Polarization and visual intensity of the inner light echo around SN 1987A

Abstract

The ESO Faint Object Spectrograph and Camera operating in the coronographic mode on the 3.6 m telescope was used with a broad-band V filter and four linear polarizers oriented at position angles 0 deg, 45 deg, 90 deg, and 135 deg to measure the total and polarized flux of radiation scattered from circumstellar material around SN 1987A. A faint arc of V band emission with a sharp inner and outer boundary centered on the supernova and of radius approximately equal to 8.3 arcsec and width approximately equal to 2.5 arcsec is clearly discernible above background, most prominently in the eastern quadrant between 45 deg and 135 deg position angle. Its azimuthally averaged surface brightness in this position angle range is 21.8 + or - 0.2 V mag/arcsec. In this region, the detected radiation is found to be partially linearly polarized with a degree of polarization 0.15 + or - 0.04 and an electric vector orientation of about 0 deg position angle. These results indicate that the reflecting material is confined to a thin sheet or shell of column density near 10 to the 18th to 10 to the 19th atoms/sq cm located at about 3-5 pc from the SN whichmore » may have arisen from a previous red giant phase of the SN progenitor. 16 refs.« less