Thursday, April 7, 2016

The Source of GW150914: Stellar Mass Black Holes

On September 14th, 2015, LIGO made the first direct detection of gravitational waves.  This event is labeled GW150914 (referring to the year, month, and day of the detection).  The objects that produced the GW150914 were a pair of stellar mass black holes that orbited each other and gradually moved closer and closer together over the course of eons.  The closer together they became, the faster they orbited around each other and the stronger the gravitational waves produced.  LIGO detected the last 0.4 seconds of these stars orbiting until they became so close they merged into a single black hole.

While we saw the death of this paired (binary) system, we didn't get to observe other parts of its life.  Where did these black holes come from?  To answer this question, we need to apply what we know about stellar evolution.


STELLAR MASS BLACK HOLES ARE CORPSES

There are several classes of black holes, determined by their mass and how they were formed: stellar mass black holes, intermediate mass black holes, and supermassive black holes.  For stellar mass black holes, they formed when the most massive of stars (more than 15-20 times the mass of our Sun) run out of nuclear fuel and gravity takes over and collapses the star.  For smaller stars, this collapse stops when the pressure from inside the atom (neutron pressure) equals the pressure from the gravitational collapse.  But for these more massive stars, there is no pressure that can stop the collapse and a black hole is formed.  It is in this way stellar mass black holes are the corpses of the most massive stars (but these kinds of black holes are among the least massive).  The newly merged GW150914 black hole now holds the record for the largest stellar mass black hole known.

There are several theories about how this happens... Sometimes this collapse is accompanied by an explosion called a hypernova and is believed to be the source for a kind of gamma-ray burst.  Sometimes the gravity of the collapsing star is so great that all of the matter and light gets sucked into it even if there was a hypernova-like explosion.   


THE EVOLUTION OF THE GW150914 SYSTEM

But how did two stellar mass black holes come to be paired together?  The most likely explanation is that they also lived their lives together as a binary star system.  This is very common as it is estimated that about 1 out of 3 stars are in systems of 2 or more stars.  This binary system would likely have formed together and lived their entire lives paired.  The more massive of the 2 stars would have died first since the more massive the star, the faster it burns through its fuel.  Once the nuclear fuel ran out, the more massive star collapsed into a black hole making the system a star/black hole system.  Eventually, the second star would run out of fuel and collapse into a black hole as well making our stellar black hole binary system.  These black holes would orbit for eons before they were close enough to merge and produce the gravitational waves LIGO detected.

In a recent paper (see reference below or read it here), simulations of millions of stars with different material compositions (specifically metalicity which, to an astronomer, is anything that isn't hydrogen or helium; the Sun is 2% 'metal') were simulated and some produced similar outcomes to what we observed.  What was found was that there were similar characteristics for the stars the went on to resemble the GW150914 binary system and this gives us estimates on the time needed for each stage in the system's evolution from birth to the gravitational-wave-generating merger.

The two stars were born about 2 billion years after the Big Bang and were each somewhere between 40 to 100 times the mass of our Sun.  These low metalicity stars (only about 0.06% 'metal') orbit each other as stars for about 4 million years until the more massive one collapses into a black hole.  The now star-black hole system orbit each other for another 1.5 million years until the other star collapses into a black hole.  Both of these stars were massive enough that there wouldn't have been a hypernova-like explosion for either of them; any material ejected would have fallen back into the black hole.  Our new black hole binary system, which is just the corpses of once very massive stars, now go on to orbit each other for over 10 billion years - that is 1000 times longer than the either star was a alive.  At the end of that time, they merge and produce the gravitational waves that LIGO detected 1.3 billion years later when they arrived at Earth.


WHAT WILL HAPPEN NOW?

The short answer: nothing.  This new single black hole is spinning (it is the first detection of a Kerr rotating black hole) but its shape and center of mass are not moving in a way that will ever produce gravitational waves again.   Gravitational waves are also the only way this system would ever have been detected since there wasn't any matter (like dust or gas) to fall into the black holes and generate X-rays.  We will never be able to observe this black hole again.

Of course, there are extremely unlikely events like another black hole flying by and crashing into it...  That may make new gravitational waves for us to see (but I wouldn't hold my breath).


Reference:

K. Belczynski, D. Holz, T. Bulik, R. O'Shaughnessy, "The origin and evolution of LIGO's first gravitational-wave source" arXive e-Print: 1602.04531 (2016).

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