Optical and near-infrared follow-up observations of four Fermi/LAT GRBs

Abstract

Fermi can measure the spectral properties of gamma-ray bursts over a very large energy range. Localizations by the instruments on Fermi in combination with follow-up by Swift provide accurate positions for observations at longer wavelengths enabling 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, GRB 090323, GRB 090328, GRB 090510, GRB 090902B. The observations were obtained in the optical/near-infraredbands with GROND which is mounted at the MPG/ESO 2.2 m telescope, and additional optical observations of GRB 090323 were obtained with the 2 m telescope in Tautenburg, Germany. Further observations of GRB 090328, GRB 090510 and GRB090902B were obtained with the VLT, Chile. Three of the events are classified as long bursts while GRB 090510 is a short GRB. The afterglows of the long bursts exhibit power-law decay indices (α) from less than 1 to ∼2.3 and spectral indices (βopt) 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 ≲ 5 × 1050 erg to the one of the highest values ever recorded, ≳ 2.2 × 1052 erg for GRB 090902B. 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. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial- ShareAlike Licence.