First optical identification of a supersoft X-ray source in M31^*

¹ Department of Astronomy, Sofia University, Bulgaria
² Isaac Newton Institute of Chile, Bulgarian branch
³ e-mail: japet@phys.uni-sofia.bg
⁴ INAF-Osservatorio Astronomico di Torino, Strada Osservatorio, 20, I10025 Pino Torinese (TO), Italy
⁵ Department of Astronomy, 474 N. Charter Str., Madison WI 53706, USA
⁶ Max-Planck-Institut für Astronomie, Königstuhl, 69117 Heidelberg, Germany
⁷ Department of Astronomy, Caltech, MC 105-24, Pasadena, CA 91125, USA
⁸ INAF-Osservatorio Astronomico di Arcetri, Firenze, Italy
⁹ Astrophysikalisches Institut, 14482 Potsdam, FRG
¹⁰ University of Hawaii, Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
¹¹ Canada France Hawaii Corporation, PO Box 1597, Kamuela, HI 96743, USA
¹² Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnyj Arkhyz, Russia

Corresponding author: M. Orio, orio@cow.physics.wisc.edu

Received: 19 December 2001
Accepted: 11 April 2002

Abstract

We propose the first association of an optical counterpart with a luminous supersoft X-ray source in M31, RX J0044.0+4118, observed with ROSAT in July 1991. The PSPC position is at 1.6´´ angular distance from a candidate nova in outburst in September of 1990. This is interesting because the incidence of classical novae among supersoft X-ray sources is an open question. The proposed optical counterpart was measured at in September of 1990, and it had faded to when it was observed again after 70 days. The light curve was too sparsely monitored for definite conclusions on the speed class of the nova. No other variable objects with were found in the ROSAT spatial error box. We evaluate that the probability that a classical or recurrent nova was in outburst in the ROSAT error box in the few years preceding the observation is very small, so the proposed identification is meaningful. We also show evidence that the associated supersoft X-ray source turned off in the third year after the outburst.