## Monday, October 16, 2017 ... /////

### LIGO-Virgo detects a collision of neutron stars

First simultaneous electromagnetic and gravitational wave observation

Please watch here, look at the LIGO Twitter account, LIGO web pages about the event (news, detection, chirp sound), and read a Nature paper or the paper on GRB 170817A in Physical Review Letters that was released exactly when the press conference began:

LIGO detected the collision first. Sadly, Virgo saw nothing. So they turned this fact into a virtue and concluded that the event had to be close enough to one of the blind spots of Virgo.

What's new is that this merger of neutron stars could have been detected both by the gravitational waves and the electromagnetic waves. The latter, electromagnetic detection was from Fermi-GBM as well as 70 observatories across the world. The electromagnetic wiggles came at all frequencies humans monitor.

Ms Cordova said "Oops, I did it again". She failed to cite Britney Spears.

David Reitze continued to talk. The collision took place on August 17th. Reitz showed a stolen golden watch and boasted that – he believes – the gold in it (just like uranium etc.) was produced in a similar merger of neutron stars, perhaps some billions of years ago. In this event, these elements are produced by rapid neutron capture.

The LIGO signal of the neutron stars was significantly longer than the signal for the black hole merger – they could detect something for over a minute! It's because the neutron stars are a bit larger geometrically, so they can't get as close. If you hear them at all, you hear them in much "earlier" stages – when it comes to their distance in the Schwarzschild radius units – which is why this stage is longer.

Another guy, David Shoemaker, talked.

Look at the time-frequency map at the bottom – it's so incredibly sharp and thin.

The collision took place 40 MPc away, plus minus some 20%. That's about 130 million light years away. So the event took place "130 million years ago" according to the cosmic time. The mass of the initial neutron stars were between 0.9 and 2.3 solar masses. That's lighter than most of the black holes. But because the source was the closest one so far, and because the signal was much longer, a minute, they could collect the much weaker signal over many periods and get a significant discovery. In fact, they collected it from 1500 orbital cycles! That huge amount of cycles allowed them to determine the parameters rather accurately.

Shoemaker says that we actually don't know what the object became after the merger – whether it's a neutron star or a black hole (or perhaps something else).

Jo van den Brand – an official from the European Virgo-covering bureaucracy – talked afterwards. The signal was closest to Virgo and they could locate it was in the Hydra constellation, as the rumors correctly predicted. While the signal in Virgo was weak and originally overlooked, the signal was nonzero and contributed some useful information.

Julie McEnery from Fermi says that the morning of August 17th started like another morning. They had a breakfast, ham and eggs, some corn flakes. She applied some lipstick etc. (I will omit this overly technical discussion). Then they saw some gamma ray burst which is boring, they see lots of those. But they immediately got some telephone calls from LIGO and within a minute or two, they knew what the great discovery was.

Ms McEnery emphasized that the gravitational and electromagnetic signals traveled for 130 million years but they came together within 2 seconds. That implies that the speed of electromagnetic and gravitational waves are the same within a huge precision, thus giving us another proof that Leslie Winkle, Lee Smolin, Sabine Hossenfelder, and all fake physicists of this kind is full of šit.

Another lady, Marica Branchesi, described the cooperation of the 70 observatories. So far, neutrinos weren't detected but it can change in the future. Because Hydra is in the Southern Hemisphere, telescopes in Chile were expected to matter. The observations continued for a week after the merger. The central object was unknown – but lots of matter was ejected around and could have been observed for a week after the merger. The Universe is enriched by gold and platinum and similar heavier "helements".

The next speaker had to be female as well, of course, especially because the Nobel prize winners were male again. Vicky Kalogera. She showed some pictures of spinning neutron stars and radio pulses coming from them (especially from pulsars). Kalogera nicely links this event to the Hulse-Taylor pulsar, the first binary system from which the gravitational waves were detected "indirectly". The Hulse-Taylor pulsar is much closer – but the components are much further from each other and won't merge for many many millions of years.

Kalogera also emphasizes the gold and platinum – I guess this is a P.R. strategy to make this physics really attractive. Elon Musk may send astronauts and in more than 280 million years, they will bring some gold from Hydra to us. ;-)

Seth Borenstein (AP) asked the first question: What was most surprising and what didn't fit? The X-rays seemed faint and delayed. Another, Asian ancestry journalist proposes automatic focusing of telescopes on the right places. Branchesi talks about robotic telescopes looking at "hobjects" but I don't think she really addresses the question. A woman from the Washington Post asks which other astrophysics questions may be answered with the multi-messenger astronomy (this event is the first "talking movie", as a metaphor puts it). David Reitze answers they want to see supernova explosions but good luck is needed – once in 15 years is the estimate.

An internet user asked whether one can estimate the frequency of such events from the observed amount of gold etc. in the Universe. Vicky Kalogera doesn't know everything that her electromagnetic partners are hiding in their pockets. But she says that there's a big uncertainty about the amount of gold produced per event. But the frequency of such events starts to be estimated now. Of course, in our galaxy, such events only take place once in tens of millions of years, so don't bet on such an event by Christmas.

