arXiv subjects

Almog Yalinewich

Publications and source records attributed to Almog Yalinewich.

At least 19 recordsLinked to original sources

arXiv2023

Observability of flashes from ejecta crashes in aspherical supernovae, with application to SN 2008D

A new class of transient, which has been hypothesized to accompany the explosion of an aspherical compact supernova, would arise when streams of ejecta collide outside the star. However, conditions that favour the prompt release of radiation from the collision, such as a diffuse stellar envelope, disfavour the creation of non-radial ejecta in the first place. To determine whether the collision can both occur and be visible, we simulate aspherical explosions using the HUJI-RICH moving-mesh hydrodynamics code and analyze them in terms of diffusion measures defined for individual fluid elements. While our simulations are highly idealized, they connect to realistic explosions via a single dimensionless parameter. Defining two measures of the importance of diffusivity (two versions of the inverse Péclet number), we find that one varies in a way that indicates colliding ejecta can release a photon flash, while the other does not. Examining the x-ray transient XT 080109 associated with supernova SN 2008D, we find that its fluence and duration are consistent with the properties of an ejecta collision in the aspherical model that is most likely to emit a flash. Our results give tentative evidence for the possibility of collision-induced flashes for a narrow and radius-dependent range of asphericity, and motivate future radiation hydrodynamics simulations.

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arXiv2022

The Moving Mirror model for Fast Radio Bursts

Recent observations of coherent radiation from the Crab pulsar (Bij et al 2021) suggest the emission is driven by an ultra - relativistic ($γ\sim 10^4$), cold plasma flow. A relativistically expanding plasma shell can compress the ambient magnetic field, like a moving mirror, and thus produce coherent radiation whose wavelength is shorter than that of the ambient medium by $γ^2$. This mechanism has been studied in the past by Colgate and Noerdelinger (1971), in the context of radio loud supernova explosions. In this work we propose that a similar mechanism drives the coherent emission in fast radio bursts. The high Lorenz factors dramatically lower the implied energy and magnetic field requirements, allowing the spin down energy of regular (or even recycled), fast spinning pulsars, rather than slow spinning magnetars, to explain FRBs. We show that this model can explain the frequency and the time evolution of observed FRBs, as well as their duration, energetics and absence of panchromatic counterparts. We also predict that the peak frequency of sub pulses decline with observation time as $ω_{\rm obs} \propto t_{\rm obs}^{-1/2}$. Unfortunately, with current capabilities it is not possible to constrain the shape of the curve $ω_{\rm obs} \left(t_{\rm obs} \right)$. Finally, we find that a variation of this model can explain weaker radio transients, such as the one observed from a galactic magnetar. In this variant, the shock wave produces low frequency photons which are then Compton scattered to the GHz range.

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arXiv2022

Plug Disintegration in GRB Jet Eruption

In this work we consider the eruption of a tenuous relativistic hydrodynamic jet from a dense baryonic envelope. As the jet moves out and away, it carries along and continues to accelerate a layer of baryonic material which we refer to as the plug. We solve the relativistic equations of motion for the trajectory of the plug, and verify it using a relativistic hydrodynamic simulation. We show that under these conditions, the plug breaks up at a radius larger by a factor of a few from the radius of the envelope, due to the onset of the Rayleigh Taylor instability. After breakup the jet continues to accelerate to higher Lorentz factors while the plug fragments maintain a moderate Lorentz factor. The presence of slower moving ejecta can explain late time features of GRBs such as X ray flares without recourse to a long lived engine.

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arXiv2021

The Propagation of Strong Shocks into Planetary and Stellar Atmospheres

In this work we present a mathematical model for the propagation of the shock waves that occur in graded density profiles. These waves can occur in a wide range of astrophysical events, such as collisions in planetary and stellar atmospheres, common envelope explosions and peculiar type Ia supernovae. The behaviour of the shock wave and its evolution can be modelled using type II self similar solutions. In such solutions the evolution of the shock wave is determined by boundary conditions at the shock front and a singular point in the shocked region. We show how the evolution can be determined for different equations of state and density profiles, and compare these results to numerical simulations. These findings are also applied to a variety of astrophysical phenomena to further test their validity.

