arXiv subjects

Ore Gottlieb

Publications and source records attributed to Ore Gottlieb.

53 records · Page 3Linked to original sources

arXiv2022

Bridging Bondi and Event Horizon Scales: 3D GRMHD Simulations Reveal X-Shaped Radio Galaxy Morphology

X-shaped radio galaxies (XRGs) produce misaligned X-shaped jet pairs and make up $\lesssim10\%$ of radio galaxies. XRGs are thought to emerge in galaxies featuring a binary supermassive black hole ($\rm SMBH$), $\rm SMBH$ merger, or large-scale ambient medium asymmetry. We demonstrate that XRG morphology can naturally form without such special, preexisting conditions. Our 3D general-relativistic magnetohydrodynamic (GRMHD) simulation for the first time follows magnetized rotating gas from outside the $\rm SMBH$ sphere of influence of radius $R_{\rm B}$ to the $\rm SMBH$ of gravitational radius $R_{\rm g}$, at the largest scale separation $R_{\rm B}/R_{\rm g} = 10^3$ to date. Initially, our axisymmetric system of constant-density hot gas contains weak vertical magnetic field and rotates in an equatorial plane of a rapidly spinning $\rm SMBH$. We seed the gas with small-scale $2\%$-level pressure perturbations. Infalling gas forms an accretion disk, and the $\rm SMBH$ launches relativistically-magnetized collimated jets reaching well outside $R_{\rm B}$. Under the pressure of the infalling gas, the jets intermittently turn on and off, erratically wobble, and inflate pairs of cavities in different directions, resembling an X-shaped jet morphology. Synthetic X-ray images reveal multiple pairs of jet-powered shocks and cavities. Large-scale magnetic flux accumulates on the $\rm SMBH$, becomes dynamically important, and leads to a magnetically arrested disk state. The $\rm SMBH$ accretes at $2\%$ of the Bondi rate ($\dot{M}\simeq2.4\times10^{-3}M_{\odot}\,{\rm yr}^{-1}$ for M87*), and launches twin jets at $η=150\%$ efficiency. These jets are powerful enough ($P_{\rm jets}\simeq2\times10^{44}\,{\rm erg\,s}^{-1}$) to escape along the spin axis and end the short-lived jets state whose transient nature can account for the rarity of XRGs

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arXiv2022

Observational signatures of stellar explosions driven by relativistic jets

The role of relativistic jets in unbinding the stellar envelope during a supernova (SN) associated with a gamma-ray burst (GRB) is unclear. To study that, we explore observational signatures of stellar explosions that are driven by jets. We focus on the final velocity distribution of the outflow in such explosions and compare its observational imprints to SN/GRB data. We find that jet driven explosions produce an outflow with a flat distribution of energy per logarithmic scale of proper velocity. The flat distribution seems to be universal as it is independent of the jet and the progenitor properties that we explored. The velocity range of the flat distribution for typical GRB parameters is $γβ\approx 0.03-3$, where $γ$ is the outflow Lorentz factor and $β$ is its dimensionless velocity. A flat distribution is seen also for collimated choked jets where the highest outflow velocity decreases with the depth at which the jet is choked. Comparison to observations of SN/GRBs rules out jets as the sole explosion source in these events. Instead, in SN/GRB the collapsing star must deposit its energy into two channels - a quasi-spherical (or wide angle) channel and a narrowly collimated one. The former carries most of the energy and is responsible for the SN sub-relativistic ejecta while the latter carries 0.01-0.1 of the total outflow energy and is the source of the GRB. Intriguingly, the same two channels, with a similar energy ratio, were seen in the binary neutron star merger GW170817, suggesting that similar engines are at work in both phenomena.

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arXiv2021

Black hole to breakout: 3D GRMHD simulations of collapsar jets reveal a wide range of transients

