next previous
Up: Identification of a

3. Optical observations

  For all X-ray positions in our sample, with the exception of a few positions for which obvious optical counterparts were known from SIMBAD, NED or other astronomical data bases (see below), we obtained optical images and spectra. These observations were carried out at the 2.15m telescope of the Guillermo Haro Observatory which is located near Cananea, Sonora, Mexico and which is operated by INAOE. For this purpose a dedicated focal reducer camera, the Landessternwarte Faint Object Spectrograph and Camera (LFOSC) was constructed at the LSW. The instrument was attached to the Cassegrain focus of the telescope and allows direct CCD imaging, filter photometry, and multi-object spectroscopy. It is equipped with an EEV P8603 CCD detector (tex2html_wrap_inline2629 pixels, 22 tex2html_wrap_inline2631m linear pixel size). With a tex2html_wrap_inline2633 field of view the image scale is 1tex2html_wrap_inline2635pixeltex2html_wrap_inline2637.

For direct imaging we used Johnson B and Cousins R filters (Bessell 1979). In this observing mode a limiting magnitude of tex2html_wrap_inline2643 can be reached within 900tex2html_wrap_inline2645. Observations in B and R were used to derive the colours for a rough first object classification.

For the spectroscopic observations we used masks with circular holes of 3tex2html_wrap2785 projected diameter in the focal plane of the telescope. These masks were produced directly from the CCD images using a computer controlled drilling device. In addition to the object holes each mask contains holes producing spectra of the sky background near the objects. Alternatively, a slit mask can be inserted in order to perform normal long slit spectroscopy. The spectra are oriented in E-W direction. Perpendicular to the direction of dispersion, i.e. in N-S direction, a minimum separation of the holes of at least 5tex2html_wrap2787 was found to be necessary to avoid overlapping spectra. Therefore more than one spectral exposure was required if several candidate objects separated by less than 5tex2html_wrap2789 in N-S direction were present in the field. Figure 4 (click here) shows an example for such a field.

Two different grisms giving reciprocal linear dispersions of 250Åmmtex2html_wrap_inline2657 and 360Åmmtex2html_wrap_inline2659, respectively, were used. For holes 2tex2html_wrap2791 5 to the east of the field centre (which is also the position of the long slit) the spectral intervals covered were 4000-7200 Å and 4200-8800 Å, respectively. With a hole diameter of 3tex2html_wrap_inline2667 the spectral resolution tex2html_wrap_inline2669 was about 13 Å and 18 Å, respectively, i.e. a resolving power of tex2html_wrap_inline2671 at central wavelengths.

For flat fielding and wavelength calibration of the spectra built-in halogen, and neon and xenon lamps were used. Flux standard stars were observed for flux calibration of the spectra. The spectra were reduced with a ESO-MIDAS based software package available at the Landessternwarte.

Accurate sky subtraction was achieved by using the intensity of the night sky emission line of [OI]tex2html_wrap_inline26735577 for calibrating small differences in the throughput of the individual mask holes.

For the classification of stellar counterparts we observed a grid of spectroscopic standard stars with spectral types between O and late M, and luminosity classes I, III, and V using both grisms. Tests showed that with these standard stars a spectral classification with an accuracy of better than 5 subclasses can be obtained for objects brighter than about tex2html_wrap_inline2675 19tex2html_wrap_inline2677 with 40tex2html_wrap_inline2679 exposure time. Therefore, even the optically faintest X-ray luminous coronal emitters expected in our subsample could be identified with LFOSC. Counterparts with emission lines, as e.g. AGN, could be identified as faint as tex2html_wrap_inline2681 to 21tex2html_wrap_inline2683.

  figure554
Figure 3: R band image of the field around the X-ray source RXJ1207.7+3148. The 90% SASS error circle (radius 35tex2html_wrap2797 ) is indicated. Many faint and diffuse objects, most likely distant galaxies, are visible. The brightness of the faintest visible objects is tex2html_wrap_inline2689. For the two brightest objects near the source position, designated by "A'' and "B'', spectra were obtained. The remaining objects are too faint for spectroscopy. Object "A'', whose spectrum is shown in the lower panel, appears to be a galaxy with tex2html_wrap_inline2691 20.3tex2html_wrap_inline2693 and tex2html_wrap_inline2695 22.3tex2html_wrap_inline2697. Absorption features most likely due to MgI b and NaI D with a redshift of tex2html_wrap_inline2699 are visible. Residuals of the night sky lines are marked by "ns'', "x'' is a cosmic. Object "B'' is a 16th magnitude F-type star. This star is visually too faint to be a plausible counterpart. On the B image no object brighter than about 22tex2html_wrap_inline2703 was visible making a QSO as counterpart unlikely. The galaxy "A'' is visually too faint to be a plausible counterpart of the X-ray source (see Sect. 4.3 (click here)). Hence, the most likely identification of this X-ray source is a distant cluster of galaxies

