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Title:
Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst
Authors:
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.; Hakala, P.
Affiliation:
AA(Department of Physics, University College Cork, Cork, Ireland), AB(Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA), AC(Department of Physics, University College Cork, Cork, Ireland), AD(Department of Physics, University College Cork, Cork, Ireland), AE(Armagh Observatory, College Hill, Armagh BT61 9DG), AF(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), AG(Astronomical Institute `Anton Pannekoek', Kruislaan 403, 1098 SJ Amsterdam, the Netherlands), AH(Physics Department, University of Michigan, 450 Church Street, Ann Arbor, MI 48109-1040, USA), AI(NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA), AJ(Department of Physics, Texas A&M University, 4242 TAMU, College Station, TX 77843-4242, USA), AK(Tuorla Observatory, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 2, pp. 884-894. (MNRAS Homepage)
Publication Date:
05/2009
Origin:
MNRAS
MNRAS Keywords:
accretion, accretion discs , binaries: close , stars: individual: V4580 Sagittarii , stars: neutron , pulsars: individual: SAX J1808.4-3658 , X-rays: binaries
DOI:
10.1111/j.1365-2966.2009.14562.x
Bibliographic Code:
2009MNRAS.395..884E

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 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 +/- 15kms-1 applying a `K-correction', we find the velocity of the secondary star projected on to the line of sight to be 370 +/- 40kms-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+0.16-0.13Msolar. 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 1.4Msolar. 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 ~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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