We present a suite of the first 3D GRMHD collapsar simulations, which extend from the self-consistent jet launching by an accreting Kerr black hole (BH) to the breakout from the star. We identify three types of outflows, depending on the angular momentum, $ l $, of the collapsing material and the magnetic field, $ B $, on the BH horizon: (i) Subrelativistic outflow (low $ l $ and high $ B $), (ii) Stationary accretion shock instability (SASI; high $ l $ and low $ B $), (iii) Relativistic jets (high $ l $ and high $ B $). In the absence of jets, free-fall of the stellar envelope provides a good estimate for the BH accretion rate. Jets can substantially suppress the accretion rate, and their duration can be limited by the magnetization profile in the star. We find that progenitors with large (steep) inner density power-law indices ($ \gtrsim 2 $), face extreme challenges as gamma-ray burst (GRB) progenitors due to excessive luminosity, global time evolution in the lightcurve throughout the burst and short breakout times, inconsistent with observations. Our results suggest that the wide variety of observed explosion appearances (supernova/supernova+GRB/low-luminosity GRBs) and the characteristics of the emitting relativistic outflows (luminosity and duration) can be naturally explained by the differences in the progenitor structure. Our simulations reveal several important jet features: (i) strong magnetic dissipation inside the star, resulting in weakly magnetized jets by breakout that may have significant photospheric emission and (ii) spontaneous emergence of tilted accretion disk-jet flows, even in the absence of any tilt in the progenitor.

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arXiv2021

The role of jet-cocoon mixing, magnetization and shock breakout in neutrino and cosmic-ray emission from short GRBs

We perform GRMHD and RMHD simulations of weakly and highly magnetized gamma-ray burst (GRB) jets propagating in binary neutron star (BNS) merger ejecta. Using the simulations, we first find that mixing between the jet and cocoon, which is present in all types of jets, inhibits the formation of subphotospheric collisionless shocks. However, we show that a mild magnetization may lead to the formation of collisionless subshocks which allow efficient proton acceleration. We consider shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and external shocks, to provide the first self-consistent estimate for neutrino and cosmic-ray (CR) signals from GRBs in BNS mergers. We find that short GRBs do not produce detectable neutrino signals with current-day facilities. Shock breakout yields $ \sim 10 $ PeV neutrinos at viewing angles $\sim20^\circ $, independent of the jet magnetization. However, a neutrino signal from shock breakout is well below detection limits of current detectors. Such a signal would allow a coincident neutrino-$γ-$ray detection, providing a testable prediction for shock breakout as a neutrino production site. Using the numerical modeling that fits GW170817 afterglow emission, we find that blast waves in BNS mergers can account for 5%-10% of the Galactic CR luminosity in the PeV-EeV energy range. Based on these estimates, the observed level of CR anisotropy places a constraint on the distance of the latest Galactic binary neutron star merger to $\lesssim3$ kiloparsecs.

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arXiv2021

Intermittent hydrodynamic jets in collapsars do not produce GRBs

Strong variability is a common characteristic of the prompt emission of gamma-ray bursts (GRB). This observed variability is widely attributed to an intermittency of the central engine, through formation of strong internal shocks in the GRB-emitting jet expelled by the engine. In this paper we study numerically the propagation of hydrodynamic jets, injected periodically by a variable engine, through the envelope of a collapsed star. By post-processing the output of 3D numerical simulations, we compute the net radiative efficiency of the outflow. We find that all intermittent jets are subject to heavy baryon contamination that inhibits the emission at and above the photosphere well below detection limits. This is in contrast to continuous jets that, as shown recently, produce a highly variable gamma-ray photospheric emission with high efficiency, owing to the interaction of the jet with the stellar envelope. Our results challenge the variable engine model for hydrodynamic jets, and may impose constraints on the duty cycle of GRB engines. If such systems exist in nature, they are not expected to produce bright gamma-ray emission, but should appear as X-ray, optical and radio transients that resemble a delayed GRB afterglow signal.

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arXiv2021

Intermittent mildly magnetized jets as the source of GRBs

Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of timescales - from $ \sim $ ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e., unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site is $σ\sim 0.1$, the initial magnetization is amplified by shocks formed in the flow to the point where it strongly suppresses baryon loading. We estimate that a significant contamination can be avoided if the magnetic energy at injection constitutes at least a few percent of the jet energy. The variability timescales of the jet after it breaks out of the star are then governed by the injection cycles rather than by the mixing process, suggesting that in practice jet injection should fluctuate on timescales as short as $ \sim 10 $ ms in order to account for the observed light curves. Better stability is found for jets with shorter modulations. We conclude that for sufficiently hot jets, the Lorentz factor near the photosphere can be high enough to allow efficient photospheric emission. Our results imply that jets with $ 10^{-2} < σ< 1 $ injected by a variable engine with $ \sim 10 $ ms duty cycle are plausible sources of long GRBs.

