LIGO may have spotted a black hole-neutron star merger

On April 1, the teams behind the three gravitational wave detectors started them up for a new observational run, the first with all three operating in parallel for the full run. With the benefit of three detectors and some upgrades that were done during the downtime, we're seeing a flood of new data. In just one month, LIGO/VIRGO has seen five gravitational wave events. Three of those are from merging black holes, one was the second neutron star merger, and another may have been the first instance of a neutron star-black hole merger.

A new season

The two LIGO detectors have been a work in progress for years, starting with an early version that everyone acknowledged was unlikely to pick up gravitational waves. But each iteration has allowed scientists to understand the sources of error in their detectors, and they've been taken down for regular upgrades. The international collaboration also benefits from the fact that two additional detectors, Europe's VIRGO and Japan's KAGRA, have similar designs, and all the teams share what they're learning about the hardware.

VIRGO joined LIGO for roughly a month in 2017 before its second observational run came to a close. According to Caltech's Jess McIver, a LIGO team member, work in the intervening time went into "pushing down the quantum noise limits in the detectors." As a result of the lowered noise, McIver said that the detectors can pick up gravitational wave events farther out into space than was ever possible before. Having three detectors helps provide better spatial information as to where the event actually originated, necessary for doing follow-up observations with traditional observatories. And, as one of the events described today makes clear, three detectors let us continue to take data even if one detector is down temporarily.

McIver added that the KAGRA detector is expected to come online later in this run, providing even better resolution.

It's been #AwesomeApril as the 1st month of #O3 produces 5 candidate #GravitationalWaves events! Are they merging #BlackHoles #NeutronStars or maybe even a #NS-BH first detection? @LIGO and @ego_virgo teams are chasing down the answers! (ps the @ehtelescope image was v cool too) pic.twitter.com/ZwyjG47MRZ

— LIGO (@LIGO) April 30, 2019

So, what have we seen since the run started on April 1? Three events appear to be mergers of two black holes, similar to the merger that provided the first detection of gravitational waves. The identity is determined by matching the waves picked up by the detectors to models of different masses colliding; neutron stars and black holes exist in different mass ranges, allowing the detectors to infer what's doing the colliding. Salvatore Vitale of MIT, however, described how there's a mass gap between about two solar masses (the upper limit of neutron stars) and the smallest known black holes, which are about five solar masses. Vitale said events that seem to have an object in that mass gap are high priority for follow-up observations so we can better understand what's going on there.

The other two events are more intriguing. One, which occurred on April 25, appears to be a neutron star collision that occurred 500 million light years away. Unfortunately, LIGO-Hanford in Washington was offline that day, so it was only registered in two detectors, which provided very poor spatial resolution. There's no word yet on whether any optical telescopes picked up information on the event, but they'd have to search roughly a quarter of the sky to do so.

A day later, all three detectors were online when an extremely distant event occurred roughly 1.2 billion light years away. This has tentatively been identified as the first merger we've seen between a black hole and a neutron star. It's not clear whether we should expect an optical counterpart to this sort of merger, but its position was narrowed down to only three percent of the total sky, suggesting we should have some good data on it if there's anything to see.

Going public

Earlier discoveries by LIGO/VIRGO went through the usual scientific publishing process before the press was notified that they'd be announced. Today's press conference and the tweet above (which was written two days ago) are indications that we've entered a rather different approach to public disclosure of gravitational wave events. In fact, LIGO/VIRGO has opened up its notification process to the public; formerly, it was used to alert astronomers of possible locations for follow-up observations. Now, anyone can sign up for alerts, or simply follow the @LIGO twitter account.

As #O3 enters its 4th week @LIGO and @ego_virgo have another #GravitationalWaves candidate event: S190421ar. This looks like another binary #BlackHole merger, with a false alarm rate of about once every two years. More info at https://t.co/cCkdei2n4v #O3ishere 1/2 pic.twitter.com/Yjqw4n85P0

— LIGO (@LIGO) April 22, 2019

There's also now an iPhone app that lets you follow the event announcements (an Android version is in the works). In short, anyone who's interested can find out what LIGO is seeing within a day of when the LIGO scientists themselves do. While most of us won't be able to do detailed follow-up observations, one of the LIGO team members suggested that amateurs can potentially help by identifying galaxies within the area of interest.

LIGO spokesman Patrick Brady of the University of Wisconsin-Milwaukee suggested that LIGO is gradually normalizing what had been "first of its kind" events just a couple of years ago. We've now seen enough black hole mergers that they won't do dedicated publications for them; instead, they'll be published as part of a catalog following every six months of observation (each observing run is expected to last a year). Unusual and distinctive events should be flagged quickly and appear in a publication roughly three months afterwards.

Right now, with only one neutron star merger having been definitively identified, these would probably fall into the "unusual" category. But that will probably end up being a temporary designation. If the upgraded LIGO works as well as it's expected to, we may be seeing one of these events every month over the next year.

The consensus seems to be that, over time, gravitational wave science will shift toward a focus on events that are either very rare—and simple mergers won't be among those—and analyses that require lots of events in order to provide some statistical certainty. But everyone held out the hope that, either through luck or further refinement of instruments, we'll be able to see something that we hadn't planned to find.