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arXiv · 0901.3991

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
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Abstract

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.

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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. 2009-01-26. Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst. https://doi.org/10.1111/j.1365-2966.2009.14562.x

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