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Digging the grave: In search of the remains of merging black holes and neutron stars

Read time: 5 mins
14 Sep 2020
Digging the grave: In search of the remains of merging black holes and neutron stars

Snapshot of simulation showing two black holes colliding with each other. [Image Credits: Wikimedia Commons]

Astronomers detect gravitational waves from the merging of neutron stars and black holes, but no electromagnetic waves.

A century back, Albert Einstein proposed the general theory of relativity. This theory predicted the existence of exotic objects called black holes—objects so dense that even light cannot escape them. It also predicted ripples in space-time termed gravitational waves. In 2015, with the help of a dedicated observatory, astronomers for the first time detected these waves, emitted by two black holes colliding and merging. Such a merger cannot emit electromagnetic waves, however, because neither black hole emits them.

Neutron stars are not as massive as black holes, but very dense. The mass of a cubic centimetre of a neutron star would be 1 followed by 14 zeros (100,000,000,000,000) kilograms. In 2017, astronomers for the first time detected gravitational waves from two neutron stars merging. Astrophysicists had earlier predicted such mergers to also emit electromagnetic waves, which they call counterparts. Hence, to see them, astronomers turned their optical and infrared telescopes to this merger, and detected them.

Now, an interesting question would be to know if astronomers could detect electromagnetic waves when a neutron star merges with a black hole to form a bigger black hole. A new study, published in the journal Nature Astronomy, and conducted by an international consortium of researchers including those from India, aims to answer this question.

On the 5th of January, 2020, gravitational wave detectors found a neutron star and a black hole spiralling and merging into each other. They sent automatic alerts to an international collaboration of optical astronomers, known as the Global Relay of Observatories Watching Transients Happen, or GROWTH. It includes astronomers from the Indian Institute of Astrophysics (IIA), Bengaluru, and the Indian Institute of Technology Bombay (IIT Bombay). Harsh Kumar, a graduate student at IIT Bombay and one of the co-authors of the study, usually undertakes the observations by the GROWTH-India telescope at the Indian Astronomical Observatory at Hanle, Ladakh.

Since the gravitational wave detectors can estimate only an approximate direction of the merger, it leaves a large part of the sky to hunt for the electromagnetic counterpart. The astronomers first used the Zwicky Transient Factory or ZTF, an optical telescope located in the Palomar Observatory at Palomar Mountain in San Diego, California, which scans a large region of the sky, and detects astronomical events called ‘transients’. These events live for a short while before disappearing. ZTF catches such transients routinely, from distant supernovae to flashes of light reflected by asteroids in our Solar System.

Within minutes of detecting the gravitational waves from the merger, the astronomers shortlisted 22 possible transients with ZTF and observed them further with a combination of optical telescopes around the world. “Unfortunately, it was snowing at Hanle and we could not open the GROWTH-India telescope during that time. So I focussed on examining the ZTF data,” said Harsh. By studying how the brightness of these objects changed with time and also their spectra, they found that all these sources were routine transients and not the merger they were looking for.

On the 15th of January, 2020, astronomers detected another similar merger. They estimated that in this case, the merged black hole had not swallowed a large chunk of the former neutron star. They could also estimate the possible direction of the sky from which the merger had originated, which gave them a better shot at detecting the counterpart. Within minutes, they started scanning this part of the sky using ZTF and shortlisted six transients for careful study. Upon further observations of their spectra, they figured that none of the six could come from the merger.

“While the non-detections are a bit disappointing, they also inform us about the nature of the sources,” says Michael Coughlin, an assistant professor at the University of Minnesota, who co-led the study with Shreya Anand, a graduate student at the California Institute of Technology. Conducting a thorough search but not finding the source is different from not looking and hence not detecting. “It can imply that the black hole swallowed the neutron star completely. This tells us that the black hole is much heavier than the neutron star,” he adds.

As gravitational wave detectors prepare for more advanced searches, the difference of mass between the heaviest neutron stars and the lightest black holes is puzzling astronomers. Till date, the number of such mergers that astronomers have detected is eight. In the other six, preliminary calculations had revealed that the merged black hole had swallowed all the available matter from the neutron star. If they detect gravitational waves from more mergers but not their electromagnetic counterpart, it could be because of a wrong assumption. The lighter component could be another black hole instead of a neutron star. Since black holes do not emit electromagnetic waves, their search could be futile.

Or, the telescopes could have been plain unlucky. Clouds over Palomar on a few nights of observations reduced the odds of detection this time. “Having more such opportunities can help us in the future,” says Shreya. “Continuing such searches is useful because in some cases you may be lucky,” says A. R. Rao, visiting professor at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, who was not involved in this study. “One such discovery is certainly worth many failed attempts,” he adds. “In recent times, astronomy has been driven forward equally by non-detection of exciting events, as it’s been by their detections,” says Navin Sridhar, astrophysics graduate student at Columbia University, New York, who was also not involved in this study.

The elusive electromagnetic counterparts rapidly decay in time, so the gravitational-wave physicists are analysing the gravitational wave detectors to send them faster alerts. They are also training ZTF to make informed decisions on which of the prospective sources may originate from mergers. “We need automated technologies because the counterpart can easily disappear within a single night,” Shreya concludes.

The hunt, it seems, has just begun.

This article has been run past the researchers, whose work is covered, to ensure accuracy. 

Editor's Note: There was an error in the hyperlink of the study published. This has been rectified. The error is regretted.