arXiv subjects

Re'em Sari

Publications and source records attributed to Re'em Sari.

At least 55 records · Page 3Linked to original sources

arXiv2019

Recombination Effects on Supernova Light Curves

The light curves of type-II supernovae (SNe) are believed to be highly affected by recombination of hydrogen that takes place in their envelopes. In this work, we analytically investigate the transition from a fully ionized envelope to a partially recombined one and its effects on the SN light curve. The motivation is to establish the underlying processes that dominate the evolution at late times when recombination takes place in the envelope, yet early enough so that $^{56}$Ni decay is a negligible source of energy. We consider the diffusion of photons through the envelope while analyzing the ionization fraction and the coupling between radiation and gas, and find that the main effect of recombination is on the evolution of the observed temperature. Before recombination the temperature decreases relatively fast, while after recombination starts it significantly reduces the rate at which the observed temperature drops with time. This behaviour is the main cause for the observed flattening in the optical bands, where for a typical red supergiant explosion, the recombination wave affects the bolometric luminosity only mildly during most of the photospheric phase. Moreover, the plateau phase observed in some type-II SNe is not a generic result of recombination, and it also depends on the density structure of the progenitor. This is one possible explanation to the different light curve decay rates observed in type II (P and L) SNe.

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arXiv2018

Cooling off with a kilonova - Lower Limit on the Expansion Velocity of GW170817

GW170817 was the first detection of a binary neutron star merger via gravitational waves. The event was observed over a wide range of the electromagnetic spectrum, revealing a thermal kilonova dominating the optical signal during the first $\sim$15 days, and a non-thermal synchrotron emission that has continued to rise $\sim$200 days post-merger, dominating the radio and x-ray emission. At early times, when the kilonova is still dominant, the synchrotron emitting electrons can efficiently cool by up-scattering the kilonova photos through inverse-Compton. Yet, the cooling frequency is not observed up to the X-ray band. This can only be explained if the source is moving at least at a mildly relativistic velocity. We find a lower limit on the source's bulk Lorentz factor of $Γ> 2.1$ at $9$ days. This lower limit is model independent, and relies directly on the observed quantities, providing an additional robust evidence to the relativistic motion in this event at early times.

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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

Deep and wide gaps by super Earths in low-viscosity discs

Planets can open cavities (gaps) in the protoplanetary gaseous discs in which they are born by exerting gravitational torques. Viscosity counters these torques and limits the depletion of the gaps. We present a simple one-dimensional scheme to calculate the gas density profile inside gaps by balancing the gravitational and viscous torques. By generalizing the results of Goodman & Rafikov (2001), our scheme properly accounts for the propagation of angular momentum by density waves. This method allows us to easily study low-viscosity discs, which are challenging for full hydrodynamical simulations. We complement our numerical integration by analytical equations for the gap's steady-state depth and width as a function of the planet's to star's mass ratio $μ$, the gas disc's aspect ratio $h$, and its Shakura & Sunyaev viscosity parameter $α$. Specifically, we focus on low-mass planets ($μ<μ_{\rm th}\equiv h^3$) and identify a new low-viscosity regime, $α<h(μ/μ_{\rm th})^5$, in which the classical analytical scaling relations are invalid. Equivalently, this low-viscosity regime applies to every gap that is depleted by more than a factor of $(μ_{\rm th}/μ)^3$ relative to the unperturbed density. We show that such gaps are significantly deeper and wider than previously thought, and consequently take a longer time to reach equilibrium.

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arXiv2018

Modal Decomposition of TTV - Inferring Planet Masses and Eccentricities

Transit timing variations (TTVs) are a powerful tool for characterizing the properties of transiting exoplanets. However, inferring planet properties from the observed timing variations is a challenging task, which is usually addressed by extensive numerical searches. We propose a new, computationally inexpensive method for inverting TTV signals in a planetary system of two transiting planets. To the lowest order in planetary masses and eccentricities, TTVs can be expressed as a linear combination of 3 functions, which we call the \textit{TTV modes}. These functions depend only on the planets' linear ephemerides, and can be either constructed analytically, or by performing 3 orbital integrations of the three-body system. Given a TTV signal, the underlying physical parameters are found by decomposing the data as a sum of the TTV modes. We demonstrate the use of this method by inferring the mass and eccentricity of 6 \textit{Kepler} planets that were previously characterized in other studies. Finally we discuss the implications and future prospects of our new method.

