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.

Merry Christmas, LIGO: Another Gravitational Wave!

WE DETECTED ANOTHER GRAVITATIONAL WAVE!

On the evening of Christmas day 2015, at 9:38 pm CST (3:38 am UTC) at the LIGO Livingston Observatory in Louisiana, another gravitational wave signal was recorded.  1.1 ms later, the LIGO Hanford Observatory in Washington state also picked up the same signal.  70 seconds later, the supercomputer that runs analyses on the near real-time data noticed that there was something special in the data and sent out emails and text messages that some of us affectionately call the "Bat Signal".  This goes out to scientists primarily to summon those who evaluate candidate gravitational wave events to determine if this event should be shared with traditional astronomers (i.e. ones with telescopes).  I am on the list because I am interested in keeping up on the latest results.  I remember exactly where I was: I was in my room at my mother's house outside of Pittsburgh changing clothes after getting back from visiting the in-laws (who live within a few miles of my family's home) for Christmas.  I looked at the event record and saw that this was an extraordinary candidate gravitational wave in that its statistical significance was high but the signal wasn't as obvious in graphs as the first detection in September was.

It was decided to send out the location of the possible detection to traditional astronomers and the emails started flying discussing the evidence that this was a true detection.  It was determined that the preliminary information on the signal warranted starting the detection checklist - the large-scale investigations that try to disprove that the signal is real.  Only after a candidate passes every test and has a high statistical significance is it accepted as a detection.  The same checklist that was applied to the first detection, labeled GW150914, was applied to this candidate as well.  Once this Christmas detection was verified, it was labeled GW151226 (the number reflects the UTC date that the gravitational wave was discovered) although we had nicknamed it the "Boxing Day Event" before the verification.

(Below I will often refer to GW150914 as the "first detection" and GW151226 as the "new detection".)

Read the paper on the detection here.


THE SIGNAL & THE SOURCE

The signal is similar to the first detected gravitational wave (GW150914).  We call this kind of signal a "chirp" because initially it has a low frequency which increases over time as does its amplitude.  You've heard signals like this before if you've ever hear a slide whistle increasing in tone.  The increase in tone reflects the increase in frequency and the loudness of the whistle represents the amplitude.  The signal we detected starts at about 35 Hz (close to the frequency of the sound made by the second black key from the left on the piano) and reaches its highest frequency at about 450 Hz (very close to the A above middle C if you convert this signal into sound).

Graph of the 1-second signal of GW151226.  The red line is the prediction of what a gravitational wave from a 14.2 and 7.5 solar mass black hole merger would look like and the grey area around it is the signal that LIGO recovered from its data.  The zoomed in portions allow you to get a better look at hour the prediction (in red) and the actual signal (in grey) compare.  At the end of this signal, the frequency and amplitude both go up.  The two black holes merge at the point where the amplitude of the signal is the highest (seen in the zoomed data to the far right).

The plot above shows what we detected in our data compared to the predictions of a pair of black holes orbiting each other and merging into one.  So this is similar to the last detection in that this is also a pair of stellar-mass black holes (formed from the death of extremely massive stars) but different because the masses of these new black holes are less than the first detection.  Here, our newly detected black holes are 14.2 and 7.5 solar masses where our last detection was 36.2 and 29.1 solar masses.  That makes this signal weaker than the last (the peak amplitude of this new signal is about 1/3 that of the first detection) but we are able to observe more orbits of the system here.  We see about 27 orbits of these new black holes (corresponding to the 55 cycles of the gravitational wave we see in the figure) where we only saw about 5 orbits (or 10 cycles) in the first detection.  It is interesting to note that lower mass black hole pairs will merge at higher frequencies than higher mass black holes.  This means that the signal will stay in LIGO's most sensitive frequencies longer and that is reflected in what we see here.  This new detection's signal is about 1 second long while the first detection is less than a half second long.

