INTEGRAL observation of the high-mass X-ray transient V 0332+53 during the 2005 outburst decline

¹ INTEGRAL Science Data Centre, Ch. d'Écogia, 1290 Versoix, Switzerland
² Observatoire de Genève, Ch. des Maillettes, 1290 Versoix, Switzerland
³ Institut für Astronomie und Astrophysik – Astronomie, Sand 1, 72076 Tübingen, Germany
⁴ School of Physics and Astronomy, University of Southampton, SO171BJ, UK
⁵ Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093-0424, USA
⁶ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁷ CEA Saclay, DSM/DAPNIA/SAp(CNRS UMR 7158 AIM), 91191 Gif-Sur-Yvette, France
⁸ Stockholm Observatory, Alba Nova, 106 91 Stockholm, Sweden

Received: 21 September 2005
Accepted: 14 December 2005

Abstract

The decline of the high mass X-ray transient V 0332+53 during the Dec. 2004 to Feb. 2005 outburst is analysed from the data recorded by INTEGRAL. The flux is shown to decrease exponentially until 2005 Feb. 10, with a decay time scale of ~30 days above 20 keV and ~20 days at lower energies, and to decrease linearly thereafter. The energy spectrum is well modelled throughout the decay by a power law with a folding energy of ~7.5 keV, and with two cyclotron absorption features. The folding energy does not vary significantly over the decay, but the spectrum becomes harder with time. Most importantly, we show that the parameters describing the fundamental cyclotron line around 27 keV do vary with time: its energy and depth increase (by about 17% for the energy in ~6 weeks), while its width decreases. These changes of the cyclotron line parameters are interpreted as resulting from a change in the extent of the cyclotron scattering region. Two quasi-periodic oscillations are also observed at various times during the observations, one at 0.05 Hz and another one near the pulsation frequency around 0.23 Hz.