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ATEL # 1653; D.Steeghs (Warwick/CfA), C.Knigge (Southampton), J.Drew (Herts), Y.Unruh (Imperial), R.Greimel (Graz) and the IPHAS Consortium (www.iphas.org)
on 11 Aug 2008; 10:27 UT
Password Certification: Danny Steeghs (email@example.com)
Subjects: Optical, X-ray, Binaries, Cataclysmic Variables
We report on optical observations of the galactic plane containing the X-ray source IGR J19267+1325=1RXS J192626.8+132153 (ATEL #1649,#1323) using data from the INT Photometric H-Alpha Survey (IPHAS; http://www.iphas.org ).
The field containing the X-ray source was observed in good observing conditions on August 22 2004 using the Isaac Newton Telescope at La Palma Observatory and its Wide Field Camera. All IPHAS data are astrometrically tied to the 2MASS frame and photometrically calibrated using regular standard star observations (see Drew et al. 2005 for a discussion of the IPHAS survey and its point source photometry). A point source is detected at the recently announced Chandra X-ray position (ATEL #1649), matching to a few tenths of an arc-second (see finder chart). Two sets of r', i' and narrow-band H-alpha measurements indicate modest short-term variability, with r'=17.70+/-0.03, i'=16.74+/-0.05 and significant H-alpha excess with (r' - H-alpha) = 1.1-1.2 mag.
Tomsick et al. 2008 (ATEL #1649) report a hydrogen column of N_H ~ 2.1 x10^21 cm^-2, which corresponds to E(B-V)~0.4. At this reddening, field stars with colours similar to our point source would have a (r'-H-alpha) colour index of only +0.4-0.5. The source is indeed an extreme outlier in the (r' - i')/(r' - H-alpha) colour-colour diagram populated by the sources in the field of view (see colour-colour diagram). Such a large H-alpha excess is therefore evidence for strong intrinsic H-alpha emission and the source is listed in the first IPHAS catalogue of emission line sources (Witham et al. 2008).
The large H-alpha excess together with its proximity to the ROSAT source 1RXS J192626.8+132153 led to its inclusion in our program of spectroscopic follow-up of IPHAS sources. An optical spectrum was obtained in July 2007 using the 4.2m William Hershel Telescope and its dual-beam ISIS spectrograph. This revealed a rich emission line spectrum containing strong and broad lines from the Balmer and Paschen series as well as HeI and HeII in emission. H-alpha was detected with an emission EW of ~120 Angstrom and a FWHM of 750 km/s. Such a spectrum is typical of a cataclysmic variable and the presence of significant HeII emission at a strength comparable to the higher Balmer lines is suggestive of a CV containing a magnetic white dwarf.
An association with a magnetic CV is also not surprising given the detection in the ROSAT bright source catalogue as well as with Integral. Magnetic CVs have been shown to make a significant contribution to the population of hard X-ray sources in the galactic plane.
We conclude that the X-ray source detected by ROSAT, SWIFT and Integral is associated with a nearby, probably magnetic, cataclysmic variable.
IPHAS finder chart plus colour-colour diagram