Elebert, P. and Reynolds, M. T. and Callanan, P. J. and Hurley, D. J. and Ramsay, G. and Lewis, F. and Russell, D. M. and Nord, B. and Kane, S. R. and 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. ISSN 0035-8711 http://resolver.caltech.edu/CaltechAUTHORS:20090824-095405124
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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 iiiλ4640.64 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.
|Additional Information:||© 2009 The Authors. Journal compilation © 2009 RAS. Accepted 2009 January 26. Received 2009 January 26; in original form 2008 November 7. Part of this work is based on observations made at the European Southern Observatory, Chile. We thank the ESO Director General for a generous allocation of Director’s Discretionary Time (DDT 281.D-5060, 281.D-5061). The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). We thank the staff and students of Glenlola Collegiate, South Downs Planetarium, Oundle School, Dartford Grammar School and Portsmouth Grammar School for performing some of the Faulkes Telescope observations. Thanks to C. Izzo and S. Bagnulo for advice on applying the skyline correction to our spectra. This research made use of NASA’s Astrophysics Data System, and the SIMBAD data base, operated at CDS, Strasbourg, France. We thank J. A. Orosz for use of the ELC code. We acknowledge the use of MOLLY and DOPPLER software packages developed by T. R. Marsh, University of Warwick. X-ray quick-look results provided by the ASM/RXTE team. We thank Ricardo Schmidt and Marco Bonati of CTIO for building the Dark Energy Camera CCD system and Juan Estrada and the entire CCD production effort at Fermilab for creating the CCD detector. Fermilab is operated by the Fermi Research Alliance, LLC under contract no. DE-AC02-07CH11359 with the United States Department of Energy. PE and PJC acknowledge support from Science Foundation Ireland. FL would like to acknowledge support from the Dill Faulkes Educational Trust.|
|Subject Keywords:||accretion, accretion discs binaries: close stars: individual: V4580 Sagittarii stars: neutron pulsars: individual: SAX J1808.4−3658 X-rays: binaries|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||24 Aug 2009 17:31|
|Last Modified:||26 Dec 2012 11:14|
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