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arXiv2021

Crater Morphology of Primordial Black Hole Impacts

In this work we propose a novel campaign for constraining relativistically compact MACHO dark matter, such as primordial black holes (PBHs), using the moon as a detector. PBHs of about $10^{19} \textrm{ g}$ to $10^{22} \textrm{ g}$ may be sufficiently abundant to have collided with the moon in the history of the solar system. We show that the crater profiles of a PBH collision differ from traditional impactors and may be detectable in high resolution lunar surface scans now available. Any candidates may serve as sites for in situ measurements to identify high pressure phases of matter which may have formed near the PBH during the encounter. While we primarily consider PBH dark matter, the discussion generalises to the entire family of MACHO candidates with relativistic compactness. Moreover, we focus on the Moon since it has been studied well, but the same principles can be applied to other rocky bodies in our solar system without an atmosphere.

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arXiv2020

Self Similar Adiabatic Strong Explosion in a Medium Gravitationally Free Falling to a Point Mass

We develop a generalisation to the classical Sedov Taylor explosion where the medium free falls to a point mass at the centre of the explosion. To verify our analytic results, we compare them to a suite of numerical simulations. We find that there exists a critical energy below which, instead of propagating outward the shock stalls and collapses under gravity. Furthermore, we find that the value of the critical energy threshold decreases when the adiabatic index increases and material is more evenly distributed within the shocked region. We apply this model to the problem of a shock bounce in core collapse supernova, in which the proto neutron star serves as the point mass. The relation between the threshold energy and the distribution of mass in the shock might help explain how turbulence prevents shock stalling and recession in a core collapse supernova explosion.

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arXiv2020

Nekhoroshev Estimates for the Survival Time of Tightly Packed Planetary Systems

$N$-body simulations of non-resonant tightly-packed planetary systems have found that their survival time (i.e. time to first close encounter) grows exponentially with their interplanetary spacing and planetary masses. Although this result has important consequences for the assembly of planetary systems by giants collisions and their long-term evolution, this underlying exponential dependence is not understood from first principles, and previous attempts based on orbital diffusion have only yielded power-law scalings. We propose a different picture, where large deviations of the system from its initial conditions is due to few slowly developing high-order resonances. Thus, we show that the survival time of the system $T$ can be estimated using a heuristic motivated by Nekhoroshev's theorem, and obtain a formula for systems away from overlapping two-body mean-motion resonances as: $T/P=c_1 \frac{a}{Δa} \exp \left(c_2 \frac{Δa}{a} /μ^{1/4}\right)$, where $P$ is the average Keplerian period, $a$ is the average semi major axis, $Δa\ll a$ is the difference between the semi major axes of neighbouring planets, $μ$ is the planet to star mass ratio, and $c_1$ and $c_2$ are dimensionless constants. We show that this formula is in good agreement with numerical N-body experiments for $c_1=5 \cdot 10^{-4}$ and $c_2=8$.

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arXiv2019

Self similar Shocks in Atmospheric Mass Loss due to Planetary Collisions

We present a mathematical model for the propagation of the shock waves that occur during planetary collisions. Such collisions are thought to occur during the formation of terrestrial planets, and they have the potential to erode the planet's atmosphere. We show that under certain assumptions, this evolution of the shock wave can be determined using the method of self similar solutions. This self similar solution is of type II, which means that it only applies to a finite region behind the shock front. This region is bounded by the shock front and the sonic point. Energy and matter continuously flow through the sonic point, so that energy in the self similar region is not conserved, as is the case for type I solutions. Instead, the evolution of the shock wave is determined by boundary conditions at the shock front and at the sonic point. We show how the evolution can be determined for different equations of state, allowing these results to be readily used to calculate the atmospheric mass loss from planetary cores made of different materials.

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arXiv2019

Optical Transient from an Explosion Close to the Stellar Surface

We study the hydrodynamic evolution of an explosion close to the stellar surface, and give predictions for the radiation from such an event. We show that such an event will give rise to a multi-wavelength transient. We apply this model to describe a precursor burst to the peculiar supernova iPTF14hls, which occurred in 1954, sixty year before the supernova. We propose that the new generation of optical surveys might detect similar transients, and they can be used to identify supernova progenitors well before the explosion.

