2009A&A...495L...1C

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. 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±20km/s (1σ confidence), which represents a strict lower limit to K₂. Also, the Doppler map of HeII 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. The lower limit on K₂ leads to a lower limit on the mass function of f(M₁)≥0.10M_☉. Applying the maximum K-correction gives 228<K₂<322 km/s and a mass ratio of 0.051<q<0.072. 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.