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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

Tidal Disruption of Inclined or Eccentric Binaries by Massive Black Holes

Binary stars that are on close orbits around massive black holes (MBH) such as Sgr A* in the center of the Milky Way are liable to undergo tidal disruption and eject a hypervelocity star. We study the interaction between such a MBH and circular binaries for general binary orientations and penetration depths (i.e. binaries penetrate into the tidal radius around the BH). We show that for very deep penetrators, all binaries are disrupted when the binary rotation axis is roughly oriented toward the BH or it is in the opposite direction. The surviving chance becomes significant when the angle between the binary rotation axis and the BH direction is between π/4 and 3 π/4. The surviving chance is as high as $\sim$ 20$\%$ when the binary rotation axis is perpendicular to the BH direction. The angular dependence is opposite for very shallow penetrators where coplanar prograde orbits have the lowest surviving chance (or equivalently most vulnerable). We provide numerical fits to the disruption probability and energy gain at the the BH encounter as a function of the penetration depth. The latter can be simply rescaled in terms of binary masses, their initial separation and the binary-to-BH mass ratio to evaluate the ejection velocity of a binary members in various systems. We also investigate the disruption of coplanar, eccentric binaries by a MBH. It is shown that for highly eccentric binaries retrograde orbits have a significantly increased disruption probability and ejection velocities compared to the circular binaries.

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arXiv2018

Core-powered mass loss and the radius distribution of small exoplanets

Recent observations identify a valley in the radius distribution of small exoplanets, with planets in the range $1.5-2.0\,{\rm R}_{\oplus}$ significantly less common than somewhat smaller or larger planets. This valley may suggest a bimodal population of rocky planets that are either engulfed by massive gas envelopes that significantly enlarge their radius, or do not have detectable atmospheres at all. One explanation of such a bimodal distribution is atmospheric erosion by high-energy stellar photons. We investigate an alternative mechanism: the luminosity of the cooling rocky core, which can completely erode light envelopes while preserving heavy ones, produces a deficit of intermediate sized planets. We evolve planetary populations that are derived from observations using a simple analytical prescription, accounting self-consistently for envelope accretion, cooling and mass loss, and demonstrate that core-powered mass loss naturally reproduces the observed radius distribution, regardless of the high-energy incident flux. Observations of planets around different stellar types may distinguish between photoevaporation, which is powered by the high-energy tail of the stellar radiation, and core-powered mass loss, which depends on the bolometric flux through the planet's equilibrium temperature that sets both its cooling and mass-loss rates.

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arXiv2017

Steeper stellar cusps in galactic centers from binary disruption

The relaxed distribution of stars around a massive black hole is known to follow a cusp profile $ρ(r)\propto r^{-α}$ with characteristic slope $α=7/4$. This follows from energy conservation and a scattering rate as given by two body encounters. However, we show that injection of stars close to the black hole, i.e. a source term in the standard cusp picture, modifies this profile. In the steady-state configuration, the cusp develops a central region with typical slope $α=9/4$ in which stars diffuse outward. Binary disruption by the intense tidal field of the massive black hole is among the phenomena that take place in the Galactic Center. In such disruption, one of the binary members remains bound to the black hole, thus providing a source term of stars close to the black hole. Assuming a binary fraction of $0.1$ and an orbital circularization efficiency of $0.35$, we show that this source is strong enough to modify the cusp profile within $\approx 0.07$ pc in the Galactic Center. If the binary fraction at the influence radius is of order unity and the orbits of all the captured stars are efficiently circularized, the steeper cusp extends almost as far as the radius of influence of the black hole.

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arXiv2017

A spectral approach to transit timing variations

The high planetary multiplicity revealed by Kepler implies that Transit Time Variations (TTVs) are intrinsically common. The usual procedure for detecting these TTVs is biased to long-period, deep transit planets whereas most transiting planets have short periods and shallow transits. Here we introduce the Spectral Approach to TTVs technique that allows expanding the TTVs catalog towards lower TTV amplitude, shorter orbital period, and shallower transit depth. In the Spectral Approach we assume that a sinusoidal TTV exists in the data and then calculate the improvement to $χ^2$ this model allows over that of linear ephemeris model. This enables detection of TTVs even in cases where the transits are too shallow so individual transits cannot be timed. The Spectral Approach is more sensitive due to the reduced number of free parameters in its model. Using the Spectral Approach, we: (a) detect 131 new periodic TTVs in Kepler data (an increase of ~2/3 over a previous TTV catalog); (b) Constrain the TTV periods of 34 long-period TTVs and reduce amplitude errors of known TTVs; (c) Identify cases of multi-periodic TTVs, for which absolute planetary mass determination may be possible. We further extend our analysis by using perturbation theory assuming small TTV amplitude at the detection stage, which greatly speeds up our detection (to a level of few seconds per star). Our extended TTVs sample shows no deficit of short period or low amplitude transits, in contrast to previous surveys in which the detection schemes were significantly biased against such systems.

