A large spin-up rate measured with INTEGRAL in the high mass X-ray binary pulsar SAX J2103.5+4545

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Milano – IASF/INAF, 20133 Milano, Italy e-mail: sidoli@mi.iasf.cnr.it
² Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden
³ INTEGRAL Science Data Center, 1290 Versoix, Switzerland
⁴ Institut für Astronomie und Astrophysik – Astronomie, Univ. of Tübingen, 72076 Tübingen, Germany
⁵ ESA – European Space Astronomy Center, 28080 Villafranca del Castillo, Spain
⁶ Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Palermo – IASF/INAF, 90146 Palermo, Italy
⁷ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France

Received: 1 March 2005
Accepted: 20 May 2005

Abstract

The High Mass X-ray Binary Pulsar SAX J2103.5+4545 has been observed with INTEGRAL several times during the last outburst in 2002–2004. We report a comprehensive study of all INTEGRAL observations, allowing a study of the pulse period evolution during the recent outburst. We measured a very rapid spin-up episode, lasting ~130 days, which decreased the pulse period by ~1.8 s. The spin-up rate,  ~ -1.5  10^-7 s s^-1, is the largest ever measured for SAX J2103.5+4545, and it is among the fastest for an accreting pulsar. The pulse profile shows evidence for temporal variability, apparently not related to the source flux or to the orbital phase. The X-ray spectrum is hard and there is significant emission up to 150 keV. A new derivation of the orbital period, based on RXTE data, is also reported.