2010A&A...516A..71M

Fermi can measure the spectral properties of gamma-ray bursts over a very large energy range and is opening a new window on the prompt emission of these energetic events. Localizations by the instruments on Fermi in combination with follow-up by Swift provide accurate positions for observations at longer wavelengths leading to the determination of redshifts, the true energy budget, host galaxy properties and facilitate comparison with pre-Fermi bursts. Multi-wavelength follow-up observations were performed on the afterglows of four bursts with high energy emission detected by Fermi/LAT: GRB090323, GRB090328, GRB090510 and GRB090902B. They were obtained in the optical/near-infrared bands with GROND mounted at the MPG/ESO 2.2m telescope and additionally of GRB090323 in the optical with the 2m telescope in Tautenburg, Germany. Three of the events are classified as long bursts while GRB090510 is a well localized short GRB with GeV emission. In addition, host galaxies were detected for three of the four bursts. Spectroscopic follow-up was initiated with the VLT for GRB090328 and GRB090510. The afterglow observations in 7 bands are presented for all bursts and their host galaxies are investigated. Knowledge of the distance and the local dust extinction enables comparison of the afterglows of LAT-detected GRBs with the general sample. The spectroscopic redshifts of GRB090328 and GRB090510 were determined to be z=0.7354±0.0003 and z=0.903±0.001 and dust corrected star-formation rates of 4.8M_☉/yr and 0.60M_☉/yr were derived for their host galaxies, respectively. The afterglows of long bursts exhibit power-law decay indices (α) from less than 1 to ∼2.3 and spectral indices (β_opt) values from 0.65 to ∼1.2 which are fairly standard for GRB afterglows. Constraints are placed on the jet half opening angles of ≲2.1° to >6.4°, which allows limits to be placed on the beaming corrected energies. These range from ≲5x10⁵⁰ erg to the one of the highest values ever recorded, >2.2x10⁵²erg for GRB 090902B, and are not consistent with a standard candle. The extremely energetic long Fermi bursts have optical afterglows which lie in the top half of the brightness distribution of all optical afterglows detected in the Swift era or even in the top 5% if incompleteness is considered. The properties of the host galaxies of these LAT detected bursts in terms of extinction, star formation rates and masses do not appear to differ from previous samples.