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arXiv2017

Probing the Gas Density in our Galactic Center: Moving Mesh Simulations of G2

The G2 object has recently passed its pericenter passage in our Galactic Center. While the $Br_γ$ emission shows clear signs of tidal interaction, the change in the observed luminosity is only of about a factor of 2, in contention with all previous predictions. We present high resolution simulations performed with the moving mesh code, RICH, together with simple analytical arguments that reproduce the observed $Br_γ$ emission. In our model, G2 is a gas cloud that undergoes tidal disruption in a dilute ambient medium. We find that during pericenter passage, the efficient cooling of the cloud results in a vertical collapse, compressing the cloud by a factor of $\sim5000$. By properly taking into account the ionization state of the gas, we find that the cloud is UV starved and are able to reproduce the observed $Br_γ$ luminosity. For densities larger than $\approx500\;\mathrm{cm}^{-3}$ at pericenter, the cloud fragments, due to cooling instabilities and the emitted radiation is inconsistent with observations. For lower densities, the cloud survives the pericenter passage intact and its emitted radiation matches the observed lightcurve. From the duration of $Br_γ$ emission which contains both redshifted and blueshifted components, we show that the cloud is not spherical but rather elongated with a size ratio of 4 at year 2001. The simulated cloud's elongation grows as it travels towards pericenter and is consistent with observations, due to viewing angles. The simulation is also consistent with having a spherical shape at apocenter.

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arXiv2017

A VLA Study of High-redshift GRBs II - The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

We present detailed multi-frequency, multi-epoch radio observations of GRB 140304A at $z=5.283$ from 1 to 86 GHz and 0.45 d to 89 d. The radio and mm data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multi-wavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to $E_{\rm K, iso}\approx4.9\times10^{54}\,$erg, $A_* \approx 2.6\times10^{-2}$, $ε_{\rm e}\approx2.5\times10^{-2}$, $ε_{\rm B}\approx5.9\times10^{-2}$, $p\approx2.6$, and $θ_{\rm jet}\approx1.1^{\circ}$, yielding a beaming corrected $γ$-ray and kinetic energy, $E_γ\approx2.3\times10^{49}\,$erg and $E_{\rm K}\approx9.5\times10^{50}\,$erg, respectively. We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a re-brightening in the X-rays at $\approx0.26\,$d, and either a standard RS or diffractive interstellar scintillation. Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, $t_{\rm dec}\approx100\,$s, the ejecta Lorentz factor, $Γ(t_{\rm dec})\approx300$, and a low RS magnetization, $R_{\rm B}\approx0.6$. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines, and the challenge presented by interstellar scintillation in conclusively identifying RS emission in GRB radio afterglows.

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arXiv2017

Hot-Jupiter Core Mass from Roche-lobe Overflow

The orbits of many observed hot Jupiters are decaying rapidly due to tidal interaction, eventually reaching the Roche limit. We analytically study the ensuing coupled mass loss and orbital evolution during the Roche-lobe overflow and find two possible scenarios. Planets with light cores $M_c\lesssim 6M_\oplus$ (assuming a nominal tidal dissipation factor $Q\sim 10^6$ for the host star) are transformed into Neptune-mass gas planets, orbiting at a separation (relative to the stellar radius) $a/R_\star\approx 3.5$. Planets with heavier cores $M_c\gtrsim 6M_\oplus$ plunge rapidly until they are destroyed at the stellar surface. Remnant gas-Neptunes, which are stable to photo-evaporation, are absent from the observations, despite their unique transit radius ($5-10R_\oplus$). This result suggests that $M_c\gtrsim 6M_\oplus$, providing a useful constraint on the poorly-known core mass that may distinguish between different formation theories of gas giants. Alternatively, if one assumes a prior of $M_c\approx 6 M_\oplus$ from the core-accretion theory, our results suggest that $Q$ does not lie in the range $10^6\lesssim Q\lesssim 10^7$.

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arXiv2017

How empty is an empty loss cone?

We consider two body relaxation in a spherical system with a loss cone. Considering two-dimensional angular momentum space, we focus on "empty loss cone" systems, where the typical scattering during a dynamical time $j_{d}$ is smaller than the size of the loss cone $j_{\rm lc}$. As a result, the occupation number within the loss cone is significantly smaller than outside. Classical diffusive treatment of this regime predict exponentially small occupation number deep in the loss cone. We revisit this classical derivation of occupancy distribution of objects in the empty loss cone regime. We emphasize the role of the rare large scatterings and show that the occupancy does not decay exponentially within the loss cone, but it is rather flat, with a typical value $\sim [(j_d/j_{\rm lc})]^2\ln^{-2}(j_{\rm lc}/j_{\min})$ compared to the occupation in circular angular momentum (where $j_{\min}$ is the smallest possible scattering). Implication are that although the loss cone for tidal break of Giants or binaries is typically empty, tidal events which occurs significantly inside the loss cone ($β\gtrsim 2$), are almost as common as those with $β\cong 1$ where $β$ is the ratio between the tidal radius and the periastron. The probability for event with penetration factor $>β$ decreases only as $β^{-1}$ rather than exponentially. This effect has no influence on events characterized by full loss cone, such as tidal disruption event of $\sim 1m_\odot$ main sequence star.

