Letter to the Editor

Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst^*

¹ Instituto de Astrofisica de Canarias, Calle via Lactea S/N, 3805 La Laguna, Spain e-mail: corneli@iac.es
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

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 (K₂), and derive estimates for the pulsar mass.

Methods. Using Doppler images of the Bowen region, we find a significant (≥8σ) 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 (1σ confidence), which represents a strict lower limit to K₂. Also, the Doppler map of He II λ4686 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 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 K_em 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.