2. Observational results
2.1. RASS observations
RX J1028.6-0844 is one of the 674 RASS X-ray sources studied by us. Position, X-ray flux, etc. as derived from the RASS data base (Voges et al. 1997a (Paper I), Paper III) are summarized in Tab. 1. In the re-processed SASS II data base (Voges et al. 1997b ) the position shifted slightly by about 22 towards the SE (, ) relative to the position given in Tab. 1.
Table 1. X-ray properties of the RASS source RX J1028.6-0844. For the conversion of count rate to flux a photon index of and galactic was assumed (Dickey & Lockman 1990 ). The hardness ratios are defined as and , where , etc. are the count rates in the respective energy bands keV, keV, keV, and keV (cf. Zimmermann et al. 1994 ).
2.2. Optical observations
We obtained direct images in the Cousins R and Johnson B bands of the field around RX J1028.6-0844 in May 1992 and December 1996 with the 2.15 m telescope at the Guillermo Haro Observatory near Cananea, Sonora, Mexico. The telescope was equipped with a focal reducer camera (LFOSC) which has a field of view of and an image scale of 1 px-1. A detailed description of the instrumentation is given in Paper II. In March 1996 we obtained a further R image and spectra of 3 possible optical counterparts of RX J1028.6-0844 at the ESO/MPIA 2.2 m telescope at La Silla, Chile. The observations were obtained with the EFOSC2 spectrometer (cf. ESO Users Manual) equipped with a Thomson CCD chip (ESO CCD #19). EFOSC2 gave a higher spatial resolution than LFOSC. The image scale of 0.336 px-1 resulted in a field of view of .
Fig. 1 shows the R band image obtained with EFOSC2 in March 1996. The position of the RASS X-ray source as given in Tab 1 is at the center of the 90% error circle for which a radius of 27 has been determined by Zickgraf et al. (1997 ) for sources with low X-ray count rates. The revised SASS-II position (see above) is indicated in Fig. 1 by a small circle. For the objects labelled A to C spectra were obtained with EFOSC2 using grism #1 and a 1 slit yielding a spectral resolution of Å. A spectrum of object "d" was obtained with LFOSC in December 1996. The spectral resolution was about 18 Å.
The spectrum of the object marked "A" in Fig. 1 is displayed in Fig. 2. This object is a QSO for which Ly gives a redshift of . From the emission line of C IV 1548/1551 we measured . However, this line appears to be affected on the red side by the broad atmospheric absorption feature around 8200 Å. We therefore adopt the redshift from Ly in the following. The Ly forest is clearly visible. Also the Ly-continuum absorption edge at 912 Å is indicated at Å.
Object B in Fig. 1 is a late F to early G-type star, and object C is an elliptical galaxy at redshift . Object "d" (see Fig. 3) is a galaxy without emission lines in the spectrum. Due to the very low S/N-ratio of the spectrum the redshift cannot be determined.
We estimated the brightnesses of the objects visible near the X-ray position from our CCD images. Unfortunately, the B and R image obtained at the Guillermo Haro Observatory were observed during non-photometric conditions and our programme for photometric calibration of the deep images is not yet finished. In order to obtain an approximate calibration for the B image we therefore made use of the COSMOS UK Schmidt (UKST) plate scan data base of the Royal Observatory Edinburgh which is available at the Max-Planck-Institut für extraterrestrische Physik. Object A was barely visible on our B image. The estimated statistical error of the B magnitude measured from this image is of the order of . This does not include a systematic error due to the calibration of the COSMOS data which is probably also of the order of , yielding an approximate total error of . For the calibration of the R image observed at ESO under good conditions we determined the instrumental zero point from a photometrically calibrated R image of the field around the RASS source RX J0720-3125 (Haberl et al. 1996 ). We estimated the resulting error to about . The B and R magnitudes for the four marked objects in Fig. 1 derived in this way are summarized in Tab. 2. Because object #d is quite faint and diffuse on our R image its brightness is rather uncertain.
Table 2. B and R magnitudes of the objects #A to #d near the X-ray position of RX J1028.6-0844.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998