Multiwavelength monitoring of BD +53°2790, the optical counterpart to 4U 2206+54

¹ Institut de Ciència dels Materials, Universidad de Valencia, PO Box 22085, 46071 Valencia, Spain e-mail: pere.blay@uv.es
² Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, EPSA, Universidad de Alicante, PO Box 99, 03080 Alicante, Spain e-mail: ignacio@dfists.ua.es
³ Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
⁴ Physics Department, University of Crete, 710 03 Heraklion, Crete, Greece e-mail: pau@physics.uoc.gr
⁵ School of Physics and Astronomy, Southampton University, Southampton SO17 1BJ, UK e-mail: M.J.Coe@soton.ac.uk
⁶ X-ray Astrophysics Laboratory, Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Universities Space Research Association e-mail: corbet@milkyway.gsfc.nasa.gov
⁸ Observatori Astronómic, Universidad de Valencia, PO BOX 22085, 46071 Valencia, Spain e-mail: juan@pleione.oauv.uv.es
⁹ Crimean Astrophysical Observatory, Nauchny, Crimea, 334413, Ukraine e-mail: tarasov@crao.crimea.ua

Received: 29 July 2005
Accepted: 5 October 2005

Abstract

We present the results of our long-term monitoring of BD +53°2790, the optical counterpart to the X-ray source 4U 2206+54. Unlike previous studies that classify the source as a Be/X-ray binary, we find that its optical and infrared properties differ from those of typical Be stars: the variability of the V/R ratio is not cyclical; there are variations in the shape and strength of the Hα emission line on timescales less than 1 day; and no correlation between the EW and the IR magnitudes or colors is seen. Our observations suggest that BD +53°2790 is very likely a peculiar O9.5V star. In spite of exhaustive searches we cannot find any significant modulation in any emission line parameter or optical/infrared magnitudes. Spectroscopy of the source extending from the optical to the K-band confirms the peculiarity of the spectrum: not only are the He lines stronger than expected for an O9.5V star but also there is no clear pattern of variability. The possibility that BD +53°2790 is an early-type analogue to He-strong stars (like Ori C) is discussed.