## Friday, August 25, 2017

### LIGO/Virgo may have detected a merger of neutron stars

On August 18th, a Texan astronomer started a wave of speculation by this tweet:

And as far as I can say, this tweet – along with somewhat more specific tweets by astronomer Peter Yoachim hours later – may be the only "original" reason for the speculations so far. Nevertheless, Ethan Siegel at Forbes wrote the following two days ago:
Beyond Black Holes: Could LIGO Have Detected Merging Neutron Stars For The First Time?
And other sources that promoted the rumor include: The Daily Mail, Nude Socialist, Telescoper, Starts With A Bang, Wired, National Geographic, Quanta Magazine, The Independent, Science Alert, Nature, SciAm, NY Post, others.

That would really be cool.

Brian Greene joined this rumor mania minutes before me:

So I am joining, too. ;-) I was looking in the sky and my guess is that a detectable collision of neutron stars took place on August 17th in NGC 4993, a galaxy in Constellation Hydra, about 130 million light years away.

I hope that X-Chandra has looked there, too. In that case, they should have named the event SGRB170817A.

Well, as Davide Castelvecchi and others observed or helped to make true, all the relevant telescopes are suddenly looking at NGC 4993. For this reason, the secrecy looks totally silly to me and J. Craig Wheeler and other originators of the rumors shouldn't be criticized at all.

Note that so far, LIGO has announced the discovery of three black hole mergers (pairs that become one black hole). In each case, the masses of the black holes were comparable to a dozen or dozens of solar masses and the signal lasted for a fraction of a second.

Now, the black hole mergers are the cleanest and most extreme sources of gravitational waves but any violent process in the Universe creates gravitational waves as well – although they are less extreme.

One such process is a collision of different heavy stars, the neutron stars. But because they're less compressed than black holes, they don't get that close to each other and the waves never get that strong. On the other hand, the neutron stars may orbit each other for minutes or hours – while the signal is already near-maximal.

Because the signal is weaker, they must be closer. The signal may be about 10 times weaker so the event should better be about 10 times closer. For the frequency of such signals to be the same as for black hole mergers, the density of pairs of neutron stars must be about 100 times higher than for the pairs of black holes, but the reality may be close to it.

What would be unprecedented about this discovery would be the likely existence of the observation detected through the electromagnetic waves – probably through gamma-ray bursts as well as X-rays. Gamma-ray bursts have had a notoriously uncertain origin – a fellow Fellow in the Society of Fellows was a champion of a particular theory and I forgot whether that theory got stronger or whether he switched. Maybe I had never known it, anyway.

A simulated sound from a merger of two neutron stars.

But with such observations, you could observe an extreme exotic process in a "multimedia" fashion. A new shape of the gravitational wave "sound" would be provided. The electromagnetic signal would be synchronized. And a part of the gamma-ray bursts could be explained and studied by the additional gravitational-wave information. Note that the neutron star collisions could be the source of some heavy elements around us.

It would be cool – and other qualitatively new processes could be detected by LIGO later, too.