So, what did this new detection look and sound like?  As far as what it looked like, there was no light that we are aware of that was produced from this system.  But we can visualize the black holes as they orbit around each other and track the corresponding progression through the signal to the merger.  [Credit: SXS Collaboration/www.black-holes.org]:

We can also "listen" to gravitational waves by taking the signal, and converting it into sound through your speakers.  Below is a comparison of what the new detection "sounded" like compared to the first detection.  The actual "sounds" are quite low in tone so that they sound more like thumps.  We also have shifted the sounds up to a higher tone so that you can hear more of the detail in the signals.  That will play after the original lower tones.  The background graph shows how the frequency changes on the vertical axis (you will see that it increases for both signals) as time progresses on the horizontal axis:

The next question is where in the sky are these black holes?  We primarily determine this using the delay in detection time between the two detectors.  When the delay is large, there is a smaller area in the shape of a ring on the sky where the gravitational wave could have come from.  The detection delay for the new detection is much shorter than the first detection, so our uncertainty is going to be larger.  Below is an illustration of the areas on the sky where the new detection (the area to the left) and the first detection (the area to the right) are likely to have come from.  Note that for the new detection on the left, there is another similar area on the opposite side of the sky that cannot be seen in this image.

The location of both the new GW151226 detection (on the left) and the first detection, GW150914 (on the right).  These are pictured on a star map (you can see the center part of the Milky Way galaxy on the left and extending right).  There is another similar area for the new detection on the opposite side of the sky (not pictured here).  The outer purple area is where we are 90% confident where the sources are located.  The inner circles each have decreasing certainty.

We will be better able to determine the location of a gravitational wave source on the sky when we have more than two detectors in operation.  Fortunately, Advanced Virgo has completed their upgrades and is currently testing their new detector.  LIGO's next observing run is expected in the 4th quarter of this year and Advanced Virgo will likely join the search before the completion of that run.  When we detect more gravitational waves (which we expect since we will be even more sensitive than we were for the two detections we have already made and the run will be longer in duration) together with Virgo, we will know even more about what it is that we are seeing.

This is an exciting time to be a scientist!

Read the official LIGO "Science Summary" on this new detection, GW151226.

LIGO Makes the First Direct Detection of Gravitational Waves

On morning of 14 September 2015 at almost 4:51 am in Louisiana (09:50:45 UTC) the LIGO detectors in Livingston, LA and Hanford, WA detected a gravitational-wave signal we've labeled GW150914 (based on the date).  The online (near real-time) data analyses alerted scientists about 3 minutes later that there was something of substantial interest in the data.  While vetting this signal (that only lasted about a half of a second) took a substantial amount of time, it opened the new field of gravitational-wave astronomy.  We had not only made the first direct detection of gravitational waves but we also made the first direct detection of a black hole binary (pair) system and proved that these kinds of systems really do exist (it was contentious because the formation of one of the black holes was expected to have destroyed the star that would have made its partner).

At the time of the posting of this blog, the press conference making the announcement is going on and I am working the satellite event being held at the Livingston Observatory.  I will be sure to update this post with the link to the recording or the announcement later (update: see the bottom of this post).  There is too much to talk about in just this post, so I am going to keep this to the basics: what did we see and what does it mean?  I will be doing a series of posts about what we did to make sure that this is a real gravitational wave, the astrophysics of the source, how we detected it, the creation of black holes and why finding a pair like we did is important to astronomy.

Update: Read the Physical Review Letters journal article here.


THE SIGNAL

This gravitational-wave detection was seen as a common signal between the two LIGO sites:

This image shows the data (top row), signal (middle row), and what's left over after the signal is subtracted from the data (bottom row).  Detailed discussion on each image is provided below.

What you see here is a series of images (above and in detail below) that picks apart the signal that was detected.  In the left column is information focusing on the Hanford Observatory and on the right the Livingston Observatory.

TOP ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

In the top row is the signal that was seen.  However, this is not the raw data as it was collected.  What you see here is data that has been filtered to 1) reduce noise and 2) to include only frequency components that are around the frequency range of the signal itself.  The red graph on the left is the signal as seen at Hanford and on the left the blue trace is as seen at Livingston.  For comparison, the light red line under the blue Livingston line is the Hanford signal that has been shifted in time to account for the travel time between detectors and flipped (multiplied by -1) to match the orientation of the arms (the arms of each site have a opposite orientation compared to each other so the positive signal in one detector will be negative in the other).  The common signal can be seen with the noise in this comparison.

MIDDLE ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

These plots compare the signal predicted by numerical relativity (which are results of computer simulations where the predictions of general relativity cannot be solved by in explicit mathematical expressions) for a pair of black holes with one mass 36 times the mass of our Sun and the other 29 times.  (The red line in the left plot for Hanford and the blue line on the right for Livingston.)  Beneath each of these lines are grey shadowed areas that show the signal as detected from actual LIGO data with two different independent data analysis methods (wavelet and template).  Here again, we can see that the predictions and observations match well.

BOTTOM ROW:

The vertical (Y-axis) units are strain with a scale of 10^-21.

These are plots of residual signals which are the noise that this left behind when the gravitational-wave signal is removed.  Seeing that there is no pattern left in these plots supports that what was seen was a real common signal - a real gravitational wave (this is necessary for a gravitational wave detection but not sufficient - the extra investigations performed will be the subject of a future post).


THE SPECTROGRAM

A powerful tool in signal analysis is breaking up a signal into its frequency components in a graph called a spectrogram.  It allows us to see how much of a signal is made up different frequencies at different times.  If you can hear, then you do this everyday.  It is how you are able to pick apart the sound of a tuba from the sound of a flute when you listen to a symphony.  Both are playing at the same time, but you don't confuse their sounds as coming from anything else.

Below is the spectrogram of this gravitational wave detection:

The horizontal (X-axis) is the progression of time (like above) and the vertical (Y-axis) is showing the contribution of each possible frequency.  The more yellow at a frequency, the stronger that frequency's contribution to the signal at that time.  Our gravitational wave starts at a low frequency (about 35 Hz) and increases to higher frequency (about 250 Hz) near the end of the signal.  This is similar to a signal a slide whistle increasing tone would produce.


WHAT WOULD THIS SOUND LIKE?

As I've mentioned in a previous post, the frequencies of gravitational waves that LIGO is sensitive to would be audible if they were sound waves (which they aren't).  Because of this, we can make them into sound waves by putting the signal through a speaker.  So we did!

Because the starting frequency of the gravitational wave is very low, it is difficult to hear.  The frequency is audible, but at that low of a frequency we tend to feel the sound vibration more than we hear it.  So unless you have a truly great subwoofer, you will probably only hear the end "whoop" of the signal.  In order to make the entire signal more audible, we shifted all of the frequencies up in the above sound up so you can hear the whole thing.  This is not unlike the false-color images made in astronomy for light that our eyes cannot see.

Now that you've heard the detected gravitational wave, you can see that when the tone of it becomes higher toward the end of the signal, the frequency in the spectrogram also goes up.


WHERE DID THE SIGNAL COME FROM?

Because the two LIGO detectors were the only detectors operating at the time of the event (Virgo in Italy is finishing their advanced detector upgrades and KAGRA in Japan is under construction with similar advanced instrumentation) it isn't easy to state precisely where the signal came from.  We can narrow it down to an area on the sky based on how long it took the gravitational wave to travel between the two LIGO detectors, and other factors like the strength of the signal in each detector (there is a slightly different response for each detector for different sky locations).  The most probable location is in the southern hemisphere around the constellations Volans and Carina:

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.]

 
WHAT MADE THIS GRAVITATIONAL WAVE?

Two different data analysis methods that look at the data in fundamentally different ways not only detected this event, but provided the same results for what the source of it was.  This gravitational wave was made by two stellar mass black holes (these are the remnants of extremely massive stars that have expended their fuel and collapsed under their own gravity).  As quoted above, their masses were about 29 and 36 times the mass of our Sun.  They orbited around each other for hundreds of thousands to millions of years before they come close enough together to start orbiting very quickly (much like an ice skater spins faster as they draw their arms into themselves).  LIGO was only sensitive to the very end of this process right before the two black holes merged into one black hole.  At the end, the stars had a relative velocity of about 1.8x10⁸ m/s, or 60% the speed of light (the universe's "speed limit").  Imagine that...  Two black holes that were each the size of cities but each about 30 times as massive as our Sun whirling around each other at more than half the speed of light!  The animation below shows what it may have looked like to see these black holes merge together.  Note that since they are black holes, no light come from them directly but they do bend the light that is coming from behind them in a process called gravitational lensing:

Based on how strong we know these gravitational waves were at their source as predicted by general relativity and how strong they were once they reached Earth, we estimate that this system is located about 1.3 billion light years (~410 Mpc) away.  That distance is about 10% of the way to the edge of the observable universe!  It also means that the gravitational waves we just detected have been traveling into the universe and toward us for 1.3 billion years.  When these gravitational wave were created the Earth was in the Proterozoic eon of Precambrian time, after when multicellular life developed but before animal life.

