The optical counterpart of the superluminal source GRO J1655 - 40

Abstract

THE accretion of gas from a companion star onto a compact object, such as a neutron star or black hole, can release large amounts of energy. Episodic accretion is generally thought to explain transient X-ray emission, such as that associated with the recently discovered<SUP>1</SUP> Galactic superluminal source GRO J1655 - 40, and to be connected with the ejection of material at relativistic velocities<SUP>2</SUP>. The only other known Galactic superluminal source<SUP>2</SUP> is difficult to study because it is obscured by intervening gas and dust. Here we report the discovery of the optical counterpart to GRO J1655 - 40, which we estimate to lie at a distance of only 3 kpc. We have identified the precursor on historical photographic plates; at the time of the X-ray burst in August 1994<SUP>1</SUP>, it brightened by 3 mag in the V bandpass. Although the 12-day delay between the X-ray burst<SUP>1</SUP> and the main radio outburst which marked the beginning of superluminal motion<SUP>3</SUP> poses a puzzle that remains to be explained, our results show that at optical wavelengths the characteristics of GRO J1655 - 40 are similar to those of other accreting-black-hole candidates<SUP>4-7</SUP>.