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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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arXiv2019

Atmospheric Mass Loss from High Velocity Giant Impacts

Using moving mesh hydrodynamic simulations, we determine the shock propagation and resulting ground velocities for a planet hit by a high velocity impactor. We use our results to determine the atmospheric mass loss caused by the resulting ground motion due to the impact shock wave. We find that there are two distinct shock propagation regimes: In the limit in which the impactor is significantly smaller than the target ($R_i<< R_t$), the solutions are self-similar and the shock velocity at a fixed point on the target scale as $m_i^{2/3}$, where $m_i$ is the mass of the impactor. In addition, the ground velocities follow a universal profile given by $v_g/v_i=(14.2x^2-25.3x+11.3)/(x^2-2.5x+1.9) +2\ln{R_i/R_t}$, where $x=\sin\left(θ/2\right)$, $θ$ is the latitude on the target measured from the impact site, and $v_g$ and $v_i$ are the ground velocity and impact velocity, respectively. In contrast, in the limit in which the impactor is comparable to the size of the target ($R_i \sim R_t$), we find that shock velocities decline with the mass of the impactor significantly more weakly than $m_i^{2/3}$. We use the resulting surface velocity profiles to calculate the atmospheric mass loss for a large range of impactor masses and impact velocities and apply them to the Kepler-36 system and the Moon forming impact. Finally, we present and generalise our results in terms of the $v_g/v_i$ and the impactor to target size ratio ($R_i/R_t$) such that they can easily be applied to other collision scenarios.

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arXiv2019

Fast Radio Bursts from Terraformation

Fast radio bursts (FRBs) are, as the name implies, short and intense pulses of radiation at wavelengths of roughly one metre. FRBs have extremely high brightness temperatures, which points to a coherent source of radiation. The energy of a single burst ranges from $10^{36}$ to $10^{39}$ erg. At the high end of the energy range, FRBs have enough energy to unbind an earth-sized planet, and even at the low end, there is enough energy to vaporise and unbind the atmosphere and the oceans. We therefore propose that FRBs are signatures of an artificial terraformer, capable of eradicating life on another planet, or even destroy the planet entirely. The necessary energy can be harvested from Wolf-Rayet stars with a Dyson sphere ($\sim 10^{38}$ erg s$^{-1}$) , and the radiation can be readily produced by astrophysical masers. We refer to this mechanism as Volatile Amplification of a Destructive Emission of Radiation (VADER). We use the observational information to constrain the properties of the apparatus. We speculate that the non-repeating FRBs are low-energy pulses used to exterminate life on a single planet, but leaving it otherwise intact, and that the stronger repeating FRB is part of an effort to destroy multiple objects in the same solar system, perhaps as a preventative measure against panspermia. In this picture, the persistent synchrotron source associated with the first repeating FRB arises from the energy harvesting process. Finally we propose that Oumuamua might have resulted from a destruction of a planet in this manner.

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arXiv2019

Statistical inference of the distance to ASKAP FRBs

The distances to fast radio bursts (FRBs) are crucial for understanding their underlying engine, and for their use as cosmological probes. In this paper, we provide three statistical estimates of the distance to ASKAP FRBs. First, we show that the number of events of similar luminosity in ASKAP does not scale as distance cubed, as one would expect, when directly using the observed dispersion measure (DM) to infer distance. Second, by comparing the average DMs of FRBs observed with different instruments, we estimated the average redshift of ASKAP FRBs to be $z\sim 0.01$ using CHIME and ASKAP, and $z\lesssim0.07$ using Parkes and ASKAP. Both values are much smaller than the upper limit $z\sim0.3$ estimated directly from the DM. Third, we cross-correlate the locations of the ASKAP FRBs with existing large-area redshift surveys, and see a 3$σ$ correlation with the 2MASS Redshift Survey and a 5$σ$ correlation with the HI Parkes All Sky Survey at $z\sim0.007$. This corresponds well with the redshift of the most likely host galaxy of ASKAP FRB 171020, which is at $z=0.00867$. These arguments combined suggest an extremely nearby origin of ASKAP FRBs and a local environment with accumulated electrons that contribute a DM of several hundred pc/cm$^3$, which should be accounted for in theoretical models.

