First Observation of a Neutron Star with Gravitational Waves and Light!

THE OBSERVATIONS

At about 8:41 am EDT on 17 August 2017, both LIGO observatories recorded a 100-second long gravitational wave (GW) signal that appeared to be coming from two neutron stars orbiting each other and merging together (this detection is called GW170817).  1.7 seconds later, the Fermi Gamma Ray Burst Monitor detected a short gamma-ray burst (GRB).  LIGO's online data analysis, along with Virgo not detecting the event due to its location in a part of the sky it isn't sensitive to, narrowed down the possible location of the gravitational wave source to the constellation Hydra (in the Southern Hemisphere) which overlapped with the area determined from the GRB detection.  Both the LIGO and GRB detectors put out circulars alerting the astronomy community to the independent discoveries; an event like this was unprecedented and became a priority target for observation.  LIGO also produced a luminosity distance (a method of estimating the distance to the source) of about 130 million light-years.

11 hours after the gravitational wave and GRB detections (the delay was caused by the time it took for the Earth to rotate observatories in South America into their nighttime sky), the 1-m Swope Telescope in Chile observed a new point of light (referred to as a transient) from galaxy NGC 4993, also about 130 million light-years away!  Approximately 70 other observatories, both on our planet and orbiting it, observed this new source of light: 11.5 hours after GW/GRB detection, infrared light was observed; 15 hours later, ultraviolet light was observed; as the days went on, the optical light from the source became redder (longer wavelength) and dimmer than the early days; 9 days later, x-rays were observed; 16 days later, radio waves were observed.

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Left: GRB data (top) collected by Fermi and gravitational wave data (bottom) collected by LIGO.
Right: Source localization on the sky from the INTEGRAL GRB satillite (light blue band), Fermi (the dark blue disk), LIGO alone (green ovals), and LIGO and VIRGO data combined (dark green oval).  Notice that all sources identified the LIGO-Virgo area.  A new source of light (located at the intersection of the tic marks) was identified inside the overlapping area by the Swope telecope (top image inset). The image below the inset is of the same area but 20.5 days before this event showing that this source was not present. [Credit: LIGO, Virgo, Fermi, Swope, DLT40]

TURNING OBSERVATIONS INTO MEANING

The cause(s) of short GRBs have been the subject of much theoretical research; the predominant theory is that they are caused by the merger of two neutron stars or a neutron star and a black hole.  The problem is that light isn't capable of bringing us information about these kinds of systems.  So, we've seen many short GRBs, we have ideas about how they are made based on these observations, and have been waiting for evidence to either support or refute the theory.  Fortunately, gravitational waves bring us the information about the system that light can't, like the masses of the objects and how they moved around each other.  This is the first conclusive evidence that at least some of the short GRBs are created by neutron star mergers.

This visualization shows the coalescence of two orbiting neutron stars. The left panel contains a visualization of the matter of the neutron stars. The different colored layers are different densities, which have been made transparent to show more structure. The right panel shows how space-time is distorted near the collisions. The spiral wave distortions at the end of the merger propagate to Earth and are measured as gravitational waves. [Credit: (taken from ligo.org) Christopher W. Evans/Georgia Tech]

Light observations allowed us to observe the rapid fading of the brightness and gradual lengthening of the wavelength (reddening) of the light which is a signature of a kilonova explosion.  Kilonovae are also thought to be the source of most of the heavy elements in the universe.  (I have previouslyattributed these to supernova here; further research has shown that while a supernova can make some heavy elements, it can't make enough of the heavier ones like gold to account for the amounts we have.)  Breaking the light down into its different wavelengths (colors) tells us about the composition of the source or of elements being created.  Measurements like this showed that heavy elements were indeed created as predicted in this kilonova and supports that neutron star mergers may very well be the source of most of the heavy elements in the universe.

Periodic table indicating the sources of the elements.  Orange indicates formation from the merger of neutron stars like the source of GW170817. [Credit: Jenifer Johnson]

The timing of the arrival of gravitational waves and the gamma-ray burst (1.7 seconds later) are our strongest support yet that gravitational waves travel at the speed of light.  Detecting gravitational waves first is not unexpected since this system has been producing gravitational waves its whole life and we only saw the last 100 seconds of it.  It is the dynamics of what happens during the merger that produces the light in the form of a GRB, so we should expect a delay.  Even when considering different possible ways and times for light to be made in a system like this, our observations are consistent with the prediction that gravitational waves travel at the speed of light.


