On April the 25th, 2019, the network of
gravitational-wave (GW) detectors formed by the European Advanced Virgo, in
Italy, and the two Advanced LIGO, in the US, detected a signal, named
GW190425. This is the second observation of a gravitational-wave signal
consistent with the merger of a binary-neutron-star system after GW170817.
GW190425 was detected at 08:18:05 UTC; about 40 minutes later the LIGO
Scientific Collaboration and the Virgo Collaboration sent an alert to trigger
follow-up telescope observations.
The source of GW190425 is estimated to be at a distance of 500 million
light years from the Earth. It is localized in the sky within an area about
300 times broader than was the case for the BNS observed by LIGO and Virgo in
2017, the famous GW170817, which gave birth to multi-messenger astrophysics.
However, unlike GW170817, no counterpart (electromagnetic signals, neutrinos
or charged particles) has been found to date.
There are a few explanations for the origin of GW190425. The most likely
is the merger of a BNS system. Alternatively, it might have been produced by
the merger of a system with a black hole (BH) as one or both components, even
if light BHs in the mass-range consistent with GW190425 have not been
observed. Yet, on the basis solely of GW data, these exotic scenarios cannot
be ruled out. The estimated total mass of the compact binary is 3.4 times the
mass of the Sun. Under the hypothesis that GW190425 originated from the
merger of a BNS system, the latter would have been considerably different to
all known BNS in our galaxy, the total mass range of which is between 2.5 and
2.9 times the mass of the Sun. This indicates that the NS system that
originated GW190425 may have formed differently than known galactic BNSs.
"After the surprise of the initial results", says Alessandro Nagar of the
Istituto Nazionale di Fisica Nucleare (INFN) of Turin, Italy, "we have
finally reached a reliable understanding of this event. Although predicted
theoretically, heavy binary systems like those that might have originated
GW190425 may be invisible through electromagnetic observations."
"While we did not observe the object formed by the coalescence, our
computer simulations based on general relativity predict that the probability
that a BH is formed promptly after the merger is high, about 96%", says
Sebastiano Bernuzzi of the University of Jena, Germany.