HST/STIS Image and UV Spectrum of GRB 000301C

The HST/STIS UV spectrum of the transient believed associated with GRB 000301C. The strong spectral break at about 2700 A is well modelled by Lyman limit absorption at z=1.95

The Spectrum

We have observed the proposed optical counterpart of GRB 000301C (Fynbo et al GCN 570, Bernabei et al. GCN 571) HST/STIS-PRISM mode during four orbits totaling 8000s from 6.26 to 6.49 March 2000 (UT). The MAMA NUV detector with sensitivity from 1175A to 3400A was used. Data reductions were performed with STIS GTO IDL tools using prelaunch spectral dispersion constants and on-orbit sensitivity calibrations.

A spectrum is detected with a peak S/N ~ 10 beginning at ~2650A, extending to ~3250A with an average flux of ~4.6+/-0.2E-18 ergs/cm2/sec. The emission is 2.2 pixels FWHM (0.055 arcseconds) along the slit indicating a point source with no obvious diffuse extension. The spectrum appears relatively flat (in f_lambda) from 3200A to 2750A and then presents a sharp drop at about 2700A.

Within the present calibration uncertainties, the observed spectrum is well fitted by a model using the power-law index determined by Halpern et al. (GCN 585), the observed V = 21.9 magnitude from the STIS CCD images obtained on 6.2 March (Fruchter et al. GCN 602) a moderate amount of extinction in the restframe of the OT, and a Lyman limit break at ~2700A.

Assuming the Lyman break is caused by neutral hydrogen in the host galaxy the redshift of GRB 000301C is 1.95 +/- 0.1 where the large error bar is due almost entirely to the present uncertainty in the STIS UV prism calibration. We expect a more accurate calibration shortly.

The present citation for this work is Smette et al. GCN 603 (2000).