WHAT DID THIS NEUTRON STAR MERGER CREATE?

A remaining question that the observations we've made hasn't been able to answer is: What did this neutron star merger create: a very large neutron star or a black hole?  We simply don't know and the reason why is that we don't have a firm understanding of the equation of state (EOS) for a neutron star (EOS is a technical term for describing matter and how it behaves).  Depending on different possible EOSs, we can get either a small black hole or a very massive neutron star at some time after the merger.  We also looked for gravitational wave evidence of which it is since a small black hole would have produced gravitational waves at about 6000 Hz and a very massive neutron star would produce them up to 4000 Hz.  The LIGO detectors are not very sensitive at high frequencies, making finding evidence for a resultant black hole impossible.  We did search for gravitational waves consistent with the formation of a very massive neutron star until the end of the run on 25 August (8 days later) and didn't find anything.  We found nothing to support either possibility so we simply don't know!

The masses of stellar remnants are measured in many different ways. This graphic shows the masses for black holes detected through electromagnetic observations (purple); the black holes measured by gravitational-wave observations (blue); neutron stars measured with electromagnetic observations (yellow); and the masses of the neutron stars that merged in an event called GW170817, which were detected in gravitational waves (orange). The remnant of GW170817 is unclassified, and labeled as a question mark. [Credit: LIGO-Virgo/Frank Elavsky/Northwestern University]

THIS IS JUST THE START!

This is the true beginning of multi-messenger astronomy!  As was referred to in this post, gravitational waves, light, and neutrinos (looked for during this event but none were found) bring us different information about the universe.  Gravitational waves tell us about how mass moves around and how much of it there is; light tells us about temperature, and composition; neutrinos can bring us information about the nuclear reactions happening deep within a star.  The effort to make and share all of these observations require not just scientific knowledge, but cooperation on a large scale.  There are many things that divide us in our societies; this is something we should be proud to unite us!

Artist’s illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted. The clouds glow with visible and other wavelengths of light. [Credit: NSF/LIGO/Sonoma State University/A. Simonnet]

SNEWS and LIGO: Neutrinos Tell of Possible Gravitational Wave

When I start off a tour at the LIGO Observatory, I usually start by talking about how gravitational waves will open a new window to view the Universe.  I've done this so many times that I have the talking point pretty much memorized:

"Up until recently, we've only been able to observe the Universe using light and its different forms.  Visible light, X-rays, and microwaves are just a few different kinds of light and every time we have looked at the Universe in a new way, we discovered something unexpected that revolutionized our understanding of the Universe.

Well, light has the inconvenient property of being fairly easily absorbed or reflected away from its path.  However, the Universe is transparent to gravitational waves; meaning that they can go through matter and come out the other side unchanged.  There is no such thing as a gravitational-wave shadow!"

Note that I start by saying that until recently all of astronomy has used light as its tool.  This is because there is another medium that has been used: neutrinos.  I've talked a bit about neutrinos previously here, namely when I discussed the debunking of the "faster-than-light neutrino" claim last year and how neutrinos are used in multi-messenger astronomy.  Quoting the important part from the multi-messenger astronomy post:

"Today, we can do astronomy with means other than light.  For example, neutrinos.  These are subatomic particles that have no electric charge, have nearly no mass, travel very near the speed of light and are able to pass through matter almost undisturbed.  However, these properties add up to make it very hard to detect neutrinos (did you know that there are billions of neutrinos from the Sun passing through your body every second?!).  Neutrinos are also emitted when a star dies in an explosion called a supernova.  That means we may observe the optical burst of light AND the neutrinos from a single supernova.  Any time that you can observe the same event in multiple ways, you almost always learn more than if you only observed it one way."

- 14 October 2010

What I didn't go into is that LIGO is working to detect gravitational waves from a supernova as well.  While we can do this without complementary detections from traditional and neutrino observatories, having that information from them will make it easier for us to find the signal buried under the detector noise that dominates what LIGO records.  This is done through the Supernova Early Warning System (SNEWS).

