiet radio transient sky is of great advantage when searching for the radio counterparts.

astro-ph.HE↗

arXiv2016

Natal Kicks and Time Delays in Merging Neutron Star Binaries - Implications for r-process nucleosynthesis in Ultra Faint Dwarfs and in the Milky Way

Merging neutron star binaries are prime candidate sources for heavy r-process nucleosynthesis. The amount of heavy r-process material is consistent with the mass ejection and rates of mergers, and abundances of relic radioactive materials suggest that heavy r-process material is produced in rare events. Observations of possible macronovae provide further support for this model. Still, some concerns remain. One is the observation of heavy r-process elements in Ultra Faint Dwarf (UFD) galaxies. The escape velocities from UFDs are so small that the natal kicks, taking place at neutron stars birth, might eject such binaries from UFDs. Furthermore the old stellar populations of UFDs requires that r-process nucleosynthesis must have taken place very early on, while it may take several Gyr for compact binaries to merge. This last problem arises also within the Milky Way where heavy r-process materials has been observed in some low metallicity stars. We show here that since a significant fraction of neutron star binaries form with a very small proper motion those won't be ejected even from a UFD. Furthermore, a significant fraction of these binaries merge within a few hundred Myr and in some cases the merger time is even shorter than a hundred Myr. This population of "rapid mergers" explains the appearance of heavy r-process material in both UFDs and in the Milky Way.

astro-ph.HE↗

arXiv2016

Evolution of Supernova Remnants near the Galactic Center

Supernovae near the galactic center evolve differently from regular galactic supernovae. This is mainly due to the environment into which the supernova remnants propagate. Instead of a static, uniform density medium, SNRs near the galactic center propagate into a wind swept environment with a velocity away from the galactic center, and a graded density profile. This causes these SNRs to be non - spherical, and to evolve faster than their galactic counterparts. We develop an analytic theory for the evolution of explosions within a stellar wind, and verify it using a hydrodynamic code. We show that such explosions can evolve in one of three possible morphologies. Using these results we discuss the association between the two SNRs (SGR East and SGR A's bipolar radio/X-ray Lobes) and the two neutron stars (the cannonball and SGR J1745-2900) near the galactic center. We show that, given the morphologies of the SNR and positions of the neutron stars, the only possible association is between SGR A's bipolar radio/X-ray Lobes and SGR J1745-2900. The compact object created in the explosion of SGR East remains undetected, and the SNR of the supernova that created the cannonball has already disappeared.

astro-ph.HE↗

arXiv2016

The Macronova in GRB 050709 and the GRB/macronova connection

GRB 050709 was the first short Gamma-ray Burst (sGRB) with an identified optical afterglow. In this work we report a re-analysis of the publicly available data of this event and the discovery of a Li-Paczynski macronova/kilonova that dominates the optical/IR signal at t $>$ 2.5 days. Such a signal would arise from ~0.05 M$_\odot$ r-process material launched by a compact binary merger. The implied mass ejection supports the suggestion that compact binary mergers are significant and possibly main sites of heavy r-process nucleosynthesis. We have re-analyzed all afterglow data from nearby short and hybrid GRBs. A statistical study of sGRB/macronova connection reveals that macronova may have taken place in all these GRBs though the fraction as low as 0.18 cannot be ruled out. The identification of two of the three macronova candidates in the I-band implies a more promising detection prospect for ground-based surveys.

astro-ph.HE↗

arXiv2016

Beaming of particles and synchrotron radiation in relativistic magnetic reconnection

Relativistic reconnection has been invoked as a mechanism for particle acceleration in numerous astrophysical systems. According to idealised analytical models reconnection produces a bulk relativistic outflow emerging from the reconnection sites (X-points). The resulting radiation is therefore highly beamed. Using two-dimensional particle-in-cell (PIC) simulations, we investigate particle and radiation beaming, finding a very different picture. Instead of having a relativistic average bulk motion with isotropic electron velocity distribution in its rest frame, we find that the bulk motion of particles in X-points is similar to their Lorentz factor gamma, and the particles are beamed within about 5/gamma. On the way from the X-point to the magnetic islands, particles turn in the magnetic field, forming a fan confined to the current sheet. Once they reach the islands they isotropise after completing a full Larmor gyration and their radiation is not strongly beamed anymore. The radiation pattern at a given frequency depends on where the corresponding emitting electrons radiate their energy. Lower energy particles that cool slowly spend most of their time in the islands, and their radiation is not highly beamed. Only particles that quickly cool at the edge of the X-points generate a highly beamed fan-like radiation pattern. The radiation emerging from these fast cooling particles is above the burn-off limit (about 100 MeV in the overall rest frame of the reconnecting plasma.) This has significant implications for models of GRBs and AGNs that invoke beaming in that frame at much lower energies.

