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ATEL # 1369; M.A.P. Torres (CfA), P. Rodriguez-Gil (ING), D. Steeghs (Warwick/CfA), P. G. Jonker (SRON/CfA)
on 25 Jan 2008; 15:02 UT
Password Certification: Danny Steeghs (firstname.lastname@example.org)
Subjects: Optical, X-ray, Transients, Variables, Stars
We report optical spectroscopy of LSPM J1528+5937, the proposed optical counterpart to the recently discovered X-ray transient XMMSL1 J152835.4+593754 (Saxton et al. 2008, ATel #1361).
A 900s spectrum was acquired starting on 2008 Jan 20 05:43 UT using the 4.2m William Herschel Telescope on La Palma equipped with the dual-beam ISIS spectrograph. The observations were performed using the R300B and EEV12 CCD in the blue arm and the R158R grating and EEV4482 CCD in the red arm. A 1.5" slit width yielded a spectral resolution of 11 Angstrom and 19 Angstrom FWHM in the blue and red, respectively. The extracted spectra covered the wavelength ranges 3230 - 5310 Angstrom and 5120-10500 Angstrom.
The optical spectrum closely resembles a late-type stellar spectrum with TiO bands being the dominant absorption features. The strength of these diatomic molecular bands are consistent with those expected in a dwarf with spectral type in the range M5 +/ 1. Narrow emission lines are also detected. Additional constraints are provided by the broadband colors of the counterpart after correcting for the reddening. Using the 2MASS magnitudes and the estimated apparent optical magnitude (Lepine & Shara,2005,AJ,129.1483) we derive (V-K,J-H,H-K)o=(5.41,0.57,0.27) based on a E(B-V)=0.016 mag (Schlegel et al.,1998,ApJ,500,525). The uncertainty in the infrared colors is 0.03 mag. The above colors support a M4V-M5V dwarf (see e.g. Bessel & Brett,1988,PASP,100,1134). For comparison, M4 and M5 dwarfs have colors (5.26 0.60,0.28) and (6.12,0.62,0.32), respectively. We can then infer a distance of ~90 pc to XMMSL1 J152835.4+593754 by using Mv=12.3 (appropriate for a M5 V star), V=17.11 and Av=0.054.
The Balmer lines (up to Hdelta) and CaII K & H are clearly detected in pure emission. These lines are single-peaked and dominated by the instrumental profile. Their equivalent widths (in Angstroms) are: 4.6 (Halpha), 7.5 (Hbeta), 8.1 (Hdelta), 12 (CaII/Hepsilon), 21 (Ca II K). Uncertainties are ~10%, except for CaII K and CaII/Hepsilon which carry larger uncertainties due to the difficulty in choosing the underlying continuum.
The optical spectrum is indicative of a chromospherically active M dwarf, which is further supported by the broadband colors of the counterpart. X-ray flares are common in such dMe stars and the observed X-ray activity of XMMSL1 J152835.4+593754 is most likely due to such an event.