A selection of my works are described below. A full list of my publications is here.

Fast Radio Bursts

Tidal Disruption Events

Main-sequence stars are shredded by tidal forces when they get sufficiently close to a supermassive black hole (BH). After disruption, the star is stretched into a long thin stream (of aspect ratio ~1000) in highly eccentric orbits (1-e ~ 0.01) which undergo general relativistic precession. Such precession causes the stream to self-intersect. Then, the shocked gas expanding from the intersection point will eventually form an accretion disk, which powers multi-wavelength emission. A few dozen of these tidal disruption event (TDE) candidates have been found in recent surveys designed to find variable sources in the optical/UV and soft X-rays.

Global numerical simulation of the hydrodynamic evolution of the thin fallback stream is prohibitively expensive. In Lu & Bonnerot (2020), we made a key simplification of the problem by first calculating the location where the stream self-intersects according to general relativistic geodesic motion and then performing hydrodynamic simulations of the stream collision process in a local region near the intersection point. We found that the self-crossing shock redistributes energy and angular momentum of the gas in the fall-back stream. For sufficiently deeply penetrating orbits (pericenter radius Rp <~ 15Rg), the shocks at the intersection are able to unbind a large fraction (up to 50%) of the fallback gas. We call the unbound gas the “collision-induced outflow” or CIO. We found that the CIO covers a large fraction of the sky viewed from the BH and is able to reprocess the hard (EUV/soft X-ray) photons from the disk into the optical band. This provides an explanation of the bright optical emission seen in many TDEs. Viewing angle effects then cause the X-ray luminosity to vary by orders of magnitude from one event to another.

A schematic picture of the stream-collision process is show to the right.

In Bonnerot, Lu & Hopkins (2021), we conduct the first radiation hydrodynamics simulation of a realistic TDE (a 0.5Msun main-sequence star disrupted by a 2.5x10^6 Msun BH), using the code GIZMO. The multi-panel figure to the right shows the time evolution of the radiation temperature defined based on the local radiation energy density (U_rad = a T_r^4) inside the equatorial plane. The time unit is t_{min} = 40d --- the minimum period of the bound stellar debris. In the t=0 panel (and all others), the BH is represented by a white circle in the lower half of the figure, and the circle above that is the position of the self-crossing shock where we inject gas into the computational domain. We found rapid formation of an accretion disk after the onset of stream self-crossing. Moreover, the region near the self-crossing point has very little radiation as a result of strong shielding, so the gas there can maintain a low ionization fraction and hence high opacity --- this provides the necessary ingredients for reprocessing of EUV/X-ray photons into the optical band.

Formation of GW190814 (most likely a BBH merger)

The LIGO-Virgo Collaboration recently reported a puzzling event, GW190814, with component masses of 23 and 2.6 Msun --- a surprising mass ratio of q ~ 0.1. In Lu, Beniamini & Bonnerot (2021), we propose a 2nd-generation merger scenario where the 2.6Msun object was from a previous binary neutron star (BNS) coalescence and the remnant was able to merge again with the 23 Msun BH tertiary. This occurs when the remnant (most likely a low-mass BH) receives a kick of about 100 km/s in the direction of the "loss cone" shaped like a flying saucer, as shown in the figure to the right. This model was motivated by the relatively small rate (1 to 23 per Gpc^3 per yr) inferred from GW190814 and the secondary mass being close to the total masses of known BNS systems. We show that about 1% of the BNS coalescence occurring in triple systems should give rise to 2nd-generation mergers, provided that a massive BH tertiary is located at a separation less than a few AU. Since the total BNS coalescence rate is of the order 10^3 per Gpc^3 per yr, this model requires that at least 10% BNS mergers occur in triple systems. Since the typical delay time for the 2nd-generation merger is about a Hubble time, these BNS mergers in triples occurred in the distant past when the Universe was less metal-enriched. Low-metallicity (< 0.1 solar) is also favored for the formation of the 23 Msun BH. This model has a number of testable predictions, one of which is that the secondary of GW190814-like events should have dimensionless spin of about 0.7. The other one is that future extreme mass-ratio (q ~ 0.1) events should also have seconary mass in the narrow range of 2.5 to 2.8 Msun (based on Galactic BNS systems).

Neutron Star Mergers and Gamma-Ray Bursts

The overall picture of GW170817/AT2017gfo, as inferred from its gravitational wave (GW) emission, gamma-ray flash, kilonova, and broadband afterglow from a relativistic off-axis jet, confirmed the conjecture that neutron star (NS) mergers are a significant contributor to the r-process nucleosynthesis as well as the sources of short gamma-ray bursts (GRBs).

In Lu & Quataert (2022), we study the long-term (>> 10 sec) evolution of the accretion disk in NS mergers (previous works focused on short-term evolution within the first few seconds). It is shown that the radioactive heating by r-process nuclei eventually exceeds the disk's binding energy roughly 10^2 sec after the merger. This causes the disk to rapidly evaporate leaving nothing behind. The figure to the right schematically shows the evolution of the disk mass with time since merger. At t ~ 1 sec, the disk loses most of its mass due to nuclear recombination forming alpha-particles. At t ~ 10^2 sec, radioactive decay causes another episode of mass loss from the disk.

We suggest that the extended emission (EE) observed in many short GRBs is powered by late-time disk accretion and that the steep flux decline seen at the end of the EE is due to rapid disk evaporation (which shuts off the jet).

We also suggest that the jets from NS mergers have two components --- a short-duration narrow one corresponding to the prompt gamma-ray emission and a long-lasting wide component producing the EE. Observers at different viewing angles (marked by 1, 2, 3 in the figure to the right) would see different phenomena. This leads to a prediction that "orphan EE" (without the prompt gamma-rays, from viewing angle "2") may be a promising electromagnetic counterpart for NS mergers observable by future wide-field X-ray surveys.

The long-lived disk produces a slow ejecta component (~0.01 c). We predict that future JWST near-IR spectroscopy of nearby (<~ 100 Mpc) NS mergers will detect narrow line features a few weeks after the merger, which provides a powerful probe of the atomic species formed in these events.