On the hunt for `hierarchical' black holes

Date:
July 27, 2021
Source:
University of Birmingham
Summary:
Black holes, detected by their gravitational wave signal as they
collide with other black holes, could be the product of much earlier
parent collisions. Such an event has only been hinted at so far,
but scientists believe we are getting close to tracking down the
first of these so- called 'hierarchical' black holes.



FULL STORY ========================================================================== Black holes, detected by their gravitational wave signal as they collide
with other black holes, could be the product of much earlier parent
collisions. Such an event has only been hinted at so far, but scientists
at the University of Birmingham in the UK, and Northwestern University
in the US, believe we are getting close to tracking down the first of
these so-called 'hierarchical' black holes.


==========================================================================
In a review paper, published in Nature Astronomy, Dr Davide Gerosa, of the University of Birmingham, and Dr Maya Fishbach of Northwestern University, suggest that recent theoretical findings together with astrophysical
modelling and recorded gravitational wave data will enable scientists
to accurately interpret gravitational wave signals from these events.

Since the first gravitational wave was detected by the LIGO and Virgo
detectors in September 2015, scientists have produced increasingly
nuanced and sophisticated interpretations of these signals.

There is now fervent activity to prove the existence of so-called
'hierarchical mergers' although the detection of GW190521 in 2019 --
the most massive black hole merger yet detected -- is thought to be the
most promising candidate so far.

"We believe that most of the gravitational waves so far detected are the
result of first generation black holes colliding," says Dr Gerosa. "But
we think there's a good chance that others will contain the remnants
of previous mergers. These events will have distinctive gravitational
wave signatures suggesting higher masses, and an unusual spin caused
by the parent collision." Understanding the characteristics of the
environment in which such objects might be produced will also help
narrow the search. This must be an environment with a large number of
black holes, and one that is sufficiently dense to retain the black
holes after they have merged, so they can go on and merge again.

These could be, for example, nuclear star clusters, or accretion disks -
- containing a flow of gas, plasma and other particles -- surrounding
the compact regions at the centre of galaxies.

"The LIGO and Virgo collaboration has already discovered more
than 50 gravitational wave events," says Dr Fishbach. "This
will expand to thousands over the next few years, giving
us so many more opportunities to discover and confirm
unusual objects like hierarchical black holes in the universe." ========================================================================== Story Source: Materials provided by University_of_Birmingham. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Davide Gerosa, Maya Fishbach. Hierarchical mergers of stellar-mass
black
holes and their gravitational-wave signatures. Nature Astronomy,
2021; DOI: 10.1038/s41550-021-01398-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210727131436.htm

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