by Mandeep  S. S. Gill

On the morn of Thursday, August 17, 2017 the LIGO-Virgo Collaboration (LVC) gravitational wave detectors saw a binary neutron star (BNS) collision in gravitational waves—and kind of blew up the astronomical world who was in on it, that day.

Why do I say this, after the announcement of the discovery of gravitational waves from a binary black hole (BBH) coalescence already made such major "ripples" among those who pay attention to things astronomical, just 1 1/2 years ago? (Resulting in the Nobel Prize being awarded for this, on October 3, 2017).

Because a BNS event is different in one fundamental respect: unlike for a BBH event, we expect to be able to see the optical counterpart of such a collision if it occurs close enough our part of the Universe. Generally, most models for what happens for when BBH merger occurs show that no optical light emission is expected to be seen, especially at the one to several billion light year distances they have been observed at. Still, heroic efforts were made by the “EM partners” (the astronomers looking for counterparts to the gravitational wave events in any part of the electromagnetic spectrum, from radio, to optical, to gamma-ray, and who had made prior partnerships with LVC to observe in the indicated region whenever LVC issued a specialized private alert to them) to go ahead and follow up the alerts they were handed and, not too surprisingly to most, nothing was seen for any of the BBH mergers that were sent out.

While this was pretty much what was expected, there was still a lingering air of slight disappointment among those who put in a lot of time and effort to point their telescope in the right areas and look, just in case something might show up, unexpectedly.

And all the while, EM astronomers awaited the notification for something they could really "sink their teeth into"—a BNS alert thatwould indicate something they should expect to see. To get some sense of why, we can think of about a black hole as having a "soft" edge, properly called an "event horizon," the ravenous maw beyond which nothing ever comes back out. But in the intense distortion of spacetime that results in the final coalescence of two massive black holes 'mushing" into each other, we can actually picture this event horizon edge as being more like the boundary of a viscous blob of jello. Normally, this boundary is very stable and keeps a strictly spherical shape as relatively small bits of matter get sucked into the black hole, but when there is a coalescence event with another black hole (which, by the way, is the most energetic event in the entire observable Universe for about a fifth of a second while it is happening), the event horizon of the resultant larger "blob of jello" actually jiggles back and forth much like the clips of jiggly blobs of water floating weightless on the space station. (Technically, this phase of the gravitational wave event is called "ringdown," and is the very final little wiggle seen in the waveform before it flatlines, indicating settling down of the resultant object, and the existence of a brand new larger mass black hole in our Universe.)

Let's contrast this with what we know happens when big massive objects with hard surfaces, like planets say, smash together: clearly, there is a lot of heat, light, and generally sturm und drang with debris flying all over the where.  There is in this case also a perturbation of spacetime that will ripple outward as gravitational waves, but these are incredibly tiny, and have no chance of being detected by *any* foreseeable gravitational wave detector of our time.

Now we get to the "intermediate" case of neutron stars—these are objects with very hard surfaces. In fact, the most adamantine surfaces that we know of in the Universe. They are objects where the gravitational pressure has overwhelmed standard "atomic pressure" and crushed all atoms together, squeezing electrons and protons together so tightly that they all end up as neutrons, packed tightly together, like in atomic nuclei. This state is so dense that we will give the standard line here: one tablespoon of a neutron star weighs about as much as Mt. Everest. That's a whole lotta mass crammed in there.

So you can imagine that when these objects crash into each other, their hard surfaces slam together in a colossally cataclysmic crash, which in fact ends up with massive numbers of neutrons sprayed out into space, see a simulation of what this might look like e.g. here:

https://www.youtube.com/watch?v=g8s81MzzJ5c

It is the neutrons recombining into radioactive nuclei that then decay down to stable nuclides, emitting gamma-rays in the process, that cascade down into visible light photons that we ultimately see with our eyes.