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arXiv2020

The structure of hydrodynamic $ γ$-ray burst jets

After being launched, GRB jets propagate through dense media prior to their breakout. The jet-medium interaction results in the formation of a complex structured outflow, often referred to as a "structured jet". The underlying physics of the jet-medium interaction that sets the post-breakout jet morphology has never been explored systematically. Here we use a suite of 3D simulations to follow the evolution of hydrodynamic long and short gamma-ray bursts (GRBs) jets after breakout to study the post-breakout structure induced by the interaction. Our simulations feature Rayleigh-Taylor fingers that grow from the cocoon into the jet, mix cocoon with jet material and destabilize the jet. The mixing gives rise to a previously unidentified region sheathing the jet from the cocoon, which we denote the jet-cocoon interface (JCI). long GRBs undergo strong mixing, resulting in most of the jet energy to drift into the JCI, while in short GRBs weaker mixing is possible, leading to a comparable amount of energy in the two components. Remarkably, the jet structure (jet-core plus JCI) can be characterized by simple universal angular power-law distributions, with power-law indices that depend solely on the mixing level. This result supports the commonly used power-law angular distribution, and disfavors Gaussian jets. At larger angles, where the cocoon dominates, the structure is more complex. The mixing shapes the prompt emission light curve and implies that typical long GRB afterglows are different from those of short GRBs. Our predictions can be used to infer jet characteristics from prompt and afterglow observations.

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arXiv2020

The structure of weakly-magnetized $ γ$-ray burst jets

The interaction of gamma-ray burst (GRB) jets with the dense media into which they are launched promote the growth of local hydrodynamic instabilities along the jet boundary. In a companion paper we study the evolution of hydrodynamic (unmagnetized) jets, finding that mixing of jet-cocoon material gives rise to an interface layer, termed jet-cocoon interface (JCI), which contains a significant fraction of the system energy. We find that the angular structure of the jet + JCI, when they reach the homologous phase, can be approximated by a flat core (the jet) + a power-law function (the JCI) with indices that depend on the degree of mixing. In this paper we examine the effect of subdominant toroidal magnetic fields on the jet evolution and morphology. We find that weak fields can stabilize the jet against local instabilities. The suppression of the mixing diminishes the JCI and thus reshapes the jet's post-breakout structure. Nevertheless, the overall shape of the outflow can still be approximated by a flat core + a power-law function, although the JCI power-law decay is steeper. The effect of weak fields is more prominent in long GRB jets, where the mixing in hydrodynamic jets is stronger. In short GRB jets there is small mixing in both weakly magnetized and unmagnetized jets. This result influences the expected jet emission which is governed by the jet's morphology. Therefore, prompt and afterglow observations in long GRBs may be used as probes for the magnetic nature at the base of the jets.

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arXiv2020

Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers

The first hours following a neutron star merger are considered to provide several UV/optical/NIR signals: $ β$-decay emission from free neutrons, radioactive decay of shocked heavy elements in the cocoon and cocoon's cooling emission. Here we consider two additional emission sources: $ β$-decay of free neutrons in the cocoon and synchrotron by the $ β$-decay electrons. We present 3D RHD simulations of jets that propagate in a multi-layer ejecta from the merger and calculate semi-analytically the resulting light curves. We find that the free neutrons emission at high latitudes is enhanced by the cocoon by a factor of a few to power a wide ($ \lesssim 60^\circ $) and brief ($ \sim 1 $ hour) UV signal that can reach an absolute magnitude of $\gtrsim$ -15, comparable with the cooling emission. If the ejected neutron matter mass is $ M_n \gtrsim 10^{-4}{\rm M_\odot} $, the synchrotron emission may yield a long ($ \sim 8 $ hours) quasi-isotropic UV/optical signal with an absolute magnitude between -12 and -15, depending on the magnetic field. Such a high mass of a mildly-relativistic component may partly obscure the cocoon's shocked r-process elements, thereby attenuating its radioactive decay emission. Future observations on these timescales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics.

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arXiv2019

Detectability of neutron star merger afterglows

VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($ θ_j \approx 4^\circ $) that dominated the afterglow peak at our viewing angle, $ θ_{\rm obs} \approx 20^\circ $. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at $ θ_{\rm obs} \gg θ_j $, and it can be modeled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, $ θ_{\rm obs} \lesssim 20^\circ $, and only $\sim 20\% $ of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $ 70\% $ of the GW detections are expected to be followed by a detectable afterglow, typically at $ θ_{\rm obs} \sim 30^\circ $. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.