astro-ph.HE↗

arXiv2016

ASASSN-14li: A Model Tidal Disruption Event

ASASSN-14li is a recently-discovered tidal disruption event with an exceptionally rich data-set: spectra and lightcurves in soft X-rays, UV, optical, and radio. To understand its emission properties in all these bands, we have extended our model for post-tidal disruption accretion and photon production to estimate both soft X-ray radiation produced by the prompt accretion phase and synchrotron emission associated with the bow shock driven through an external medium by the unbound tidal debris, as well as optical and UV light. We find that fiducial values of the stellar mass ($1 M_\odot$) and black hole mass ($10^{6.5} M_{\odot}$) yield: quantitative agreement with the optical/UV luminosity, lightcurve, and color temperature; approximate agreement with the somewhat uncertain soft X-ray spectrum and lightcurve; and quantitative agreement with the radio luminosity, spectrum and lightcurve. Equipartition analysis of the radio data implies that the radio-emitting region expands with a constant speed, and its magnitude is comparable to the speed expected for the unbound stellar ejecta. Both facts provide strong support to our model. We find that the disruption event took place in mid-September 2014. Two independent parameters, the magnitude and logarithmic radial gradient of the ambient gas density near the black hole, must be fit to the data to explain the radio emission; their inferred values are comparable to those found near both Sgr A* and the TDE candidate Swift J1644.

astro-ph.HE↗

arXiv2016

Particle acceleration, magnetization and radiation in relativistic shocks

What are the mechanisms of particle acceleration and radiation, as well as magnetic field build up and decay in relativistic shocks are open questions with important implications to various phenomena in high energy astrophysics. While the Weibel instability is possibly responsible for magnetic field build up and diffusive shock acceleration is a model for acceleration, both have problems and current PIC simulation show that particles are accelerated only under special conditions and the magnetic field decays on a short length scale. We present here a novel model for the structure and the emission of highly relativistic collisionless shocks. The model takes into account (and is based on) non-local energy and momentum transport across the shock front via emission and absorption of high-energy photons. This leads to a pre-acceleration of the fluid and pre-amplificaiton of the magnetic fields in the upstream region. Both have drastic implications on the shock structure. The model explains the persistence of the shock generated magnetic field at large distances from the shock front. The dissipation of this magnetic field results in a continuous particle acceleration within the downstream region. The model suggests two non-uniform emission zones (the downstream and the upstream), that give rise to three emission components with different spectral and temporal properties. A unique feature of the model is the existence of an "attractor", toward which any shock will evolve. This enables us to estimate from first principles the synchrotron and inverse Compton spectrum of the downstream emission. The model is applicable to any relativistic shock, but its distinctive features show up only for large compactness. We demonstrate that prompt and afterglow Gamma-Ray Bursts' shocks satisfy the relevant conditions and compare their observations with the predictions.

astro-ph.HE↗

arXiv2016

A Revised Analysis of Gamma Ray Bursts' prompt efficiencies

The prompt Gamma-Ray Bursts' (GRBs) efficiency is an important clue on the emission mechanism producing the $γ$-rays. Previous estimates of the kinetic energy of the blast waves, based on the X-ray afterglow luminosity $L_X$, suggested that this efficiency is large, with values above 90\% in some cases. This poses a problem to emission mechanisms and in particular to the internal shocks model. These estimates are based, however, on the assumption that the X-ray emitting electrons are fast cooling and that their Inverse Compton (IC) losses are negligible. The observed correlations between $L_X$ (and hence the blast wave energy) and $E_{γ\rm ,iso}$, the isotropic equivalent energy in the prompt emission, has been considered as observational evidence supporting this analysis. It is reasonable that the prompt gamma-ray energy and the blast wave kinetic energy are correlated and the observed correlation corroborates, therefore, the notion $L_X$ is indeed a valid proxy for the latter. Recent findings suggest that the magnetic field in the afterglow shocks is significantly weaker than was earlier thought and its equipartition fraction, $ε_B$, could be as low as $10^{-4}$ or even lower. Motivated by these findings we reconsider the problem, taking now IC cooling into account. We find that the observed $L_X-E_{γ\rm ,iso}$ correlation is recovered also when IC losses are significant. For small $ε_B$ values the blast wave must be more energetic and we find that the corresponding prompt efficiency is significantly smaller than previously thought. For example, for $ε_B\sim10^{-4}$ we infer a typical prompt efficiency of $\sim15\%$.

