arXiv subjects

Brian D. Metzger

Publications and source records attributed to Brian D. Metzger.

At least 217 records · Page 12Linked to original sources

arXiv2014

Red or blue? A potential kilonova imprint of the delay until black hole formation following a neutron star merger

Mergers of binary neutron stars (NSs) usually result in the formation of a hypermassive neutron star (HMNS). Whether- and when this remnant collapses to a black hole (BH) depends primarily on the equation of state and on angular momentum transport processes, both of which are uncertain. Here we show that the lifetime of the merger remnant may be directly imprinted in the radioactively powered kilonova emission following the merger. We employ axisymmetric, time-dependent hydrodynamic simulations of remnant accretion disks orbiting a HMNS of variable lifetime, and characterize the effect of this delay to BH formation on the disk wind ejecta. When BH formation is relatively prompt (~ 100 ms), outflows from the disk are sufficiently neutron rich to form heavy r-process elements, resulting in ~ week-long emission with a spectral peak in the near-infrared (NIR), similar to that produced by the dynamical ejecta. In contrast, delayed BH formation allows neutrinos from the HMNS to raise the electron fraction in the polar direction to values such that potentially Lanthanide-free outflows are generated. The lower opacity would produce a brighter, bluer, and shorter-lived ~ day-long emission (a `blue bump') prior to the late NIR peak from the dynamical ejecta and equatorial wind. This new diagnostic of BH formation should be useful for events with a signal to noise lower than that required for direct detection of gravitational waveform signatures.

astro-ph.HE↗

arXiv2014

Shocks in nova outflows. I. Thermal emission

Evidence for shocks in nova outflows include (1) multiple velocity components in the optical spectra; (2) keV X-ray emission weeks to months after the outburst; (3) early radio flare on timescales of months, in excess of that predicted from the freely expanding photo-ionized gas; and (4) ~ GeV gamma-rays. We present a 1D model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated thermal X-ray, optical, and radio emission. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. The radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray classical nova V1324 Sco, obtaining an adequate fit to the early radio maximum for reasonable assumptions about the fast nova outflow and assuming the DES possesses a velocity ~1e3 km/s and mass ~ 2e-4 M_sun; the former is consistent with the velocities of narrow line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferred independently by modeling the late radio peak. Rapid evolution of the early radio light curves require the DES possess a steep outer density profile, which may indicate that the onset of mass loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled by the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times may contribute significantly to the optical/UV emission, which we speculate is responsible for the previously unexplained `plateaus' and secondary maxima in nova optical light curves.

astro-ph.HE↗

arXiv2013

Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars

The coalescence of binary neutron stars (NSs) may in some cases produce a stable massive NS remnant rather than a black hole. Due to the substantial angular momentum from the binary, such a remnant is born rapidly rotating and likely acquires a strong magnetic field (a `millisecond magnetar'). Magnetic spin-down deposits a large fraction of the rotational energy from the magnetar behind the small quantity of mass ejected during the merger. This has the potential for creating a bright transient that could be useful for determining whether a NS or black hole was formed in the merger. We investigate the expected signature of such an event, including for the first time the important impact of electron/positron pairs injected by the millisecond magnetar into the surrounding nebula. These pairs cool via synchrotron and inverse Compton emission, producing a pair cascade and hard X-ray spectrum. A fraction of these X-rays are absorbed by the ejecta walls and re-emitted as thermal radiation, leading to an optical/UV transient peaking at a luminosity of ~1e43-1e44 erg/s on a timescale of several hours to days. This is dimmer than predicted by simpler analytic models because the large optical depth of electron/positron pairs across the nebula suppresses the efficiency with which the magnetar spin down luminosity is thermalized. Nevertheless, the optical/UV emission is more than two orders of magnitude brighter than a radioactively powered `kilonova.' In some cases nebular X-rays are sufficiently luminous to re-ionize the ejecta, in which case non-thermal X-rays escape the ejecta unattenuated with a similar peak luminosity and timescale as the optical radiation. We discuss the implications of our results for the temporally extended X-ray emission that is observed to follow some short gamma-ray bursts (GRBs), including the kilonova candidates GRB 080503 and GRB 130603B.

astro-ph.HE↗

arXiv2013

Constraints on long-lived remnants of neutron star binary mergers from late-time radio observations of short duration gamma-ray bursts

The coalescence of a binary neutron star (NS) system may in some cases produce a massive NS remnant that is long-lived and, potentially, indefinitely stable to gravitational collapse. Such a remnant has been proposed as an explanation for the late X-ray emission observed following some short duration gamma-ray bursts (GRBs) and as possible electromagnetic counterparts to the gravitational wave chirp. A stable NS merger remnant necessarily possesses a large rotational energy > 1e52 erg, the majority of which is ultimately deposited into the surrounding circumburst medium (CBM) at mildly relativistic velocities. We present Very Large Array radio observations of 7 short GRBs, some of which possessed temporally extended X-ray emission, on timescales of ~1-3 years following the initial burst. No radio sources were detected, with typical upper limits ~0.3 mJy at 1.4 GHz. A basic model for the synchrotron emission from the blast wave is used to constrain the presence of a long-lived NS merger remnant in each system. Depending on the GRB, our non-detections translate into upper limits on the CBM density n < 3e-2 - 3e-3 particles/cm^3 required for consistency with the remnant hypothesis. Our upper limits rule out a long-lived remnant in GRB 050724 and 060505, but cannot rule out such a remnant in other systems due to their lower inferred CBM densities based on afterglow modeling or the lack of such constraints. Additional VLA observations in the near future could place tighter limits on the presence of merger remnants in these system. The population of long-lived NS merger remnants will also be constrained by their (non-)detection with upcoming radio transient surveys.

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arXiv2013

X-ray Decay Lines from Heavy Nuclei in Supernova Remnants as a Probe of the r-Process Origin and the Birth Periods of Magnetars

The origin of rapid neutron capture (r-process) nuclei remains one of the longest standing mysteries in nuclear astrophysics. Core collapse supernovae (SNe) and neutron star binary mergers are likely r-process sites, but little evidence yet exists for their in situ formation in such environments. Motivated by the advent of sensitive new or planned X-ray telescopes such as the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Large Observatory for X-ray Timing (LOFT), we revisit the prospects for the detection of X-ray decay lines from r-process nuclei in young or nearby supernova remnants. For all remnants planned to be observed by NuSTAR (and several others), we conclude that r-process nuclei are detectable only if the remnant possesses a large overabundance O > 1e3 relative to the average yield per SN. Prospects are better for the next Galactic SN (assumed age of 3 years and distance of 10 kpc), for which an average r-process yield is detectable via the 10.7(9.2) keV line complexes of Os194 by LOFT at 6 sigma (5 sigma) confidence; the 27.3 keV line complex of Sb125 is detectable by NuSTAR at 2 sigma for O > 2. We also consider X-rays lines from the remnants of Galactic magnetars, motivated by the much higher r-process yields of the magneto-rotationally driven SNe predicted to birth magnetars. The ~ 3.6-3.9 keV lines of Sn126 are potentially detectable in the remnants of the magnetars 1E1547.0-5408 and 1E2259+586 by LOFT for an assumed r-process yield predicted by recent simulations. The (non-)detection of these lines can thus probe whether magnetars are indeed born with millisecond periods. Finally, we consider a blind survey of the Galactic plane with LOFT for r-process lines from the most recent binary neutron star merger remnant, concluding that a detection is unlikely without additional information on the merger location.

astro-ph.HE↗

arXiv2013

Short GRB 130603B: Discovery of a jet break in the optical and radio afterglows, and a mysterious late-time X-ray excess

