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

Cornelisse, R., D'Avanzo, P., Muñoz-Darias, T., Campana, S., Casares, J., Charles, P.A., Steeghs, D., Israel, G. and Stella, L. (2009) Phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst. Astronomy & Astrophysics, 495 (1), L1-L4. (doi:10.1051/0004-6361:200811396).

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 (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.

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