Astron. Astrophys. 359, 113-130 (2000)

2. The ROSAT HRI X-ray observations

2.1. The ROSAT HRI images

We have observed the dense cores A and F of the Oph dark cloud with the ROSAT   HRI . The detector is sensitive to the 0.1-2.4 keV energy range, but has no spectral resolution. The two observation fields were respectively centered on approximatively the WTTS DoAr21 ( [J2000]) and on the Class I protostar YLW15 = IRS43 ( [J2000]). ROSAT   HRI images have a diameter of . Fig. 1 displays the two observation fields, which include the dense DCO⁺ cores A, B, C, E, and F (Loren et al. 1990), most of the area studied by CMFA and ASCA , as well as the ISOCAM survey. The first observation field, centered approximatively on  Oph A, will be referred to as the "core A field"; the second observation field, centered on dense core F, will be called the "core F field". The core A field was observed between 1995 August 29 and 1995 September 12 with a total exposure of 51.3 ks. The core F field was observed at three different epochs (hereafter observations #1, #2, and #3): between 1995 March 9 and 14 (12.5 ks), between 1995 August 18 and 20 (27.5 ks), and between 1996 September 7 and 11 (37.2 ks). These three different epochs give a total exposure of 77.2 ks (see Table 1 for the log of ROSAT   HRI observations details).

Fig. 1. Map of the Ophiuchi dark cloud observation fields. 2o diameter ROSAT   PSPC field of view, and its central studied by Casanova et al. (1995); 40´ diameter ROSAT   HRI fields of view and YLW15 area published in Grosso et al. (1997); ASCA   GIS field of view from Koyama et al. (1994) and Kamata et al. (1997); ISOCAM survey field (central programme observations by Nordh et al.; see Abergel et al. 1996). The background image of the Ophiuchi star forming region is taken from the first Digitized Sky Survey, contrast was enhanced with Gaussian histogram specification (Canzian 1997). Regions of high visual extinction are clearly visible. DCO+(J=2-1) contours from Loren et al. (1990) show for reference the location of dense cores A, F.

Table 1. Log of ROSAT   HRI Observations.

2.2. The HRI image analysis

We have analyzed separately the four data sets. Standard source detection algorithms were used to find X-ray sources, and to search optical counterparts for each set. We then selected X-ray sources with the best position accuracy (usually the brightest not too far away from the field center) and having an unambiguous counterpart, corrected all X-ray positions from the existing offsets, and used this improved astrometry to remove possible ambiguities in the identification of the X-ray sources. Details can be found in Appendix A.

We find 63 HRI X-ray sources. Fig. 2 shows the positions of these sources, superimposed on a combined IR and optical image of the Oph cloud. Coordinates, error boxes, likelihoods of existence, and count rates of these HRI X-ray sources can be found in Appendix A (Table A1).

Fig. 2. X-ray sources in the ROSAT   HRI fields. The composite ISOCAM map of Abergel et al. (1996) (LW2 filter image [5-8.5 µm], plus LW3 filter image [12-18 µm]) is merged with the background optical image taken from the first Digitized Sky Survey. DCO+(J=2-1) contours show the location of dense cores named A, B, C, E, F by Loren et al. (1990). A scale of 0.5 pc is shown for pc. The positions of the X-ray sources are marked by -size squares. Typical X-ray error positions range between 1"-6".

2.3. HRI X-ray source identifications

Identification of the HRI X-ray sources was made by cross-correlations with published lists of confirmed or suspected cloud members (AM), IR surveys (Greene & Young 1992; BKLT; and Bontemps et al. 2000, see 3), K-band spectroscopic (Luhman & Rieke 1999), radio surveys (André, Montmerle, & Feigelson 1987; Stine et al. 1988; Leous et al. 1991), and with previously known X-ray sources (PSPC : CMFA, Martín et al. 1998; ASCA : Kamata et al. 1997). For X-ray sources without published counterparts we have used SIMBAD , ¹ and we have also searched optical counterparts on optical red band images from the Digitized Sky Survey. ²

As shown by the finding charts in Appendix B, thanks to the good angular resolution of the ROSAT   HRI (PSF FWHM on axis; due to the mirrors as well as the detector), the position accuracy (-, see Col. 6 Table A1) allows us to find counterparts almost without ambiguity. ³ Identification lists for the core A and core F fields are given in Appendix B, Table B1. We also discuss in Appendix B the identification of X-ray sources with a low statistical significance.

Nearly of the HRI X-ray sources are identified. We detect only of the PSPC X-ray sources (CMFA; Casanova 1994), but this can be explained by the difference in sensitivity between the two instruments, and the intrinsic variability of the X-ray sources (see Appendix C).

© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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