The hard X-ray transient MAXI J0655-013 is a Be/X-ray binary

ATel #15612; P. Reig (Institute of Astrophysics/FORTH), A. Tzoubanou (University of Crete), V. Pantoulas (Institute of Astrophysics/FORTH)
on 12 Sep 2022; 07:35 UT
Credential Certification: Pablo Reig (pau@physics.uoc.gr)

Tweet

MAXI J0655-013 was discovered by MAXI/GSC as a bright X-ray transient source on June 18, 2022 (Serino et al., ATel #15442). Its hard spectrum suggested that the source is an X-ray pulsar or a black hole X-ray binary in the hard state. A NuSTAR follow-up observation of MAXI J0655-013 revealed 1129s pulsations, confirming that the source is a binary X-ray pulsar (Shidatsu et al., ATel #15495). Its optical counterpart was proposed to be the 12.5 mag long-period variable star V520 Mon (ATel #15495). V520 Mon was confirmed as the optical counterpart to MAXI J0655-013 when a precise location was provided by Swift (Kennea et al., ATel#15564). Zaznobin et al. (ATel #15582) obtained an optical spectrum of V520 Mon and reported strong Halpha emission, placing MAXIJ0655-013 in the group of Be/X-ray binaries. In this telegram we report optical spectroscopic observations of V520 Mon obtained from the 1.3 m telescope at the Skinakas observatory (Crete, Greece) on the nights September 8 and 9, 2022. We used a 1302 l/mm grating, giving a nominal dispersion of ~0.8 A/pixel. The optical spectrum above 5500 A is dominated by strong Halpha line emission. Below 5000 A, the spectrum displays numerous He I and He II lines, typical of late O-type or early-type B stars. The H$\alpha$ (EW=-32+-2 A)and Hbeta (EW=-2.4+-0.1 A) line profiles are asymmetric and double peaked, with the red peak dominating over the blue peak, while Hgamma exhibits an absorption profile partially affected by emission. The He I 6678 A also shows an asymmetric shape. The presence of He II lines rules out a spectral type later than B0.5. A visual comparison of the 4000-4700 A spectrum with the Atlas by Walborn & Fitzpatrick (1990, PASP, 102, 379) results in an O9.5-B0V as the most likely spectral type of the companion. We conclude that MAXI J0655-013 is a new Be/X-ray binary.