arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

At least 91 records · Page 5Linked to original sources

arXiv2019

Jet-driven bubbles in Fanaroff-Riley type I sources

Observations of several Fanaroff-Riley (FR) type I sources reveal outflowing bipolar bubbles of hot gas surrounded by a weak forward shock. We consider the possibility that these bubbles were driven by choked relativistic jets which failed to penetrate the ambient intracluster medium (ICM). Using new results on choked jets linking the geometry of the forward shock to the jet properties, we infer robust limits on the radius $R_{ch}$ at which the jet was quenched in 5 well-studied FRI sources, finding typically $R_{ch}\sim 10$ kpc. We further show that, in order to reach this radius in less than the current age of the system, the jet must have been tightly collimated, with the jet head subtending an angle of $θ_h < 2^\circ$. The ambient pressure is not high enough to explain this collimation, suggesting that the jet was collimated by interaction with its own cocoon. Although the choking radius is well-constrained, we find a degeneracy between the initial jet opening angle before collimation, $θ_0$, and the duration of jet activity, $t_b$, with $(t_b/1\rm{Myr})(θ_0/5^\circ)^{-2} \sim 0.1$. We speculate that the working time and/or opening angle of the jet may be important factors contributing to the FR type I/type II morphology in galaxy clusters, with short-lived or wide jets being choked to form bipolar bubbles filled with diffuse radio emission, and longer-lived or narrow jets successfully escaping the cluster core to produce cocoons with radio hotspots.

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arXiv2019

The propagation of choked jet outflows in power-law external media

Observations of both gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) point to the idea that some relativistic jets are suffocated by their environment before we observe them. In these "choked" jets, all the jet's kinetic energy is transferred into a hot and narrow cocoon of near-uniform pressure. We consider the evolution of an elongated, axisymmetric cocoon formed by a choked jet as it expands into a cold power-law ambient medium $ρ\propto R^{-α}$, in the case where the shock is decelerating ($α<3$). The evolution proceeds in three stages, with two breaks in behaviour: the first occurs once the outflow has doubled its initial width, and the second once it has doubled its initial height. Using the Kompaneets approximation, we derive analytical formulae for the shape of the cocoon shock, and obtain approximate expressions for the height and width of the outflow versus time in each of the three dynamical regimes. The asymptotic behaviour is different for flat ($α\le 2$) and steep ($2 < α< 3$) density profiles. Comparing the analytical model to numerical simulations, we find agreement to within $\sim15$ per cent out to 45 degrees from the axis, but discrepancies of a factor of 2-3 near the equator. The shape of the cocoon shock can be measured directly in AGNs, and is also expected to affect the early light from failed GRB jets. Observational constraints on the shock geometry provide a useful diagnostic of the jet properties, even long after jet activity ceases.

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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

Radio Emission from the unbound Debris of Tidal Disruption Events

When a star gets too close to a supermassive black hole, it is torn apart by the tidal forces. Roughly half of the stellar mass becomes unbound and flies away at tremendous velocities - around $10^4$ km/s. In this work we explore the idea that the shock produced by the interaction of the unbound debris with the ambient medium gives rise to the synchrotron radio emission observed in several TDEs. We use a moving mesh numerical simulation to study the evolution of the unbound debris and the bow shock around it. We find that as the periapse distance of the star decreases, the outflow becomes faster and wider. A tidal disruption event whose periapse distance is a factor of 7 smaller than the tidal radius can account for the radio emission observed in ASASSN-14li. This model also allows us to obtain a more accurate estimate for the gas density around the centre of the host galaxy of ASASSN-14li.

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arXiv2018

Cosmic-Ray Anisotropy from Large Scale Structure and the effect of magnetic horizons

Motivated by the ~7 % dipole anisotropy in the distribution of ultra-high energy cosmic-rays (UHECRs) above 8 EeV, we explore the anisotropy induced by the large scale structure, using constrained simulations of the local Universe and taking into account the effect of magnetic fields. The value of the intergalactic magnetic field (IGMF) is critical as it determines the UHECR cosmic horizon. We calculate the UHECR sky maps for different values of the IGMF variance and show the effect of the UHECR horizon on the observed anisotropy. The footprint of the local ($\lesssim350$ Mpc) Universe on the UHECR background, a small angular scale enhancement in the Northern Hemisphere, is seen. At 11.5 EeV (the median value of the energy bin at which the dipole has been reported), the LSS-induced dipole amplitude is $A_1\sim10\%$, for IGMF in the range [0.3-3] nG for protons, helium and nitrogen, compatible with the rms value derived from the cosmic power spectrum. However at these energies the UHECRs are also influenced by the Galactic Magnetic Field (GMF) and we discuss its effect on the LSS-induced anisotropy.

