Two gravitational wave signals from an entirely new class of cosmic
collisions have been discovered by researchers working on the Advanced LIGO
and Advanced Virgo detectors. Each of the signals came from the merger of a
black hole with a neutron star.
The first signal was first detected on January 5, 2020. Astoundingly, just
10 days later, the gravitational waves from a second neutron star-black hole
binary merger were observed. A paper describing the findings was published
on June 29, 2021, in The Astrophysical Journal Letters.
Previously, Advanced LIGO and Virgo had observed more than 50 signals from
the merger of pairs of black holes and pairs of neutron stars, but never
before from a mixed pair. Although neutron star-black hole binaries have
been one of the most exciting theoretical predictions from astrophysicists
for many decades, they have eluded observational efforts until now.
Dr. Geraint Pratten, a member of the LIGO Team at the Institute for
Gravitational Wave Astronomy in Birmingham, worked on producing a
theoretical model of this anticipated event. He said: “Neutron star-black
hole binaries are one of the final pieces of the puzzle that we had yet to
observe. We now have convincing observational evidence of this event and to
see my theoretical model for the observed gravitational waves play a key
role in the analysis is a really special moment.”
The first of the two new observations, GW200105, resulted from the merger of
a black hole 8.9 times the mass of the sun with a neutron star 1.9 times the
mass of the sun. The second observation, GW200115, was a slightly lighter
system, consisting of two objects with masses 5.6 and 1.5 times the mass of
the sun.
“With these extraordinary discoveries gravitational-wave astronomy has
reached yet another milestone,” said Dr. Patricia Schmidt, Lecturer at the
Institute for Gravitational Wave Astronomy. “These detections have strong
repercussions for our understanding of the formation of such mixed binaries
and the role they potentially play as a source of gamma-ray bursts.”
These highly energetic flashes of light were, however, not observed for
these events. This is not surprising as the black holes were significantly
heavier than the neutron stars in both systems, and therefore no significant
shedding of matter from the neutron star that could light up these gamma ray
bursts would be expected. Additionally, astronomers from around the globe
would have had a tough job discovering them: the distance to the
systems combined with the gravitational wave instruments’ limited ability to
precisely identify the sources’ location in the sky made it particularly
difficult for telescopes to monitor the area from which such a burst could
have come.
With these new detections, astrophysicists can seriously start to get their
teeth into the many possible ways in which stars evolve, pair up and
eventually form these kinds of systems.
“These are the impossible couples,” said Riccardo Buscicchio, a PhD student
in the Birmingham LIGO team. “Pictured in one of their last dances, their
families are always divided by boundaries: with one too heavy for the other.
On one side the black holes: dangerous, attractive, dark souls. It’s all or
nothing, with them. You either let yourself go or they will tear you apart.
On the other side neutron stars: very bright at times, crusty on the
surface, but they could hide a soft, gentle spirit.”
Lucy Thomas, a PhD student working on LIGO observations at the Institute for
Gravitational Wave Astronomy in Birmingham, wonders: “How squishy are the
neutron stars? How and where do mixed binaries like this form?” This is just
the beginning of the search for answers. Studying these binaries will keep
Lucy Thomas and Natalie Williams, both PhD students at Birmingham, busy
during their studies: “By studying these mergers and hearing the final cries
of a neutron star before it plunges into a black hole, we can start piecing
together theoretical predictions with real observations and solve the many
mysteries surrounding these exotic objects,” says Natalie.
LIGO and Virgo are scheduled to resume science observations in about a year,
after a commissioning break to further improve their sensitivity. “When we
started observing in 2015, I was convinced it would take till the 2020s to
observe even a single gravitational wave,” says Alberto Vecchio, Director of
the Institute for Gravitational Wave Astronomy at Birmingham. “Six years
later, we have observed many of the different kinds of binary systems
astrophysicists have predicted: for once, it’s very pleasing to be proven so
spectacularly wrong. With even better instruments starting to listen to the
sky again next year, I think we are now on the hook to discover something
completely unexpected. I’ll stick my neck out: it’ll take more than a
decade. Of course, I’m looking forward to be wrong once more.”
Reference:
“Observation of Gravitational Waves from Two Neutron Star–Black Hole
Coalescences” by R. Abbott, T. D. Abbott, S. Abraham, F. Acernese, K.
Ackley, A. Adams, C. Adams, R. X. Adhikari, V. B. Adya, C. Affeldt […] A. B.
Zimmerman, Y. Zlochower, M. E. Zucker, J. Zweizig and the LIGO Scientific
Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, 29 June
2021, Astrophysical Journal Letters.
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