LIGO-GW170817

LIGO and Virgo found a merging binary neutron star; told astronomer colleagues where to point their telescopes; and those colleagues found a flash of light, consistent with what we’d expect from such a merger: a radiation from a small amount of hot, cooling radioactive material ejected at high speed. And a (very faint) gamma ray burst, with an X-ray and radio afterglow.

Superlatives fail. This event the Rosetta stone we’ve been waiting for. Now we can connect the investigations conducted over decades in many mostly-independent fields of astrophysics. And it’s a triumphant beginning to the era of multimessenger astronomy. We expect to find many more events like this, soon. Using the census of events we obtain, we’ll know how these cosmic explosions work (e.g., some will be bright, some not, depending on whether they ‘point’ towards us or not and just what kinds of NS take part in the merger). Combined (let us not forget!) with our growing census of black holes, we will be able to deduce how these objects can be formed from the lives and deaths of massive stars.

For those keeping score at home, let’s count how many discoveries are wrapped into one:

* Discovering gravitational waves from a merging neutron star: Neutron stars (and even pairs of neutron stars that will eventually merge) have been seen many times before.  A Nobel prize was even already awarded for observing the (slow!) inspiral of a galactic neutron star binary.  But this is the first time we've seen the waves directly, over about 100 seconds of rapidly-accelerating inspiral and finally merger.

* Discovering  (faint) gamma ray burst from a (close by!) merging neutron star: Now we know NS mergers are responsible for (at least some) short gamma ray bursts.  Despite being so close -- the closest ever found by far -- it is also  weak (as a gamma-ray burst).  Followup observations by our EM partners (e.g., the X-ray and radio emission) suggest we're seeing it off-axis.   We're finally getting the opportunity to test these burst models in detail.

* Nuclear physics:  We can't (yet) use GW alone to confirm matter is present, but we can rule out a few equations of state. We can estimate how much of that radioactive material is ejected quickly and forms gold and other heavy (r-process) elements. Based on how often these occur, we expect NS mergers  can form most of these elements.

* Cosmology: Gravitational wave sources are "standard candles", with known (but orientation-dependent) brightness.  We can use this standard candle to  measure the distance to the source and hence the expansion of the universe,  independently checking a century of work going back to Edwin Hubble!

* Astrophysics: By finding it, we know they occur fairly often: at least as often as we expected from our own galaxy, possibly more.  EM partners found it near  an old galaxy (as has been seen before with GRBs), suggesting it was born long ago and may have received a large "natal kick" when the star exploded.

For experts: Plans are useless but planning is essential. In low latency, search and sky-localization experts handled a strong glitch 1.1s before coalescence with aplomb, combining tools to provide a skymap quickly, when it mattered. That said, handling a loud glitch like this was still a pretty novel challenge for parameter inference, let alone on this timescale, and we’re not done with this event by far. In my personal opinion, I expect we (the LVC) will publish mildly refined parameter estimates and new tests of GR soon, when we have better control over systematics (e.g., a careful study of glitch subtraction and measuring tides in the strong field and modifications to gravity), but we can only do so much in 2 months.

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For more information

• GW170817 summary page, a one-stop location summarizing LIGO results on this unprecedented discovery, including our youtube channel showing representative simulations of merging NSs

• Papers from LIGO (papers.ligo.org), and many, many, many colleagues, which you can find summarized on the arxiv, ApJL, Science, and Nature.

• LIGO Open Science Center: anyone can delve into the data themselves.