What happens when two black holes smash into each another? They form an even larger black hole while driving jets of relativistic matter into the far reaches of the cosmos. New complex computer simulations have attempted to give a glimpse into the explosion of electromagnetic and gravitational waves that result. The answers may tell us whether or not we could ever detect such a merger.

A paper describing the simulations appears in the current issue of Science. It takes a look at what happens as two supermassive black holes dance around each another, accompanied by a conducting plasma and magnetic field. The emission of gravitational waves would carry both angular momentum and energy away, causing them to eventually collide and merge. The numerical simulations indicated that the particle jets that sometimes accompany black holes would align, thanks to an anchoring and alignment of the magnetic field that occurs as they merge.

An interesting side note mentioned by the authors is that stray charged particles will get caught up in the magnetic field and accelerate to enormous velocities. This acceleration will result in the particle radiating off enough energy to form electron-positron pairs from the vacuum which will be accelerated in turn. This process would repeat, producing a cascade of particles that populate the immediate region with a charged, conducting plasma.

All of the work is interesting to any theorist, but the results show what astronomers might look for as indications that an event of this sort has taken place. The electromagnetic energy of the collision can be transferred to kinetic energy through the plasma, which will bleed it off through synchrotron radiation. Those emissions could be detected with future X-ray telescopes out to a significant distance from the source (a redshift of z = 1).

Further in the future, joint X-ray-gravitational wave detectors will allow more refined observations of supermassive black hole mergers. The paper gives a scaling for gravitational wave power that should be detectable at even greater distances, back to redshifts of 5 to 10.

Science, 2010. DOI: 10.1126/science.1191766