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

摘要

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 kms-1 (1confidence), which represents a strict lower limit toK2. Also, the Doppler map of HeII 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