Letter to the Editor

Studies of orbital parameters and pulse profile of the accreting millisecond pulsar XTE J1807–294

¹ European Space Agency (ESA), Research and Scientific Support Department (RSSD), Science Operations and Data System Division (SCI-SD), Science Operations Centre, Apartado, PO Box 50727, 28080 Madrid, Spain e-mail: mkirsch@xmm.vilspa.esa.es
² Space Research Centre, Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
³ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Colaba, Mumbai–400005, India
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁵ Institut für Astronomie und Astrophysik Universität Tübingen, Abteilung Astronomie, Sand 1, 72076 Tübingen, Germany

Received: 2 February 2004
Accepted: 1 July 2004

Abstract

The accreting millisecond pulsar XTE J1807-294 was observed as a Target of Opportunity (ToO) by XMM-Newton on March 22, 2003 after its discovery on February 21, 2003 by RXTE . The source was detected in its bright phase with an observed average count rate of 33.3 cts s^-1 in the EPIC-pn camera in the 0.5–10 keV energy band (3.7 mCrab). Using the earlier established best-fit orbital period of 40.0741 ± 0.0005 min from the RXTE observations and considering a circular binary orbit as first approximation, we derived a value of 4.8 ± 0.1 lt-ms for the projected orbital radius of the binary system and an epoch of the orbital phase of MJD 52 720.67415(16). The barycentric mean spin period of the pulsar was derived as 5.2459427 ± 0.0000004 ms. The pulsar's spin-pulse profile showed a prominent (1.5 ms FWHM) pulse, with energy and orbital phase dependence in the amplitude and shape. The measured pulsed fraction in four energy bands was found to be 3.1 ± 0.2% (0.5–3.0 keV), 5.4 ± 0.4% (3.0–6.0 keV), 5.1 ± 0.7% (6.0–10.0 keV) and 3.7 ± 0.2% (0.5–10.0 keV), respectively. Studies of spin-profiles with orbital phase and energy showed significant increase in its pulsed fraction during the second observed orbit of the neutron star, gradually declining in the subsequent two orbits, which was associated with sudden but marginal increase in mass accretion. From our investigations of orbital parameters and estimation of other properties of this compact binary system, we conclude that XTE J1807-294 is very likely a candidate for a millisecond radio pulsar.