ABSTRACT In this paper, we model the observable signatures of tidal disruptions of white dwarf (WD) stars using massive black holes (MBHs) of moderate mass, 103-105 M . When the WD passes deep enough ...within the MBH's tidal field, these signatures include thermonuclear transients from burning during maximum compression. We combine a hydrodynamic simulation that includes nuclear burning of the disruption of a 0.6 M C/O WD with a Monte Carlo radiative transfer calculation to synthesize the properties of a representative transient. The transient's emission emerges in the optical, with light curves and spectra reminiscent of Type I supernovae. The properties are strongly viewing angle dependent, and key spectral signatures are 10,000 km s−1 doppler shifts, due to the orbital motion of the unbound ejecta. Disruptions of He WDs likely produce large quantities of intermediate-mass elements, offering a possible production mechanism for Ca-rich transients. Accompanying multi-wavelength transients are fueled by accretion and arise from the nascent accretion disk and relativistic jet. If MBHs of moderate mass exist with number densities similar to those of supermassive BHs, both high-energy wide-field monitors and upcoming optical surveys should detect tens to hundreds of WD tidal disruptions per year. The current best strategy for their detection may therefore be deep optical follow-up of high-energy transients of unusually long duration. The detection rate or the nondetection of these transients by current and upcoming surveys can thus be used to place meaningful constraints on the extrapolation of the MBH mass function to moderate masses.
We present the results of a systematic numerical study of the onset of mass transfer in double degenerate binary systems and its impact on the subsequent evolution. All investigated systems belong to ...the regime of direct impact, unstable mass transfer. In all of the investigated cases, even those considered unstable by conventional stability analysis, we find a long-lived mass transfer phase continuing for as many as several dozen orbital periods. This settles a recent debate sparked by a discrepancy between earlier smoothed particle hydrodynamics (SPH) calculations that showed disruptions after a few orbital periods and newer grid-based studies in which mass transfer continued for tens of orbits. The number of orbits a binary survives sensitively depends on the exact initial conditions. We find that the approximate initial conditions that have been used in most previous SPH calculations have a serious impact on all stages of the evolution from the onset of mass transfer up to the final structure of the remnant. We compare 'approximate' initial conditions where spherical stars are placed at an initial separation obtained from an estimate of the Roche lobe size with 'accurate' initial conditions that were constructed by carefully driving the binary system to equilibrium by a relaxation scheme. Simulations that use the approximate initial conditions underestimate the initial separation when mass transfer sets in, which yields a binary that only survives for only a few orbits and thus a rapidly fading gravitational wave signal. Conversely, the accurate initial conditions produce a binary system in which the mass transfer phase is extended by almost two orders of magnitude in time, resulting in a gravitational wave signal with amplitude and frequency that remain essentially constant up until merger. As we show that these binaries can survive at small separation for hundreds of orbital periods, their associated gravitational wave signal should be included when calculating the gravitational wave foreground (although expected to be below Laser Interferometer Space Antenna's sensitivity at these high frequencies). We also show that the inclusion of the entropy increase associated with shock heating of the accreted material reduces the number of orbits a binary survives given the same initial conditions, although the effect is not as pronounced when using the appropriate initial conditions. The use of accurate initial conditions and a correct treatment of shock heating allows for a reliable time evolution of the temperature, density, and angular momentum, which are important when considering thermonuclear events that may occur during the mass transfer phase and/or after merger. Our treatment allows us to accurately identify when surface detonations may occur in the lead-up to the merger, as well as the properties of final merger products.
Flares from tidal disruption events are unique tracers of quiescent black holes at the centre of galaxies. The appearance of these flares is very sensitive to whether the star is totally or partially ...disrupted, and in this paper we seek to identify the critical distance of the star from the black hole (rd) that enables us to distinguish between these two outcomes. We perform here mesh-free finite mass, traditional, and modern smoothed particle hydrodynamical simulations of star-black hole close encounters, with the aim of checking if the value of rd depends on the simulation technique. We find that the critical distance (or the so-called critical disruption parameter βd) depends only weakly on the adopted simulation method, being βd = 0.92 ± 0.02 for a γ = 5/3 polytrope and βd = 2.01 ± 0.01 for a γ = 4/3 polytrope.
We examine the consequences of a model in which relativistic jets can be triggered in quiescent massive black holes when a geometrically thick and hot accretion disk forms as a result of the tidal ...disruption of a star. We use this information, together with a two-dimensional hydrodynamic model of the structure of the relativistic flow, to study the dynamics of the jet, the propagation of which is regulated by the density stratification of the environment and by its injection history. The breaking of symmetry involved in transitioning from one to two dimensions is crucial and leads to qualitatively new phenomena. We show that small velocity gradients, induced near or at the source, steepen into internal shocks and provide a source of free energy for particle acceleration and radiation along the jet's channel. As a result, the internal energy at the jet head continues to accumulate until long after the peak feeding rate is reached.
