As most of India slept in anticipation of a national holiday and the usual Independence Day celebrations the following morning, three gravitational wave detectors in the US and Italy observed one of the strangest signals to date. The twin Advanced LIGO detectors in the US and the Virgo detector in Italy are the most precise measuring devices ever built. They can sense extremely tiny disturbances in the fabric of spacetime called gravitational waves, created when massive bodies like two black holes or neutron stars rapidly spiral in and merge. Since the first detection of a binary black hole merger in September 2015, which signalled the start of a new era of astronomy, the LIGO-Virgo collaboration has reported the observation of multiple binary black hole and binary neutron star mergers.  Officially designated GW190814, this signal originated about 800 million years ago – or 800 million lightyears away – from the inspiral and merger of two compact objects. The heavier of these objects was a black hole of 23 solar masses (i.e. 23-times as massive as the Sun), while its companion had a mass of 2.6 solar masses. The gravitational waves from this event travelled at the speed of light and passed through Earth on August 15, 2019, at around 2:40 am IST. And it immediately stood out from all previous LIGO-Virgo detections because of two outstanding features.

First, with the heavier object weighing nine times more than its companion, this is the most asymmetric system ever observed using gravitational waves. Second, the lighter object of 2.6 solar masses is either the lightest black hole or the heaviest neutron star ever observed in a system of two compact objects. And we’re not sure which one it is.

Neutron star or black hole?

Astronomers have long hard time for explaining why they haven’t been able to find black holes lighter than 5 solar masses. At that same time, current theoretical models about the internal structures of neutron stars predict that their maximum allowed mass should be around of 2 solar masses. The difference between these two limits has created the idea of a ‘mass gap’ – a region in the population of objects that were once stars, like black holes and neutron stars, a.k.a. the stellar graveyard, devoid of objects within 2 to 5 solar masses. GW190814 marks the first time astronomers have made a confident mass measurement of a compact object in the mass gap. This however brings us no closer to reducing the ambiguity about its nature. The presence of a neutron star in a binary should leave a unique signature on the gravitational wave signal, as the neutron star is deformed by the gravitational pull of its companion black hole – like how the Sun and the Moon raise tides on Earth. However, for a system where the masses of the two objects are as different as astrophysicists have found to be in GW190814, this tidal imprint is exceedingly small, and therefore exceedingly difficult to measure. On the other hand, if indeed the lighter object is a neutron star, astrophysicists will have to revise their current understanding of neutron star physics