Observing gravitational waves AND light from the same source

artist conception

It's been so exciting to see, hear and read through the physics news in the past week that I have not had time to write! More gravitational waves have been detected and this time telescopes have seen light emitted from what's believed to be the closest observed merger of compact objects to date. The collision happened 130 million years ago, when dinosaurs still roamed the Earth, in a galaxy from the Hydra constellation. LIGO has seen gravitational waves from this inspiral on August 17, 2017.  The event was made public on October 16 together with a suite of technical articles.

Edward and his goats

Those of you who know us might guess Andy has been pretty stressed out this summer, but also excited and proud. I was mostly in charge of the children, house repairs and the many animals we have acquired -- taking time off now-and-then to prepare a talk and a fellowship application. Our son, Edward, had a gravitational wave appear on his birthday (the binary black hole also seen by VIRGO) and the neutron star merger 3-days later. So, a picture of Edward next to that of the gravitational waves is somewhat appropriate. His 3-goats also sneaked in the picture. The announcement happened while we were celebrating Edward Seidel's 60th birthday.

What happened on August 17, 2017? The first light from this event was picked up by Fermi's Gamma Ray Burst monitor. Independently, LIGO Hanford saw the event, LIGO Virgo saw nothing, LIGO Livingston saw a huge glitch, which meant data would have been removed around it if there had been no event. The Hanford detector triggered. When the trigger was checked, they saw that Fermi has seen gamma rays some two seconds after LIGO saw the merger of the neutron stars.  Gravitational waves travel at the speed of light towards Earth to shake LIGO's mirrors, while light moves slower through air than vacuum (and is delayed when passing near massive objects), which makes it travel a bit slower. The short time delay meant it could be from the same event, but they had to find its location to be sure.

The waveform was long with the highest signal to noise ratio observed to date. It looked like the first neutron star - neutron star collision or the first neutron star - black hole collision was observed. The strength of the signal meant the colliding stars were close to Earth. So, it was important to get the localization as accurate as possible and look for other kinds of light with all telescopes that could see it.

After running a code developed by Andrew Lundgren at the LVC meeting in 2010 to remove huge  glitches, they uncovered the signal in LiIGO Livingston. The fact that Virgo saw nothing meant the event was near one of its blind spots.  It took four more hours of work to obtain sky localization. When the location was out it was not night in Chile yet, and so scientist spent time planning observations. 

Telescopes might have ignored  the event altogether if LIGO had not seen it because the burst of radiation from the colliding neutron stars was off-axis, i.e., it was not pointed towards Earth. This is why this very close collision was not bight in Gamma rays. It may be that we have missed other sources like this before. Yet only when gravitational wave observatories run for longer we'll be able to tell.

Days later Chandra saw X-rays at this sky location, and telescopes are still observing radio waves. So, stayed tuned for more!!! It will also take time to analyze and understand the data.

The US LIGO detectors are upgrading. If they succeed in reaching design sensitivity, they could see up to an event a week in a year or so. The down side is that we'll miss detections in this period. I wish they could let one detector running in the US and LIGO Virgo, while upgrading the other.

All gravitational waves signals seen to date (LIGO collaboration)

Where was Andy when the universe burped emitting gravitational waves? We were celebrating Edward's seventh birthday belatedly. So, Andy was in Chizatau - in Transylvania. He was watching his computer screen while cooking in our lightless kitchen. This kitchen has no window, but we can leave the door open for light. If the door is open, we can hear noise from the main road (lots of trucks) and the goats. So, the lightless state is at times preferred. I took the children swimming to let him work and cook. When we returned, he was very animated. Nothing was burning in the kitchen. So, while I knew he was not allowed to tell me until the official announcement, I thought this must have been the first gravitational wave LIGO has seen while we were in Chizatau.  We've seen gravitational waves from a black hole binary on August 14, but black holes are...well... black... and this time the universe thought we needed light!  

To remember: Let your children dream! Today one can observe the universe while cooking in Transylvania, and lead/work with a multi-national collaboration after-and-before goat milking! (Many thanks to LouAnne Lundgren for pointing this out).

Note:  The LIGO collaboration has about 1000 people -- many of whom I am proud to call my friends. Work was done in collaboration with observers and with theorists (more friends and people I respect) for this event. I don't mean to take credit or discredit work done by others. This is simply meant to be a fun personal post.