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arXiv2018

Positron flux and gamma-ray emission from Geminga pulsar and pulsar wind nebula

Nearby pulsars have been suggested as sources of $\sim$TeV $e^+/e^-$ Cosmic Ray (CR) excess on Earth. The High-Altitude Water Cherenkov Observatory (HAWC) detected extended TeV emission regions in the direction of two nearby middle-aged pulsars, Geminga and PSR B0656+14. By modeling the TeV emission as inverse Compton emission from electron-positron pairs diffusing in the interstellar medium (ISM), the HAWC collaboration derives a diffusion coefficient much smaller than the standard value in the vicinity of the two pulsars, which make them unlikely the origin of the positron excess. We propose that the observed $γ$-ray emission originate from the relic pulsar wind nebula. A two zone diffusion model with a slow diffusion in the nebula and a fast diffusion in the ISM can explain the HAWC surface brightness profile and the positron excess simultaneously. Inefficient diffusion in the $γ$-ray emission region surrounding a middle-aged pulsar maybe a common phenomenon that can be tested by future observation. The implied diffusion coefficient in the ISM is smaller than the one suggested by the standard CR propagation model, but it is fully consistent with the predictions of the spiral arm model.

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arXiv2018

Constraints on the emitting region of the gamma-rays observed in GW170817

The gravitational waves from the neutron star merger event GW170817 were accompanied by an unusually weak short GRB 170817A, by an optical/IR macronova/kilonova and by a long lasting radio to X-rays counterpart. While association of short GRBs with mergers was predicted a long time ago, the luminosity of this prompt γ-ray emission was weaker by a few orders of magnitude than all known previous sGRBs and it was softer than typical sGRBs. This raise the question whether the γ-rays that we have seen were a regular sGRB viewed off-axis. We revisit this question following recent refined analyses of the γ-ray signal and the VLBI observations that revealed the angular structure of the relativistic outflow: observing angle of $\sim\,20^\circ$, a narrow jet with core $\lesssim5^\circ$ and $E_{\rm iso}>10^{52}\,{\rm ergs}$. We show here that: (i) The region emitting the observed $γ$-rays must have been moving with a Lorentz factor $Γ\gtrsim5$; (ii) The observed γ-rays were not "off-axis" emission (viewing angle $>1/Γ$) emerging from the core of the jet, where a regular sGRB was most likely produced; (iii) The $γ$-ray emission region was either "on-axis" (at an angle $<1/Γ$) or if it was "off-axis" then the observing angle must have been small ($<5^\circ$) and the on-axis emission from this region was too faint and too hard to resemble a regular sGRB.

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arXiv2018

Who Ordered That? On The Origin of LIGO's Merging Binary Black Holes

LIGO's detection of gravitational waves from binary black hole mergers was an unexpected surprise that immediately raised the question - what is the origin of these black hole binaries? The "simplest" scenario is evolution of field massive stellar binaries. However, other possibilities involving capture have been proposed. We explore here one of the more interesting clues on this puzzle: the relatively modest spins of the resulting black holes that imply that the progenitor black holes were not spinning rapidly. More specifically we consider the implication of observed distribution of, $χ_{\rm eff}$, the mass weighted projected (along the orbital axis) spins on the field evolution scenario. In all cases $χ_{\rm eff}$ is small and in two of the cases the best fit value is negative. Only in one event the spin is positive at 90\% credible. These observations are puzzling within the field binary scenario in which positive higher spins ($χ_{\rm eff} \ge 0.5$) are expected. At first sight one may expect that this rules out the field evolutionary scenario. Indeed we show that with typical parameters a significant fraction ($\ge 25\%$) of the mergers should have high effective spin values. However, uncertainties in the outcome of the common envelope phase (the typical separation and whether the stars are rotating or not) and in the late stages of massive star evolution (the strength of the winds) make it impossible to rule out, at present, these scenarios. While observations of mergers with high effective spin will support this scenario, future observations of negative spin mergers would rule it out.

