Strongly Polarized Optical Afterglows of Gamma-Ray Bursts.pdf

a r X i v : a s t r o - p h / 9 9 0 5 2 7 6 v 1 2 1 M a y 1 9 9 9 Strongly Polarized Optical Afterglows of Gamma-Ray Bursts Andrei Gruzinov Institute for Advanced Study, School of Natural Sciences, Princeton, NJ 08540 ABSTRACT The optical afterglows of the gamma ray bursts can be strongly polarized, in principle up to tens of percents, if: (i) the afterglow is synchrotron radiation from an ultra- relativistic blast, (ii) the blast is beamed during the afterglow phase, i.e. the shock propagates within a narrow jet, (iii) we observe at the right time from the right viewing angle, (iv) magnetic fields parallel and perpendicular to the jet have different proper strengths. Subject headings: gamma rays: bursts – polarization – shock waves 1. Introduction, observations Gamma-ray burst (GRB) afterglows were explained by synchrotron radiation from an ultra- relativistic blast wave (Paczynski Rhoads 1993, Katz 1994, Meszaros Rees 1997, Vietri 1997, Waxman 1997). A good way to test the synchrotron emission model is to measure the polarization (Loeb Perna 1998, Gruzinov Waxman 1999). Hjorth et al (1999) found an upper limit of 2.3% on the linear polarization of the optical emission from GRB 990123. Covino et al (1999) have detected a 1.7% linear polarization of the optical transient associated to GRB 990510. A few percent polarization relative to the stars in the field can be induced by dust along the line of site in the host galaxy. To be sure that the polarization is from the GRB afterglow, one needs to look for the time variability of the polarization signal (Gruzinov Waxman 1999). While current observations are not in disagreement with the model of Gruzinov Waxman (1999), we would like to bring attention to the fact that a much stronger polarization of the optical afterglows, tens of percents, is theoretically possible. Polarization of more than a few percent would be a true signature of the synchrotron emission model. The strong polarization might be achieved if