SNEWS

Yes, this is pronounced just like you pronounce "snooze".  A SNEWS alert is sent out shortly after neutrinos from a supernova (as opposed to neutrinos from the Sun) are detected.  Since neutrinos and travel through matter with very little disturbance just like gravitational waves, that means that if we saw neutrinos, there is also a good chance that we may see the gravitational waves from that event.  Even more compelling for us in the gravitational wave community is that SNEWS only really expects to see neutrinos from a supernova if it came from within the Milky Way galaxy (or the Magellanic Clouds that are two small galaxies that orbit the Milky Way).  As far as gravitational waves are concerned, that is in our own backyard so any accompanying gravitational waves would likely be large enough for us to detect!   (When searching for gravitational waves in general, we expect that almost all of our sources will come from galaxies outside of the Milky Way.)


EARLY WARNING SYSTEM?

The reason that the detection of neutrinos is considered an "early warning system" for a supernova is that the processes that produce these neutrinos happen hours to days before the optical explosion that traditional observatories would be able to see.  The supernova explosion occurs after the mass of the star collapses in on itself; this is called a core collapse.  Neutrinos are normally produced by the nuclear fusions inside the star (our Sun produces MANY all the time), but during the core collapse many more are produced (it is estimated that over 90% of the energy in the collapse is expended as neutrinos).  It is also during the core collapse, when so much of the star's mass is in motion, that gravitational waves are produced.  If there is a SNEWS alert, that means that there is a higher probability of a gravitational wave detection at that time.


WHAT HAPPENS AT LIGO DURING A SNEWS ALERT?

First off, let me say that there has not been a SNEWS alert yet since these supernovae in our galaxy are rare (they happen about every 50 years or so that we are aware of).  But if a SNEWS alert comes through while LIGO is looking for gravitational waves, the protocol is quite simple: don't do anything that would cause the quality of the data to be degraded.  More specifically, don't create vibrations.  Don't walk close to the detector (your footsteps appear to be little earthquakes to the detector), don't leave the site in an automobile (any acceleration by that a car or larger vehicle will create a little wave in the ground that will affect the detector).  This has brought up the question of what to do with the FedEx guy if he is on site making a delivery...  While we cannot hold anyone against their will, I am sure that he would be asked to stick around for a while. 

This may sound a little harsh, but considering the rarity of these events and what is to be gained, sitting around isn't all that bad!


WHAT IS TO BE GAINED?

First, to the traditional astronomy community (telescopes detecting light), it is exceedingly rare to see a supernova from its beginning and doing so can tell astronomers more about what kind of supernova it is (see this Wikipedia page for more information about the different types of supernovae).

Also, if the gravitational waves from the core collapse of a star were to be detected, this will allow us to "see" what went on inside the star - something that can never be done with traditional astronomy.  Knowing what goes on inside the star will allow us to use the dying star as a nuclear reactor unlike any we could ever create on Earth.  This may be able to tell us more about nuclear physics which could have implications for technology in the future (I have no idea what those may be).


NEUTRINOS AND SUPERNOVA IN THE PAST

So far, there are only two detected sources of neutrinos other than those produced by nuclear reactions on Earth: those from the Sun and those from the supernova known as SN 1987A.

NASA image of 1987A supernova remnant near the center.  Inset: a close up of the supernova  [Source: Wikipedia]

SN 1987A happened on 23 February 1987 (hence the name) and was located in the (relatively) nearby Large Magellanic Cloud and could be seen from the Southern Hemisphere.  About 2-3 hours before the star exploded (as seen from Earth), neutrinos were detected at 3 different neutrino detectors.  This detection not only was the birth of neutrino astronomy, but also allowed for the early observation of the light from the supernova.

Also, this supernova is thought by some to be the instigator of the LIGO concept.  This was when Joseph Weber made his claims of the first detection of gravitational waves (which was debunked - but that is a discussion for another blog post).  Weber used a method of looking for gravitational waves called a resonant bar gravitational-wave detector (a.k.a. Weber Bar).  Even though there wasn't a gravitational-wave detection, his claims and SN 1987A made scientists begin to consider other way to look for gravitational waves and that the technology needed was within reach.  So, that February day in 1987 was also the birth of LIGO in a way!