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arXiv2019

High efficiency photospheric emission entailed by formation of a collimation shock in gamma-ray bursts

The primary dissipation mechanism in jets of gamma-ray bursts (GRBs), and the high efficiency of the prompt emission are long standing issues. One possibility is strong collimation of a weakly magnetized relativistic jet by the surrounding medium, which can considerably enhance the efficiency of the photospheric emission. We derive a simple analytic criterion for the radiative efficiency of a collimated jet showing that it depends most strongly on the baryon loading. We confirm this analytic result by 3D numerical simulations, and further find that mixing of jet and cocoon material at the collimation throat leads to a substantial stratification of the outflow as well as sporadic loading, even if the injected jet is uniform and continuous. One consequence of this mixing is a strong angular dependence of the radiative efficiency. Another is large differences in the Lorentz factor of different fluid elements that lead to formation of internal shocks. Our analysis indicates that in both long and short GRBs a prominent photospheric component cannot be avoided when observed within an angle of a few degrees to the axis, unless the asymptotic Lorentz factor is limited by baryon loading at the jet base to $Γ_\infty <100$ (with a weak dependence on outflow power). Photon generation by newly created pairs behind the collimation shock regulates the observed temperature at $\sim 50~θ_0^{-1}$ keV, where $θ_0$ is the initial jet opening angle, in remarkable agreement with the observed peak energies of prompt emission spectra. Further consequences for the properties of the prompt emission are discussed at the end.

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arXiv2018

A Hubble constant measurement from superluminal motion of the jet in GW170817

The Hubble constant ($H_0$) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure $H_0$. Notably, Planck cosmic microwave background (CMB) and the local Cepheid-supernovae distance ladder measurements determine $H_0$ with a precision of $\sim 1\%$ and $\sim 2\%$ respectively. A $3$-$σ$ level of discrepancy exists between the two measurements, for reasons that have yet to be understood. Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer a completely independent standard siren (the GW analogue of an astronomical standard candle) measurement of $H_0$, as demonstrated following the discovery of the neutron star merger, GW170817. This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of $H_0=74^{+16}_{-8}$ km/s/Mpc (median and symmetric $68\%$ credible interval). In this analysis, the degeneracy in the GW signal between the source distance and the weakly constrained viewing angle dominated the $H_0$ measurement uncertainty. Recently, Mooley et al. (2018) obtained tight constraints on the viewing angle using high angular resolution imaging of the radio counterpart of GW170817. Here we obtain a significantly improved measurement $H_0=68.9^{+4.7}_{-4.6}$ km/s/Mpc by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more localized GW170817-like events (comparable signal-to-noise ratio, favorable orientation), having radio images and light curve data, will potentially bring resolution to the tension between the Planck and Cepheid-supernova measurements, as compared to 50-100 GW events without such data.

astro-ph.CO↗

arXiv2018

A cocoon shock breakout as the origin of the $ γ$-ray emission in GW170817

The short Gamma-Ray Burst, GRB170817A, that followed the binary neutron star merger gravitational waves signal, GW170817, is not a usual sGRB. It is weaker by three orders of magnitude than the weakest sGRB seen before and its spectra, showing a hard early signal followed by a softer thermal spectrum, is unique. We show, first, that the $γ$-rays must have emerged from at least mildly relativistic outflow, implying that a relativistic jet was launched following the merger. We then show that the observations are consistent with the predictions of a mildly relativistic shock breakout: a minute $γ$-ray energy as compared with the total energy and a rather smooth light curve with a hard to soft evolution. We present here a novel analytic study and detailed numerical 2D and 3D relativistic hydrodynamic and radiation simulations that support the picture in which the observed $γ$-rays arose from a shock breakout of a cocoon from the merger's ejecta (Kasliwal 2017). The cocoon can be formed by either a choked jet which does not generate a sGRB (in any direction) or by a successful jet which generates an undetected regular sGRB along the system's axis pointing away from us. Remarkably, for the choked jet model, the macronova signal produced by the ejecta (which is partially boosted to high velocities by the cocoon's shock) and the radio that is produced by the interaction of the shocked cocoon material with the surrounding matter, agree with the observed UV/optical/IR emission and with current radio observations. Finally, we discuss the possibility that the jet propagation within the ejecta may photodissociate some of of the heavy elements and may affect the composition of a fraction of ejecta and, in turn, the opacity and the early macronova light.

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arXiv2018

Numerically calibrated model for propagation of a relativistic unmagnetized jet in dense media

Relativistic jets reside in high-energy astrophysical systems of all scales. Their interaction with the surrounding media is critical as it determines the jet evolution, observable signature, and feedback on the environment. During its motion the interaction of the jet with the ambient media inflates a highly pressurized cocoon, which under certain conditions collimates the jet and strongly affects its propagation. Recently, Bromberg et al. (2011) derived a general simplified (semi)analytic solution for the evolution of the jet and the cocoon in case of an unmagnetized jet that propagates in a medium with a range of density profiles. In this work we use a large suite of 2D and 3D relativistic hydrodynamic simulations in order to test the validity and accuracy of this model. We discuss the similarities and differences between the analytic model and numerical simulations and also, to some extent, between 2D and 3D simulations. Our main finding is that although the analytic model is highly simplified, it properly predicts the evolution of the main ingredients of the jet-cocoon system, including its temporal evolution and the transition between various regimes (e.g., collimated to uncollimated). The analytic solution predicts a jet head velocity that is faster by a factor of about 3 compared to the simulations, as long as the head velocity is Newtonian. We use the results of the simulations to calibrate the analytic model which significantly increases its accuracy. We provide an applet that calculates semi-analytically the propagation of a jet in an arbitrary density profile defined by the user at http://www.astro.tau.ac.il/~ore/propagation.html.

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arXiv2018

From $γ$ to Radio - The Electromagnetic Counterpart of GW 170817

The gravitational waves from the first binary neutron star merger, GW170817, were accompanied by a multi-wavelength electromagnetic counterpart, from $γ$-rays to radio. The accompanying gamma-rays, seems at first to confirm the association of mergers with short gamma-ray bursts (sGRBs). The common interpretation was that we see an emission from an sGRB jet seen off-axis. However, a closer examination of the sub-luminous $γ$-rays and the peculiar radio afterglow were inconsistent with this simple interpretation. Here we present results of 3D and 2D numerical simulations that follow the hydrodynamics and emission of the outflow from a neutron star merger form its ejection and up to its deceleration by the circum-merger medium. Our results show that the entire set of $γ$-rays, X-rays and radio observations can be explained by the emission from a mildly relativistic cocoon material (Lorentz factor $\sim$2-5) that was formed while a jet propagated through the material ejected during the merger. The $γ$-rays are generated when the cocoon breaks out from the engulfing ejecta while the afterglow is produced by interaction of the cocoon matter with the interstellar medium. The strong early uv/optical signal may be a Lorentz boosted macronova/kilonova. The fate of the jet itself is currently unknown, but our full-EM models define a path to resolving between successful and choked jet scenarios, outputting coupled predictions for the image size, morphology, observed time-dependent polarization and light curve behavior from radio to X-ray. The predictive power of these models will prove key in interpreting the on-going multi-faceted observations of this unprecedented event.

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arXiv2017

The cocoon emission - an electromagnetic counterpart to gravitational waves from neutron star mergers

Short Gamma-Ray Bursts (SGRBs) are believed to arise from compact binary mergers (either neutron star-neutron star or black hole-neutron star). If so their jets must penetrate outflows that are ejected during the merger. As a jet crosses the ejecta it dissipates its energy, producing a hot cocoon which surrounds it. We present here 3D numerical simulations of jet propagation in mergers' outflows and we calculate the resulting emission. This emission consists of two components: the cooling emission, the leakage of the thermal energy of the hot cocoon, and the cocoon macronova that arises from the radioactive decay of the cocoon's material. This emission gives a brief (~ one hour) blue, wide angle signal. While the parameters of the outflow and jet are uncertain, for the configurations we have considered the signal is bright (~ -14 $-$ -15 absolute magnitude) and outshines all other predicted UV-optical signals. The signal is brighter when the jet breakout time is longer and its peak brightness does not depend strongly on the highly uncertain opacity. A rapid search for such a signal is a promising strategy to detect an electromagnetic merger counterpart. A detected candidate could be then followed by deep IR searches for the longer but weaker macronova arising from the rest of the ejecta.

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arXiv2017

The gamma-rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta

We present the first relativistic MHD numerical simulation of a magnetic jet that propagates through and emerges from the dynamical ejecta of a binary neutron star merger. Generated by the magnetized rotation of the merger remnant, the jet propagates through the ejecta and produces an energetic cocoon that expands at mildly relativistic velocities and breaks out of the ejecta. We show that if the ejecta has a low-mass ($\sim10^{-7} M_\odot$) high-velocity ($v\sim0.85$ c) tail, the cocoon shock breakout will generate $γ$-ray emission that is comparable to the observed short GRB170817A that accompanied the recent gravitational wave event GW170817. Thus, we propose that this GRB, which is quite different from all other short GRBs observed before, was produced by a different mechanism. We expect, however, that such events are numerous and many will be detected in coming LIGO-Virgo runs.

astro-ph.HE↗