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Astron. Astrophys. 323, L21-L24 (1997)

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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 [FORMULA] towards the SE ([FORMULA], [FORMULA]) relative to the position given in Tab. 1.


[TABLE]

Table 1. X-ray properties of the RASS source RX J1028.6-0844. For the conversion of count rate to flux [FORMULA] a photon index of [FORMULA] and galactic [FORMULA] was assumed (Dickey & Lockman 1990 ). The hardness ratios are defined as [FORMULA] and [FORMULA], where [FORMULA], etc. are the count rates in the respective energy bands [FORMULA] keV, [FORMULA] keV, [FORMULA] keV, and [FORMULA] 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 [FORMULA] and an image scale of 1 [FORMULA] 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 [FORMULA] CCD chip (ESO CCD #19). EFOSC2 gave a higher spatial resolution than LFOSC. The image scale of 0.336 [FORMULA] px-1 resulted in a field of view of [FORMULA].

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 [FORMULA] 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 [FORMULA] slit yielding a spectral resolution of [FORMULA] Å. A spectrum of object "d" was obtained with LFOSC in December 1996. The spectral resolution was about 18 Å.

[FIGURE] Fig. 1. R image of the field of RX J1028.6-0844. The 90% error circle of the RASS position from Paper III is indicated. The revised SASS-II position is given by the small circle. The position of the radio source PKS B1026-084 is marked by [FORMULA]. The PMN 4.85 GHz source is indicated by [FORMULA].

The spectrum of the object marked "A" in Fig. 1 is displayed in Fig. 2. This object is a QSO for which Ly [FORMULA] gives a redshift of [FORMULA]. From the emission line of C IV [FORMULA] 1548/1551 we measured [FORMULA]. 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 [FORMULA] in the following. The Ly [FORMULA] forest is clearly visible. Also the Ly-continuum absorption edge at [FORMULA] 912 Å is indicated at [FORMULA] Å.

[FIGURE] Fig. 2. Spectrum of object #A in Fig. 1. It is a quasar with redshift of [FORMULA] as measured from Ly [FORMULA].

Object B in Fig. 1 is a late F to early G-type star, and object C is an elliptical galaxy at redshift [FORMULA]. 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.

[FIGURE] Fig. 3. Iso-contour plot (on a logarithmic intensity scale, step size is 0.01 between contour lines) of the region around the QSO #A shown in Fig. 1. The seeing was [FORMULA]. Object #d is clearly extended.

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 [FORMULA]. This does not include a systematic error due to the calibration of the COSMOS data which is probably also of the order of [FORMULA], yielding an approximate total error of [FORMULA]. 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 [FORMULA]. 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]

Table 2. B and R magnitudes of the objects #A to #d near the X-ray position of RX J1028.6-0844.


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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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