2. The Cygnus area
2.1. Selection from the ROSAT all-sky survey
The main scientific goal justifying the selection of this particular area in Cygnus was the study of the X-ray source population at very low galactic latitudes. At the time the optical observations started (1991 May) the Standard Analysis Software System (SASS; Voges et al. 1992) had analyzed a limited fraction of the all-sky survey. The only low galactic latitude areas then available were comprised between ecliptic longitude = and = . Two years later, a larger field embracing the former one was interactively analyzed using the Extended Scientific Analysis System (EXSAS) developed at MPE (Zimmermann et al. 1992).
As a result of this two step X-ray reduction process the completeness of the optical investigations carried out in these two regions is slightly different and we shall define an 'inner' area corresponding to the part in which the first automatic analysis took place and a 'full' area corresponding to the EXSAS analysis.
Another complication arises from the existence of intense diffuse emission from the northern part of the Cygnus super bubble (see Fig. 2). The high number of spurious point-like sources produced by both the SASS and EXSAS analysis in regions of enhanced background and the resulting reduced sensitivity for detecting point sources led us to discard these specific areas amounting to a total of 13.5 deg2.
Both the 'inner' and 'full' areas are roughly centered at l = and b = . The SASS analysis estimates the background from data collected within wide strips parallel to the scan direction of the satellite in survey mode, whereas the interactive analysis allows to handle in one run all X-ray photons detected from the entire selected sky region. This difference of approach means that the source parameters, in particular, count rates, derived from the interactive run are in principle more reliable than those given by the automatic process and we shall not consider the latter in this paper. Also, a slightly lower source acceptance maximum likelihood (ML) for the 'full' area yielded the detection of several more sources in the 'inner' area.
Table 1. Characteristics of the investigated areas
The selected area is located in a range of galactic longitude which is closest to the north ecliptic pole. Near the ecliptic poles the exposure time in the ROSAT all-sky survey reached the maximum and therefore a strong gradient in exposure time is present in the selected field from about 500 s at to 1000 s at . However, our sample is rather homogeneous since 84% of the sources have exposure times ranging from 700 to 900 s. Other areas in the galactic plane have lower exposures, like e.g. the galactic center region with only a few hundred seconds (see the exposure map of the ROSAT all-sky survey in Snowden et al. 1995).
The background level in the PSPC image, determined from source-free areas outside the diffuse emission regions shows no strong variations and was between 0.7 and 0.8 counts per square arcmin for the full energy band of 0.1 - 2 keV. The source detection using the maximum likelihood technique from the EXSAS package was done in three energy bands, namely 0.1 - 0.4 keV, 0.5 - 2.0 keV and 0.1 - 2.0 keV. Sources were formally accepted above a maximum likelihood value of 7. Because of the high expected level of spurious sources among the very low ML detections (up to 0.29 sources per square degree or 19 spurious detections with ML 7 over the 'full' area, see Sect. 9.3) we only considered sources with ML 8 for most statistical purposes. Each of these sources, listed in Table 11, has an associated running index ranging from 1 to 128 and increasing with decreasing count rates. Since some of the detections with ML between 7 and 8 had established optical counterparts of scientific interest, we separately listed in Table 12 these additional sources which were given index numbers in the range of 129 to 158.
The area analyzed here is about 1/4 of that covered by the whole Einstein Galactic Plane Survey (Hertz & Grindlay 1984, 1988). However, the flux completeness level of our sample in Cygnus is 4 times fainter and accordingly the total amount of sources studied in this paper is comparable to that in the total Einstein survey.
2.2. Astrophysical characteristics
In the direction of l = , b = , the line of sight first crosses the local spiral arm during the first 1 kpc and then reaches the Perseus arm at a distance of about 4 kpc (e.g., Vogt & Moffat 1975, Georgelin & Georgelin 1976). Further away, the HI Cygnus arm (Kulkarni et al. 1982) is encountered at a distance of 11 kpc.
Two molecular cloud systems dominate the interstellar absorption and are well visible on Fig. 4. At l the edge of the Cygnus Rift cloud complex appears. However, the main structure is the cloud related to the CYG OB7 association (Dame & Thaddeus 1985) located in the l ; b part of our selected region. From CO measurements Dame & Thaddeus (1985) derive distances of 700 - 800 pc for these two molecular clouds, well within the local spiral arm. These two structures are also visible in the optical absorption map of Neckel & Klare (1980) at a comparable distance. Finally, a similar structure may be seen in the dark cloud map of Feitzinger & Stüwe (1986).
Our sample region overlaps with the north-east part of the X-ray super-bubble discovered by Cash et al. (1980). This ring-shaped soft X-ray diffuse emission is well seen in Snowden et al. (1995). This hot bubble is thought to be located at a distance of about 2 kpc and its total angular extent of implies a diameter of 450 pc. The total X-ray energy radiated is 5 1036 erg s-1 at a temperature of 2 106 K and the total energy content of the bubble is estimated to be of the order of 1051 ergs (Cash et al. 1980). The origin of this large structure is unknown but probably related to the Cyg OB2 association. A series of 30-100 supernovae during the last 3-10 million years or hot winds emanating from OB associations and interacting with the interstellar medium could explain the X-ray emission.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998