At redshift z = 0.03, the recently discovered SN 2017egm is the nearest Type I superluminous supernova (SLSN) to date and first near the center of a massive spiral galaxy (NGC 3191). Using SDSS ...spectra of NGC 3191, we find a metallicity ∼2 at the nucleus and ∼1.3 for a star-forming region at a radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a star formation rate of ∼15 yr−1 (with ∼70% dust obscured), which can account for a Swift X-ray detection and a stellar mass of . We model the early UV-optical light curves with a magnetar central-engine model, using the Bayesian light curve fitting tool MOSFiT. The fits indicate an ejecta mass of 2-4 , a spin period of 4-6 ms, a magnetic field of G, and a kinetic energy of erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al., although the derived mass and spin are toward the low end, possibly indicating an enhanced loss of mass and angular momentum before explosion. This has two implications: (i) SLSNe can occur at solar metallicity, although with a low fraction of ∼10%, and (ii) metallicity has at most a modest effect on their properties. Both conclusions are in line with results for long gamma-ray bursts. Assuming a monotonic rise gives an explosion date of MJD 57889 1. However, a short-lived excess in the data relative to the best-fitting models may indicate an early-time "bump." If confirmed, SN 2017egm would be the first SLSN with a spectrum during the bump phase; this shows the same O ii lines seen at maximum light, which may be an important clue for explaining these bumps.
Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One ...potential outcome of this "dynamically driven double-degenerate double-detonation" (D6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its >1000 km s−1 pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km s−1, and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is >1000 km s−1, the radial velocities of the other two stars are puzzlingly consistent with 0. The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.
Upon formation, degenerate He core white dwarfs are surrounded by a radiative H-rich layer primarily supported by ideal gas pressure. In this Letter, we examine the effect of this H-rich layer on ...mass transfer in He+C/O double white dwarf binaries that will eventually merge and possibly yield a Type Ia supernova (SN Ia) in the double detonation scenario. Because its thermal profile and equation of state differ from the underlying He core, the H-rich layer is transferred stably onto the C/O white dwarf prior to the He core's tidal disruption. We find that this material is ejected from the binary system and sweeps up the surrounding interstellar medium hundreds to thousands of years before the SN Ia. The close match between the resulting circumstellar medium profiles and values inferred from recent observations of circumstellar absorption in SNe Ia gives further credence to the resurgent double detonation scenario.
ABSTRACT The kinetic energy of a star in orbit about a supermassive black hole is a significant fraction of its rest mass energy when its periapse is comparable to its tidal radius. Upon its ...destruction, a fraction of this energy is extracted and injected into the stellar debris, half of which becomes unbound from the black hole, with the fastest material moving at . In this paper, we present a formalism for determining the fate of these unbound debris streams (UDSs) as they depart from the black hole and interact with the surrounding gas. As the density and velocity varies along the length of a UDS, we find that hydrodynamical drag quickly shapes UDSs into loop-like structures, with the densest portions of the streams leading portions of lower density. As UDSs travel outwards, their drag against the ISM increases quadratically with distance, which causes UDSs to deposit their momentum and energy into the ambient medium before the surrounding shocked ISM has a chance to cool. This sudden injection of erg into the ambient medium generates a Sedov-like unbound debris remnant (UDR) that mimics supernova remnants (SNRs) in energetics and appearance, accelerates particles which will produce cosmic rays and synchrotron emission, and provides momentum feedback into the molecular clouds surrounding a black hole. We estimate that a few of these UDRs might be present within a couple degrees of the Galactic Center masquerading as SNRs, and that the UDR scenario is a plausible explanation for Sgr A east.
We describe a three-dimensional simulation of a 1 M solar-type star approaching a 106 M black hole on a parabolic orbit with a pericenter distance well within the tidal radius. While falling toward ...the black hole, the star is not only stretched along the orbital direction but also even more severely compressed at right angles to the orbit. The overbearing degree of compression achieved shortly after pericenter leads to the production of strong shocks that largely homogenize the temperature profile of the star, resulting in surface temperatures comparable to the initial temperature of the star's core. This phenomenon, which precedes the fallback accretion phase, gives rise to a unique double-peaked X-ray signature that, if detected, may be one of the few observable diagnostics of how stars behave under the influence of strong gravitational fields. If ~106 M black holes were prevalent in small or even dwarf galaxies, the nearest of such flares may be detectable by EXIST from no further away than the Virgo Cluster.
The discovery of the gas cloud G2 on a near-radial orbit about Sgr A* has prompted much speculation on its origin. In this Letter, we propose that G2 formed out of the debris stream produced by the ...removal of mass from the outer envelope of a nearby giant star. We perform hydrodynamical simulations of the returning tidal debris stream with cooling and find that the stream condenses into clumps that fall periodically onto Sgr A*. We propose that one of these clumps is the observed G2 cloud, with the rest of the stream being detectable at lower Br gamma emissivity along a trajectory that would trace from G2 to the star that was partially disrupted. By simultaneously fitting the orbits of S2, G2, and ~2000 candidate stars, and by fixing the orbital plane of each candidate star to G2 (as is expected for a tidal disruption), we find that several stars have orbits that are compatible with the notion that one of them was tidally disrupted to produce G2. If one of these stars were indeed disrupted, it last encountered Sgr A* hundreds of years ago and has likely encountered Sgr A* repeatedly. However, while these stars are compatible with the giant disruption scenario given their measured positions and proper motions, their radial velocities are currently unknown. If one of these stars' radial velocity is measured to be compatible with a disruptive orbit, it would strongly suggest that its disruption produced G2.
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