ESO VLT observations of MAXI J1836-194
ATel #3640; Manfred W. Pakull and Christian Motch (Observatoire de Strasbourg)
on 8 Sep 2011; 15:59 UT
Credential Certification: Manfred W. Pakull (pakull@astro.u-strasbg.fr)
Subjects: Optical, Transient
Referred to by ATel #: 3652
Several medium resolution (2.4 A FWHM) optical spectra of the counterpart of X-ray transient and black hole candidate MAXI J1836-194 (ATel #3611, #3613, #3614, #3619, #3628) were obtained on Sept. 1 and 2 UT with the FORS2 instrument on the ESO VLT UT1 telescope and using grisms1200B and 1200R. Total exposure times were 90 min in the blue range (3700 A to 5200 A) and 45 min in the red range (5900A to 7300 A). No spectral changes were detected between the two nights. The spectra are dominated by broad Balmer absorption lines (FWHM of Hbeta ~ 20 A) with superimposed narrow emission cores (FWHM ~ 4.0 A for Hbeta) confirming the results reported in ATel 3614. Furthermore, we detect the hallmarks of X-ray binaries in the form of HeII 4686 emission (0.49 A Equivalent Width, EW, and 6.0 A FWHM) and the presence of CIII/NIII Bowen complex at 4630-50 A (0.57 A EW). In view of the large optical variability and spectral features we agree with the conclusion of Rau et al. (ATel 3619) that the optical light of MAXI J1836-194 is not dominated by a Be star mass donor. Interstellar CaII H & K absorption lines and the diffuse 4430 A feature indicate moderate IS reddening. The latter feature is consistent with E(B-V)~0.6 using the relation of Krelowski et al. 1987, ApJ 316, 449.
