Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst
Elebert, Patrick; Reynolds, M. T.; Callanan, Paul J.; Hurley, Daniel Jason; Ramsay, G.; Lewis, F.; Russell, D. M.; Nord, B.; Kane, S. R.; DePoy, D. L.; Hakala, P.
Date:
2009
Copyright:
© 2009, the Authors. Journal compilation © 2009, RAS
Citation:
Elebert, P., Reynolds, M. T., Callanan, P. J., Hurley, D. J., Ramsay, G., Lewis, F., Russell, D. M., Nord, B., Kane, S. R., DePoy, D. L. and Hakala, P. (2009) 'Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst', Monthly Notices of the Royal Astronomical Society, 395(2), pp. 884-894. doi: 10.1111/j.1365-2966.2009.14562.x
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 NIII lambda 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(-1); applying a 'K-correction', we find the velocity of the secondary star projected on to the line of sight to be 370 +/- 40 km s-1. Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044(-0.004)(+0.005), and the mass function for the pulsar to be 0.44(-0.13)(+0.16)M(circle dot). 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 1.4M(circle dot). Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the CaII H and K interstellar absorption lines suggest that the distance to the source is similar to 2.5 kpc. This is consistent with previous distance estimates based on type-IX-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.
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