An Explosive Merger … Maybe

On August 16, 2019, both the Fermi Gamma-ray Burst Monitor (GBM) and the Laser Interferometer Gravitational-wave Observatory (LIGO) detected faint blips that didn’t quite register as events. But could these ghost signals actually correspond to the first collision we’ve detected of a black hole with a neutron star?

Neutron Stars and Black Holes: Mix and Match

LIGO’s first detection of gravitational waves was GW150914, the collision of a pair of black holes. In the years since, LIGO has partnered with its European counterpart, Virgo, to make another dozen confirmed detections of binary black holes merging. The collaboration also spotted two instances of binary neutron stars colliding — one of which, GW170817, was accompanied by a short gamma-ray burst and emission spanning the electromagnetic spectrum.

But LIGO/Virgo’s mix-and-match collection is incomplete: we’re still waiting to detect a definite neutron-star–black-hole collision. In particular, we’d like to spot a merger in which the neutron star is tidally destroyed by the black hole, lighting up the sky with accompanying electromagnetic emission.

Could it be that such an event is actually hidden in the reject data from LIGO/Virgo and Fermi?

A Pair of Intriguing (Non-?)Events

The results from LIGO-Virgo’s third observing run, cut short by the pandemic in March 2020, are still being carefully analyzed by the collaboration. The O3 alert data, however, is publicly available — and a team of scientists have taken advantage of this to do some independent analysis, recently detailed in a publication led by Yi-Si Yang (Nanjing University, China).

Yang and collaborators take note of two faint signals that occurred on August 16, 2020:

1. A subthreshold gravitational-wave event in the LIGO/Virgo data — i.e., an event with a signal-to-noise ratio below 12, the threshold to qualify as a significant candidate.
2. A subthreshold gamma-ray burst, GBM-190816, that was picked up by Fermi/GBM just 1.57 seconds after the gravitational-wave event.

If these two signals are both real and related, then GBM-190816 represents a short gamma-ray burst emitted from the merger of two compact objects — and Yang and collaborators’ analysis shows that, with a mass ratio of q ~ 2.26, the system is most likely a neutron-star–black-hole binary. In the simplest explanation, the neutron star was torn apart before the bodies ultimately merged, producing the pair of signals.

Identifying What’s Real

So are these subthreshold events real? We can’t say, yet! The public alerts from LIGO/Virgo only contain a portion of the signal information, so Yang and collaborators had to make a number of assumptions to analyze the event.

That said, the faintness of both signals is reasonable given the parameters of this potential merger: if real, it took place at a distance of 1.4 billion light-years, roughly nine times farther away than GW170817. The gamma-ray spike was also extremely short — just ~0.1 seconds, compared to the ~2 second duration of GW170817 — which is what caused it to register below the Fermi/GBM trigger threshold.

If confirmed, this event could provide interesting insight into how the light emitted by such a merger escapes and travels to us. Now we just have to wait for the official joint analysis from the LIGO/Virgo/Fermi team!

Citation

“Physical Implications of the Subthreshold GRB GBM-190816 and Its Associated Subthreshold Gravitational-wave Event,” Yi-Si Yang et al 2020 ApJ 899 60. doi:10.3847/1538-4357/ab9ff5