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ESO VLT spectral observations of SAX J1808.4-3658 in outburst

ATel #1752; Gavin Ramsay (Armagh Observatory), Patrick Elebert (University College Cork), , Paul Callanan, (University College Cork), Mark Reynolds (University of Michigan), Pasi Hakala (Tuorla Observatory)
on 1 Oct 2008; 20:21 UT
Credential Certification: Paul Callanan (paulc@ucc.ie)

Subjects: Optical, Binary

We report on observations made using the ESO VLT of the ongoing outburst of the accreting millisecond pulsar SAX J1808.4-3658 (ATEL #1728). Spectra were obtained using Kueyen+FORS1 (1200B Grism, ~370-510 nm) and Antu+FORS2 (600B Grism, ~330-621 nm) on 27 Sept 2008 with a series of exposures starting at 01:03 UT (FORS1) and 00:10 UT (FORS2) and lasting for a total of ~1hr 40 each. Using the acquisition image, we determine the source was at R~17.2 (consistent with that reported in ATEL #1749). Analysis of the 1200B grism data reveals deep, broad Balmer absorption lines with weak emission features near the line cores. However, weak emission features from the Bowen blend near 4640 Ang (EW~-0.3 Ang) and He II at 4686 Ang (EW~-0.8 Ang) are also present. Although the trailed spectra are noisy, Doppler tomography provides tentative evidence for radial velocity shifts in the Bowen blend in both the 600B and 1200B Grism data. The phasing of the Bowen blend shifts is consistent with an origin on the secondary, with a velocity of ~300 km/s. This yields a mass function of ~0.2 Msolar: however, this is likely a lower limit, as it does not include the `K-correction' (Munoz-Darias et al. 2005, ApJ, 635, 502). We note that such a limit, when combined with the known value of asin(i) (e.g. Chakrabarty and Morgan 1998 Nature 394 346) would yield a lower limit to the system inclination of ~35 degrees, for a 1.4 Mo neutron star. The He II Doppler map shows circular emission associated with an accretion disc, with a brighter region of emission near the expected location of the hotspot. These preliminary estimates are subject to confirmation. We thank the DG of ESO for approving these observations.