A torque reversal of 4U 1907+09

¹ Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany e-mail: fritz@astro.uni-tuebingen.de
² INTEGRAL Science Data Centre, 16 Ch. d'Écogia, 1290 Versoix, Switzerland
³ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁴ CASS, University of California at San Diego, La Jolla, CA 92093-0424, USA
⁵ CEA Saclay, Unité Mixte de Recherche CEA – CNRS – Université Paris 7 (UMR 7158/AIM) – DSM/DAPNIA/SAp, 91191 Gif-sur-Yvette, France

Received: 5 May 2006
Accepted: 8 August 2006

Abstract

Aims.We present an analysis of the accreting X-ray pulsar system 4U 1907+09 based on INTEGRAL data. The main focus of this analysis is a study of the timing behavior of this source. In addition we also show an analysis of the 5–90 keV spectrum.

Methods.The data were extracted using the official INTEGRAL software OSA 5.1. Timing analysis was performed using epoch folding and pulsar pulse phasing.

Results.We have measured 12 individual pulse periods for the years 2003 to 2005. We confirm earlier RXTE results that during 2003 the spin down became slower and show furthermore that after this phase 4U 1907+09 started to spin up with in 2004. The similarity of the pulse period histories of 4U 1907+09  and 4U 1626-26 suggests that accretion onto an oblique rotator, as recently proposed by Perna et al., is a possible explanation for this change.