Monday, July 27, 2015

A Population of Extremely Long Duration Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are among the most energetic events in the universe. There are three classes of GRBs - short gamma-ray bursts (SGRBs) with durations less than ~2 seconds, long gamma-ray bursts (LGRBs) with durations extending to ~1,000 seconds and ultra-long gamma-ray bursts (ULGRBs) with durations of ~10,000 seconds. SGRBs originate from the mergers of neutron star binaries or neutron star-black hole binaries, while LGRBs are created when the cores of massive stars collapse to form black holes.

ULGRBs have such long burst durations that their progenitors are unlikely to be the same as those for LGRBs. Instead, ULGRBs probably originate from the collapse of giant or supergiant stars into black holes. These stars are orders of magnitude larger than the progenitors of LGRBs, resulting in much longer collapse times. Additionally, these stars have lower densities, resulting in lower mass in-fall rates. The continuous in-fall of material into the nascent black hole drives a GRB with an extremely long duration, leading to an ULGRB. Alternatively, ULGRBs can also be created when white dwarfs get tidally shredded by intermediate mass black holes (IMBH).

Figure 1: Artist’s impression of a gamma-ray burst.

Figure 2: Parameter space for GRBs and other high energy transient phenomena plotted as a function of burst duration versus average luminosity. The classes of events are - soft gamma repeaters (SGRs), short gamma-ray bursts (SGRBs), low-luminosity and long gamma-ray bursts (LLGRBs), long gamma-ray bursts (LGRBs), ultra-long gamma-ray bursts (ULGRBs) and tidal disruption events (TDEs). Andrew Levan (2015).

Recent observations by NASA’s Swift Gamma-Ray Burst Mission have revealed what could be a new population of GRBs with extremely long durations that exceed 100,000 seconds. These extremely long duration GRBs may represent the tidal disruption of main sequence stars by supermassive black holes (SMBHs). When a main sequence star comes too close to a SMBH, the gravitational pull on the star’s outer layers from the SMBH can be stronger than the star’s own gravity. This can cause the star to be completely or partly disrupted.

Material stripped from the star forms an accretion disk around the SMBH and a small fraction of the material may be expelled at relativistic velocities, driving a tidal disruption flare (TDF) that is observed as an extremely long duration GRB. Tidal disruption events (TDEs) involving the tidal shredding of main sequence stars by SMBHs are likely to be the progenitors of extremely long duration GRBs.

Andrew Levan (2015), “Swift discoveries of new populations of extremely long duration high energy transient”, arXiv:1506.03960 [astro-ph.HE]