astro-ph.HE↗

arXiv2016

The B/C and sub-Iron/Iron Cosmic ray ratios - further evidence in favor of the spiral arm diffusion model

The Boron to Carbon (B/C) and sub-Fe/Fe ratios provides an important clue on Cosmic Ray (CR) propagation within the Galaxy. These ratios estimate the grammage that the CR traverse as they propagate from their sources to Earth. Attempts to explain these ratios within the standard CR propagation models require ad hoc modifications and even with those these models necessitate inconsistent grammages to explain both ratios. As an alternative, physically motivated model, we have proposed that CR originate preferably within the galactic spiral arms. CR propagation from dynamic spiral arms has important imprints on various secondary to primary ratios, such as the B/C ratio and the positron fraction. We use our spiral arm diffusion model with the spallation network extended up to Nickel to calculate the sub-Fe/Fe ratio. We show that without any additional parameters the spiral arm model consistently explains both ratios with the same grammage, providing further evidence in favor of this model.

astro-ph.HE↗

arXiv2016

The dynamics of radiation driven, optically thick winds

Recent observation of some luminous transient sources with low color temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass loss rate regimes ($\dot{M} > L_{\rm Edd\,}/c^2$). In the large total luminosity regime the solution resembles an adiabatic wind solution. Both the radiative luminosity, $L$, and the kinetic luminosity, $L_k$, are super-Eddington with $L < L_k$ and $L \propto L_k^{1/3}$. In the lower total luminosity regime most of the energy is carried out by the radiation with $L_k < L \approx L_{\rm Edd\,}$. In a third, low mass loss regime ($\dot{M} < L_{\rm Edd\,}/c^2$), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind, therefore they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.

astro-ph.HE↗

arXiv2016

Cosmic explosions, life in the Universe and the Cosmological Constant

Galactic Gamma-Ray Bursts (GRBs) are copious sources of gamma-rays that can pose a threat to complex life. Using recent determinations of their rate and the probability of GRBs causing massive extinction, we explore what type of universes are most likely to harbour advanced forms of life. For this purpose we use cosmological N-body simulations to determine at what time and for what value of the cosmological constant ($Λ$) the chances of life being unaffected by cosmic explosions are maximised. We find that $Λ-$dominated universes favour the survival of life against GRBs. Within a $Λ$CDM cosmology, the parameters that govern the likelihood of life survival to GRBs are dictated by the value of $Λ$ and the age of the Universe. We find that we seem to live in a favorable point in this parameter phase space which minimises the exposure to cosmic explosions, yet maximises the number of main sequence (hydrogen-burning) stars around which advanced life forms can exist.

astro-ph.CO↗

arXiv2016

On the maximal efficiency of the collisional Penrose process

The center of mass (CM) energy in a collisional Penrose process - a collision taking place within the ergosphere of a Kerr black hole - can diverge under suitable extreme conditions (maximal Kerr, near horizon collision and suitable impact parameters). We present an analytic expression for the CM energy, refining expressions given in the literature. Even though the CM energy diverges, we show that the maximal energy attained by a particle that escapes the black hole's gravitational pull and reaches infinity is modest. We obtain an analytic expression for the energy of an escaping particle resulting from a collisional Penrose process, and apply it to derive the maximal energy and the maximal efficiency for several physical scenarios: pair annihilation, Compton scattering, and the elastic scattering of two massive particles. In all physically reasonable cases (in which the incident particles initially fall from infinity towards the black hole) the maximal energy (and the corresponding efficiency) are only one order of magnitude larger than the rest mass energy of the incident particles. The maximal efficiency found is ~ 13.92 and it is obtained for the scattering of an outgoing massless particle by a massive particle.

gr-qc↗

arXiv2016

Formation of Double Neutron Star systems as implied by observations

Double Neutron Stars (DNS) have to survive two supernovae and still remain bound. This sets strong limits on the nature of the second collapse in these systems. We consider the masses and orbital parameters of the DNS population and constrain the two distributions of mass ejection and kick velocities directly from observations with no a-priori assumptions regarding evolutionary models and/or the types of the supernovae involved. We show that there is strong evidence for two distinct types of supernovae in these systems, where the second collapse in the majority of the observed systems involved small mass ejection ($ΔM\lesssim 0.5M_{\odot}$) and a corresponding low-kick velocity ($v_{k}\lesssim 30 $km\,s$^{-1}$). This formation scenario is compatible, for example, with an electron capture supernova. Only a minority of the systems have formed via the standard SN scenario involving larger mass ejection of $\sim 2.2 M_{\odot}$ and kick velocities of up to $400$km\,s$^{-1}$. Due to the typically small kicks in most DNS (which are reflected by rather low proper motion), we predict that most of these systems reside close to the galactic disc. In particular, this implies that more NS-NS mergers occur close to the galactic plane. This may have non-trivial implications to the estimated merger rates of DNS and to the rate of LIGO / VIRGO detections.

