Phase resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst
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 on the pulsar mass. methods: Using Doppler images of the Bowen region we find a significant (>8sigma) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure Kem=248+/-20 km/s (1sigma confidence) which represents a strict lower limit to K2. Also, the Doppler map of He II lambda4686 shows the characteristic signature of the accretion disk, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disk 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)>0.10Msun. Applying the maximum K-correction gives 228<K2<322 km/s 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 warranted to confirm this. If the derived Kem is correct, the largest uncertainty in the determination of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.