# GRB 021211 as a Faint Analog of GRB 990123: Exploring the Similarities and Differences in the Optical Afterglows

### Abstract

We present BVR$_C$JHK$_s$ photometry of the optical afterglow of the gamma-ray burst GRB 021211 taken at the Magellan, MMT, and WIYN observatories between 0.7 and 50 days after the burst. We find an intrinsic spectral slope at optical and near-infrared wavelengths of 0.69+/-0.14 at 0.87 days. The optical decay during the first day is almost identical to that of GRB 990123 except that GRB 021211's optical afterglow was intrinsically approximately 38 times fainter and the transition from the reverse shock to the forward shock may have occurred earlier than it did for GRB 990123. We find no evidence for a jet break or the cooling break passing through optical frequencies during the first day after the burst. There is weak evidence for a break in the J-band decay between 0.89 and 1.87 days, which may be due to a jet. The optical and infrared data are consistent with a relativistic fireball where the shocked electrons are in the slow cooling regime and the electron index is 2.3+/-0.1. The forward shock appears to have been expanding in a homogeneous ambient medium during the first day after the burst. Our analysis suggests that the jet of GRB 021211 may have a small opening angle and that the total gamma-ray energy is likely to be much less than the canonical value of 1.33×10$^51$ ergs. If this is the case, then it is possible that most of the energy of the burst is in another form such as a frozen magnetic field, supernova ejecta, or a second jet component. The host galaxy of GRB 021211 is subluminous and has a star formation rate of at least 1 M$_solar$ yr$^-1$.

Some of the observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. Some of the observations reported here were obtained with the Magellan 2 (Landon Clay) telescope at the Las Campanas Observatory. Some of the observations reported here were taken at the WIYN Observatory, a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National Optical Astronomy Observatory.

Publication
Astronomical Journal