One of the key goals of gravitational wave astronomy is to understand and
characterize binary black hole spins, according to Vijay Varma, a Klarman
Postdoctoral Fellow in physics in the College of Arts and Sciences.
By measuring the masses and spin rates of binary black hole systems in which
two of the super-compact astronomical objects orbit each other, using
gravitational waves emitted as the objects merge, researchers can gain
insight into larger questions in astrophysics, including general relativity
and the lifespan of stars.
In “New spin on LIGO-Virgo binary black holes,” published in Physical Review
Letters on April 29, 2021, Varma and collaborators proposed a novel way of
studying binary black holes by identifying each of their individual
component black holes by their spins – rather than their masses – which
leads to an improved measurement of the spins. The researchers applied the
new method to analyze binary black hole data gathered by the LIGO and Virgo
gravitational wave detectors.
“Rather than attempting to identify the spin of the heaviest and lightest of
the two objects, as is usually done, we infer the properties of the objects
with the highest and lowest spin,” the researchers wrote. This refocus on
the black holes’ spins, rather than their masses, gives a new importance to
spin measurements in binaries in which the masses of the two black holes are
nearly equal – “which appear to be the majority,” they wrote.
Their finding potentially changes the way scientists study black holes,
which provide insight into general relativity and our knowledge of the
evolution of stars, among other large questions.
“We realized that for systems where the two black holes in the binary have
equal masses or close to equal masses, it’s hard to measure the spin,”
Biscoveanu said. The team reframed the question to look directly at the spin
of the black hole with the highest spin and the black hole with the lowest
spin.
Varma and collaborators (lead author Sylvia Biscoveanu, Maximiliano Isi and
Salvatore Vitale, all from the Massachusetts Institute of Technology) were
inspired to pursue this line of research while studying data from GW190521,
a binary black hole system detected by LIGO, a very sensitive instrument
which detects gravitational waves from astronomical objects, including black
holes. This system is particularly interesting, the researchers said,
because it is the most massive detected to date, and it also demonstrates
evidence for a unique spin signature that hadn’t previously been observed.
“We are especially interested in systems that have spins because they carry
a lot of astrophysical information that can tell us how these binaries were
formed in the first place,” said Varma, an expert on developing ‘surrogate
models’, which allow researchers to determine characteristics of black holes
based on supercomputer simulations.
Black holes are incredibly heavy and dense, Varma said, typically 10 to 30
times more massive than the sun, sometimes heavier, but packed into a space
about the size of Hawaii.
Biscoveanu compared measuring the mass and the spin of a binary black hole
system to measuring the temperature and the sweetness of two juices. “You
would measure the temperature of the coldest juice that you’re tasting and
the sweetness of the sweetest juice,” she said. “You wouldn’t try to measure
the sweetness of the coldest juice because that’s a convoluted question,
especially if both of them are the same temperature.”
The researchers said that inquiring about the fastest spinning black hole
helps researchers learn more about individual binary black hole systems, or
a whole population of binary black holes, such as those observed via
gravitational waves by the LIGO-Virgo collaboration.
“That has implications for how stars evolve and form black holes,” Varma
said. “We can go back to the earlier stages of the evolution and try to
understand the secrets of black hole astrophysics.”
Reference:
“New Spin on LIGO-Virgo Binary Black Holes” by Sylvia Biscoveanu,
Maximiliano Isi, Salvatore Vitale and Vijay Varma, 29 April 2021, Physical
Review Letters. DOI:
10.1103/PhysRevLett.126.171103
Tags:
Space & Astrophysics