PRESS CONFERENCE RECORDING

Note:  Fast forward to 26:30.  It's just waiting before that. 

Next post: On the formation of stellar mass black hole and why this pair of them are interesting to astronomy...

How We Monitor Data Collection with Advanced LIGO

The first Advanced LIGO observing run (O1) started in mid-September and will end in mid-January.  Today I want to tell you about how we collect our data.  On the surface this is obvious: with computers and sensitive electronics.  But how do we keep the detector working so that we can collect data and how do we know that our data is good?

The LIGO Livingston control room on 3 November 2015 (during O1).

OPERATORS

The most important step in collecting data is that the detector is working.  This is the primary job responsibility of the roughly 10 operators who work at the site.  There are 3 10-hour shifts a day, each one overlapping with the previous operator's shift by 2 hours so that the incoming operator can be brought up to speed on any issues that may be ongoing.  Since O1 will last into mid-January, that means that there will be at least the operator in the control room every night, weekend, and holiday - even during Thanksgiving dinner, Christmas morning, and New Years at midnight!

During their shift, they monitor various things like the power of the laser, local vibrations, and a multitude of other readings from all over the detector that tend to drift over time.  This work is mainly to prevent a fault in one of the systems that would interrupt data collection.  When everything is working the way it is supposed to, this part of their job can be boring - and we love boring days and nights. 

Excitement happens when we are no longer able to keep the light bouncing back and forth between the mirrors (we call this "breaking lock").  The operator's job now is to respond by discovering if the lock was lost due to an environmental issue we can't control (like an earthquake anywhere on the planet) or due to an detector issue.  If there is a malfunction in the detector, the operator identifies what subsystem caused the problem and then uses their training to fix it and get the detector up and running again.  Through my conversations with them, one of the harder parts of their job is identifying which part of the detector isn't working properly since there are so many subsystems that need to work all at the same time for us to be able to collect data.

SCIENTISTS

During the Initial LIGO science runs, there were always 2 people in the control room: the operator and the "scimon" (short for science monitor).  The scimon's job was to ensure the quality of the data that was being collected and give feedback to the operator.  Scimons came from institutions across the country who would usually spend a week or two at the observatory before returning to their home.  This meant that there were a lot of people passing though the observatory (which isn't bad) and by the time they really got comfortable in their job it was time for them to go home (this isn't good). 

We are doing the science monitoring differently for Advanced LIGO: we have longer-term (several months) visiting scientists (LSC Fellows) working on site to monitor the data as it is collected and we have data quality scientists (we call them "DQ Shifters") who remotely monitor the properties of the data for a period of 3-4 days.

SCIENTISTS: LSC FELLOWS

These scientists are on-site to monitor the data as it is collected and they also each have a project related to improving the instrument.  There is almost always a fellow on-site except for the earliest hours of the day (they are not as necessary as the operator and their instrument research is best done when other scientists are also around).  The fellows work with the operators to identify subsystems that may be causing issues and they work to resolve them.  Basically, these are the Advanced LIGO version of the scimon but with the benefit of having the visiting scientist being able to apply what they learn while on site.

SCIENTISTS: DQ SHIFTERS

The DQ shifter is a scientist who monitors the quality of the data that has already been taken (within about a day or so).  Sometimes, patterns only become evident after a significant amount of data has been collected.  Because this work is not expected to have immediate feedback to the operators and fellows, this work can be done remotely.  We have created automated web pages that have all the plots needed to look at how the different parts of the detector are working.  There are about 40 or so of us (including me) who have been trained on how to interpret all of the graphs that appear on these pages and what specific things we should be watching for.  We communicate with the fellows at the site we are monitoring on a daily basis so that they can use the feedback to improve the quality of the data.  When our shift is done (we usually cover 3-4 days in a shift), we document our findings, report to the data quality group who specializes in studying collected data, and we enter an entry in the detector log with a summary of our shift.

Summary pages used by DQ Shifters to evaluate the quality of data already taken.  These plots specifically show how the ground was moving in different frequency bands throughout the day on 2 November 2015.

First Science Data With Advanced LIGO is Near!

