GRB010222: Afterglow emission from a rapidly decelerating shock

Abstract

Abstract. The GRB010222 optical and near-infrared (NIR) afterglow was monitored at the TNG and other Italian telescopes starting ∼1 day after the high-energy prompt event. The BV R light curves, which are the best sampled, are continuously steepening and can be described by two power laws, f (t) ∝ t −α , of indices α1 ∼ 0.7 and α2 ∼ 1.3 before and after a break occurring at about 0.5 days after the GRB start time, respectively. This model accounts well also for the flux in the U , I and J bands, which are less well monitored. The temporal break appears to be achromatic. The two K-band points are not consistent with the above behaviour, and rather suggest a constant trend. A low-resolution optical spectrum has also been taken with TNG. In the optical spectrum we found three absorption systems at different redshifts (0.927, 1.155 and 1.475), the highest of which represents a lower limit to, and probably coincides with, the redshift of the GRB. The broad-band optical spectral energy distributions do not appear to vary with time, consistently with the achromatic behaviour of the light curves. We compare our measurements with different afterglow evolution scenarios and we find that they favor a transition from relativistic to non-relativistic conditions in the shock propagation.

Hubble Space Telescopeand Ground‐based Optical and Ultraviolet Observations of GRB 010222

“…Optical observations have been presented by Cowsik et al (2001), Lee et al (2001), Masetti et al (2001), Sagar et al (2001), and Stanek et al (2001). Although there is good agreement among these authors that the optical light curves exhibit an achromatic break about 0.5 days after the burst, no consensus has been reached regarding the origin of this steepening.…”

“…Absorption lines yielded a redshift of z ¼ 1:4768AE 0:0002 (Garnavich et al 2001;Jha et al 2001;Bloom et al 2001;Castro et al 2001;Masetti et al 2001) for the highest redshift absorber. This system shows kinematic substructure, with two distinct systems at z 1 ¼ 1:47667 AE 0:00005 and z 2 ¼ 1:47755 AE 0:00005, corresponding to a rest-frame velocity separation of 106 km s À1 , typical for internal motions in galaxies .…”

“…In this case, the break results from a collimated (jetted) outflow (Sari, Piran, & Halpern 1999;Rhoads 1999); the synchrotron cooling frequency is below the optical passband, and the index p of the electron energy distribution is unusually hard, p $ 1:4 (Cowsik et al 2001;Sagar et al 2001;Stanek et al 2001;Panaitescu & Kumar 2002). Other authors prefer to interpret the break as a dynamical transition to nonrelativistic expansion (in 't Zand et al 2001;Masetti et al 2001), requiring a very energetic burst ($10 54 ergs) expanding into a dense ($10 6 cm À3 ) circumburst medium. More recently, Bjö rnsson et al (2002) have argued in favor of the jet model but invoke continuous energy injection to explain why the standard model may erroneously be deriving a hard electron energy index.…”

“…We supplement the HST WFPC2 and ground-based optical observations from Tables 2 and 3 with optical and infrared observations from Cowsik et al (2001), Lee et al (2001), Masetti et al (2001), Sagar et al (2001), Stanek et al (2001), and Watanabe et al (2001), and with those reported in several GRB Coordinate Network (GCN) circulars (Orosz 2001;Massi, Dolci, & Carlo 2001;Oksanen et al 2001;Veillet 2001aVeillet , 2001bHolland et al 2001). We restricted the data set to observations for which magnitudes were reported relative to field stars and converted those to the calibration of Henden (2001b) and corrected for a small foreground Galactic extinction (E BÀV ¼ 0:023 mag from Schlegel, Finkbeiner, & Davis 1998).…”

“…We restricted the data set to observations for which magnitudes were reported relative to field stars and converted those to the calibration of Henden (2001b) and corrected for a small foreground Galactic extinction (E BÀV ¼ 0:023 mag from Schlegel, Finkbeiner, & Davis 1998). The infrared measurements of the OT by Masetti et al (2001) were taken in poor seeing (3 00 -4 00 ), and the authors therefore used an aperture of 5 00 in radius. Keck K-band observations presented in Frail et al (2002) show that there are several other (relatively bright) near-infrared (NIR) sources nearby.…”