We present radio, optical/NIR, and X-ray observations of the afterglow of the short-duration 130603B, and uncover a break in the radio and optical bands at 0.5 d after the burst, best explained as a jet break with an inferred jet opening angle of 4-8 deg. GRB 130603B is only the third short GRB with a radio afterglow detection to date, and the first time that a jet break is evident in the radio band. We model the temporal evolution of the spectral energy distribution to determine the burst explosion properties and find an isotropic-equivalent kinetic energy of (0.6-1.7) x 10^51 erg and a circumburst density of 5 x 10^-3-30 cm^-3. From the inferred opening angle of GRB 130603B, we calculate beaming-corrected energies of Egamma (0.5-2) x 10^49 erg and EK (0.1-1.6) x 10^49 erg. Along with previous measurements and lower limits we find a median short GRB opening angle of 10 deg. Using the all-sky observed rate of 10 Gpc^-3 yr^-1, this implies a true short GRB rate of 20 yr^-1 within 200 Mpc, the Advanced LIGO/VIRGO sensitivity range for neutron star binary mergers. Finally, we uncover evidence for significant excess emission in the X-ray afterglow of GRB 130603B at >1 d and conclude that the additional energy component could be due to fall-back accretion or spin-down energy from a magnetar formed following the merger.

astro-ph.HE↗

arXiv2013

Ionization Break-Out from Millisecond Pulsar Wind Nebulae: an X-ray Probe of the Origin of Superluminous Supernovae

Magnetic spin-down of a millisecond neutron star has been proposed as the power source of hydrogen-poor "superluminous" supernovae (SLSNe-I). However, producing an unambiguous test that can distinguish this model from alternatives, such as circumstellar interaction, has proven challenging. After the supernova explosion, the pulsar wind inflates a hot cavity behind the expanding stellar ejecta: the nascent millisecond pulsar wind nebula. Electron/positron pairs injected by the wind cool through inverse Compton scattering and synchrotron emission, producing a pair cascade and hard X-ray spectrum inside the nebula. These X-rays ionize the inner exposed side of the ejecta, driving an ionization front that propagates outwards with time. Under some conditions this front can breach the ejecta surface within months after the optical supernova peak, allowing ~0.1-1 keV photons to escape the nebula unattenuated with a characteristic luminosity L_X ~ 1e43-1e45 erg/s. This "ionization break-out" may explain the luminous X-ray emission observed from the transient SCP 06F, providing direct evidence that this SLSN was indeed engine-powered. Luminous break-out requires a low ejecta mass and that the spin-down time of the pulsar be comparable to the photon diffusion timescale at optical maximum, the latter condition similar to that required for a supernova with a high optical fluence. These relatively special requirements may explain why most SLSNe-I are not accompanied by detectable X-ray emission. Global asymmetry of the supernova ejecta increases the likelihood of an early break-out along the direction of lowest density. Atomic states with lower threshold energies are more readily ionized at earlier times near optical maximum, allowing UV break-out across a wider range of pulsar and ejecta properties than X-ray break-out, possibly contributing to the blue/UV colors of SLSNe-I.

astro-ph.HE↗

arXiv2013

Delayed outflows from black hole accretion tori following neutron star binary coalescence

Expulsion of neutron-rich matter following the merger of neutron star (NS) binaries is crucial to the radioactively-powered electromagnetic counterparts of these events and to their relevance as sources of r-process nucleosynthesis. Here we explore the long-term (viscous) evolution of remnant black hole accretion disks formed in such mergers by means of two-dimensional, time-dependent hydrodynamical simulations. The evolution of the electron fraction due to charged-current weak interactions is included, and neutrino self-irradiation is modeled as a lightbulb that accounts for the disk geometry and moderate optical depth effects. Over several viscous times (~1s), a fraction ~10% of the initial disk mass is ejected as a moderately neutron-rich wind (Y_e ~ 0.2) powered by viscous heating and nuclear recombination, with neutrino self-irradiation playing a sub-dominant role. Although the properties of the outflow vary in time and direction, their mean values in the heavy-element production region are relatively robust to variations in the initial conditions of the disk and the magnitude of its viscosity. The outflow is sufficiently neutron-rich that most of the ejecta forms heavy r-process elements with mass number A >130, thus representing a new astrophysical source of r-process nucleosynthesis, distinct from that produced in the dynamical ejecta. Due to its moderately high entropy, disk outflows contain a small residual fraction ~1% of helium, which could produce a unique spectroscopic signature.

astro-ph.HE↗

arXiv2013

Swift J1644+57 gone MAD: the case for dynamically-important magnetic flux threading the black hole in a jetted tidal disruption event

The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet powered by accretion onto a massive black hole (BH) following the tidal disruption (TD) of a star. Several mysteries cloud the interpretation of this event: (1) extreme flaring and `plateau' shape of the X-ray/gamma-ray light curve during the first 10 days after the gamma-ray trigger; (2) unexpected rebrightening of the forward shock radio emission months after trigger; (3) no obvious evidence for jet precession, despite misalignment typically expected between the angular momentum of the accretion disk and BH; (4) recent abrupt shut-off in jet X-ray emission after 1.5 years. Here we show that all of these seemingly disparate mysteries are naturally resolved by one assumption: the presence of strong magnetic flux Phi threading the BH. Initially, Phi is weak relative to high fall-back mass accretion rate, Mdot, and the disk and jets precess about the BH axis = our line of sight. As Mdot drops, Phi becomes dynamically important and leads to a magnetically-arrested disk (MAD). MAD naturally aligns disk and jet axis along the BH spin axis, but only after a violent rearrangement phase (jet wobbling). This explains the erratic light curve at early times and the lack of precession at later times. We use our model for Swift J1644+57 to constrain BH and disrupted star properties, finding that a solar-mass main sequence star disrupted by a relatively low mass, M~10^5-10^6 Msun, BH is consistent with the data, while a WD disruption (though still possible) is disfavored. The magnetic flux required to power Swift J1644+57 is too large to be supplied by the star itself, but it could be collected from a quiescent `fossil' accretion disk present in the galactic nucleus prior to the TD. The presence (lack of) of such a fossil disk could be a deciding factor in what TD events are accompanied by powerful jets.[abridged]

astro-ph.HE↗

arXiv2012

Nuclear Dominated Accretion Flows in Two Dimensions. I. Torus Evolution with Parametric Microphysics

We explore the evolution of radiatively inefficient accretion disks in which nuclear reactions are dynamically important (`Nuclear Dominated Accretion Flows', or NuDAFs). Examples of such disks are those generated by the merger of a white dwarf with a neutron star or black hole, or by the collapse of a rotating star. Here we present two-dimensional hydrodynamic simulations that systematically explore the effect of adding a single nuclear reaction to a viscous torus. The equation of state, anomalous shear stress, and nuclear reactions are given parametric forms. Our results point to the existence of two qualitatively different regimes of NuDAF evolution: (1) steady accretion with quiescent burning; or (2) detonation of the disk. These outcomes are controlled primarily by the ratio of the nuclear energy released to the enthalpy at the burning radius. Disks detonate if this ratio exceeds a critical value (~1), and if burning occurs in regions where neutrino cooling is unimportant. Thermonuclear runaways are seeded by the turbulent mixing of hot ash with cold fuel at the burning front. Subcritical disks do not explode, but instead power a persistent collimated outflow of unbound material composed primarily of ash, with a mass-loss rate that increases with the nuclear binding energy released. We discuss the implications of our results for supernova-like counterparts from astrophysical events in the NuDAF regime. In particular, detonations following a white dwarf - neutron star merger could account for some subluminous Type Ia supernovae, such as the class defined by SN 2002cx.

astro-ph.HE↗

arXiv2012

Late Time Radio Emission from X-ray Selected Tidal Disruption Events

We present new observations with the Karl G. Jansky Very Large Array of seven X-ray-selected tidal disruption events (TDEs). The radio observations were carried out between 9 and 22 years after the initial X-ray discovery, and, thus, probe the late-time formation of relativistic jets and jet interactions with the interstellar medium in these systems. We detect a compact radio source in the nucleus of the galaxy IC 3599 and a compact radio source that is a possible counterpart to RX J1420.4+5334. We find no radio counterparts for five other sources with flux density upper limits between 51 and 200 microJy (3 sigma). If the detections truly represent late radio emission associated with a TDE, then our results suggest that a fraction >~ 10% of X-ray-detected TDEs are accompanied by relativistic jets. We explore several models for producing late radio emission, including interaction of the jet with gas in the circumnuclear environment (blast wave model), and emission from the core of the jet itself. Upper limits on the radio flux density from archival observations suggest that the jet formation may have been delayed for years after the TDE, possibly triggered by the accretion rate dropping below a critical threshold of ~ 10^{-2} -- 10^{-3} Mdot_Edd. The non-detections are also consistent with this scenario; deeper radio observations can determine whether relativistic jets are present in these systems. The emission from RX J1420.4+5334 is also consistent with the predictions of the blast wave model, however the radio emission from IC 3599 is substantially underluminous, and its spectral slope is too flat, relative to the blast wave model expectations. Future radio monitoring of IC 3599 and RX J1420.4+5334 will help to better constrain the nature of the jets in these systems.

astro-ph.HE↗

arXiv2012

Comparing H-alpha and HI Surveys as Means to a Complete Local Galaxy Catalog in the Advanced LIGO/Virgo Era

Identifying the electromagnetic counterparts of gravitational wave (GW) sources detected by upcoming networks of advanced ground-based interferometers will be challenging due in part to the large number of unrelated astrophysical transients within the ~10-100 square degree sky localizations. A potential way to greatly reduce the number of such false positives is to limit detailed follow-up to only those candidates near galaxies within the GW sensitivity range of ~200 Mpc for binary neutron star mergers. Such a strategy is currently hindered by the fact that galaxy catalogs are grossly incomplete within this volume. Here we compare two methods for completing the local galaxy catalog: (1) a narrow-band H-alpha imaging survey; and (2) an HI emission line radio survey. Using H-alpha fluxes, stellar masses (M_star), and star formation rates (SFR) from galaxies in the Sloan Digital Sky Survey (SDSS), combined with HI data from the GALEX Arecibo SDSS Survey and the Herschel Reference Survey, we estimate that a H-alpha survey with a luminosity sensitivity of L_H-alpha = 1e40 erg/s at 200 Mpc could achieve a completeness of f_SFR ~ 75% with respect to total SFR, but only f_Mstar ~ 33% with respect to stellar mass (due to lack of sensitivity to early-type galaxies). These numbers are significantly lower than those achieved by an idealized spectroscopic survey due to the loss of H-alpha flux resulting from resolving out nearby galaxies and the inability to correct for the underlying stellar continuum. An HI survey with sensitivity similar to the proposed WALLABY survey on ASKAP could achieve f_SFR ~ 80% and f_Mstar ~ 50%, somewhat higher than that of the H-alpha survey. Finally, both H-alpha and HI surveys should achieve > 50% completeness with respect to the host galaxies of short duration gamma-ray bursts, which may trace the population of binary neutron star mergers.

astro-ph.CO↗

arXiv2012

Nuclear-dominated accretion and subluminous supernovae from the merger of a white dwarf with a neutron star or black hole

We construct one dimensional steady-state models of accretion disks produced by the tidal disruption of a white dwarf (WD) by a neutron star (NS) or stellar mass black hole (BH). At radii r <~ 1e8.5-1e9 cm the midplane density and temperature are sufficiently high to burn the initial white dwarf material into increasingly heavier elements (e.g. Mg, Si, S, Ca, Fe, and Ni) at sequentially smaller radii. When the energy released by nuclear reactions is comparable to that released gravitationally, we term the disk a nuclear-dominated accretion flow (NuDAF). At small radii <~1e7 cm Fe photo-disintegrates into He and then free nuclei, and cooling by neutrinos may be efficient. At the high accretion rates of relevance ~ 0.1-1e-4 Msun/s, most of the disk is radiatively inefficient and prone to outflows powered by viscous dissipation and nuclear burning. Outflow properties are calculated by requiring that material in the midplane be marginally bound (Bernoulli constant <~ 0), due (in part) to cooling by matter escaping the disk. For reasonable assumptions regarding the properties of disk winds, we show that a significant fraction >50-80 per cent of the total WD mass is unbound. The ejecta composition is predominantly O, C, Si, Mg, Ne, Fe, and S [He, C, Si, S, Ar, and Fe], in the case of C-O [He] WDs, respectively, along with a small quantity ~1e-3-1e-2 Msun of radioactive Ni56 and, potentially, a trace amount of H. We use our results to evaluate possible EM counterparts of WD-NS/BH mergers, including optical transients powered by the radioactive decay of Ni56 and radio transients powered by the interaction of the ejecta with the interstellar medium. We address whether recently discovered subluminous Type I supernovae result from WD-NS/BH mergers. Our results also have implications for accretion following the core collapse of massive stars in collapsar models for gamma-ray bursts.

astro-ph.HE↗

arXiv2012

Afterglow Model for the Radio Emission from the Jetted Tidal Disruption Candidate Swift J1644+57

The recent transient event Swift J1644+57 has been interpreted as emission from a collimated relativistic jet, powered by the sudden onset of accretion onto a supermassive black hole following the tidal disruption of a star. Here we model the radio-microwave emission as synchrotron radiation produced by the shock interaction between the jet and the gaseous circumnuclear medium (CNM). At early times after the onset of the jet (t < 5-10 days) a reverse shock propagates through and decelerates the ejecta, while at later times the outflow approaches the Blandford-McKee self-similar evolution (possibly modified by additional late energy injection). The achromatic break in the radio light curve of J1644+57 is naturally explained as the transition between these phases. We show that the temporal indices of the pre- and post-break light curve are consistent with those predicted if the CNM has a wind-type radial density profile n ~ 1/r^2. The observed synchrotron frequencies and self-absorbed flux constrain the fraction of the post-shock thermal energy in relativistic electrons epsilon_e ~ 0.03-0.1; the CNM density at 1e18 cm ~ 1-10 1/cm^3; and the initial Lorentz factor Gamma_j ~ 10-20 and opening angle theta_j ~ (0.1-1)Gamma_j^(-1) ~ 0.01-0.1 of the jet. Radio modeling thus provides robust independent evidence for a narrowly collimated outflow. Extending our model to the future evolution of J1644+57, we predict that the radio flux at low frequencies (<~ few GHz) will begin to brighten more rapidly once the characteristic frequency crosses below the self-absorption frequency on a timescale of months (indeed, such a transition may already have begun). Our results demonstrate that relativistic outflows from tidal disruption events provide a unique probe of the conditions in distant, previously inactive galactic nuclei, complementing studies of normal AGN.

astro-ph.HE↗

arXiv2012

Optical and X-ray Transients from Planet-Star Mergers

We evaluate the prompt observational signatures of the merger between a massive close-in planet (a `hot Jupiter') and its host star, events with an estimated Galactic rate of ~0.1-1/yr. Depending on the ratio of the mean density of the planet rho_p to that of the star rho_star, a merger results in three possible outcomes. If rho_p/rho_star > 5, then the planet directly plunges below the stellar atmosphere before being disrupted by tidal forces. Dissipation of orbital energy creates a hot wake behind the planet, producing a EUV/soft X-ray transient as the planet sinks below the stellar surface. The peak luminosity L_X ~ 1e36 erg/s is achieved weeks-months prior to merger, after which the stellar surface is enshrouded by an outflow. The final inspiral is accompanied by an optical transient powered by the recombination of hydrogen in the outflow, which peaks at L~1e37-38 erg/s on a timescale ~days. If instead rho_planet/rho_star < 5, then Roche Lobe overflow occurs above the stellar surface. For rho_p/rho_star < 1, mass transfer is stable, resulting the planet being accreted on a relatively slow timescale. However, for 1 < rho_p/rho_star < 5, mass transfer may instead be unstable, resulting in the planet being dynamically disrupted into an accretion disk around the star. Super-Eddington outflows from the disk power an optical transient with L~1e37-38 erg/s and characteristic duration ~week-months. The disk itself becomes visible once the accretion rate become sub-Eddington, resulting in a bolometric brightening and spectral shift to the UV. Optical transients from planet merger events may resemble classical novae, but are distinguished by lower ejecta mass and velocity ~100s km/s, and by hard pre- and post-cursor emission, respectively. Promising search strategies include combined optical, UV, and X-ray surveys of nearby massive galaxies with cadences from days to months.

