Bailyn, Charles D.Charles D.BailynOrosz, Jerome A.Jerome A.OroszGirard, Terrence M.Terrence M.GirardJogee, ShardhaShardhaJogeeDELLA VALLE, MassimoMassimoDELLA VALLEBegam, Michael C.Michael C.BegamFruchter, Andrew S.Andrew S.FruchterGonzález, RosaRosaGonzálezLanna, Philip A.Philip A.LannaLayden, Andrew C.Andrew C.LaydenMartins, Donald H.Donald H.MartinsSmith, MalcolmMalcolmSmith2022-10-042022-10-0419950028-0836http://hdl.handle.net/20.500.12386/32687THE 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>.STAMPAenArticle10.1038/374701a02-s2.0-0040347374https://www.nature.com/articles/374701a0https://api.elsevier.com/content/abstract/scopus_id/00403473741995Natur.374..701BFIS/05 - ASTRONOMIA E ASTROFISICA