  figure559
Figure 4: R image of the X-ray source RXJ0747.3+6822. The 90% SASS error circle is indicated (radius 44tex2html_wrap2801 ). With two exposures spectra of all objects within 60tex2html_wrap2803 radius around the RASS position could be obtained. "r'' denotes reflex images of bright stars in the field. In the lower panel the spectrum of object "A'' is displayed which is the likely counterpart of the RASS source. It is a Sy 1 galaxy with tex2html_wrap_inline2711 18tex2html_wrap_inline2713 at redshift z = 0.120

  figure564
Figure 5: R image of the position of the X-ray source RXJ0403.5+0837. In the the 90% SASS error circle (radius 44tex2html_wrap2807 ) several possible candidates for the optical counterpart are visible. The objects observed spectroscopically are designated by S1, "2'', "5'', and "8''. The bright object S1 is a 13th magnitude G to K-type star. Object #5 with tex2html_wrap_inline2721 18.4tex2html_wrap_inline2723 is a QSO with Htex2html_wrap_inline2725+[OIII ] and MgII tex2html_wrap_inline2727 at redshift z = 0.589. Htex2html_wrap_inline2731 partly falls into the atmospheric band at 7600Å. Each of the two objects could be the X-ray source or at least contribute to the observed X-ray flux (see text). The remaining objects are faint stars which can be excluded for being the counterpart of the X-ray source

Examples of observations obtained with LFOSC are shown in Figs. 3 (click here), 4 (click here), and 5 (click here). Usually possible counterparts within a circle of about 50tex2html_wrap2809 to 60tex2html_wrap2811 radius around the X-ray position were observed (see below). This is typically the 3tex2html_wrap_inline2737 error circle as calculated by the standard ROSAT reduction software (SASS) (see Paper I). The average SASS 90% error circles which are indicated in the figures are of the order of 40tex2html_wrap2813 to 45tex2html_wrap2815. As noted below, our identifications discussed in Sect. 5 (click here) showed that the true error circles are, in fact, smaller.

The observations collected in Cananea were supplemented by spectroscopic observations with the 2.2m telescope at ESO, La Silla, and with the 72 cm Waltz telescope at the Landessternwarte Heidelberg. In March 1996 we observed part of the sources in area III at the ESO/MPIA 2.2m telescope. These observations were obtained with the EFOSC2 spectrometer which was equipped with a Thomson tex2html_wrap_inline2743 pixel CCD chip (ESO CCD #19). The spectral resolution obtained with grism #1 and the 1tex2html_wrap2817 slit (cf. ESO Users Manual) was tex2html_wrap_inline2747 Å and hence comparable to that achieved with the lower resolution grism of LFOSC. A sample of bright stellar counterparts previously identified with LFOSC were observed with higher spectral resolution between March and October 1993 at the 72 cm Waltz telescope in order to study the spectra in more detail. A Boller & Chivens spectrograph attached to the Nasmyth focus and equipped with an EEV P8603 CCD chip with tex2html_wrap_inline2749 22 tex2html_wrap_inline2751m pixels was used. The grating with 1200 lines mmtex2html_wrap_inline2753 yielded a reciprocal linear dispersion of 44 Å tex2html_wrap_inline2755, and (with a 2.4tex2html_wrap2819 slit) a spectral resolution of about 2Å. The observed spectral region was 6250-6750 Å.

Because of varying weather conditions most of the direct images could not be directly calibrated photometrically. Therefore we started an additional observing program to obtain secondary photometric sequences for the fields around each X-ray source. These photometric observations are being carried out in Cananea and at the Calar Alto 1.23m telescope in Spain. Details will be described elsewhere. At this time and throughout the present paper we use photometry taken from the HST Guide Star Catalog (Lasker et al. 1988) (GSC) and from the APM catalogue (Irwin & McMahon 1992). For the APM photometric data V magnitudes can be estimated from O and O-E by using the relation tex2html_wrap_inline2767 by (Irwin & McMahon 1992) and the colour transformation determined for the POSS plates by Humphreys et al. (1991) who found O-B to be nearly independent of B-V to within 0.3tex2html_wrap_inline2773 for tex2html_wrap_inline2775. Likewise, an estimate for R can be obtained from E and O.

Since Sep. 1990 we observed more than 800 RASS positions and we obtained in this way more than 3500 spectra of suspected optical counterparts. Meanwhile, observations or literature identifications (SIMBAD and NED databases) exist for nearly all sources in the count-rate and area limited complete subsample described above. In the next section we discuss the method of optical identification.


next previous
Up: Identification of a

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr