Abstract
If the envelope of a massive star is not entirely removed during common envelope (CE) interaction with an orbiting compact (e.g., black hole (BH) or neutron star (NS)) companion, the ...residual bound material eventually cools, forming a centrifugally supported disk around the binary containing the stripped He core. We present a time-dependent height-integrated model for the long-term evolution of post-CE circumbinary disks (CBDs), accounting for mass and angular momentum exchange with the binary, irradiation heating by the He core, and photoevaporation wind mass loss. A large fraction of the CBD’s mass is accreted prior to its outwards viscous spreading and wind dispersal on a timescale of ∼10
4
–10
5
yr, driving significant orbital migration, even for disks containing ∼10% of the original envelope mass. Insofar that the CBD lifetime is comparable to the thermal (and, potentially, nuclear) timescale of the He core, over which a second mass-transfer episode onto the companion can occur, the presence of the CBD could impact the stability of this key phase. Disruption of the core by the BH/NS would result in a jetted energetic explosion into the dense gaseous CBD (≲10
15
cm) and its wind (≳10
16
cm), consistent with the environments of luminous fast blue optical transients like AT2018cow. Evolved He cores that undergo core collapse still embedded in their CBD could generate Type Ibn/Icn supernovae. Thousands of dusty wind-shrouded massive-star CBDs may be detectable as extragalactic luminous infrared sources with the Roman Space Telescope; synchrotron radio nebulae powered by the CBD-fed BH/NS may accompany these systems.
We construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear ...reactions in the disc mid-plane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. A model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the mid-plane to a fixed value ≲0. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of ∼109 cm s−1. The outflows from WD-NS merger discs contain 10−4–3 × 10−3 M⊙ of radioactive 56Ni, resulting in fast (∼ week long) dim (∼1040 erg s−1) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ∼1043 erg s−1. The accreted mass on to the NS is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of ∼10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars.
Abstract “Quasiperiodic eruptions” (QPE) are recurrent nuclear transients with periods of several hours to almost a day, which thus far have been detected exclusively in the X-ray band. We have shown ...that many of the key properties of QPE flares (period, luminosity, duration, emission temperature, alternating long-short recurrence time behavior, and source rates) are naturally reproduced by a scenario involving twice-per-orbit collisions between a solar-type star on a mildly eccentric orbit, likely brought into the nucleus as an extreme mass-ratio inspiral (EMRI), and the gaseous accretion disk of a supermassive black hole (SMBH). The flare is generated by the hot shocked debris expanding outwards from either side of the disk midplane, akin to dual miniature supernovae. Here, we consider the conditions necessary for disk–star collisions to generate lower-temperature flares that peak in the ultraviolet (UV) instead of the X-ray band. We identify a region of parameter space at low SMBH mass M • ∼ 10 5.5 M ⊙ and QPE periods P ≳ 10 hr for which the predicted flares are sufficiently luminous L UV ∼ 10 41 erg s −1 to outshine the quiescent disk emission at these wavelengths. The prospects to discover such “UV QPEs” with future satellite missions such as ULTRASAT and Ultraviolet Explorer depend on the prevalence of very low-mass SMBHs and the occurrence rate of stellar EMRIs onto them. For gaseous disks produced by the tidal disruption of stars, we predict that X-ray QPEs will eventually shut off, only to later reappear as UV QPEs as the accretion rate continues to drop.
If at least one neutron star (NS) is magnetized in a binary NS merger, then the orbital motion of the conducting companion during the final inspiral induces a strong voltage and current along the ...magnetic field lines connecting the NSs. If a modest fraction ... of the extracted electromagnetic power extracted accelerates relativistic particles, the resulting gamma-ray emission a compact volume will result in the formation of an electron-positron pair fireball. Applying a steady-state pair wind model, we quantify the detectability of the precursor fireball with gamma-ray satellites. For ... ~ 1 the gamma-ray detection horizon of D sub( max) ... 10(B sub( d)/10 super( 14) G) super( 3/4) Mpc is much closer than the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo horizon of 200 Mpc, unless the NS surface magnetic field strength is very large, Bd~>1015 G. Given the quasi-isotropic nature of the emission, mergers with weaker NS fields could contribute a nearby population of short gamma-ray bursts. Power not dissipated close to the binary is carried to infinity along the open field lines by a large-scale Poynting flux. Reconnection within this outflow, well outside of the pair photosphere, provides a potential site for non-thermal emission, such as a coherent millisecond radio burst. (ProQuest: ... denotes formulae/symbols omitted.)
