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MAXI J1305-704: Optical Spectroscopy with SALT

ATel #4105; Phil Charles (Southampton/UCT), Remon Cornelisse (IAC), Deatrick Foster (SAAO/UCT/Vanderbilt), Jorge Casares (IAC), Marissa Kotze (SAAO/UCT), Andrzej Zdzarski (CAMK)
on 9 May 2012; 17:04 UT
Credential Certification: Phil Charles (pac@saao.ac.za)

Subjects: Optical, X-ray, Binary, Black Hole, Transient

We report on optical spectroscopy of the black hole candidate MAXI J1305-704 (ATel #4024; #4030) using SALT and the Robert Stobie Spectrograph. On 2012 April 15 (20:40-23:00 UTC) and 16 (22:30-01:40 UTC) we obtained 2 hrs of continuous spectroscopy using a slit width of 0.6 arcsec, resulting in a wavelength coverage of 4050-5100 Angstrom and a resolving power of approximately 6000. According to the online MAXI monitoring of the source, it was in the high soft state.

The resulting spectra show typical features of an LMXB, i.e. a blue continuum with superimposed Balmer and He II 4686 A emission lines, but little evidence of the presence of He I. Furthermore strong emission from the Bowen blend is present, as well as an absorption feature in the red wing of Hbeta. We note that all emission lines are broad (e.g. FWHM of Hbeta is ~1300 km/s), variable and double peaked (with a velocity separation of ~1000 km/s for He II 4686 A).

These spectra are very reminiscent of those observed from the short period black hole binary XTE J1118+480 (e.g. Torres et al. 2002, ApJ, 569, 423; Dubus et al. 2001, ApJ, 553, 307). However, our spectra show no clear indication of orbital motion in any of the emission lines of MAXI J1305-704, suggesting an orbital period much longer than 2 hrs. Confirmation of the true orbital period (see e.g. ATel #4044, but also see ATel #4073) remains therefore crucial for our further understanding of this black hole candidate.