Friday, April 10, 2015

GRBs: Astro FOTD

Gamma Ray Bursts, during their brief (seconds or so) existence are the brightest objects in the universe, hundreds or thousands of times as bright as a quasar and millions of times as bright as a supernova. Brilliance and brevity both present related puzzles. If the gamma rays released are thermal, the implied temperatures (trillions of K) are implausible, and the brevity of the emission time is also hard to explain, if we assume that emitting shock wave cools by collision with the interstellar medium.

Both effects are believed to be explained by the ultrarelativistic character of the shock wave, moving at a speed just barely lower than the speed of light. Gamma = sqrt(1/(1-v^2/c^2)) is 1000 or more. This produces a relativistic Doppler effect, which can be considered to be the combination of an ordinary Doppler effect with relativistic time dilation. Because the source is coming rapidly toward us the emitted radiation doesn't get very far ahead of the shock wave that emitted it. This compresses both radiated wavelength and pulse length. As a result, energy released while the shock wave traveled light weeks is compressed into seconds or minutes, and energy radiated at X-ray wavelengths is blue shifted to gamma radiation.