Gamma-Ray Bursts

Short (sub-second to minutes) flashes of gamma- rays, for ~ 30 years not associated with any counterparts in other wavelength bands First dedicated GRB detector: BATSE on the Compton Gamma-Ray Observatory

GRB Light Curves Long GRBs (duration > 2 s)Short GRBs (duration < 1 s) Possibly two different types of GRBs: Long and short bursts

General Properties Random distribution in the sky Approx. 1 GRB per day observed No repeating GRB sources

BeppoSAX (1996 – 2003) Wide-field Camera and Narrow-Field Instrument (NFI; X-ray telescope) allowed localization of GRBs to arc-minute accuracy First identification of X-ray and optical afterglows of  -ray bursts in 1997

Afterglows of GRBs Most GRBs have gradually decaying afterglows in X-rays, some also in optical and radio. X-ray afterglow of GRB (GRBs are named by their date: Feb. 28, 1997) On the day of the GRB3 days after the GRB

Optical afterglow of GRB (May 10, 1999) Optical afterglows of GRBs are extremely difficult to localize: Very faint (~ 18 – 20 mag.); decaying within a few days. 1 day after GRB2 days after GRB

Optical Afterglows of GRBs Optical afterglow of GRB , observed with Hubble Space Telescope (HST/STIS) Long GRBs are often found in the spiral arms (star forming regions!) of very faint host galaxies Host Galaxy Optical Afterglow

Energy Output of GRBs Observed brightness combined with large distance implies huge energy output of GRBs, if they are emitting isotropically: E ~ erg L ~ erg/s Energy equivalent to the entire mass of the sun (E = mc 2 ), converted into gamma-rays in just a few seconds! … another one, observed by us with the MDM 1.3 m telescope on Kitt Peak!

Beaming Evidence that GRBs are not emitting isotropically (i.e. with the same intensity in all directions), but they are beamed: E.g., achromatic breaks in afterglow light curves. GRB

Models of GRBs (I) Hypernova: There’s no consensus about what causes GRBs. Several models have been suggested, e.g.: Supernova explosion of a very massive (> 25 M sun ) star Iron core collapse forming a black hole; Material from the outer shells accreting onto the black hole Accretion disk => Jets => GRB! GRB!

Models of GRBs (II) Supranova: If a neutron star is rotating extremely rapidly, it could escape collapse (for a few months) due to centrifugal forces. Neutron star will gradually lose angular momentum, then collapse into a black hole => collapse triggers the GRB

Results of the BeppoSAX Era During the BeppoSAX era, X-ray, optical, radio afterglows were only found for long GRBs. In afterglows and host galaxies, redshifts, clustered around ~ 1, were measured; unambiguously established the cosmological origin of GRBs. Association with star-forming region, similarities of some optical light curves and spectra with type Ic supernovae provided strong support for hypernova/collapsar model for long GRBs

Swift (launched 2004) Localization of the first short GRBs; Some with significant offsets from host galaxies, favoring binary-compact- object merger models: Dedicated GRB misssion with on-board soft  -ray, X-ray, and optical telescopes; Rapid automated localization and electronic distribution of GRBs with arc-second precision

Models of GRBs (II) Black-hole – neutron-star merger: Black hole and neutron star (or 2 neutron stars) orbiting each other in a binary system Neutron star will be destroyed by tidal effects; neutron star matter accretes onto black hole => Accretion disk => Jets => GRB! Model works probably only for short GRBs.

Most successful model: