On V520 Mon - the optical counterpart of MAXI J0655-013/SRGA J065513.5-012846

ATel #15582; Zaznobin I. A., Sazonov S. Yu., Mereminskiy I. A., Burenin R. A., Lutovinov A. A., Semena A. N., Molkov S. V., Lyapin A. R. (IKI RAS, Moscow, Russia), Eselevich M. V. (ISTP, SB RAS, Irkutsk, Russia)
on 1 Sep 2022; 12:29 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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On June 19, 2022, the MAXI all-sky monitor reported on the new, bright X-ray transient MAXI J0655-013 (ATel #15442), which was later independently confirmed by Swift/BAT (ATel#15443). A long follow-up NuSTAR observation revealed coherent pulsations at 1129s (ATel#15495) and a typical X-ray pulsar spectrum, strongly suggesting that the source is a new Galactic HMXB undergoing an outburst. The source had been previously detected in 2020-2021 during the SRG/ART-XC all-sky X-ray survey (SRGA J065513.5-012846) and identified as an HMXB candidate based on its likely association with the long-period star V520 Mon (Pavlinsky et al. 2022). This association has now been firmly established thanks to an improved localization of the X-ray source by Swift/XRT (ATel#15561, #15564).

As part of our identification program of new X-ray sources from the SRG/ART-XC all-sky survey (Pavlinsky et al. 2022) we obtained an optical spectrum of V520 Mon on 2021-12-04 with the ADAM spectrometer on the 1.6m AZT-33IK telescope of the Sayan observatory. The spectrum with 8.6Å resolution was accumulated for 600s. Its most prominent feature is an Hα line, which has an equivalent width of EW=-36±2Å. There are also Hβ in emission and Hγ, Hδ in absorption as well as plenty of HeI lines. Overall, from observed optical properties we could preliminarily propose that the optical star in this system is B1-3e III-V.

From the optical spectrum and other observed properties we can conclude that the source MAXI J0655-013/SRGA J065513.5-012846 is a representative of the BeXRB class. Both the Corbet and Porb-EW diagrams suggest that the orbital period should be around 100-200 days. Given that the outburst lasted for ≈50 days and reached a luminosity of a few x 10³⁷ erg s^-1 we suggest that the source experienced a Type-I outburst during summer 2022. Detections at a lower luminosity level (≈10³⁴ erg s^-1) could then correspond to stable accretion from cold accretion disk (Tsygankov et al., 2017).