Text Box:

Ongoing Research

Gamma-ray Bursts

             Gamma-ray bursts are the most energetic events occurring in the universe today.  Roughly the rest-mass of the sun (modulo beaming factors which may reduce this by orders of magnitude) is liberated in ~100 s in the form of gamma-rays.  For both long (greater than ~2 s) and short (less than ~2 s) bursts  X-ray afterglows have now been observed.  From these it is possible to infer that (a) the bursts are cosmological and (b) that the emission is likely to be highly beamed.   In at least three long bursts a supernova light curve was detected (and all three look surprisingly alike).  Despite being first discovered more than 30 yrs ago, there is still is not a complete explanation for what causes these extreme events!

             I have developed a competitor to the leading models (the collapsar and internal shock models) in which the burst is triggered by a helium star supernova in a helium star—black hole or (maybe) helium star—neutron star binary.   In this scenario a Poynting dominated jet is formed when the supernova ejecta begins to accrete onto the compact object.  Thermal photons entrained in the ejecta (or created in shocks around the jet) are then inverse Compton scattered to produce the prompt gamma-ray emission.   As a consequence the jet in decelerated and ultimately stalls, at which point a Compton thick head develops, ending the burst.  At this point the now trans-relativistic jet head essentially expands as a fireball and produces the X-ray afterglow in the standard way.  In this model, the incredible variety in burst spectra and light curves are due to variations in the orbital distance, orbit-jet alignment, and characteristic fragment size in the supernova ejecta.


Papers on this topic:

          Broderick, A.E., 2005, MNRAS, 361, 955
Supernovae in helium star—compact object binaries: a possible gamma-ray burst mechanism