The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy

Aj. Castro-tirado et al., The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy, ASTRON ASTR, 370(2), 2001, pp. 398-406
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-6361 → ACNP
Year of publication
398 - 406
SICI code
Broad-band optical observations of the extraordinarily bright optical after glow of the intense gamma-rag burst GRB 991208 started similar to2.1 days a fter tile event and continued until 4 Apr. 2000. The flux decay constant of the optic al afterglow in the R-band is -2.30 +/- 0.07 up to similar to5 d ays. which is very likely due to the jet effect. and it is followed by a mu ch steeper decay with constant -3.2 +/- 0.2. tile fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films take n simultaneously with the er ent. that otherwise would have reached naked e ye brightness, implies either a previous additional break prior to similar to2 days after tile occurrence of the GRB (as expected from the jet effect) or a maximum, as observed in GRB 970508. The existence of a second break m ight indicate a steepening in the electron spectrum or the superposition of two events, resembling GRB 000301C. Once the afterglow emission vanished, contribution of a bright underlying supernova was found on the basis of the late-time R-band measurements. but tile light curve is not sufficiently we ll sampled to rule out a dust echo explanation. Our redshift determination of z = 0.706 indicates that GRB 991208 is at 3.7 Gpc (for H-0 = 60 km s(-1) Mpc(-1), Omega (0) = 1 and Lambda (0) = 0), implying an isotropic energy r elease of 1.15 10(53) erg which may. be relaxed by beaming by a factor >10( 2). Precise astrometry indicates that the GRB coincides within 0.2 " with t he host galaxy, thus supporting a massive star origin. The absolute magnitu de of the galaxy is M-B = -18.2, well below the knee of the galaxy luminosi ty function and we derive a star-forming rate of (11.5 +/- 7.1) M-circle do t yr(-1), which is much larger than tile present-day rate in our Galaxy. Th e quasi simultaneous broad-band photometric spectral energy distribution of tile afterglow was determined similar to3.5 day after the burst (Dec. 12.0 ) implying a cooling frequency ve below the optical band, i.e. supporting a jet model with p = -2.30 as the index of the power-law electron distributi on.