SALT spectroscopy of the accreting millisecond pulsar binary SAX J1808-3658 during its recent increased activity
ATel #13006; D. A. H. Buckley (South African Astronomical Observatory), E. Kotze (SAAO/SALT), D. de Martino (INAF)
on 8 Aug 2019; 20:40 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)
Subjects: Optical, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 13026
Following the announcement of increased optical activity of the accreting millisecond pulsar binary (AMXP) SAX J1808-3658 on July 30 (ATel# 12964) and of an abrupt brightening beginning August 5 (ATel #12993) and August 6 (ATel #13000), we carried out long-slit optical spectroscopy on August 2, 3 and 6 with the Southern African Large Telescope (SALT). We used the Robert Stobie Spectrograph (RSS) with the PG900 VPH grating, covering the region 4060-7120Å at a mean resolution of 4.7Å, using frame transfer mode with repeated 200 sec exposures.
No spectrum was registered in an accumulated 2400 s exposure on August 2.964 UTC (JD 2458698.4640), although there was a focus issue. On August 3.958 UTC (JD 2458699.4576), a weak spectral trace was detected in an accumulated 2200 s exposure. Starting on August 6.938 UTC (JD 2458702.4381) the object was considerably brighter and 12 repeat 200 s exposures were obtained, revealing Balmer absorption lines and strong interstellar NaD absorption.
The summed spectrum (2400 sec) reveals absorption lines of Hα, Hβ and Hγ, typical of
an optically thick accretion disc seen at moderately low inclination and very similar to the spectrum seen by Cornelisse et al. 2009 (A&A, 495, L1) during the 2008 outburst . The Hα and Hβ lines also show central emission reversals, almost filling in the former. He II 4686Å is weakly detected in emission, while there is no evidence of Bowen emission (NIII 4634Å /CIII 4647-4659Å).
SALT spectrum of SAX J1808.6-3658 6 Aug2019
