Cornelisse, R., D'Avanzo, P., Muñoz-Darias, T., Campana, S., Casares, J., Charles, P. A., Steeghs, D. (Danny), 1972-, Israel, G. L. and Stella, L.. (2009) Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst. Astronomy & Astrophysics, Vol.495 (No.1). L1-L4. ISSN 0004-6361

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Abstract

Aims. We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K-2), and derive estimates for the pulsar mass. Methods. Using Doppler images of the Bowen region, we find a significant (>= 8 sigma) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure K-em = 248 +/- 20 km s(-1) (1 sigma confidence), which represents a strict lower limit to K-2. Also, the Doppler map of He II lambda 4686 shows the characteristic signature of the accretion disc, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disc that are expected in very low mass-ratio interacting binaries. Results. The lower limit on K-2 leads to a lower limit on the mass function of f (M-1) >= 0.10 M-circle dot. Applying the maximum K-correction gives 228 < K-2 < 322 km s(-1) and a mass ratio of 0.051 < q < 0.072. Conclusions. Despite the limited S/N of the data, we were able to detect a signature of the donor star in SAXJ1808.4-3658, although future observations during a new outburst are still needed to confirm this. If the derived K-em is correct, the largest uncertainty in determining of the mass of the neutron star in SAXJ1808.4-3658 using dynamical studies lies with the poorly known inclination.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science > Physics
Journal or Publication Title:	Astronomy & Astrophysics
Publisher:	EDP Sciences
ISSN:	0004-6361
Date:	February 2009
Volume:	Vol.495
Number:	No.1
Number of Pages:	4
Page Range:	L1-L4
Identification Number:	10.1051/0004-6361:200811396
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	STFC Advanced Fellowship, Spanish MCYT, Spanish MEC
Grant number:	AYA2007-66887, CSD2006-00070
URI:	http://wrap.warwick.ac.uk/id/eprint/28491

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