Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst |
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| Authors: | 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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| Institution: | Department of Physics, University College Cork, Cork, Ireland;Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA;Armagh Observatory, College Hill, Armagh BT61 9DG;Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff CF24 3AA;Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA;Las Cumbres Observatory Global Telescope, 6740 Cortona Drive, Goleta, CA 93117, USA;Astronomical Institute 'Anton Pannekoek', Kruislaan 403, 1098 SJ Amsterdam, the Netherlands;Physics Department, University of Michigan, 450 Church Street, Ann Arbor, MI 48109-1040, USA;NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA;Department of Physics, Texas A&M University, 4242 TAMU, College Station, TX 77843-4242, USA;Tuorla Observatory, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland |
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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 λ4640.64 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+0.005−0.004 , and the mass function for the pulsar to be 0.44+0.16−0.13 M⊙ . 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 M⊙ . 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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| Keywords: | accretion accretion discs binaries: close stars: individual: V4580 Sagittarii stars: neutron pulsars: individual: SAX J1808 4−3658 X-rays: binaries |
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