The true identity of last week’s purported neutron star-black hole merger may never be known, as follow-up searches for a source of the signal have turned up nothing.
Scientists affiliated with the LIGO detector announced last week that they’d spotted, in the form of gravitational waves, what appeared to be a black hole gobbling up a neutron star. But without an electromagnetic counterpart to the detection, it’s difficult to distinguish such an event from a pair of black holes merging. Either possibility is exciting.
“We are not aware of any black holes in the Universe with masses less than about five solar masses,” Susan Scott, professor at the Australian National University, told Gizmodo in an email. “Since we currently estimate the smaller mass to be less than three solar masses, if it is a black hole, then it would be significantly lighter than any other black hole that we know about.”
On August 14, scientists operating the Laser Interferometer Gravitational Wave Observatory’s two machines, as well as the Virgo gravitational wave interferometer, all reported spotting gravitational waves. The detectors determined that this event, now named S190814bv, was highly unlikely to be a false alarm. The astrophysicists calculated that the gravitational waves they detected would have been created by two masses, one larger than five times the mass of the Sun and one smaller than three times the mass of the Sun, colliding nearly 900 million light-years away.
Initial calculations suggested this was the strongest evidence yet of a black hole, a super-dense object whose gravity warps space such that light can’t escape it, colliding with a neutron star, a stellar corpse slightly less dense than a black hole, which packs huge amounts of mass into an object just a few miles across. Theory predicts that such a collision would come with an emission of electromagnetic radiation called a kilonova, similar to the one that accompanied the first detected colliding neutron stars, as the victim neutron star gets shredded up. After the recent LIGO detection, telescopes scanned the sky looking for an electromagnetic counterpart to S190814bv.
One of those missions is already reporting its results on the arXiv physics paper server: An instrument on a 6.5-meter telescope at Las Campanas Observatory in Chile failed to find any evidence of accompanying electromagnetic radiation. This complicates things for those hoping that LIGO had spotted a black hole eating a neutron star.
“Not seeing something leaves this ambiguity,” Edo Berger, a professor of astronomy at Harvard University who worked on this follow-up mission, explained to Gizmodo. “We’re going to have to wait a few months until LIGO and Virgo publish their final results, but my suspicion is that [this detection’s identity] is going to stay ambiguous.”
The absence of the signal in telescopes doesn’t mean the signal wasn’t there. Though LIGO supplied follow-up missions with coordinates of where to look, 900 million light-years is much farther away than the August 2017 neutron star collision. Some models that predict what a black hole colliding with a neutron star look like predict a much fainter resulting explosion. Maybe, if the mass difference between the neutron star and black hole is great enough, there would be no emission at all as the star gets swallowed whole, Scott said.
All of this means that conclusive evidence of a black hole colliding with a neutron star will require more searching, and that this event is more than likely another “maybe.”
But the alternative, that LIGO has actually spotted yet another pair of colliding black holes, is still exciting. The lighter of the two objects would then be the lightest black hole ever found. “Either way, the science from this event will be amazing, a black hole swallowing whole a neutron star, or the discovery potentially of a very lightweight black hole,” Scott said.
You might wonder why we haven’t seen knockout detections of electromagnetic radiation accompanying gravitational waves since the August 2017 discovery. Unfortunately, we probably just got lucky that time. “It was nearby, well-localized in space, and had everything going for it,” Berger said. “We got spoiled a little bit.” Given the vastness of the universe, ambiguous detections like S190814bv are likely to be the norm for this new, multimessenger era of astronomy, in which scientists are able to use more than just incoming light to observe the cosmos.
LIGO’s third observing run, ongoing now, is scheduled to continue until April 2020.