No "Faster Than Light" Neutrinos

SCIENCE AS A PROCESS

Most people see science as purporting itself to be infallible and they can twist this perception for many reasons (e.g. "See, they didn't see what they thought they saw so science cannot be trusted.").  The truth is that science is a process.  It must be reproducible by others.  Sometimes, an experiment comes around that seems to defy the current understanding of science and people are quick to jump and accuse science of being unreliable.  Really, when results like this come to light, it is the duty of other scientists to scrutinize the results: to try to reproduce them and, if they cannot, try to find where the errors in the original experiment occurred.  Most of the time, radical findings are disproved.  When they are not, this is an exciting time for science to learn more about the world around us!  We scientists often spend as much time trying to disprove things as we spend trying to prove them.  Truly revolutionary results often exploit a subtlety in a theory (which in science means a highly tested and verified description of how something works and NOT a hypothesis or guess as it is sometimes used in everyday language) or law that opens the way to a deeper understanding.  Science is not created or invented by scientists - the Universe has its properties and we simply pursue the discovery of them so we can understand better how it works.

THE "FASTER THAN LIGHT" NEUTRINOS

While at the APS April Meeting this past week, there was a lot of excitement (see the talk abstracts in this session) about the "faster than the speed of light" neutrinos that the OPERA collaboration claimed to have observed.  There was extra excitement since there was a final resolution at the beginning of the meeting along with a little drama.  There were even talks on how to use this new coverage as a great outreach opportunity to illustrate science as a process (don't think of the scientific method that you were taught in school - science almost never follows that prescription but it is a good starting point).  I've had many people bring this up to me when I talk about how gravitational waves are expected to travel at the speed of light but could travel slower - never faster.  Then there is usually someone who asks about the new neutrino results and this is when I get to talk about how science is a process.  So, I've decided that I would dedicate today's blog post to the subject matter.  Spoiler alert: there are NO "faster than light" neutrinos!  If you are interested in a very good discussion of these results, disproof, and aftermath, read more about it here.

***  What is a neutrino?  ***

A neutrino is a virtually massless particle that interacts so weakly with matter that it can travel right through any matter with only a few (of billions and billions) interacting with matter.  The neutrino has never been directly detected but we know when one interacts with matter because it produces other subatomic particles or radiation.  Every second, about 10,000,000,000,000 (that's 10 trillion) neutrinos from our Sun pass through every square foot when the Sun is directly overhead.  Those neutrinos pass right through you and, since they so rarely interact with anything, you don't notice a thing. 

Because neutrinos are virtually massless (I say virtually because there is evidence they they do indeed have mass, but it is so small that it hasn't been accurately measured) they can travel at or so near the speed of light that we haven't measured evidence of them traveling slower.  This agrees with special relativity: only massless particles can travel the speed of light and massive particles can only travel slower (there are theoretical particles called tachyons that can only travel as slow as the speed of light and travel faster otherwise - these have never been observed).

***  What is the OPERA experiment?  ***

The OPERA experiment used a beam of neutrinos created at CERN on the Franco-Swiss border to send to the OPERA detector in Gran Sasso, Italy.  That's right, the beam of neutrinos was shot right through the intervening earth between these 2 sites.  Since the distance is known to high precision, the time it takes the neutrinos to arrive at OPERA is directly related to their speed.  It appeared that they were measuring their arrival about 60 nanoseconds (0.00000006 seconds) before they should have if they traveled at the speed of light. 

***  What did we know about the speed of neutrinos before OPERA?  ***

There have been many experiments that have observed neutrinos traveling at the speed of light.  These experiments have been both Earth-sourced (where we create and then detect the resulting neutrinos) and Universe-sourced.  A spectacular example of using neutrinos from space was the detection of neutrinos that preceded the supernova 1987a.  They arrived 3 hours before the light from the stellar explosion did.  This is what is expected because neutrinos are created when the matter in the star collapses before the supernova explosion.  If neutrinos traveled as fast as the OPERA collaboration claimed to have observed them traveling, then after traveling the more than 160,000 light years to Earth they would have arrived 4 years before the accompanying light we observed.

***  Should OPERA have published their result?  ***

So, was the OPERA collaboration wrong to publish their observations?  Absolutely not (in my opinion at least)!  Nowhere in their paper did they claim that they have found a fault with the current understanding of the physics - they simply couldn't disprove their own observations so they opened their experiment up to the scrutiny of the scientific community.  They even recognize the controversial results and their desire for scrutiny of their experiment in their paper (which can be read in full here):

"Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results. "

THE RESOLUTION TO THE CONTROVERSY AND THE FALLOUT

In the end, it was found that a loose fiber optic cable and an error in their timing produced the superluminal (fancy way of saying 'faster than the speed of light') observations.  THERE IS NO EVIDENCE TO SUPPORT THAT NEUTRINOS CAN TRAVEL FASTER THAN THE SPEED OF LIGHT.  Also, the ICARUS experiment (located in Gran Sasso with OPERA) independently reproduced the experiment and found no faster than light neutrinos.

