Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst

P. Elebert, M. T. Reynolds, P. J. Callanan, D. J. Hurley, G. Ramsay, F. Lewis, D. M. Russell, B. Nord, S. R. Kane, D. L. DePoy, P. Hakala

Published 2009-01-26Version 1

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4-3658, obtained during the 2008 September/October outburst. Doppler tomography of the N III 4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 +/- 15 km/s; applying a "K-correction", we find the velocity of the secondary star projected onto the line of sight to be 370 +/- 40 km/s. Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044^{+0.005}_{-0.004}, and the mass function for the pulsar to be 0.44^{+0.16}_{-0.13} Msun. Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4-3658 is more massive than the canonical value of 1.4 Msun. Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca II H and K interstellar absorption lines suggest that the distance to the source is ~2.5 kpc. This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.