It has been a very exciting time for Advanced LIGO recently.  A few weeks ago we completed a test run of the instrument to identify any remaining bugs in the instrument or other stability issues.  The commissioners (instrumental scientists who work on making LIGO more sensitive) have been busy adjusting various settings in a multitude of subsystems to increase our sensitivity to gravitational waves.  We are continuously learning more about how all of these subsystems react to one another and to the environment.  And learning is never without its own pains.  Some bugs have been bigger than others. We've had to actually touch the new instrumentation - meaning we had to seal off the chamber the part was in, let the air back in (since almost all of the instrument is in a vacuum), fix it, close up the chamber, and pump the air back out.  This is rare but it has happened.  Once the instrument was performing well, that's when we decided to stop tinkering with it and use it like we would if we were looking for gravitational waves.  More subtle issues in stability and other bugs will make themselves apparent only after you use it the way it's meant to be used - all the time.

Installing one of Advanced LIGO's seismic isolation platforms at the Hanford observatory in 2013.

ENGINEERING RUNS

These test runs are called engineering runs.  We abbreviate them ER followed by the number of the run.  The last one was called ER8.  I've already talked about the first one (ER1) back when almost everything was being simulated since the installation of the instruments was just getting off the ground.  The purpose of those early engineering runs was to test out the ability of our data analysis systems to handle the large amount of data we will collect.  As parts of aLIGO were installed, we replaced the simulated data from that component with real data.  ER8 was our first test of all of the instrument without anything being simulated.  While the purpose of this data is to test the stability of the whole system and to find other small bugs, we are still running all of our data analysis methods over the collected data.  We don't expect to find a gravitational wave in this data, but if we have compelling reason to believe that we really did see something we will certainly pursue it as a real detection.  Don't get too excited, though, since there are no indications that we collected a gravitational wave.

OBSERVING RUNS

What is really exciting is that we are preparing to make that first detection.  We don't really expect to detect a gravitational wave with our first science data (which will be called O1 - observation 1) with aLIGO but it is not as improbable as it was with Initial LIGO.  We are talking about what we learned from the blind injection in our last iLIGO data set (otherwise known as the "Big Dog" event) and what our detection validation should entail.  We are talking about writing the paper that we will publish announcing the first detection and its details.  We are even talking about how we will engage the public with this announcement.  Don't misunderstand me - we have not seen anything yet, but we are preparing ourselves for the possibility of detection.

DETECTION IS POSSIBLE...

You really don't have any idea how exciting this is especially for those of us who have been around a while (I have been working on LIGO since starting grad school in 1999 and I'm a youngster).  I have been working on this project that is so much bigger than myself since before we took our first data with Initial LIGO.  I remember when the collaboration was a couple hundred scientists (there are now almost 1000 of us).  I remember when we analyzed our first data and debated how to interpret our detection candidates when we almost sure that everything we had was noise (i.e. garbage).  Now we are talking about confidently making a detection, and doing astronomy with it.  This is the dawn of a new age in astronomy and I'm proud to be here to see it.

Distance in parsecs (1 pc = 3.26 light years) Initial LIGO was able to detect its standard source of 2 neutron stars orbiting each other just before they merge into one body.  (Read more here.)
aLIGO wil be able to "see" up to 200 Mpc (about 650 million light years).

Remember, we don't expect a detection, but it is possible.  To give you an idea of how possible, once we have aLIGO working at the sensitivity it was designed to work at, it will observe as much of the universe in several hours as Initial LIGO did in an entire year.  We won't be at design sensitivity for O1, but we can already detect our standard source 4 times farther away than we could on our best days with Initial LIGO.

An image of light that was filtered out of the laser before entering the LIGO detector.  Bend your neck to the right and you should be able to see a smiley face.  This is just a chance configuration and has no significance, but we thought it was cool.

Advanced LIGO is Here!

I've been away from all of you for a little over a year due to many factors including teaching new courses, starting new research projects, and more than a few personal reasons.  However, I wanted to let all of you know about the status of Advanced LIGO (spoiler: it's done) and that I will be back to posting on this blog on a regular basis.

ADVANCED LIGO

On 20 October 2010, Initial LIGO (iLIGO) recorded its last bits of science data [read the blog post here].  At that time, we were taking some of the most sensitive gravitational wave data and we thought we may have recorded a real gravitational wave (it was a fake signal purposefully placed in the data to test our ability to find real gravitational waves, but we didn't know that at the time [you can read all about it here]).  The metaphorical "keys" to the detector were transferred from operations to the aLIGO installation team.

In the nearly 5 years since iLIGO, we've removed all of the old instrumentation, much of which had been designed 15 years ago (remember what cell phones looked like back then? - we've come a long way) and replaced it with newly redesigned instruments.  You won't notice anything different by flying over LIGO (there was not real estate expansion) but we gutted at very intricate and technical instrument and replaced it with more sophisticated hardware.  The details on the upgrades could make a whole series of blog posts, but a few of them included improved seismic (ground vibration) isolation, better ways to hang our mirrors like pendula, a more powerful laser, more massive mirror, better coatings on the mirrors, and new ways to reuse laser light to increase the laser power in the the arms.  All of this will combine to make aLIGO over 10 times as sensitive as it was before allowing us to observe 1000 times more of the universe than with the original observations we made.

The illustration above shows the anticipated "reach" of Advanced LIGO (the purple sphere) compared to Initial LIGO (the orange sphere).  Each small dot in the figure represents a galaxy.  Since the volume of space that the instrument can see grows as the cube of the distance, this means that the event rates will be more than 1,000 times greater.  Advanced LIGO will equal the 1-yr integrated observation time of Initial LIGO in roughly 3 hours. (Galaxy map credit: R. Powell, www.atlasoftheuniverse.com.)

On 19 May 2015, aLIGO was dedicated at the Hanford, WA observatory (since I am at the Livingston, LA site and generally unimportant, I missed out).  The "keys" are now back with the operations team at both sites (the Livingston site was scheduled to be 'done' before Hanford and has been 'working' for several months now).  Why did I put done and working in quote in my parenthetical comment?  Well, now is the time for commissioning.  The detector can turn on and operate as in interferometer but all of the new components aren't yet optimized to work together resulting in the detector being less sensitive than it was designed to be.  The work that is currently gong in with the detector is commissioning work that seeks to work on individual subsystems so that the detector works better as a whole.  In short, this is our version of tuning up our car.

Break time at the Advanced LIGO dedication at the LIGO Hanford Observatory on 19 May 2015.  [Source: LIGO Scientific Collaboration's Facebook page]

Even though neither detector is working at the sensitivity it was designed to, we are regularly setting sensitivity records when we do turn on the detector to test the commissioning work.  One of the ways we measure our sensitivity is to determine the farthest distance away a standard source of gravitational waves could be for us to just be able to detect it.  The standard source we use is two neutron stars orbiting each other and merging into one.  (We picked this because it is a simple system were we can predict how big the gravitational waves will be and what shape the waves will have.)  We call this the inspiral range.  Below is the insprial range for each aLIGO detector (Livingston is the blue squares line and Hanford is the red dots line) given the number of days since the aLIGO installation was declared complete (there are more data points for Livingston since we were scheduled to be done a little before Hanford).

The distance into the universe we would be able to detect a gravitational wave from our reference source of two neutron stars orbiting each other and merging into one.  [Source: Talk given by David Shoemaker at the aLIGO Dedication on 19 May 2015]

Our best data with iLIGO was able to detect out to about 20 Mpc (a little over 65 million light years away).  Currently, the Livingston's inspiral range is at 65 Mpc (212 million light years) and Hanford's is at 57 Mpc (almost 186 million light years).  So, even though we are still commissioning the detectors, we are already gathering the most sensitive gravitational-wave data ever!


IN MEMORY:

Cristina Torres

I lost a very good friend a few moths ago.  Cristina and I were both postdocs at LIGO Livingston until 2012 when she took a position at the University of Texas at Brownsville as a professor.  We shared a passion for engaging others in our science, but she always had an openness to others that I have admired.  She was a better friend to me than I ever was to her, but if she was here to read this she would argue with me since she did exactly that in one of our last emails. 

The last time I saw her in person was when I was at UT Brownsville earlier this year to speak about work/life balance, which I don't really have figured out, at a regional Conference for Undergraduate Women in Physics (there is a beautiful tribute to her at the bottom of this page).  She was so stressed since much of the local organization and logistics was on her shoulders but the meeting went very well!  If I had any idea that I wouldn't be seeing her again, I would have made more of an effort to spend time with her (instead of just trying to stay out of her hair).

This is a picture of Cristina with a prototype of the new mirror suspension system at LIGO Livingston in 2012.  We use this display to show visitors some of the upgrades that they aren't able to see inside of aLIGO. 

