On the cyclotron line in Cepheus X-4

¹ School of Physics & Astronomy, University of Southampton, Highfield, SO17 1BJ, UK e-mail: vanessa@astro.soton.ac.uk
² Dr. Karl Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁴ INTEGRAL Science Data Centre, 16 Ch. d'Écogia,1290 Versoix, Switzerland
⁵ Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA
⁶ European Space Astronomy Centre (ESAC), European Space Agency, PO Box 50727, 28080, Madrid, Spain

Received: 5 February 2007
Accepted: 19 May 2007

Abstract

Context.Accreting X-ray pulsars provide us with laboratories for the study of extreme gravitational and magnetic fields, hence accurate descriptions of their observational properties contribute to our understanding of this group of objects.

Aims.We aim to detect a cyclotron resonance scattering feature in the Be/X-ray binary Cep X-4 and to investigate pulse profile and spectral changes through the outburst.

Methods.Spectral fitting and timing analysis are employed to probe the properties of Cep X-4 during an outburst in 2002 June.

Results.A previously announced cyclotron feature at 30.7 keV is confirmed, while the source shows spectral behaviour and luminosity related changes similar to those observed in previous outbursts. The long-term X-ray lightcurve shows a periodicity at 20.85 d, which could be attributed to the orbit in this Be system.