astro-ph.EP↗

arXiv2012

Global Models of Runaway Accretion in White Dwarf Debris Disks

A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are accompanied by excess infrared emission, indicating that they are encircled by a compact dusty disk of solid debris. Such `WD debris disks' are thought to originate from the tidal disruption of asteroids or other minor bodies, but the precise mechanism(s) responsible for transporting matter to the WD surface remains unclear, especially in those systems with the highest inferred metal accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent calculations of the coupled evolution of the gaseous and solid components of WD debris disks. Solids transported inwards (initially due to PR drag) sublimate at tens of WD radii, producing a source of gas that accretes onto the WD surface and viscously spreads outwards in radius, where it overlaps with the solid disk. If the aerodynamic coupling between the solids and gaseous disks is sufficiently strong (and/or the gas viscosity sufficiently weak), then gas builds up near the sublimation radius faster than it can viscously spread away. Since the rate of drag-induced solid accretion increases with gas density, this results in a runaway accretion process, during which the WD accretion rate reaches values orders of magnitude higher than can be achieved by PR drag alone. We explore the evolution of WD debris disks across a wide range of physical conditions and calculate the predicted distribution of observed accretion rates dM_Z/dt, finding reasonable agreement with the current sample. Although the conditions necessary for runaway accretion are at best marginally satisfied given the minimal level of aerodynamic drag between circular gaseous and solid disks, the presence of other stronger forms of solid-gas coupling---such as would result if the gaseous disk is only mildly eccentric---substantially increase the likelihood of runaway accretion.

astro-ph.EP↗

arXiv2011

What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger?

The final inspiral of double neutron star and neutron star-black hole binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic (EM) counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), "orphan" optical and radio afterglows, and ~day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta ("kilonovae"). We assess the promise of each counterpart in terms of four "Cardinal Virtues": detectability, high fraction, identifiability, and positional accuracy. Taking into account the search strategy for typical error regions of ~10s degs sq., we conclude that SGRBs are the most useful to confirm the cosmic origin of a few GW events, and to test the association with NS mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows will be detectable for at most ~10% of all events, while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium, and even then will require hundreds of hours of EVLA time per event. Our main recommendations are:(i) an all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma-ray triggered events; (ii) LSST should adopt a 1-day cadence follow-up strategy, ideally with ~0.5 hr per pointing to cover GW error regions (the standard 4-day cadence and depth will severely limit the probability of a unique identification); and (iii) radio searches should only focus on the relativistic case, which requires observations for a few months.

astro-ph.HE↗

arXiv2011

Relic proto-stellar disks and the origin of luminous circumstellar interaction in core collapse supernovae

A small fraction of core collapse supernovae (SNe) show evidence that the outgoing blast wave has encountered a substantial mass ~ 1-10 M_sun of circumstellar matter (CSM) at radii ~100-1000 AU, much more than can nominally be explained by pre-explosion stellar winds. In extreme cases this interaction may power the most luminous, optically-energetic SNe yet discovered. Interpretations for the origin of the CSM have thus far centered on explosive eruptions from the star just ~ years to decades prior to the core collapse. Here we consider an alternative possibility that the inferred CSM is a relic disk left over from stellar birth. We investigate this hypothesis by calculating the evolution of proto-stellar disks around massive stars following their early embedded phase using a self-similar accretion model. We identify an initial gravitationally-unstable ("gravito-turbulent") phase, followed by a much longer period of irradiation-supported accretion during which less effective non-gravitational forms of angular momentum transport dominate. Although external influences, such as the presence of a wide binary companion, may preclude disk survival in many systems, we find that massive (~1-10 M_sun) disks can preferentially survive around the most massive stars. Reasons for this perhaps counterintuitive include (1) the shorter stellar lifetimes and (2) large photo-evaporation radii (~ 1000 AU) of very massive stars; (3) suppression of the magneto-rotational instability due to the shielding from external sources of ionization; and (4) relative invulnerability of massive disks to lower mass stellar collisions and luminous blue variable eruptions. Because very luminous SNe are rare, testing the relic disk model requires constraining the presence of long-lived disks around a small fraction of very massive stars.

astro-ph.HE↗