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arXiv2017

Mass loss through the L2 Lagrange point - Application to Main Sequence EMRI

We consider stable mass transfer from the secondary to the primary of an extreme mass ratio binary system. We show that when the mass transfer is sufficiently fast, mass leakage occurs through the outer Lagrange point L2, in addition to the usual transfer through L1. We provide an analytical estimate for the mass leakage rate through L2 and find the conditions in which it is comparable to the mass transfer rate through L1. Focusing on a binary system of a main-sequence star and a super-massive black hole, driven by the emission of gravitational radiation, we show that it may sustain stable mass transfer, along with mass loss through L2. If such a mass-transferring system occurs at our Galactic Centre, it produces a gravitational wave signal detectable by future detectors, such as eLISA. The signal evolves according to the star's adiabatic index and cooling time. For low mass stars, the evolution is faster than the Kelvin-Helmholtz cooling rate driving the star out of the main-sequence. In some cases, the frequency and amplitude of the signal may both decrease with time, contrary to the standard chirp of a coalescing binary. Mass loss through L2, when occurs, decreases the evolution timescale of the emitted gravitational wave signal by up to a few tens of per cent. We conclude that L2 mass ejection is a crucial factor in analyzing gravitational waves signals produced by such systems.

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arXiv2017

Analytic heating rate of neutron star merger ejecta derived from Fermi's theory of beta decay

Macronovae (kilonovae) that arise in binary neutron star mergers are powered by radioactive beta decay of hundreds of $r$-process nuclides. We derive, using Fermi's theory of beta decay, an analytic estimate of the nuclear heating rate. We show that the heating rate evolves as a power law ranging between $t^{-6/5}$ to $t^{-4/3}$. The overall magnitude of the heating rate is determined by the mean values of nuclear quantities, e.g., the nuclear matrix elements of beta decay. These values are specified by using nuclear experimental data. We discuss the role of higher order beta transitions and the robustness of the power law. The robust and simple form of the heating rate suggests that observations of the late-time bolometric light curve $\propto t^{-\frac{4}{3}}$ would be a direct evidence of a $r$-process driven macronova. Such observations could also enable us to estimate the total amount of $r$-process nuclei produced in the merger.

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arXiv2016

An Update on Monitoring Stellar Orbits in the Galactic Center

Using 25 years of data from uninterrupted monitoring of stellar orbits in the Galactic Center, we present an update of the main results from this unique data set: A measurement of mass of and distance to SgrA*. Our progress is not only due to the eight year increase in time base, but also due to the improved definition of the coordinate system. The star S2 continues to yield the best constraints on the mass of and distance to SgrA*; the statistical errors of 0.13 x 10^6 M_sun and 0.12 kpc have halved compared to the previous study. The S2 orbit fit is robust and does not need any prior information. Using coordinate system priors, also the star S1 yields tight constraints on mass and distance. For a combined orbit fit, we use 17 stars, which yields our current best estimates for mass and distance: M = 4.28 +/- 0.10|stat. +/. 0.21|sys. x 10^6 M_sun and R_0 = 8.32 +/- 0.07|stat. +/- 0.14|sys. kpc. These numbers are in agreement with the recent determination of R_0 from the statistical cluster parallax. The positions of the mass, of the near-infrared flares from SgrA* and of the radio source SgrA* agree to within 1mas. In total, we have determined orbits for 40 stars so far, a sample which consists of 32 stars with randomly oriented orbits and a thermal eccentricity distribution, plus eight stars for which we can explicitly show that they are members of the clockwise disk of young stars, and which have lower eccentricity orbits.

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arXiv2016

Tidal Heating of Young Super-Earth Atmospheres

Short-period Earth to Neptune size exoplanets (super-Earths) with voluminous gas envelopes seem to be very common. These gas atmospheres are thought to have originated from the protoplanetary disk in which the planets were embedded during their first few Myr. The accretion rate of gas from the surrounding nebula is determined by the ability of the gas to cool and radiate away its gravitational energy. Here we demonstrate that heat from the tidal interaction between the star and the young (and therefore inflated) planet can inhibit the gas cooling and accretion. Quantitatively, we find that the growth of super-Earth atmospheres halts for planets with periods of about 10 days, provided that their initial eccentricities are of the order of 0.2. Thus, tidal heating provides a robust and simple mechanism that can simultaneously explain why these planets did not become gas giants and account for the deficit of low-density planets closer to the star, where the tides are even stronger. We suggest that tidal heating may be as important as other factors (such as the nebula's lifetime and atmosphere evaporation) in shaping the observed super-Earth population.

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