Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738^⋆

¹ IESL, Foundation for Reseach and Technology-Hellas, 71110 Heraklion, Greece
e-mail: pau@physics.uoc.gr
² Physics Department, University of Crete, 71003 Heraklion, Greece
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA02138, USA

Received: 5 March 2013
Accepted: 3 November 2013

Abstract

Context. Present X-ray missions are regularly discovering new X-ray and γ-ray sources. The identification of their counterparts at other wavelengths allows us to determine their nature.

Aims. The main goal of this work is to determine the properties of the optical counterpart to the INTEGRAL source IGR J21343+4738, and to study its long-term optical variability. Although its nature as a Be/X-ray binary has been suggested, little is known about its physical parameters.

Methods. We have been monitoring IGR J21343+4738 since 2009 in the optical band. We present BVRI optical photometric and spectroscopic observations covering the wavelength band 4000–7500 Å. The photometric data allowed us to derive the colour excess E(B − V) and estimate the distance. The blue-end spectra were used to determine the spectral type of the optical companion, while the spectra around the Hα line allowed us to study the long-term structural changes in the circumstellar disc.

Results. We find that the optical counterpart to IGR J21343+4738 is a V = 14.1 B1IVe shell star located at a distance of ~8.5 kpc. The Hα line changed from an absorption-dominated profile to an emission-dominated profile, and then back again into absorption. In addition, fast line profile asymmetries were observed once the disc developed. Although the Balmer lines are the most strongly affected by shell absorption, we find that shell characteristics are also observed in He I lines.

Conclusions. The optical spectral variability of IGR J21343+4738 is attributed to the formation of an equatorial disc around the Be star and the development of an enhanced density perturbation that revolves inside the disc. We have witnessed the formation and dissipation of the circumstellar disc. The strong shell profile of the Hα and He I lines and the fact that no transition from the shell phase to a pure emission phase is observed imply that we are seeing the system near edge-on.