Coordinated TeV gamma-ray and optical polarization study of BL Lac object Mkn 501

Abstract

Coordinated TeV gamma-ray and optical polarization observations of the BL Lac object Mkn 501, carried out during 1997 with the TACTIC imaging Cerenkov telescope of the BARC and the 1.2 m Gurushikhar optical/IR telescope at Mt. Abu, have revealed a significant increase in the degree of polarization of the optical radiation in association with a Tev gamma-ray flare. The observations indicate a possible leptonic origin of the TeV gamma-ray signal as against a hadronic origin proposed in some models.