A Greg from a radio asked about the final state. No firm knowledge, Shoemaker said. It could be a black hole (by the way, it would be the smallest black hole we know so far), some observers said, or a neutron star or a supernova? Branchesi tried to answer a question about the equation of state. Adam Smith asked whether the invisible hand of the free markets did something with the gold and platinum that was produced. ;-) They mostly recommend the later, second panel, but a few words about the "helements" were said, anyway, you know by whom.

Shoemaker emphasized that in late August, they declaread a One Year Vacation which should be used to increase the LIGO sensitivity by a factor of two – the visible volume could therefore octuple (eight times larger). The future is bright.

A break between 16:51 and 17:15, Prague Summer Time. That break was used for some LIGO/Virgo Q&A YouTube session to be answered by some young people. Let me mention that the Nature paper uses the event to estimate the Hubble constant – as 70 kilometers per second per megaparsec, fully consistent with other estimates (the error is some 10-15%). The Hubble constant – the expansion of the Universe – is measured in analogy with the "standard candles" (supernovae that always shine the same) except that in this case, one uses "standard sirens" (a cute term for the merging neutron stars i.e. kilonovas that always do the same sound via gravitational waves). These standard sirens, like standard candles, are combined with the speed extracted from the red shift.

The young folks said lots of things, including plans to build a LIGO in India and many others. Let's not forget about the space.

The second panel began at 17:15 my time, indeed. Jim Ulvestad of NSF told us that already "hundreds" of papers are already submitted, including 67 papers on the arXiv just today. Lots of observatories are involved. The multi-messenger paper has 3,500 authors, slightly beating both LHC and ATLAS separately. Hmm, I am not sure whether these 3,500 physicists feel special enough after this remarkable discovery – the number is high, indeed. No idea who could get a Nobel prize for this highly collective success of science. Laura Cadonati is showing some nice colorful spiraling simulations from GR.

Andy Howell of LCO Global (robotic telescopes) presents the neutron stars as really hard rocks. When they crash, it's a really harsh collider that beats the LHC. ;-) He said that "there really was a pot of gold at the end of the rainbow." Ryan Foley said that when he heard it from a student, he thought it was a prank. But after some time, he saw it was real so he went to save science on his bike. After several images, he identified the NGC 4993 galaxy because some new source was added on a picture relatively to one year ago. Charlie was the first man who saw the optical photon from a gravitational wave, whoever is Charlie. ;-)

Ms Marcelle Soares-Santos talked about their task to find the source – which is faint to start with and fading. It's just like looking for the needle in the haystack except that the needle is fading and the haystack is moving. ;-) Some trick with overlapping exposures helped. She did it even though she's actually a cosmology – dark stuff – expert. OK, they eliminated 1,500 needle candidates. And they're certain now. David Sand says that a 16-inch robotic telescope in Chile normally looks for supernovae but it got a new task. They saw this "kilonova" – a sibling of a supernova (others have used the word before; they can see a gradual change through all colors). Nial Tanvir talks about the "fireball" that was created. Do we believe this theory? He described that they picked the infrared where a longer event (relatively to visible light) was expected – three weeks of observation. The theory passed the test with flying infrared colors. ;-)

Edo Berger of Harvard says that this was the most quickly fading known transient in astronomy. Dozens of Earth masses of gold were created. A whole history of the production of all elements was seen in that event in one way or another. Eleonora Troja of NASA's Goddard center (whose boss is, outrageously, Gavin Schmidt who has no clue what's going on today) says that they immediately pointed out their UV and X-ray telescopes and Swift only saw UV, no X-rays. They waited for X-rays, anyway. Her team used Chandra but they weren't guaranteed to see anything, it was a lottery ticket. They won the lottery – Chandra was the only one that could see the weak signal and it did. She believes that the X-rays prove that there is a black hole at the end. After some time, the Sun got in the path and they didn't know how to shoot the Sun away to continue their observations.

Alessandra Corsi of LIGO saw some dot – in her radio telescope, too. If you were listening to radio, you could have heard the neutron star merger, too. They're still looking there (again, the merger was seen on August 17th). I guess that the lower frequencies are generally more long-lived after the merger. So these new observations tell us about the interactions of the jet etc. with the interstellar environment. Corsi ended with a trained monologue ending with "welcome to the era of multi-messenger astronomy".

Borenstein suggests that without LIGO, it wouldn't be multi-messenger. Indeed. ;-) Has there been something similar minus the gravitational wave detection? The answer is basically No – it's still first – because they didn't know where to look. Second question, from the Asian ancestry man: How bright is the galaxy? Some 1,000 times fainter than what is seen by the naked eye. How much time they lost with some telescopes? Up to 11 hours. Edo Berger also says that it's a similar-size as ours but elliptic, not spiral, and the stars there are therefore older. Ulvestad says that they were lucky because both the LIGO upgrade and the Sun in between could have made the observation impossible.

Some questions repeated what has been said – and written above. Someone obviously asked how to exploit the black hole and how to pick the precious metals over there. ;-) The electromagnetic young folks say that they must be grateful to the gravitational wave industry – more people and students will be joining, too. After all this celebration of LIGO, a journalist asked why LIGO was useful at all, oops. They just wouldn't focus on this GRB because it didn't look exceptional. The gravitational waves made them know it was a neutron star merger.

This blog post was written spontaneously, during the press conference, and updated many times. Some imperfections follow from this violent origin of the text. I don't plan to edit it too much. If I wanted to make it more organized, I would probably prefer to start from scratch LOL.