It is a massively powerful event by any normal standards—however the primary issue as far as observing them is that compared to the BBH events, BNS collisions release much less energy in both gravitational waves and in EM signals, because the amount of mass converted into either of these signals is at least an order of magnitude smaller than for BBHs. And while one order less in signal would correspond roughly to these events being visible by LVC only to a tenth of the distance (in this case the reduction in reach is actually linear, vs. an inverse square law, because it is amplitude that matters in this case), this translates to a factor of 1000 less in volume that can be searched for signal events (because volume goes as a factor of distance cubed). And while overall in the Universe, we know there are many more neutron stars than black holes (because of the mass ranges involved, and our knowledge of stellar physics and the end states of stars), because of astrophysical uncertainties, we still do not knowexactly how many of these will be found in BNS systems that are imminently headed for a collision, and as no BNS collision events had been seen in either the first observing run of LVC (Sept 2015-Jan 2016), nor in the early part of the second run (Nov 2016–Aug 2017), most astronomers were feeling fairly glum about the possibilities, and assumed we would need to wait until the third run, starting sometime around the middle of 2018. This was especially true as the end of the second observing run approached, scheduled for Aug 25, 2017. I for one felt I was only being realistic that the gravitational wave detectors were not going to see anything new before the approximately year-long shutdown (for improving the detectors) closed in. I even commented on this during a DES-gravitational wave team videocon on Aug 14, 2017 when the fourth clear BBH gravitational wave event by LVC (this time including detection by the VIRGO detector in Italy) was sent out, saying we might as well use up the rest of our allotted special "target of opportunity” follow-up time for O2 on this event, since the likelihood of anything super-exciting showing up in the remaining week was fairly miniscule.

Shows just how pessimistic I can be!

Because astronomers who are on the joint “lv-em-observers” email list for alerts LVC sends out to its EM partners awoke the morning of Thursday, August 17, to a whole series of emails beginning with this message at 6.31am PDT:

The LIGO Scientific Collaboration and the Virgo Collaboration report:

The online CBC pipeline (gstlal) has made a preliminary identification of a gravitational wave candidate associated with the time of Fermi GBM trigger 524666471/170817529 at gps time 1187008884.47 (Thu Aug 17 12:41:06 GMT 2017) with RA=186.62deg Dec=-48.84deg and an error radius of 17.45deg.

The candidate is consistent with a neutron star binary coalescence with False Alarm Rate of ~1/10,000 years.”

There is a lot of technical language in there, but let's pull out the most essential bit, at the very end—the alert says this is clearly a BNS event (seen at 5:41am PDT on 8/17/2017) with such a high signal to noise ratio that it would be expected to show up in random data streams only once every 10,000 years.

In other words: sit up folks, this is real, no fire drill here.

Further, note the earlier words about the event being coincident with a Fermi GBM trigger in the overlap region—that is, the Gamma-ray Burst Monitor instrument on the Fermi Gamma-ray Space Telescope. To already, this event had been seen in gamma-rays by the morning of Aug 17 (in fact, two seconds after the gravitational wave event registered in LVC, which is in accordance with most models for a BNS). This was it—the very first time in human history that an event that resulted in gravitational wave emission had also emitted a much more "prosaic" kind of EM radiation we have long been familiar with.

Actually, this is exactly what we were expecting if the event were coming from a BNS collision, as it had long been theorized that these events resulted in a class of observations called “Short Gamma-Ray Bursts” or sGRBs. But no one had known for sure.

Thus, it was clear that it was an extremely promising event to be searched for in optical light as soon as possible.

And so it was that many major optical telescopes on the Earth eagerly anticipated nightfall on that Thursday night, to aim into the "localization region" that LVC and Fermi had narrowed down in the sky, in the constellation Hydra (the sea monster), to just several hand-spans worth of angular area in the sky, vs. the huge wide swaths which is all that could be done previously before any input from other detectors.

For DES, the team was led by Marcelle Soares-Santos, with major contributions by Jim Annis, Ken Herner, Dillon Brout, Zoheyr Doctor, Phil Cowperthwaite, Hsin-Yu Chen and a handful of others from the DES gravitational wave group. This team had worked long and hard on setting up the protocols and technology to rapidly follow up the LVC detections, working with the BBH events as practice—and on the morning of August 17, with the BNS announcement from LVC, it was now all systems go and all hands on deck. Dozens of emails rolled through the des-gravitational wave list, people communicating all over the US along with observers down in the control room at CTIO high in the Andes about where the telescope ought be pointed, what filters to use, time settings, software scripts and on and on—there was an intense sense of anticipation in the air.

