Gamma-Ray Bursts

The main scientific goal of the Watcher telescope is to make detailed observations of the optical light which accompanies and follows the universe’s most powerful cosmic explosions, called gamma-ray bursts or ‘GRBs’.

GRB120711A

Optical light curve from a GRB that happened on July 11th, 2012 (GRB120711A). This GRB occurred about 7.5 billion light years away and was observed by Watcher within 80 seconds of its onset. Credit: A. Martin-Carrillo et al. 2014

Gamma ray bursts (GRBs) are cosmic explosions that occur once or twice per day in the universe. The energy released over such a short time is so vast that they can only be produced by exotic events such as the death of massive stars to form a black hole, or the merger of compact objects such as neutron stars.

GRB

Artist’s impression of a GRB
Image courtesy of NASA

GRBs were discovered in 1967 by the Vela satellites and have been observed since then by many space observatories including: the Compton Gamma-ray Observatory (CGRO), Beppo-SAX, HETE-2, Konus-WIND, Suzaku, INTEGRAL, Swift and Fermi. GRBs appear as intense bursts of gamma-ray radiation, lasting from a few seconds to several hundred seconds, outshining all other gamma-ray sources during that time.

Based on the duration of this phase, known as the ‘prompt emission’ phase, GRBs can be divided in two groups: long GRBs have durations longer than 2 seconds, and short GRBs have durations less than 2 seconds. The emission rapidly fades away and the source normally disappears in the gamma-ray instruments. However, a rapidly fading afterglow is typically seen at lower energies such as X-rays, optical and radio, lasting from days to months.

Example of a long GRB observed by the INTEGRAL space observatory

Example of a long GRB observed by the ESA INTEGRAL space observatory

Hundreds of models were theorized in order to help explain these bursts. However little information was available to constrain these models until in 1997, when the first X-ray, optical afterglows and direct measurement of their redshifts using optical spectroscopy was detected, confirming their cosmological origin.

Why are Gamma-Ray Bursts interesting?

GRBs are a significant topic in astrophysics for 2 main reasons. Firstly, they involve extreme physical conditions, such as highly relativistic outflows, and hence pose a challenge to physical theory. For example, GRBs have been used to test for violations of Lorentz Invariance.

Secondly, GRBs are important tools for cosmology since they are luminous enough to probe the early universe. The current redshift record holder, GRB090429 at z=9.04, is located close to the boundary of the reionised universe, on a par with the most distant known quasars and galaxies known. Variable optical emission from GRB 050904 at z=6.295 (I-band mag. of ~14) was detected by the 25cm TAROT telescope, underlining the potential of robotic technology to contribute significantly to our understanding of the high redshift universe. Notably, the BOOTES system did not detect emission, due to its use of an R-band filter. At such a high redshift, most photons were in the 880-1000 nm range.

What causes Gamma-Ray Bursts?

One model for the formation of long duration GRBs is the so-called collapsar model. The name comes from the fact that the process involves the collapse of the core of a massive star to form a black hole. The process believed to be responsible for the production of the burst and afterglow emission is illustrated below.

Schematic of the different phases of the Fireball model.  Image courtesy of NASA

Schematic of the Fireball model, illustrating the production of the ‘prompt’ and ‘afterglow’ emission.
Image courtesy of NASA

Further reading on GRBs

A short explanation for the general public
NASA’s ‘Imagine the Universe’ description
BBC articles and videos
NASA’s Swift GRB Mission