Thursday, November 16, 2017

Gravity waves, Gamma-rays and gold jewelery

It has been a bumper month for astrophysicists.

On October 16th, the combined LIGO/VIRGO collaborations announced the observation of gravitational waves from an even that occurred on August 17th.  Unlike the previous observations, these waves came from relatively 'light' objects, reflecting the collisions of two presumed neutron stars, with masses around 1.1 to 1.6 times the mass  of the sun, forming a black hole with a mass around 2.74 times the mass of the sun.   Previous gravitational wave events had come from the collisions of much heavier objects.

But, that's not all.  Two seconds later, the FERMI observatory, a satellite containing a large gamma-ray detector, and the INTEGRAL satellite both observed pulses of gamma-rays coming from the same direction.   This is the classical signature of a 'gamma-ray burst' (GRB).  GRBs were first observed in the 1960's by the VELA satellites, built to monitor gamma-rays from possible atmospheric or space-based nuclear weapons tests.   VELA did not observe these, but it did find mysterious bursts of gamma-rays coming from space.     These bursts have been the subject of scientific speculation for decades, and the conventional wisdom was that some GRBs came from the merger of neutron stars or black-hole on neutron star mergers.  That theory has now been amply confirmed by the LIGO/VIRGO/FERMI/INTEGRAL observation.   The graphic above, from the LIGO collaboration, shows the process.

Of course, this collision site was studied by many many other astronomical instruments.  IceCube looked, but we didn't see anything.   However, the optical studies were very fruitful.  Multiple telescopes observed an optical signal that lasted for a few days, plus an infrared signal that lasted for nearly two weeks.  These signals were consistent with some predictions made by my LBNL colleague Dan Kasen and his collaborators.  Kasen made a detailed model of the graviational, nuclear and atomic processes that would occur in a collision of two neutron stars, and, from that, predicted the optical and infrared light emission.  His model predicts considerable production of heavy elements (heavier than iron) via rapid neutron capture (the 'r-process').   The shorter-lived broadband optical emission comes from an initial ejection of lighter nuclei. The long-lived infrared component comes from a secondary emission which is powered by the radioactive decay of heavy elements which heat the plasma that surrounds the newly formed black hole.  Heavy elements (Z between 58 and 90) scatter the light strongly, so it takes longer to escape from the plasma.

This agreement is of great interest to nuclear physicists, since it may provide a new answer to the question: where do the heavy elements in the universe come from?  Previously, it was thought that they were mostly produced in supernovae, explosions that occur when heavy stars reach the end of their livetime and collapse.  However, Dan's simulations  shows that GRBs produce heavy elements, and could account for much or all of the gold used in our jewelry, along with all of the other heavy elements.

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