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arXiv2019

The Signature of a Windy Radio Supernova Progenitor in a Binary System

Type II supernova progenitors are expected to emit copious amounts of mass in a dense stellar wind prior to the explosion. When the progenitor is a member of a binary, the orbital motion modulates the density of this wind. When the progenitor explodes, the high-velocity ejecta collides with the modulated wind, which in turn produces a modulated radio signal. In this work we derive general analytic relations between the parameters of the radio signal modulations and binary parameter in the limit of large member mass ratio. We use these relations to infer the semi major axis of SN1979c and a lower bound for the mass of the companion. We further constrain the analytic estimates by numerical simulations using the AMUSE framework. In these calculations we simulate the progenitor binary system including the wind and the gravitational effect of a companion star. The simulation output is compared to the observed radio signal in supernova SN1979C. We find that it must have been a binary with an orbital period of about 2000 year. If the exploding star evolved from a $\sim 18 M_{\odot}$ zero-age main-sequence at solar metalicity, we derive a companion mass of $5$ to $12 M_{\odot}$ in an orbit with an eccentricity lower than about 0.8.

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arXiv2018

Shock Breakouts from Tidal Disruption Events

Tidal disruption events of stars by supermassive black holes have so far been discovered months to years after the fact. In this paper we explore the short, faint and hard burst of radiation is emitted at maximum compression, as a result of shock breakout. The detection of this burst can be used to capture tidal disruption events in real time. We verify that shock breakout from main sequence stars produces radiation in the X-ray range, but find that it is difficult to detect using all sky X-ray surveying telescopes. In the case of shock breakout from red giants, most of the radiation is emitted in the UV and visible range, which is significantly easier to detect. A similar burst of UV/optical radiation will also be emitted by stars puffed by tidal heating from a previous passage close to the central black hole. This radiation can be detected by surveys like ZTF and LSST. We calculate detection rates for different types of galactic nuclei. For the case of a very full loss cone we predict a detection rate of once per month with LSST, whereas for the case of a very empty loss cone we predict a rate of once per year with LSST. Evidence from a recent tidal disruption event, ASASSN-14li, seems to favour a very full loss cone, in which case LSST is expected to detect one such event every month.

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arXiv2018

Dark Passengers in Stellar Surveys

We develop stellar population models to predict the number of binaries with a single luminous member is Gaia and Hipparcos. Our models yield dozens of detections of black hole - luminous companion binaries (BHLC) and hundreds to thousands of neutron star - luminous companion binaries (NSLC) with Gaia. Interestingly, our models also yield a single detection of BHLC binary with Hipparcos, and a few NSLC binaries. We also show how the statistical distribution of detected binaries with a single luminous companion can be used to constrain the formation process of neutron stars and black holes.

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arXiv2018

A Generalised Bondi Accretion Model for the Galactic Centre

We develop an analytic, steady-state model for the gas environment in quiescent galactic nuclei. We assume that the mass is constantly supplied by a spherically symmetric distribution of wind emitting stars, and that gravity is solely due to a central supermassive black hole. We show that at some finite radius, where the Keplerian velocity is comparable to the wind velocity, the bulk velocity vanishes. Matter generated below that radius will be accreted onto the black hole, while matter outside it will escape the system. Under certain conditions, the flow may become supersonic at both domains. We obtain radial profiles of the hydrodynamic variables and verify them using a time-dependent hydrodynamic simulation. We delineate the conditions under which radiative cooling can be neglected, and predict the luminosity and spectrum of the free-free X-ray emission from such a system. We discuss applications of our solution to our own Galactic Centre and other quiescent galactic nuclei.

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arXiv2018

Ignition of detonation in accreted helium

Sub-Chandrasekhar CO white dwarfs accreting helium have been considered as candidates for Type Ia supernova(SNIa) progenitors since the early 1980s (helium shell mass $> 0.1 M_\odot $). These models, once detonated did not fit the observed spectra and light curve of typical SNIa observations. New theoretical work examined detonations on much less massive ($< 0.05 M_\odot $) envelopes. They find stable detonations that lead to light curves, spectra and abundances that compare relatively well with the observational data. The exact mechanism leading to the ignition of helium detonation is a key issue, since it is a mandatory first step for the whole scenario. As the flow of the accreted envelope is unstable to convection long before any hydrodynamic phenomena develops, a multidimensional approach is needed in order to study the ignition process. The complex convective reactive flow is challenging to any hydrodynamical solver. To the best of our knowledge, all previous 2D studies ignited the detonation artificially. We present here, for the first time, fully consistent results from two hydrodynamical 2D solvers that adopt two independent accurate schemes. For both solvers an effort was made to overcome the problematics raised by the finite resolution and numerical diffusion by the advective terms. Our best models lead to the ignition of a detonation in a convective cell. Our results are robust and the agreement between the two different numerical approaches is very good.

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