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arXiv2018

Detecting Black Hole Binaries by Gaia

We study the prospect of the Gaia satellite to identify black hole binary systems by detecting the orbital motion of the companion stars. Taking into account the initial mass function, mass transfer, common envelope phase, interstellar absorption and identifiability of black holes, we estimate the number of black hole binaries detected by Gaia and their distributions with respect to the black hole mass for several models with different parameters. We find that $\sim 200-1000$ black hole binaries will be detected by Gaia during its 5 years operation. The shape of distribution function of the black hole mass is affected most severely by the relation between the zero-age main sequence (ZAMS) stellar mass and the black hole mass in parameters adopted in this paper, which implies that black holes detected with Gaia enables us to constrain the mass relation.

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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

Implications of the radio and X-ray emission that followed GW170817

The radio and X-rays that followed GW170817 increased gradually over $\sim~100$ days, resembling the radio flare predicted to arise from the interaction of a binary neutron star merger outflow with the ISM \citep{nakar2011}. Considering a blast wave moving with a Lorentz factor $Γ$, we show that an off-axis observer, namely an observer at $θ_{obs}>1/Γ$, sees a light curve rising faster than $F_ν\propto t^3$. Therefore, the observed rise, $F_ν\propto~t^{0.78}$, implies that at all times we have seen an on-axis emission. Namely, the emitting matter was within $θ_{obs}<1/Γ$ at the time of observations (even if it was off-axis beforehand). The observations tightly constrain the blast wave Lorentz factor: $Γ\sim(1.5-7)(t/10~{\rm day})^{-0.21}$.The isotropic equivalent energy in the observed region increases with time, $E_{iso}\sim~10^{50}{\rm~erg} ~(t/150~{\rm day})^{1.3}$, implying that the outflow was structured. The energy increase can arise from a slower material moving behind the blast wave or from a matter moving at larger angles that has slowed down. Such a structure could have different origins, however, the only physically motivated one, proposed so far, is the interaction of a relativistic jet with the ejecta and the resulting cocoon. The jet could have been choked or successful. In the latter case, it has produced a short GRB pointing elsewhere (this successful jet-cocoon system is sometimes called a "structured jet"). Although circumstantial evidence disfavors a successful jet, the jet fate cannot be decisively determined from current observations. The light curve alone may not be sufficient to resolve this question, since both chocked and successful jets can lead to a gradual rise to a peak, followed by a decay. Therefore, the recent turnover of the light curve does not necessitate a successful jet.

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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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arXiv2018

Magnetorotational instability in eccentric disks

Eccentric disks arise in such astrophysical contexts as tidal disruption events, but it is unknown whether the magnetorotational instability (MRI), which powers accretion in circular disks, operates in eccentric disks as well. We examine the linear evolution of unstratified, incompressible MRI in an eccentric disk orbiting a point mass. We consider vertical modes of wavenumber $k$ on a background flow with uniform eccentricity $e$ and vertical Alfvén speed $v_\mathrm A$ along an orbit with mean motion $n$. We find two mode families, one with dominant magnetic components, the other with dominant velocity components; the former is unstable at $(1-e)^3f^2\lesssim3$, where $f\equiv kv_\mathrm A/n$, the latter at $e\gtrsim0.8$. For $f^2\lesssim3$, MRI behaves much like in circular disks, but the growth per orbit declines slowly with increasing $e$; for $f^2\gtrsim3$, modes grow by parametric amplification, which is resonant for $0<e\ll1$. MRI growth and the attendant angular momentum and energy transport happen chiefly near pericenter, where orbital shear dominates magnetic tension.

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arXiv2018

Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers

We find, using high resolution numerical relativistic simulations, that the tail of the dynamical ejecta of neutron star mergers extends to mildly relativistic velocities faster than $0.7c$. The kinetic energy of this fast tail is $\sim 10^{47}$--$10^{49}$ erg, depending on the neutron star equation of state and on the binary masses. The synchrotron flare arising from the interaction of this fast tail with the surrounding ISM can power the observed non-thermal emission that followed GW170817, provided that the ISM density is $\sim 10^{-2}\,{\rm cm^{-3}}$, the two neutron stars had roughly equal masses and the neutron star equation of state is soft (small neutron star radii). One of the generic predictions of this scenario is that the cooling frequency crosses the X-ray band on a time scale of a few months to a year, leading to a cooling break in the X-ray light curve. If this dynamical ejecta scenario is correct, we expect that the synchrotron radio flare from the ejecta that have produced the macronova/kilonova emission will be observable on time scales of $10^3$ to $10^5$ days. Further multi-frequency observations will confirm or rule out this dynamical ejecta scenario.

