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.

CLICK TO ENLARGE!
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]

Second Anniversary of the First Detection and a New Job!

While posts here have been long awaited, I've been busy doing research, teaching, and changed my job!  I've just started a new job as an assistant professor at Villanova University and an academic job search is quite intensive (perhaps I will write about finding a faculty job soon).  Also, I've been writing some other things.  I've written a TED-Ed video/lesson on gravitational waves and it premiers today (you can view it here), I have a PhysicsWorld Discovery text on "Gravitational Waves" coming soon, and have written a few other things here and there.  So, now that I've moved from Baton Rouge to the suburbs of Philadelphia, I have some time to talk with all of you again.



ANNIVERSARY OF GW150914:  WHERE IS IT NOW?

Today is the second anniversary of the first detection of gravitational waves.  That got me thinking of about where the front of that wave is now...

The colored area on this map shows the most probable source of the detected gravitational wave where red is more likely than purple.  The location is shown against a map of the night sky centered on the Milky Way galaxy with constellations outlined.
[Credits: NASA Deep Star Maps (Visualization Credits, Ernie Wright (USRA): Lead Animator, Tom Bridgman (GST): Animator) by NASA/Goddard Space Flight Center Scientific Visualization Studio with constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg), and the source location based on Gravoscope screen grabs (LIGO & Nick Risinger, skysurvey.org), all in galactic coordinates. Composition by University of Florida / S. Barke.]

The source of GW150914 was from the general vicinity of the constellations Volans and Carina.  That means that it is traveling towards the stars in the constellation Draco.  It hasn't encountered much.  Since it has traveled a distance of 2 light years from Earth, it is still in our Milky Way galaxy (the radius of the Milky Way 60,000 is ly and its disk is 2000 ly).  It has not encountered any other stars (the closest star in Draco is Struve 2398, a binary system of red dwarf stars 11.6 ly away) and therefore no other planets.  That means that no other life forms have detected GW150914 and won't reach Struve until early 2027 (give or take for the error in our understanding of its distance).


THE THIRD DETECTION: GW170104

Since I wrote last, we announced the discovery of a third detection of gravitational waves from another binary black hole system dubbed GW170104.  A black hole 32 times the mass of our Sun merged with another black hole 19 times the mass of our Sun resulting in a single 49 solar mass black hole after radiating away 3 solar masses in gravitational waves.  Among other things, this detection helps to fill in the range of masses we've observed; gaps would imply that there is something preventing the formation of those kinds of systems and that would be unexpected.

Graphic representation of the known stellar mass black holes observed through X-ray observations (purple) and gravitational waves (blue).  [credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)]

TESTING GENERAL RELATIVITY 

This system originated from about 3 billion light-years away: farther out in the universe than any of our previous detections.  Using a gravitational wave that has traveled such a distance allows us to test a part of general relativity that we haven't been able to before: do gravitational waves disperse?  For example, when white light enters a prism, the different colors (frequencies) of light travel at slightly different speeds causing them to separate or disperse.  General relativity predicts that different frequencies of gravitational waves should not disperse.  There are alternate theories of gravity that make predictions of how dispersion will affect a gravitational wave.  We compared our observation to standard general relativity and the alternate theories' predictions and found our observations to be consistent with general relativity.  That is, we did not observe any significant dispersion of our gravitational wave!  We've also done all of the tests of general relativity that were done for the previous detections and this gravitational wave continues to affirm that general relativity is correct.

MEASURING SPIN TO INVESTIGATE BLACK HOLE FORMATION

Artist's conception shows two merging black holes similar to those detected by LIGO. The black holes are spinning in a non-aligned fashion, which means they have different orientations relative to the overall orbital motion of the pair. LIGO found hints that at least one black hole in the system called GW170104 was non-aligned with its orbital motion before it merged with its partner. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

We also were able to investigate the spin of the black holes.  For as mind-blowing as a black hole is, it is completely described by three numbers: 1) its mass, 2) its spin, 3) its charge (this is postulated by the no-hair theorem).  Since we believe the electric charge of astrophysical black holes to be negligible, physicists get very excited about the mass and the spin.  The mass is not too hard to measure (we can get that pretty accurately from the waveform shape and frequency evolution), but the spin is all in the details of the signal which makes it more difficult to estimate.  But we always still mention it because any information we get about it will tell us more about the other half of the back hole's story.

For this detection, we were able to extract information about how the combined spin of the black hole system compares to the direction of its orbital angular momentum.  Basically, does the direction the effective spin of the black holes (on their internal axes) go in the same direction as their orbit?  For example, the Earth spins on its axis in the same direction as it orbits the Sun, so this is a positive alignment, but the Earth's axis is tilted so it isn't a perfect alignment.

If most of our black hole systems have small misalignments that would support their formation through something we call "common envelope evolution" (I wrote about this here), which is a complicated way of saying that the stars that formed the black holes were always paired together and once they both died you end up with a binary black hole system.  The interactions between those original stars will cause their spins to align giving the resulting binary black hole system only small misalignments.

Another formation mechanism for binary black holes is that they just happened to pass one another while drifting through space and became gravitationally bound together (this is called dynamical assembly).  We expect things like this to happen in dense stellar clusters or near the centers of galaxies.  Since these would have had no interaction with each other before they became a system, we expect random spin alignments from the black holes.

Ultimately we found that our system likely had a low total spin and was likely not aligned with the orbital angular momentum of the system.  It is also possible that our black holes had no spin to begin with.  So this isn't definitive, but we are starting to assemble the story of how black hole systems like this form.


THE BIG PICTURE

With this discovery, we are adding to our understanding of the universe and testing general relativity in ways we've never been able to before.  The importance is that LIGO is truly operating as an observatory (that's what the 'O' in LIGO stands for after all) and building a database of observations.  In astronomy, you can never make a single observation and understand a system's history.  You need to take a large sample of observations from similar systems and find out what the patterns are.  That's how we understand how stars evolve since a human life, or even all of human existence, isn't long enough to have followed a single star's life cycle.  But because we have observed many stars in different stages of their lives we've discovered patterns.  That's how we know that the black holes we just observed are the collapsed corpses of the extremely massive stars.  Now we can collect observations of many of these black hole systems to learn more about black holes in general and how these pairs of black holes form.