The heads of the collaboration resigned their post on March 30 (just a few days ago) after a vote of no confidence.  There were scientists in the collaboration who felt the publication of the results was premature, and that not everything that was done was good experimental procedure.  It seems that the resignations were the result of their rush to publish the paper, more than what they published.

Multi-Messenger Astronomy

Right now, I am getting to participate in the growing field of multi-messenger astronomy.  Yes, right now!  "How can you possibly write a blog post while doing this exciting science?", you ask...  Well, because there is a lot of waiting involved.  But before I get into exactly what I am doing right this second, let me tell you a little bit more about multi-messenger astronomy...

Humans have been doing astronomy from the dawn of time by simply looking into the night sky and observing the stars and the Moon and the Sun.  But the largest advances in astronomy have come when we've devised new ways of observing our Universe.  Galileo used a telescope for the first time to discover moons around Jupiter, phases of Venus, and Sun spots.  All of these observations add up to prove Copernicus' theory that the Earth is not the center of the Universe (and this proof ended up landing Galileo under house arrest for the rest of his life).  In more modern times, we've observed the Universe with different forms of light: we've discovered pulsars when we observed the Universe with radio waves, we discovered the Cosmic Microwave Background (CMB) the first time we turned on a microwave telescope (which won the 1978 Nobel Prize in Physics for evidence supporting the Big Bang) and we discovered extraordinarily energetic gamma-ray bursts from deep space (by accident when satellites orbiting Earth were looking for gamma rays from atomic bomb detonations) whose origin we still don't fully understand.  The moral here is that every time we looked at the Universe in a new way, we discovered something we didn't expect to that revolutionized our understanding of the Universe.

Today, we can do astronomy with means other than light.  For example, neutrinos.  These are subatomic particles that have no electric charge, have nearly no mass, travel very near the speed of light and are able to pass through matter almost undisturbed.  However, these properties add up to make it very hard to detect neutrinos (did you know that there are billions of neutrinos from the Sun passing through your body every second?!).  Neutrinos are also emitted when a star dies in an explosion called a supernova.  That means we may observe the optical burst of light AND the neutrinos from a single supernova.  Any time that you can observe the same event in multiple ways, you almost always learn more than if you only observed it one way.

LIGO has been participating in multi-messenger astronomy for years by looking for gravitational wave counterparts from observed gamma-ray bursts (observing gravitational waves from a gamma-ray burst would tell us much about their mysterious origin).  Now, we are also participating by analyzing our gravitational wave data in real time to notify several partner telescopes when we think we may have detected something.  If we could observe an optical counterpart to a gravitational wave detection, not only would that be a gold plating on a first detection but we would be able to combine the information learned from both means of observation to have a deeper understanding of what we just detected.

So, what I am doing right now is waiting for a candidate gravitational wave to come through LIGO's data analysis pipeline so that I can look at how the detectors were working and make the decision on whether to send this event out for optical observation (those of us who specialize in this take 8 hour shifts for continuous 24/7 coverage).  Since LIGO has not made a direct detection of gravitational waves yet (and we didn't expect to - more on that in another post) we expect everything we send out to be a false alarm and our telescope partners know that.  But the potential of an event being real... well, that's why I do what I do; I want to be on the front lines when gravitational wave astronomy revolutionizes the way we understand our Universe!

P.S.  The is the view outside my office window right now.  The building to the left contains the input, output and corner optics of LIGO (LIGO is an 'L' shaped detector).  The concrete tunnel that goes off to the right and out of the frame is one of the arms of LIGO (the other arm comes out of the back of the corner building from this view and through the area of no trees in the back - you can see it if you look close).  Each of the arms are 4 km long (in our everyday units in the United States, that is about almost 2.5 miles).  The white silo in the middle of the view is filled with liquid nitrogen (at a temperature of −321 °F) that we use to help maintain the vacuum inside of LIGO (there is over 300,000 cubic feet of vacuum at one trillionth the atmospheric pressure inside of LIGO).