RapidUBVRIFollow‐up of the Highly Collimated Optical Afterglow of GRB 010222

Abstract

We present the earliest optical observations of the optical counterpart to the gamma-ray burst (GRB) 010222, obtained with the Fred L. Whipple Observatory 1.2 m telescope in UBV RI passbands, starting 3.64 hr after the burst (0.4 hr after public notiÐcation of the burst localization). We also present late R-band observations of the afterglow obtained with the 1.8 m Vatican Advanced Technology Telescope D25 days after the burst. The temporal analysis of our data joined with published data indicates a steepening decay, independent of wavelength, asymptotically approaching at early times F l P t~0.80B0.05 (t > 1 day) and at late times, with a sharp break at days. This is the F l P t~1.30B0.05 t b \ 0.72^0.10 second earliest observed break of any afterglow (after GRB 980519), which clearly indicates the importance of rapid multiband follow-up for GRB afterglow research. The optical spectral energy distribution, corrected for small Galactic reddening, can be Ðtted fairly well by a single power law with F l P l~1.07B0.09. However, when we Ðt using our BV RI data only, we obtain a shallower slope of [0.88^0.10, in excellent agreement with the slope derived from our low-resolution spectrum ([0.89^0.03). The spectral slope and light-curve decay slopes we derive are not consistent with a jet model despite the presence of a temporal break. SigniÐcant host dust extinction with a starburst reddening law would Ñatten the spectral index to match jet predictions and still be consistent with the observed spectral energy distribution. We derive an opening angle of smaller than any listed in the 2¡ .1, recent compilation of Frail et al. The total beamed energy corrected for the jet geometry is 4 ] 1050 ergs, very close to the "" standard ÏÏ value of 5 ] 1050 ergs found by Frail et al. for a number of other bursts with light-curve breaks.

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.…”

“…Several authors have interpreted the optical afterglow in terms of the standard relativistic blast wave model (Sari, Piran, & Narayan 1998). 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.…”

“…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).…”

“…Following previous efforts (Cowsik et al 2001;Sagar et al 2001;Stanek et al 2001;Panaitescu & Kumar 2002), we chose to fit the data in terms of the standard afterglow model (e.g., Sari et al 1998Sari et al , 1999). We do not favor an interpretation in which the break is due to a transition of the blast wave to the nonrelativistic phase (in 't Zand et al 2001;Masetti et al 2001).…”

“…Previous fits to the GRB 010222 light curves have treated these two parameters as independent (e.g., Stanek et al 2001). We take 1 ¼ Àð3p À 2Þ=4, 2 ¼ Àp, and ¼ Àp=2 (where p is the power-law index of the electron energy distribution), a relation that applies in the standard jet model when the optical band lies above the synchrotron cooling frequency (Sari et al 1999).…”