Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst

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.

12 pages, 10 figures; Accepted for publication in MNRAS

References in corpus (8)

• Empirical Color Transformations Between SDSS Photometry and Other Photometric Systems
• Broad relativistic iron emission line observed in SAX J1808.4-3658
• Detection of the Irradiated Donor in the LMXBs 4U 1636-536 (=V801 Ara) and 4U 1735-444 (=V926 Sco)
• The light curve of the companion to PSR B1957+20
• Optical Observations of SAX J1808.4-3658 During Quiescence
• Swift observations of SAX J1808.4-3658: monitoring the return to quiescence
• Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455
• A transient I band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4-3658

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