They knew a region as large as the outside white ellipse in the below figure would need to be covered fully to find where the new "transient object" was. 

Then, at 5.42pm PDT (9.42pm in Chile, a couple of hours after darkness, and after most of the potential field had been scanned), Ryan Chornock wrote the DES-gravitational wave list with a message which has become known in some circles as the “Holy Cripes email,” prefacing his email with an exclamation, then saying:

Check out NGC 4993 in DECam_00668440.fits.fz[N5]

Attached is tonight's image + ps1-3pi.

Galaxy is at 40 Mpc.

R

And attached these two images:

That is—this long, long hunt of the last two years (and in some sense, the 50 before that, going back to the early searches for gravitational waves, and then the founding of LIGO itself), was at an end. We little humans on an insignificant little planet in some obscure corner of the Universe had done it again, dared greatness, banded together into collaborations with an ability to go above and beyond our individual selves and explore our vast Universe, reach for the stars, and "touch the face of God" for a brief moment in our lives.

For even just those of us not active in the search, but just watching the live emails come in, in those heady hours, there was a sense of shock, elation, flabbergasted amazement, and pure joyousness that it had really happened. These kinds of periods are rather few and far between in the lives of scientists. But this was one that will not be forgotten by those who shared in seeing it happen in real time.

In fact, within about two hours, some six major observatories saw it—booming out, bright as day, that dazzling point of light right next to that otherwise obscure and unremarkable elliptical galaxy.

Another image of it shows the DECam picture from that night on the left, and on the right the observation two weeks after, when the object had totally faded away:

Because galaxies generally have dim extensions with stars still bound to them far beyond their bright edge, astronomers associate things that close to the edge with the galaxy, but also try to look for spectroscopic confirmation when possible—that is, an independent determination of the distance of the object from lines in its spectrum that are shifted from its motion away (usually) from us.

Now, as to what this is all good for, even in astronomical terms, we could say first it is the very first time we are able to associate gravitational waves with the type of observations astronomers have been making for centuries—with the light we would see with our very own eyes (if close enough to the event, here on Earth we need to boost the signal with telescopes of course). And every time humans have opened a new way to see the Universe, we have discovered amazing new things: things that blow up, shoot out huge jets, flares, move in the sky, gulp down matter, hiccup, and even take long naps. Basically—there is a whole Universe of events constantly happening out there that are fascinating to observe, study, learn about in their own right to figure out the wild menagerie of possible incarnations of matter out there.

But, further, they also allow us to dig deeper into the structure of matter and in extreme states, and even the laws that govern our Universe. A few examples of this are the potential to study the neutron star equation of state which is actually ultimately connected to the study of quantum chromodynamics (i.e., the strong nuclear force, which is what binds protons and neutrons together inside of atomic nuclei, and also powers the Sun to shine). Another is using this event as a "standard siren" to extract the expansion rate of our Universe from a totally different method than has ever been used before, which DES is also releasing a paper about today, led by Daniel Holz, here. And perhaps most exotically, even to probe our best current theory of gravity, Einstenian general relativity (GR) in regions it has never been tested before (if interested in following a technical discussion see e.g. Sampson et al. 2014, Berti et al. 2015, or Sagunski et al. 2017 for three recent papers on this general subject).

Because one thing we feel very sure of is that this is an incredibly powerful tool that is that lets us probe the most profound secrets of the Universe -- and  somewhere, somehow, somewhen, very likely -- allows us to go even beyond Einstein.

Further reading

Shorter version of this blogpost, at DES DArchive site (10/16/2017)

Stanford Press Release on the BNS (10/16/2017)

LVC Press Release for the BNS event (10/16/2017)

Hulse Taylor binary (Wikipedia)

Rumors swirl that LIGO snagged gravitational waves from a neutron star collision (Science News -- 8/25/2017)

Q&A: Rainer Weiss on LIGO’s origins (MIT News -- 2/11/2016)

Gravitational-Wave Announcement Coming on Oct. 16: What Could It Be? (Space.com -- 10/5/2017)