On 1994 May 1 UT we obtained spectra of several stars near the PSPC position (Table 1) in an effort to identify the optical counterpart. We used the Hiltner 2.4m telescope at MDM Observatory on Kitt Peak, the Mark III spectrograph, and a Tektronix thinned CCD detector. The spectra covered from 4700 to 6700 Å with a resolution of Å. Because the aim was a quick search, exposure times were only 180 s, and the signal-to-noise was low. None of the stars observed (stars A-C and number 2 in Fig. 1) showed any sign of being the counterpart; all appeared to be ordinary late-type stars. The spectrograph had a long slit, and the field was rather crowded, so we examined the 2-dimensional CCD spectra in case a more promising candidate had fortuitously landed anywhere in the 1" slit. Amazingly, in the frame containing the spectrum of star `A' we found a spectrum showing the broad Balmer emission characteristic of a cataclysmic! On the following night we obtained h of time series spectra, which were sufficient to show an orbital period of h (Sect. 4).
Table 1. X-Ray and Optical Positions
We derived the position of the cataclysmic using a direct image taken with the MDM 1.3 m McGraw-Hill telescope. A fit to six stars from the HST Guide Star Catalog (version 1.2) yielded the optical position listed in Table 1, with an uncertainty (estimated from the fit residuals) of ".
The optical position is 83" from the PSPC position, which is much worse than typical location errors for a source this strong (0.20 cts s-1). To clarify the identification, we observed the source with the HRI, once for 22.9 ksec in 1997 April and again for 5.2 ksec in 1997 November (Sect. 5). In both observations we found two HRI sources near the position (see Table 1). Source 1, the stronger of the two, was located well within the error circle of the PSPC source RX J1910.8+2856, while the position of the weaker Source 2 was nearly coincident with that of the cataclysmic binary. We assign the name RX J191059.6+285639 to the cataclysmic variable (source 2) derived from the position found in the longer HRI observation. The PSPC source was apparently the superposition of two sources, the weaker of which is the cataclysmic, so the cataclysmic's discovery is essentially accidental!
What about Source 1, the stronger of the two? The HRI position error is dominated by the " uncertainty in the satellite aspect. Assuming that Source 2 is the cataclysmic, for which we have an accurate position, we correct for the aspect uncertainty and derive a position for Source 1 accurate to ". This position (Table 1), derived using the longer HRI observation, is bracketed by the open cross-hairs in Fig. 1. There is no bright star at the position, but a object is 1.2" NE of the position, and a object is 3.4" W. The fainter of the two appears to be a good candidate for the identification based on its position, while the brighter one is less likely.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000