Space-time ripples flip our understanding of black holes & neutron stars upside down

Katie Ramirez
June 28, 2020

The mystery object involved in the merger detected a year ago might have been an unexpectedly small black hole or an unexpectedly large neutron star. It's never been clear if cosmic objects with a mass in that range could exist. Both found a fusion of objects with masses between 23 and 2.6 times greater than that of the Sun.

The Advanced Virgo detector at the European Gravitational Observatory (EGO) in Italy and two wave observatories in the United States discovered the object previous year and calculated it to weigh around 2.6 times our own Sun. In the absence of light, they can only identify such mergers by detecting their gravitational waves - ripples in spacetime created by the collisions of massive objects. And that's where the classification problem arises.

"I couldn't believe it the first time I saw it, it's stunning".

The newly reported detection, described today in a paper published in The Astrophysical Journal Letters, fills in what was thought to be a "mass gap" dictated by physics and stellar evolution. "We are really pushing our knowledge of low-mass compact objects", Vicky Kalogera, study coauthor was quoted as saying. This size falls into what scientists call the mass gap: an object significantly smaller than any black hole studied to date (about 5 times the mass of the sun), but also probably larger than any known neutron star (about 2.5 times the mass of the sun). "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities". Altogether, the observatories have detected likely gravitational waves more than 30 times.

In the case of this latest discovery, researchers first detected gravitational waves emanating from the quasar J124942.3+344929-an active supermassive black hole 100 million times more massive than our sun-in May 2019.

LIGO has a double dose of detectors in Hanford, Wash., and in Livingston, La., to guard against spurious signals that could be caused, for example, by seismic activity. This event, called S190521g, at first seemed to produce no visible light. The instruments are sensitive enough to measure changes in distance that are 10,000 times smaller than a proton's width.

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Both LIGO experiments and Virgo consist of two 2.5-mile-long arms. That leads the detector to record some flashes of light. Gordon Baym of the University of IL recalled that a collision between two neutron stars resulted in a single neutron star of 2.7 solar masses. If a neutron star was involved in last August's crash, there might have been a similar flash. "When the masses are highly asymmetric, the smaller neutron star can be eaten in one bite".

"At the center of most galaxies lurks a supermassive black hole".

If it is a light black hole then there is no established theory for how such an object could develop. By sensing only the gravitational waves from the collision, LIGO and Virgo can not tell for sure what the object is, she says. The pattern of the waves and their intensity is what provided researchers with the data about the objects that crashed together. "So, people who are looking at exotic equations that explain what goes on inside them might be thinking, "maybe this is evidence that we can get much heavier neutron stars".

However, the signal raises hopes for future observations to be encountered.

"The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it", said Pedro Marronetti, program director for gravitational physics at the National Science Foundation.

"We don't know how the nuclear strong force operates under the extreme conditions you need inside a neutron star".

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