Optical spectroscopic observations of A 0535+262 during its ongoing giant outburst

ATel #14171; M. Ozbey Arabaci (Univ. of Southampton), T. Saguner Rambaldi (METU), P. Ochner (INAF OAPd - Univ. Padova), D. Altamirano (Southampton), E. Sonbas (Univ. of Adiyaman), M. Zejmo (University of Zielona Gora)
on 12 Nov 2020; 22:19 UT
Credential Certification: Mehtap Ozbey Arabaci (m.ozbey-arabaci@soton.ac.uk)

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Following the detection of an increase in the X-ray activity of the Be/X-ray binary A 0535+262 (ATel #14157), we have performed optical follow-up observations of the system with the B&C spectrograph mounted on the 1.22-m Galileo telescope at Asiago Astrophysical Observatory. The spectra were obtained between November 7-11, 2020 (MJD ~ 59160.95-59164.114), using two different gratings (1200 ln/mm) blazed at 4000 Å and 6825 Å in order to cover the wavelength range of 3820–7000 Å. Preliminary analysis shows that Hα and Hβ lines are in emission with an average EW of -16.56±0.35 and -2.42±0.03 Å, respectively. Both profiles are single-peaked but slightly asymmetric with a possible weak hump on the blue side. HeI λ4713 Å and λ5016 Å lines are in double-peaked emission whereas λ5876 and λ6678 emission profiles have rather complicated shapes with weak multi-components may indicate a warping episode of the Be decretion disk (Moritani et al. 2013).

Since its discovery, A 0535+262 has undergone eight type II outbursts (so-called giant; Lx ≥ 10³⁷ erg/s), several type I (normal) outbursts, and some peculiar brightening periods which could not be fit in the classical outburst definition. We note that the system has been in an active phase since October 2018 (ATel #12092) and exhibited consecutive outbursts in Swift/BAT light curve. Using the ephemeris from Camero-Arranz et al. (2012), we found these outbursts showed shifts relative to the periastron passages. The peak of the previous outburst (type I) corresponds to the time of periastron passage (MJD 59055.43), which occurred 13-day after the onset. Similarly, the current outburst started approximately 15 days before the next periastron passage on MJD ~59166.5 (13-14 November 2020). Given that the orbital phase shifts might be related to the structural changes in the decretion disk and can be investigated through the optical spectroscopic observations, further multiwavelength observations during the outburst are highly encouraged.

The daily average BAT flux of the source as of writing this report (MJD 59165.9) has increased to 0.611±0.022 count/cm2/s (~ 3 Crab) and evolved to a giant outburst.

We thank Luca Zampieri for providing observing time from his schedule on November 11, 2020.

Optical spectra of A 0335+262