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A&A
Volume 436, Number 2, June III 2005
Page(s) 647 - 652
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20042575


A&A 436, 647-652 (2005)
DOI: 10.1051/0004-6361:20042575

INTEGRAL spectroscopy of the accreting millisecond pulsar XTE J1807-294 in outburst

M. Falanga1, J. M. Bonnet-Bidaud1, J. Poutanen2, R. Farinelli3, A. Martocchia4, P. Goldoni1, J. L. Qu5, L. Kuiper6 and A. Goldwurm1

1  CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France
    e-mail: mfalanga@cea.fr
2  Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
3  Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
4  Observatoire Astronomique, 11 rue de l'Université, 67000 Strasbourg, France
5  Laboratory for Particle Astrophysics, Institute of High Energy Physics, CAS, Beijing 100039, PR China
6  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

(Received 20 December 2004 / Accepted 7 March 2005 )

Abstract
The transient X-ray accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satellites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature ~40 keV, considerably higher than the value (~10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of $1.3 \times 10^{37}$ erg s-1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to $3.6 \times 10^{36}$ erg s-1. A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source, with shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass $M_{\rm c} < 0.022~M_{\odot}$, assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.


Key words: accretion, accretion disks -- X-rays: binaries -- stars: individual: XTE J1807-294 -- stars: neutron

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