Optical Spectropolarimetry of the GRB 020813 Afterglow

Abstract

The optical afterglow of gamma-ray burst GRB 020813 was observed for 3 hr with the LRIS spectropolarimeter at the Keck I telescope, beginning 4.7 hr after the burst was detected by the HETE-2. The spectrum reveals numerous metal absorption lines that we identify with two systems at and . We also detect z p 1.223 z p 1.255 an [O ii] l3727 emission line at , and we identify this galaxy as the likely host of the GRB. After a z p 1.255 correction for Galactic interstellar polarization, the optical afterglow has a linear polarization of 1.8%-2.4% during 4.7-7.9 hr after the burst. A measurement of on the following night by Covino et p p 0.80% ‫ע‬ 0.16% al. demonstrates significant polarization variability over the next 14 hr. The lack of strong variability in the position angle of linear polarization indicates that the magnetic field in the jet is likely to be globally ordered rather than composed of a number of randomly oriented cells. Within the framework of afterglow models with collimated flows, the relatively low observed polarization suggests that the magnetic field components perpendicular and parallel to the shock front are only different by about 20%.

Dissecting the Circumstellar Environment of γ‐Ray Burst Progenitors

“…Although the results are directed to the gas in this GRB host galaxy, neutral species have been reported in other GRB afterglow spectra (e.g., Barth et al 2003;Foley et al 2006) and our conclusions are qualitatively applicable to those cases. One can also generalize the results to additional atomic species, e.g., Si 0 and C 0 .…”

Evolution of the polarization of the optical afterglow of the γ-ray burst GRB030329

“…Previous single measurements found low-level (1-3 %) polarization 8−15 in optical afterglows, and the only reports on variable polarization 16,17 were based on few measurements with different instruments and modest signal-to-noise. We have overcome the previous sampling limitations with 31 polarimetric observations of the afterglow of GRB 030329 obtained with the same instrumentation (plus few more with different instruments) over a time period of 38 days for a total of an unprecedented ∼50 hours of observations with an 8 m telescope.…”

Linear and circular polarimetry observations of gamma-ray burst afterglows

“…The case of 030329 in particular shows some correlated behaviour between the light curve bumps and polarisation behaviour (Greiner et al 2003), which makes interpretation in terms of simple polarimetry models difficult (Granot & Königl 2003). Data of GRB 020813 may also obey this correlation between light curve and polarisation variability: its polarisation curve is smooth (Fig 1; Barth et al 2003;Gorosabel et al 2004;Lazzati et al 2004) just like the optical light curve (Laursen & Stanek 2003).…”