Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are short flashes of photons thought to originate from collapsing stars. These strong pulses have energies in the X-ray to gamma-ray ranges and time scales ranging from a few milliseconds to around 100 seconds. The separation between "short" and "long" bursts is around two seconds. Though short-lived, gamma-ray bursts are extremely powerful and can emit as much energy in a few seconds as the Sun will in its entire lifespan.

The spectral energy distribution of GRBs typically peaks in the range of a few hundred keV, but a number of bursts have been detected at higher energies by the Fermi Large Area Telescope.  These bursts occur at energy levels in the GeV range and have an extra power-law component distinct from the main sub-MeV emission.

GRB light curves can be very short, composed of several millisecond pulses, or they can be comprised of single, longer pulses lasting tens of seconds.  The shortest pulses observed in the most rapidly varying bursts suggest that they originate in compact regions.  Because of the opacity to photon-photon pair production that is prevalent in such small regions, large-bulk Lorentz factors are required for the emitting material in order for the gamma-ray emission to escape.  Typically, Lorentz factors of 100-1000 have been inferred, suggesting that these objects must produce the most highly relativistic outflows known.  Scientists have postulated that pulses of gamma-ray emission in GRB light curves are produced by relativistic particles accelerated in shocks formed when non-uniform regions – known as inhomogeneitites – in the bulk outflow collide.

GRBs have also been observed on much longer time scales -- up to tens of kiloseconds after the "prompt" gamma-ray phase of the burst.  This so-called "afterglow" emission has been seen over a broad range of wavebands, from radio wavelengths to GeV energies, and is believed to be generated when relativistic material flowing interacts with the ambient material surrounding the source of the GRB.  Through observations of GRB afterglow emission, researchers have determined that gamma-ray bursts are at cosmological distances and redshifts ranging from z~1-8.  At these distances, the apparent energy release for the brightest bursts are in excess of 10^54 ergs, making GRBs the most luminous explosions in the universe.

Such energetic explosions almost certainly must be powered by the release of gravitational potential energy by stellar mass objects. For the longer duration bursts, it is believed that the emission comes from jets produced in the aftermath of the core collapse of very massive stars.  These stellar collapses are thought to be similar to supernovae, except that a relativistic jet is produced by the accretion of stellar material onto a compact object formed at the center of the collapsing star.  This jet, theoretically, could emerge from the stellar envelope and produce the emissions we see as GRBs.  By contrast, shorter duration bursts are thought to be produced the coalescing of compact object binaries – either neutron star-neutron star or neutron star-black hole binaries.

At KIPAC, research to understand GRBs is occurring on several fronts: The Fermi Gamma-ray Space Telescope is continuously monitoring the gamma-ray sky for bursts with its two instruments, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT). The GBM, which is sensitive to sub-MeV emissions, detects GRBs on nearly a daily basis and can localize GRBs to within a few degrees.  The LAT is sensitive to higher energy emission (> 100 MeV) and can localize bright bursts to 0.1 degrees, which is sufficiently accurate to enable other observatories to perform follow-up observations to find counterparts as well as redshifts. However, less than five percent of GRBs produce GeV emission that can be detected by the LAT. In addition to detecting GRBs, KIPAC researchers are also building models of particle acceleration and jet formation from GRBs.