My Research

Electromagnetic Signals from Neutron Star Mergers

Binary neutron stars (BNS) and black hole–neutron star (BHNS) mergers are believed to be progenitors of short gamma-ray bursts and kilonovae. We developed a detailed viewing-angle-dependent model of BHNS kilonovae with two viewing angles defined by the latitudinal and longitudinal angles. We used our BHNS kilonova model to evaluate the detectability of electromagnetic counterparts in connection with GW detections, GW-triggered target-of-opportunity, and serendipitous observations. The relevant explorations have been extended to BNS kilonovae and afterglows.

References: (1) Zhu et al. 2020, ApJ, 897, 20; (2) Zhu et al. 2021, ApJ, 917, 24; (3) Zhu et al. 2022, ApJL, 936, L10; (4) Zhu et al. 2022, ApJ, 938, 147; (5) Zhu et al. 2022, ApJ, 942, 88.

Gravitational-wave Events and Their Formation Channel

I am deeply interested in the properties and formation mechanisms of gravitational-wave (GW) NS mergers, espically for BHNS mergers). My research focus on both the characteris- tics of individual BHNS events and the GW population of these events. Recently, we employed population synthesis simulations with COMPAS to simulate the event rate densities and mass distribution of mass-gap BHNS mergers, finding consistency with observed values based on the discovery of GW230529. We also suggested that future observable kilonovae will be from mass-gap BHNS mergers. Additionally, we used MESA to examine unique formation scenarios for BHNS mergers, such as BHNS binary mergers formed through super- Eddington stable mass transfer, and to explore mechanisms for enhancing the spin of BHs in these binaries, thereby increasing the likelihood of bright electromagnetic signals during mergers.

References: (1) Zhu et al. 2021, ApJ, 921, 156; (2) Zhu et al. 2022, ApJ, 928, 167; (3) Zhu et al. 2024, MNRAS, 529, 4554; (4) Zhu et al. 2024, ApJ, 974, 211; (5) Hu et al. 2022, ApJ, 928, 163; (6) Wang et al. 2024, ApJ, 965, 177.

Multimessenger Signals from Compact Object Coalescences and Supernovae in Active Galactic Nuclues Accretion Disks

AGN disks are believed to harbor a large population of stars and compact objects, indicating that SN and compact object coalescences frequently occurring within the disk atmosphere. The potential association between the binary BH GW190521 and an EM flare in an AGN has exploded widespread interest in catastrophic explosions occurring in AGN disks. Due to the dense environment around these AGN catastrophic explosions, their EM signals should be unique. Since BNS and BHNS mergers can occur in AGN disks, we proposed for the first time that launched GRB jets could be choked by the disk, powering a hot cocoon. The cocoon and the ejecta would break out from the disk, powering two shock breakout signals. Since white dwarfs and their binaries are predicted to exist in AGN disks, we predicted AGN thermonuclear explosion (accretion-induced collapse resuling in a millisecond magnetar) can power ejecta (magnetar-driven) shock breakouts, shock cooling signals, and slow-rising, relatively dim SNe Ia (magnetar-powered transients). Furthermore, we also showed that the choked GRB jets in AGN disks, nonrelastivistic jets from binary BH mergers in AGN disks, and the interaction between shock-accelerated cosmic rays and disk materials after the shock breakouts can effectively produce detectable TeV–PeV neutrinos without production of high-energy gamma-rays. We explored the detectability of neutrino emissions from catastrophic explosions by IceCube and predicted that AGN catastrophic explosions can be potential hidden sources to significantly contribute to the diffuse neutrino background.

References: (1) Zhu et al. 2021, ApJL, 906, L11; (2) Zhu et al. 2021, ApJL, 911, L19; (3) Zhu et al. 2021, ApJL, 914, L19; (4) Zhu et al. 2021, ApJL, 917, L28; (5) Zhu 2024, MNRASL, 528, L88.

Magnetar-powered SNe and Their Binary Origins.

One of the most popular central engine models for type Ic superluminous SNe (SLSNe), broad-lined type Ic SNe (SNe Ic-BL) associated or unassociated with long GRBs, and some fast blue optical transients (FBOTs) is the magnetar central engine. By combing results of ejecta masses and magnetar rotational energy for these magnetar-driven SNe, we found a strong universal relationship between them, indicating magnetar-driven SNe could share a common origin. With detailed binary simulations and population synthesis simulations, we found that helium stars in close binaries can be tidally spun up by main-sequence or compact-object companions and these systems can form via the classic common-envelope channel.

Observations have revealed that the lightcurves and spectra of SLSNe are diverse, showing characteristics such as multiple post-peak bumps, late-time broad hydrogen lines, and polarization evolution. We proposed that a close binary origin could naturally explain this diversity. Specifically, we suggested that the wind from a newborn magnetar may significantly evaporate its main-sequence companion if the binary system is not disrupted after SN. The subsequent heating and acceleration of the evaporated star material and ejecta by the magnetar wind could produce post-peak bumps, which fits well with many bumpy SLSN lightcurves. This “magnetar–star binary engine” model may offer a possible explanation for the evolution of polarization, along with the origin and velocity broadening of late-time hydrogen broad spectral features observed in some bumpy SLSNe. The diversity in the lightcurves and spectra of SLSNe may be attributed to the wide variety of companion stars and post-SN binary systems.

References: (1) Zhu et al. 2024, ApJL, 970, L42; (2) Yu et al. 2017, ApJ, 840, 12; (3) Liu et al. 2022, ApJL, 935, L34; (4) Hu et al. 2023, submitted to Nature Communications.

Testing Fundamental Physics

The “Breakthrough Starshot” program is planning to send transrelativistic probes to travel to nearby stellar systems within decades. We improved provided simulations to solve the probe’s motion in flight by comparing the positions of three or more point sources observed in the Earth’s rest frame and in the probe’s comoving frame. Our results showed high precision in the moving direction and the velocity of the probe. This work confirmed the feasibility of solving the motion of future transrelativistic probes in theory. We also introduced a method to constrain photon mass using the light aberration effect. Besides, we explored different methods to test the special relativity and constrain photon mass using the Doppler effect.

(1) Zhu et al. 2019, ApJ, 877, 14; (2) Yang et al. 2019, ApJ, 883, 159.