Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburstR. Cornelisse1, P. D'Avanzo2, T. Muñoz-Darias1, S. Campana2, J. Casares1, P. A. Charles3, 4, D. Steeghs5, 6, G. Israel7, and L. Stella7
1 Instituto de Astrofisica de Canarias, Calle via Lactea S/N, 3805 La Laguna, Spain
2 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
3 South Africa Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
4 School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
5 Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
6 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7 INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Rome), Italy
Received 21 November 2008 / Accepted 15 December 2008
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 (K2), and derive estimates for the pulsar mass.
Methods. Using Doppler images of the Bowen region, we find a significant (8) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure = 24820 km s-1 (1 confidence), which represents a strict lower limit to K2. Also, the Doppler map of He II 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 K2 leads to a lower limit on the mass function of f(M1) 0.10 . Applying the maximum K-correction gives 228 < K2 < 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 SAX J1808.4-3658, although future observations during a new outburst are still needed to confirm this. If the derived is correct, the largest uncertainty in determining of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.
Key words: accretion, accretion disks -- X-rays: binaries -- stars: individual: SAX J1808.4-3658
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