Gravitational Waves and Their Electromagnetic Counterparts:

On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the first gravitational wave (GW) event, GW 150914, produced by the merger of two black holes. This marked the beginning of a new era of gravitational wave astronomy. Two years later on August 17, 2017, LIGO and Virgo collaboration reported the detection of the first binary neutron star merger event GW170817, which was spatially and temporarily associated with a short-duration gamma-ray burst GRB 170817A with a temporal delay of ~1.7 s. Multi-wavelength follow-up observations revealed an optical transient, AT2017gfo, which is consistent with the so-called "kilonova" powered by radioactive decay of the neutron-rich ejecta launched during the merger, as well as the broad-band (from radio to X-rays) afterglow. Humankind formaly entered the era of GW-led multi-messenger astrophysics.

Collaborating with many colleagues, I have been actively working in this emerging new field. The subjects we are studying include:

* What are the merger products (prompt black hole [BH], hypermassive neutron star [HMNS], supramassive neutron star [SMNS], stable neutron star [SNS]) of NS-NS mergers?
* What are the observational signatures of a massive neutron star (SMNS or SNS) merger product?
* What is the origin of the ~1.7 s delay of GW170817/GRB 170817A? In general, what physical/astrophysical processes define the observed delay between the GW merger time and the GRB? (see Figure on the right)
* When is the jet launched? What is the jet composition? How is the short GRB emitted?
* How does the jet launching process affect the jet structure? (see Movies on this website produced by Dr. Jin-Jun Geng, see this paper for details)
* How is kilonova emisson modified by energy injection from a massive NS?
* How are BH-NS mergers different from NS-NS mergers in terms of short GRB and kilonova signals?
* What could be other EM signals from compact binary coalescence (CBC) events? In particular, I have proposed two scenarios: One is the short-GRB-less X-ray transients associated with NS-NS mergers with a massive NS merger product, which was predicted in this paper. An X-ray transient discovered from Chandra Deep Field South (CDF-S XT2) seems to be very consistent with this scenario. The other the so-called "charged compact binary coalescence" (cCBC) signals which are predicted in these two papers: Paper 1 and Paper 2. The signals are faint, but should exist. When detected, a direct measurement on the globabl charges carried by neutron stars or even black holes can be made.