Astron. Astrophys. 355, 629-638 (2000)

2. The ROSAT observation and source detection

The 13.5 ksec ROSAT PSPC pointing observation in Cha II was centered in and and was carried out from September 10 to 17, 1993. The Cha II dark cloud has the right size, of about 1.5 square degrees, to be practically entirely covered in a single PSPC pointing.

The reduction and subsequent analysis of the X-ray data were performed following the pipelines of the Extended Scientific Analysis System (EXSAS). The sources are identified by an excess of photons in comparison with both a local and a global background using a "maximum likelihood" method. For a detailed description of the steps of the reduction we refer to Alcalá et al. (1995). We have fixed the detection threshold to the value of L=7 ¹. This might produce some spurious sources, but we can reach lower X-ray fluxes. In this way, 40 X-ray sources were detected in our ROSAT pointing in Cha II. The RXJ designation, coordinates, count rate, as well as exposure time and maximum likelihood of each X-ray source are listed in Table 1. Following the notation by Feigelson et al. (1993) for the Cha I cloud, we call these sources "CHIIXR". We find that the typical error box for bright sources has a radius of about 3", while for the faintest may be as high as 30".

Table 1. X-ray sources detected in the ROSAT pointed observation of the Chamaeleon II dark cloud.
Notes to table:
: source investigated spectroscopically; UNS: unrelated nearby star resulted from spectroscopic observations, but the corresponding error circle still contains objects fainter than the magnitude limit for our spectroscopic observations; A#: ASCA sources by Yamauchi et al. 1998; L#: IR sources by Larson et al. 1998; : for this source there is another possible counterpart, not observed spectroscopically, closer to X-ray center.

In Fig. 1 the soft (0.1-0.5 keV) and the hard (0.5-2.5 keV) ROSAT images are shown (left and right panel respectively). Except for one source, located almost at the lower border of the PSPC window and identified with the bright star HD113513 (unrelated to Cha II, see Sect. 3), almost all the X-ray sources are detected only in the hard energy band. This means that most of the soft X-ray photons are absorbed or that the sources are intrinsically hard.

Fig. 1. Soft (0.1-0.5 keV, left panel) and hard (0.5-2.5 keV, right panel) ROSAT images in the direction of the Cha II dark cloud.

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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