The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet, powered by the sudden onset of accretion on to a massive black hole (BH) following the tidal disruption (TD) ...of a star. However, several mysteries cloud the interpretation of this event, including (1) the extreme flaring and 'plateau' shape of the X-ray/γ-ray light curve during the first t − t
trig ∼ 10 d after the γ-ray trigger; (2) unexpected rebrightening of the forward shock radio emission at t − t
trig ∼ months; (3) lack of obvious evidence for jet precession, despite the misalignment typically expected between the angular momentum of the accretion disc and BH; (4) recent abrupt shut-off in the jet X-ray emission at t − t
trig ∼ 1.5 yr. Here, we show that all of these seemingly disparate mysteries are naturally resolved by one assumption: the presence of strong magnetic flux Φ* threading the BH. Just after the TD event, Φ* is dynamically weak relative to the high rate of fall-back accretion
, such that the accretion disc (jet) freely precesses about the BH axis = our line of sight. As
decreases, however, Φ* becomes dynamically important, leading to a state of 'magnetically arrested disk' (MAD). MAD naturally aligns the jet with the BH spin, but only after an extended phase of violent rearrangement (jet wobbling), which in Swift J1644+57 starts a few days before the γ-ray trigger and explains the erratic early light curve. Indeed, the entire X-ray light curve can be fitted to the predicted power-law decay
(α 5/3 − 2.2) if the TD occurred a few weeks prior to the γ-ray trigger. Jet energy directed away from the line of sight, either prior to the trigger or during the jet alignment process, eventually manifests as the observed radio rebrightening, similar to an off-axis (orphan) γ-ray burst afterglow. As suggested recently, the late X-ray shut-off occurs when the disc transitions to a geometrically thin (jetless) state once
drops below ∼the Eddington rate. We predict that, in several years, a transition to a low/hard state will mark a revival of the jet and its associated X-ray emission. We use our model for Swift J1644+57 to constrain the properties of the BH and disrupted star, finding that a solar mass main-sequence star disrupted by a relatively low-mass M
* ∼ 105-106 M BH is consistent with the data, while a white dwarf disruption (though still possible) is disfavoured. The magnetic flux required to power Swift J1644+57 is much too large to be supplied by the star itself, but it could be collected from a quiescent 'fossil' accretion disc that was present in the galactic nucleus prior to the TD. The presence (lack of) of such a fossil disc could be a deciding factor in what TD events are accompanied by powerful jets.
Abstract
Despite recent progress, the astrophysical channels responsible for rapid neutron capture (
r
-process) nucleosynthesis remain an unsettled question. Observations of the kilonova following ...the gravitational-wave-detected neutron star merger GW170817 established mergers as one site of the
r
-process, but additional sources may be needed to fully explain
r
-process enrichment in the universe. One intriguing possibility is that rapidly rotating massive stars undergoing core collapse launch
r
-process-rich outflows off the accretion disks formed from their infalling matter. In this scenario,
r
-process winds are one component of the supernova (SN) ejecta produced by “collapsar” explosions. We present the first systematic study of the effects of
r
-process enrichment on the emission from collapsar-generated SNe. We semianalytically model
r
-process SN emission from explosion out to late times and determine its distinguishing features. The ease with which
r
-process SNe can be identified depends on how effectively wind material mixes into the initially
r
-process-free outer layers of the ejecta. In many cases, enrichment produces a near-infrared (NIR) excess that can be detected within ∼75 days of explosion. We also discuss optimal targets and observing strategies for testing the
r
-process collapsar theory, and find that frequent monitoring of optical and NIR emission from high-velocity SNe in the first few months after explosion offers a reasonable chance of success while respecting finite observing resources. Such early identification of
r
-process collapsar candidates also lays the foundation for nebular-phase spectroscopic follow-up in the NIR and mid-infrared, for example, with the James Webb Space Telescope.
ABSTRACT
AT 2022cmc is a luminous optical transient (νLν ≳ 1045 erg s−1) accompanied by decaying non-thermal X-rays (peak duration tX ≲ days and isotropic energy EX,iso ≳ 1053 erg) and a long-lived ...radio/mm synchrotron afterglow, which has been interpreted as a jetted tidal disruption event (TDE). Both an equipartition analysis and a detailed afterglow model reveal the radio/mm emitting plasma to be expanding mildly relativistically (Lorentz factor $\Gamma \gtrsim \, \mathrm{ few}$ ) with an opening angle θj ≃ 0.1 and roughly fixed energy Ej,iso ≳ few × 1053 erg into an external medium of density profile n ∝ R−k with k ≃ 1.5–2, broadly similar to that of the first jetted TDE candidate Swift J1644+57 and consistent with Bondi accretion at a rate of ∼$10^{-3}\,\dot{M}_{\rm Edd}$ on to a 106 M⊙ black hole before the outburst. The rapidly decaying optical emission over the first days is consistent with fast-cooling synchrotron radiation from the same forward shock as the radio/mm emission, while the bluer slowly decaying phase to follow likely represents a separate thermal emission component. Emission from the reverse shock may have peaked during the first days, but its non-detection in the optical band places an upper bound Γj ≲ 100 on the Lorentz factor of the unshocked jet. Although a TDE origin for AT 2022cmc is indeed supported by some observations, the vast difference between the short-lived jet activity phase tX ≲ days and the months-long thermal optical emission also challenges this scenario. A stellar core-collapse event giving birth to a magnetar or black hole engine of peak duration ∼1 d offers an alternative model also consistent with the circumburst environment, if interpreted as a massive star wind.
Binary stars commonly pass through phases of direct interaction, which result in the rapid loss of mass, energy, and angular momentum. Though crucial to understanding the fates of these systems, ...including their potential as gravitational wave sources, this short-lived phase is poorly understood and has thus far been unambiguously observed in only a single event, V1309 Sco. Here we show that the complex and previously unexplained photometric behavior of V1309 Sco prior to its main outburst results naturally from the runaway loss of mass and angular momentum from the outer Lagrange point, which lasts for thousands of orbits prior to the final dynamical coalescence, much longer than predicted by contemporary models. This process enshrouds the binary in a "death spiral" outflow, which affects the amplitude and phase modulation of its light curve, and contributes to driving the system together. The total amount of mass lost during this gradual phase ( ) rivals the mass lost during the subsequent dynamical interaction phase, which has been the main focus of "common envelope" modeling so far. Analogous features in related transients suggest that this behavior is ubiquitous.
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