Swift triggered on a precursor to the main burst of GRB 061121 ( z = 1.314 ) , allowing observations to be made from the optical to gamma-ray bands .
Many other telescopes , including Konus-Wind , XMM-Newton , ROTSE and the Faulkes Telescope North , also observed the burst .
The gamma-ray , X-ray and UV/optical emission all showed a peak \sim 75 s after the trigger , although the optical and X-ray afterglow components also appear early on – before , or during , the main peak .
Spectral evolution was seen throughout the burst , with the prompt emission showing a clear positive correlation between brightness and hardness .
The Spectral Energy Distribution ( SED ) of the prompt emission , stretching from 1 eV up to 1 MeV , is very flat , with a peak in the flux density at \sim 1 keV .
The optical-to-X-ray spectra at this time are better fitted by a broken , rather than single , power-law , similar to previous results for X-ray flares .
The SED shows spectral hardening as the afterglow evolves with time .
This behaviour might be a symptom of self-Comptonisation , although circumstellar densities similar to those found in the cores of molecular clouds would be required .
The afterglow also decays too slowly to be accounted for by the standard models .
Although the precursor and main emission show different spectral lags , both are consistent with the lag-luminosity correlation for long bursts .
GRB 061121 is the instantaneously brightest long burst yet detected by Swift .
Using a combination of Swift and Konus-Wind data , we estimate an isotropic energy of 2.8 \times 10 ^ { 53 } erg over 1 keV – 10 MeV in the GRB rest frame .
A probable jet break is detected at \sim 2 \times 10 ^ { 5 } s , leading to an estimate of \sim 10 ^ { 51 } erg for the beaming-corrected gamma-ray energy .