Until again, Cristina...

Silver and Gold: Clues to the History of Our Solar System

It's that time of year when many of us are buying gifts for our loved ones.  And there's a song from the old Rudolph the Red-Nosed Reindeer TV special that comes to mind (sung by the character Yukon Cornelius):  "Silver and gold, silver and gold.  How many wonders can one cavern hold?..."    But have you ever stopped to think about where your jewelery came from?  I don't mean how your silver and gold was mined, but how was it created?  It's much more interesting than anything you've probably imagined!

SILVER & GOLD

Gold crystal (image from Wikipedia)

First, let's go back to the birth of the Universe:  BANG!  (That was the Big Bang.)  Sometime between 10 seconds and 20 minutes from now, the first atoms will be formed (this is called Big Bang nucleosynthesis).  These first atoms were of the simplest elements in the periodic table; mostly hydrogen and some helium and lithium.  Even today, the most common elements in the Universe are hydrogen and helium.  A few hundred million years later, clouds of this material will collapse to form the first stars.  This birth is marked by the ignition of nuclear fusion: the smashing of lighter elements together to make heavier ones.  All stars start by smashing 2 hydrogen (H) atoms together to make a helium (He) atom.  Eventually, the star will start to run out of hydrogen and will start to fuse helium to make beryllium (Be) and carbon (C).  The largest stars will continue fusing atoms together until the product element is iron (Fe).  For elements lighter than iron, smashing two lighter elements together to make a heavier element will release some energy.  For elements heavier than iron, breaking an atom into two lighter atoms will release energy: this is called nuclear fission.  So, the heaviest element that any star can normally make during its life is iron.  Heavier elements like silver (Ag), gold (Au), and platinum (Pt) and many other elements can only be created in the most extreme environments such as a supernova explosion that marks the death of a star.  Therefore, the metal in our jewelery is truly a relic of a star that died before our Sun was born.

Our Sun and solar system was born from the supernova explosion of an older star that died and, in doing so, seeded the material that makes up our solar system with heavier elements (this is the nebular hypothesis).  For example, since the Earth is not a star it cannot perform nuclear fusion.  Yet the core of our planet is primarily iron and nickel and had to be made in a previous star.  This is what Carl Sagan meant in his famous quote, "We are all starstuff."


WHAT ABOUT DIAMONDS?

Diamond set into a gold ring (image from Wikipedia)

 While I'm on the topic of the exotic origins of jewelery (and many other things), let me talk a little about why our Sun may turn into a giant diamond.

First off, a diamond is made from carbon which our Sun is capable of making much later in its life (more on this in a little bit).  Carbon is everywhere around us - pretty much every molecule that isn't water in your body has at least one carbon atom in it (this is the definition of an organic compound).  What makes a diamond special from all of the other common forms of carbon is that it has a very specific crystal structure that is only formed under extreme pressure and heat.  (Any mineral that has the same chemical structure but a different crystalline organization is called a polymorph.)  So, the diamonds that are formed naturally on Earth started as a carbon deposit that transformed into a diamond under the extreme pressure and temperatures found beneath the surface of the Earth.

What does this have to do with our Sun?  Well, right now the Sun is busy turning hydrogen into helium.  It will eventually (in about 5 billion years) start making heavier elements, but only much more massive stars ever have enough oomph (enough mass to create the most extreme pressures) to fuse elements all the way up to iron.  Our Sun is not one of those stars (which is good since those massive stars live comparatively short lives).  The Sun will only be able to produce up to carbon and oxygen (O) before fusion stops and the outer layers of the Sun are blown into the interstellar medium to seed future stars and solar systems.  What is left behind is called a white dwarf composed of carbon that has been exposed to intense pressure and temperatures.  That means that the white dwarf our Sun leaves behind may very well be a diamond!

A white dwarf that could be a massive diamond has already been observed and nicknamed "Lucy" (after the Beatles song "Lucy in the Sky With Diamonds").  It is more massive than the white dwarf our Sun will leave behind, but seems to be created by the same processes we expect for our Sun.  All of this is very promising for a diamond to be the memorial for our Sun (and solar system).


WHAT DOES THIS HAVE TO DO WITH GRAVITATIONAL WAVES?

Directly, not much.  However, this past August it was confirmed that a short gamma-ray burst was the result of a kilonova, the collision of two neutron stars that releases a massive amount of energy (read more about this here and see my comments towards the end of the article).  Neutron stars are the remnants of dead stars that were much more massive than our Sun (about 10 to 40 times the mass of our Sun) that died in a supernova.  Those past supernovae would have created silver, gold, and everything heavier than iron and seeded the interstellar medium for future stars and solar systems.

These kilonova sources should produce copious amounts of gravitational waves, however, there has never been a short gamma-ray burst (like the one I mentioned above) detected close enough for LIGO to see it.  Once Advanced LIGO is complete (soon!) the closest short gamma-ray burst will be just on the edge of the distance we expect to be able to detect these kinds of gravitational waves.  But the great thing about what we do is that we really don't know how strong the gravitational waves from the actual explosion will be so they could very well be detectable in the future.

Fingers crossed!

Happy New Year!

I can't believe how long it's been since I've last blogged - I've had so many ideas of stories to post, but I've also had some life issues that have kept me away.  Not to worry!  My most important resolution for 2013 is to write blog posts a few weeks ahead of time so that I can still post weekly even when life gets in the way.  I will be back in full force in 2013!  Expect posts on Thursdays, unless there is something timely I want to share before then.  I will make sure to post on Twitter when I a new post is available so if you don't follow me already, please follow @livingligo.

This is a smiley face the deicing crew at the Pittsburgh International Airport made in the snow.  As seen through the deicing fluid on the window of my plane on the evening of 29 December 2012.

2012

This year has been a year of many changes for me.  My days as a postdoc have come to an end and I now hold a dual position with Caltech as a scientist at the LIGO Livingston Observatory and as a physics instructor at LSU.  It is great being back in the classroom but that is also something that has kept me from posting as much as I would like.  It takes a lot of time to create interesting lectures for a class of 150 students and handle all of the class administration myself (office hours, grading, etc.).  This semester I am teaching the second semester of physical science (astronomy, chemistry, earth science) and will only have a 30 students.  I am very excited about the more personal instruction I will be able to do!

There have also been many changes at LIGO.  When I first started working at the Livingston observatory in 2007, there were about 25-30 people who worked there on a daily basis.  Starting with the Advanced LIGO preparations in 2010, we nearly doubled the number of daily staff.  Since the installation is well underway, we no longer need to have so many people on site (having too many people on site while we are looking for gravitational waves will cause ground vibrations that will decrease our sensitivity).  The parking lots are noticeably less full and it is starting to feel a little lonely even though we still have more people working on site than when I started.

As far as my personal life is concerned, I'm glad that 2012 is over.  It has been full of drama and uncertainty and it is one of the things that have been getting in the way of keeping up with this blog and my career in general.  But I wouldn't change a moment of it since I have so many great people around me, at home and at work, who care for me. 


2013

This coming year will prove to be exciting!  The installation of Advanced LIGO should be completed and the first commissioning (use of the detector to fine tune it to its best sensitivity) started.  This is always an interesting time when you get to use the detector for the first time and solve novel problems.  I will be sure to tell you all about them here! 

I will also continue teaching at LSU.  As I mentioned above, I will be teaching the second semester of physical science with about 30 students.  I also expect to teach a masters degree class on inquiry learning for in-service teachers this summer (I've done this class twice before with LSU).  

Of course, the most exciting events are usually the unexpected.  I look forward to sharing the professional and personal excitement with you here.

Thank you to all of my readers, followers on Twitter, and those who found me through a search engine!  Keep coming back for more!

What are you looking forward to this year?

Happy Halloween!

Wow!  I can't believe how long it has been since I've posted.  I've been horribly busy keeping up with teaching at LSU (and trying my best to make my lectures interesting), getting my LIGO work done (we are preparing for the 3rd software engineering run for Advanced LIGO [read about the first one here]), and some personal life complications that we all deal with from time to time.  I understand better why the blessing, "May you live in interesting times," is more of a curse.

So, to tide you over until my next full post (tomorrow), here is the feature presentation of the Science Education Center's monthly Science Saturday - Halloween Edition (2011):

Here, William Katzman (Science Education Center Lead) plays a laid back fellow with some paranormal explanations of "spooky phenomena".  I play a scientist who explains all of the phenomena in terms of science.  Before the day of the presentation, we decided what spooky phenomena we were going to use, but we never rehearsed the show - I'm surprised it turned out so well (if I say so myself)!