“Elusive missing piece of the family picture of compact object mergers”
From: Northwestern University
June 29, 2021 -- A long time ago, in two
galaxies about 900 million light-years away, two black holes each gobbled up
their neutron star companions, triggering gravitational waves that finally hit
Earth in January 2020. Astrophysicists' observation of the two events -- detected
just 10 days apart -- mark the first-ever detection of a black hole merging
with a neutron star.
Discovered by an international team of
astrophysicists including Northwestern University researchers, two events --
detected just 10 days apart -- mark the first-ever detection of a black hole
merging with a neutron star. The findings will enable researchers to draw the
first conclusions about the origins of these rare binary systems and how often
they merge.
"Gravitational waves have allowed
us to detect collisions of pairs of black holes and pairs of neutron stars, but
the mixed collision of a black hole with a neutron star has been the elusive
missing piece of the family picture of compact object mergers," said Chase
Kimball, a Northwestern graduate student who co-authored the study.
"Completing this picture is crucial to constraining the host of
astrophysical models of compact object formation and binary evolution. Inherent
to these models are their predictions of the rates that black holes and neutron
stars merge amongst themselves. With these detections, we finally have
measurements of the merger rates across all three categories of compact binary
mergers."
The research will be published June 29
in the Astrophysical Journal Letters. The team includes researchers
from the LIGO Scientific Collaboration (LSC), the Virgo Collaboration and the
Kamioka Gravitational Wave Detector (KAGRA) project. An LSC member, Kimball led
calculations of the merger rate estimates and how they fit into predictions
from the various formation channels of neutron stars and black holes. He also
contributed to discussions about the astrophysical implications of the
discovery.
Kimball is co-advised by Vicky Kalogera,
the principal investigator of Northwestern's LSC group, director of the Center
for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the
Daniel I. Linzer Distinguished Professor of Physics and Astronomy in the
Weinberg Colleges of Arts and Sciences; and by Christopher Berry, an LSC member
and the CIERA Board of Visitors Research Professor at Northwestern as well as a
lecturer at the Institute for Gravitational Research at the University of
Glasgow. Other Northwestern co-authors include Maya Fishbach, a NASA Einstein
Postdoctoral Fellow and LSC member.
Two events in
ten days
The team observed the two new
gravitational-wave events -- dubbed GW200105 and GW200115 -- on Jan. 5, 2020,
and Jan. 15, 2020, during the second half of the LIGO and Virgo detectors third
observing run, called O3b. Although multiple observatories carried out several
follow-up observations, none observed light from either event, consistent with
the measured masses and distances.
"Following the tantalizing discovery,
announced in June 2020, of a black-hole merger with a mystery object, which may
be the most massive neutron star known, it is exciting also to have the
detection of clearly identified mixed mergers, as predicted by our theoretical
models for decades now," Kalogera said. "Quantitatively matching the
rate constraints and properties for all three population types will be a
powerful way to answer the foundational questions of origins."
All three large detectors (both LIGO
instruments and the Virgo instrument) detected GW200115, which resulted from
the merger of a 6-solar mass black hole with a 1.5-solar mass neutron star,
roughly 1 billion light-years from Earth. With observations of the three widely
separated detectors on Earth, the direction to the waves' origin can be
determined to a part of the sky equivalent to the area covered by 2,900 full
moons.
Just 10 days earlier, LIGO detected a
strong signal from GW200105, using just one detector while the other was
temporarily offline. While Virgo also was observing, the signal was too quiet
in its data for Virgo to help detect it. From the gravitational waves, the
astronomers inferred that the signal was caused by a 9-solar mass black hole
colliding with a 1.9-solar mass compact object, which they ultimately concluded
was a neutron star. This merger happened at a distance of about 900 million
light-years from Earth.
Because the signal was strong in only
one detector, the astronomers could not precisely determine the direction of
the waves' origin. Although the signal was too quiet for Virgo to confirm its
detection, its data did help narrow down the source's potential location to
about 17% of the entire sky, which is equivalent to the area covered by 34,000
full moons.
Where do they
come from?
Because the two events are the first
confident observations of gravitational waves from black holes merging with
neutron stars, the researchers now can estimate how often such events happen in
the universe. Although not all events are detectable, the researchers expect roughly
one such merger per month happens within a distance of one billion light-years.
While it is unclear where these binary
systems form, astronomers identified three likely cosmic origins: stellar
binary systems, dense stellar environments including young star clusters, and
the centers of galaxies.
The team is currently preparing the
detectors for a fourth observation run, to begin in summer 2022.
"We've now seen the first examples
of black holes merging with neutron stars, so we know that they're out there,"
Fishbach said. "But there's still so much we don't know about neutron
stars and black holes -- how small or big they can get, how fast they can spin,
how they pair off into merger partners. With future gravitational wave data, we
will have the statistics to answer these questions, and ultimately learn how
the most extreme objects in our universe are made."
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