Millimetre Observations of Gamma-ray Bursts

Since 1997, and following our detection of the first mm afterglow (GRB 970508), we have followed-up 38 gamma-ray bursts (GRBs), mainly with the IRAM's Plateau de Bure Interferometer. Many long-duration events have been detected at 3 mm, in the range z = 0.168-8.3, the latter pointing to one of the most distant galaxies discovered so far. The fact that the synchrotron GRB afterglow spectrum peaks in the millimetre range, proves the importance of studying GRBs (the most energetic events in the Universe) at these wavelengths. This has allowed in a few cases to detect the host galaxies of dark GRBs and to compare the time evolution of the mm emission for events spanning two orders of magnitude in z with observables at other wavelengths (X-rays and optical) determining the characteristic frequencies of the afterglow spectrum, and to trace their evolution, allowing to derive the physical parameters of the circumburst medium. Here we summarize the results obtained so far.