astro-ph.HE↗

arXiv2016

Testing the Magnetar Model via Late Time Radio Observations of Two Macronova Candidates

Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infra-red flare, dubbed macronova (or kilonova), on a time scale of a week. Recently two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce a long term (months to years) radio emission due to its interaction with the surrounding ISM. In search for this emission, we observed the two macronova candidates, GRB 130603B and GRB 060614 with the Jansky very large array (VLA) and the Australia Telescope Compact Array (ATCA). Our observations resulted in null-detections, putting strong upper limits on the kinetic energy and mass of the ejecta. A possible outcome of a merger is a highly magnetized neutron star (a magnetar), which has been suggested as the central engine for GRBs. Such a magnetar will deposit a significant fraction of its energy into the ejecta leading to a brighter radio flare. Our results, therefore, rule out magnetars in these two events.

astro-ph.HE↗

arXiv2015

Linear and Circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows

Polarization measurements from relativistic outflows are a valuable tool to probe the geometry of the emission region and the microphysics of the particle distribution. Indeed, the polarization level depends on: (i) the local magnetic field orientation, (ii) the geometry of the emitting region with respect to the line of sight, and (iii) the electron pitch-angle distribution. Here we consider optically thin synchrotron emission and we extend the theory of circular polarization from a point source to an extended radially expanding relativistic jet. We present numerical estimates for both linear and circular polarization in such systems. We consider different configurations of the magnetic field, spherical and jetted outflows, isotropic and anisotropic pitch-angle distributions, and outline the difficulty in obtaining the reported high level of circular polarization observed in the afterglow of GRB 121024A. We conclude that the origin of the observed polarization cannot be intrinsic to an optically thin synchrotron process, even when the electron pitch-angle distribution is extremely anisotropic.

astro-ph.HE↗

arXiv2015

Radioactive decay products in neutron star merger ejecta: heating efficiency and $γ$-ray emission

The radioactive decay of the freshly synthesized $r$-process nuclei ejected in compact binary mergers power optical/infrared macronovae (kilonovae) that follow these events. The light curves depend critically on the energy partition among the different products of the radioactive decay and this plays an important role in estimates of the amount of ejected $r$-process elements from a given observed signal. We study the energy partition and $γ$-ray emission of the radioactive decay. We show that $20$-$50\%$ of the total radioactive energy is released in $γ$-rays on timescales from hours to a month. The number of emitted $γ$-rays per unit energy interval has roughly a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy is carried by $\sim 1$ MeV $γ$-rays. However at the peak of macronova emission the optical depth of the $γ$-rays is $\sim 0.02$ and most of the $γ$-rays escape. The loss of these $γ$-rays reduces the heat deposition into the ejecta and hence reduces the expected macronova signals if those are lanthanides dominated. This implies that the ejected mass is larger by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission heats up the ejecta and the heating rate can increase if a sufficient amount of transuranic nuclei are synthesized. Direct measurements of these escaping $γ$-rays may provide the ultimate proof for the macronova mechanisms and an identification of the $r$-process nucleosynthesis sites. However, the chances to detect these signals are slim with current X-ray and $γ$-ray missions. New detectors, more sensitive by at least a factor of ten, are needed for a realistic detection rate.

astro-ph.HE↗

arXiv2015

Short-lived $^{244}$Pu Points to Compact Binary Mergers as Sites for Heavy r-process Nucleosynthesis

Measurements of the radioactive $^{244}$Pu abundances can break the degeneracy between high-rate/low-yield and low-rate/high-yield scenarios for the production of heavy $r$-process elements. The first corresponds to production by core collapse supernovae (cc-SNe) while the latter corresponds to production by e.g. compact binary mergers. The estimated $^{244}$Pu abundance in the current interstellar medium inferred from deep-sea measurements (Wallner et al. 2015) is significantly lower than that corresponding Early Solar System abundances (Turner et al 2007). We estimate the expected median value of the $^{244}$Pu abundances and fluctuations around this value in both models. We show that while the current and Early Solar System abundances are naturally explained within the low-rate/high-yield (e.g. merger) scenario, they are incompatible with the high-rate/low-yield (cc-SNe) model. The inferred event rate remarkably agrees with compact binary merger rates estimated from Galactic neutron star binaries and from short gamma-ray bursts. Furthermore, the ejected mass of $r$-process elements per event agrees with both theoretical and observational macronova/kilonova estimates.

astro-ph.HE↗