Wenbin Lu

Infrared emission from tidal disruption events — probing the pc-scale dust content around galactic nuclei

Recent UV-optical surveys have been successful in finding tidal disruption events (TDEs), in which a star is tidally disrupted by a supermassive black hole (BH). These TDEs release a huge amount of radiation energy ~ 10^51-52 erg into the circum-nuclear medium. If the medium is dusty, most of the radiation energy will be absorbed by dust grains within ~ 1 pc from the BH and re-radiated in the infrared. We calculate the dust emission lightcurve from a 1-D radiative transfer model, taking into account the time-dependent heating, cooling and sublimation of dust grains. We show that the dust emission peaks at 3-10 microns and has typical luminosities ~ 10^42-43 erg/s (with sky covering factor of dusty clouds ranging from 0.1-1). This is detectable by current generation of telescopes. In the near future, James Webb Space Telescope will be able to perform photometric and spectroscopic measurements, in which silicate or polycyclic aromatic hydrocarbon (PAH) features may be found. Observations at rest-frame wavelength > 2 microns have only been reported from two TDE candidates, SDSS J0952+2143 and Swift J1644+57. Although consistent with the dust emission from TDEs, the mid-infrared fluxes of the two events may be from other sources. Long-term monitoring is needed to draw a firm conclusion. We also point out two nearby TDE candidates (ASSASN-14ae and -14li) where the dust emission may be currently detectable. The dust infrared emission can give a snapshot of the pc-scale dust content around weakly- or non-active galactic nuclei, which is hard to probe otherwise.

Thermal and non-thermal emission from the cocoon of a gamma-ray burst jet

We present hydrodynamic simulations of the hot cocoon produced when a relativistic jet passes through the gamma-ray burst (GRB) progenitor star and its environment, and we compute the lightcurve and spectrum of the radiation emitted by the cocoon. The radiation from the cocoon has a nearly thermal spectrum with a peak in the X-ray band, and it lasts for a few minutes in the observer frame; the cocoon radiation starts at roughly the same time as when

On the radiation mechanism of repeating fast radio bursts

\gamma

Stellar disruption events support the existence of the black hole event horizon

-rays from a burst trigger detectors aboard GRB satellites. The isotropic cocoon luminosity (

Swift J1644+57: an ideal test bed of radiation mechanisms in a relativistic super-Eddington jet

\sim 10^{47}

External inverse-Compton emission from jetted tidal disruption events

erg s

Probing Massive Stars Around Gamma-Ray Burst Progenitors

^{-1}

Radiative interaction between the relativistic jet and optically thick envelope in tidal disruption events

) is of the same order of magnitude as the X-ray luminosity of a typical long-GRB afterglow during the plateau phase. This radiation should be identifiable in the Swift data because of its nearly thermal spectrum which is distinct from the somewhat brighter power-law component. The detection of this thermal component would provide information regarding the size and density stratification of the GRB progenitor star. Photons from the cocoon are also inverse-Compton (IC) scattered by electrons in the relativistic jet. We present the IC lightcurve and spectrum, by post-processing the results of the numerical simulations. The IC spectrum lies in 10 keV--MeV band for typical GRB parameters. The detection of this IC component would provide an independent measurement of GRB jet Lorentz factor and it would also help to determine the jet magnetisation parameter.

Monte Carlo Simulations of Photospheric Emission in Relativistic Outflows

Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events occurring at cosmological distances. The properties of their progenitors are largely unknown in spite of many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which is investigated in this paper. The high brightness temperatures (>10^{35} K) of FRBs mean that the emission process must be coherent. Two general classes of coherent radiation mechanisms are considered --- maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions needed for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in either vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by coherent charge bunches where the burst is powered by magnetic reconnection near the surface of a magnetar (B > 10^{14} G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate. This model should be applicable to all FRBs that have multiple outbursts like FRB 121102.

On the Missing Energy Puzzle of Tidal Disruption Events

Many black hole (BH) candidates have been discovered in X-ray binaries and in the nuclei of galaxies. The prediction of Einsteins general relativity is that BHs have an event horizon --- a one-way membrane through which particles fall into the BH but cannot exit. However, except for the very few nearby supermassive BH candidates, our telescopes are unable to resolve and provide a direct proof of the event horizon. Here, we propose a novel observation that supports the existence of event horizons around supermassive BH candidates heavier than 10^{7.5} solar masses. Instead of an event horizon, if the BH candidate has a hard surface, when a star falls onto the surface, the shocked baryonic gas will form a radiation pressure supported envelope that shines at the Eddington luminosity for an extended period of time from months to years. We show that such emission has already been ruled out by the Pan-STARRS1 3pi survey if supermassive BH candidates have a hard surface at radius larger than 1 + 10^{-4.4} times the Schwarzschild radius. Future observations by LSST should be able to improve the limit to 1 + 10^{-6}.