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arXiv2018

Neutron Star Mergers as sites of r-process Nucleosynthesis and Short Gamma-Ray Bursts

Neutron star mergers have been long considered as promising sites of heavy $r$-process nucleosynthesis. We overview observational evidence supporting this scenario including: the total amount of $r$-process elements in the Galaxy, extreme metal poor stars, geological radioactive elemental abundances, dwarf galaxies, and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical, and near infrared observations point to $r$-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We find however that, within simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the europium abundance history of Galactic stars for [Fe/H] $\gtrsim -1$. This implies that to account for the role of mergers in the Galactic chemical evolution, we need a Galactic model with multiple populations that have different spatial distributions and/or varying formation rates.

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arXiv2017

Physics of the saturation of particle acceleration in relativistic magnetic reconnection

We investigate the saturation of particle acceleration in relativistic reconnection using two-dimensional particle-in-cell simulations at various magnetizations σ. We find that the particle energy spectrum produced in reconnection quickly saturates as a hard power law that cuts off at γ~4σ, confirming previous work. Using particle tracing, we find that particle acceleration by the reconnection electric field in X-points determines the shape of the particle energy spectrum. By analyzing the current sheet structure, we show that physical cause of saturation is the spontaneous formation of secondary magnetic islands that can disrupt particle acceleration. By comparing the size of acceleration regions to the typical distance between disruptive islands, we show that the maximum Lorentz factor produced in reconnection is γ~ 5 σ, which is very close to what we find in our particle energy spectra. We also show that the dynamic range in Lorentz factor of the power law spectrum in reconnection is < 40. The hardness of the power law combined with its narrow dynamic range implies that relativistic reconnection is capable of producing the hard narrowband flares observed in the Crab Nebula but has difficulty producing the softer broadband prompt GRB emission.

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arXiv2017

The extragalactic ultra-high energy cosmic-ray dipole

We explore the possibility that the recently detected dipole anisotropy in the arrival directions of~$>8$~EeV ultra-high energy cosmic-rays (UHECRs) arises due to the large-scale structure (LSS). We assume that the cosmic ray sources follow the matter distribution and calculate the flux-weighted UHECRs' RMS dipole amplitude taking into account the diffusive transport in the intergalactic magnetic field (IGMF). We find that the flux-weighted RMS dipole amplitude is $\sim8$% before entering the Galaxy. The amplitude in the [4-8] EeV is only slightly lower $\sim 5$%. The required IGMF is of the order of {5-30 nG}, and the UHECR sources must be relatively nearby, within $\sim$300 Mpc. The absence of statistically significant signal in the lower energy bin can be explained if the same nuclei specie dominates the composition in both energy bins and diffusion in the Galactic magnetic field (GMF) reduces the dipole of these lower rigidity particles. Photodisintegration of higher energy UHECRs could also reduce somewhat the lower energy dipole.

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arXiv2017

Recovering the observed B/C ratio in a dynamic spiral-armed cosmic ray model

We develop a fully three dimensional numerical code describing the diffusion of cosmic rays in the Milky Way. It includes the nuclear spallation chain up to Oxygen, and allows the study of various cosmic ray properties, such as the CR age, grammage traversed, and the ratio between secondary and primary particles. This code enables us to explore a model in which a large fraction of the cosmic ray acceleration takes place in the vicinity of galactic spiral arms and that these spiral arms are dynamic. We show that the effect of having dynamic spiral arms is to limit the age of cosmic rays at low energies. This is because at low energies the time since the last spiral arm passage governs the Cosmic Ray (CR) age, and not diffusion. Using the model, the observed spectral dependence of the secondary to primary ratio is recovered without requiring any further assumptions such as a galactic wind, re-acceleration or various assumptions on the diffusivity. In particular, we obtain a secondary to primary ratio which increases with energy below about 1 GeV.

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