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Astron. Astrophys. 359, 960-966 (2000) 2. X-ray observationsThe X-ray observations were obtained with the ROSAT X-Ray Telescope
(Trümper et al. 1991) in combination with the High-Resolution
Imager (HRI, David et al. 1995). The log of the observations is given
in Table 1 ; the last entry in that table is the one obtained
near the BeppoSAX observations, the other entries refer to earlier
observations in the ROSAT data archive. The standard data reduction
was done with the Extended Scientific Analysis System (Zimmermann et
al. 1996). To take into account the re-calibration of the pixel size
(Hasinger et al. 1998), we multiply the
Table 1. Log of the ROSAT HRI observations of NGC 6440. For each observation we give the JD at start and end of the exposures and the effective exposure time, and the number of counts and the countrate for the central source, according to the standard reduction. Errors for detections are 1- 2.1. The 1998 observationNo source is detected in the 1998 Sep 8 observation. For a point source at the center of the image, 90% of the photons arrive within a circle with a 5" radius, in stable HRI pointings (David et al. 1995). At the time of observation the ROSAT satellite pointing was experiencing difficulties, effectively extending the radius of the point spread function by a few arcseconds. We therefore search a circle with a 10" radius around the center of NGC 6440 (according to Picard & Johnston 1995; see Table 3), only five photons are detected. The maximum of five photons detected remains if we move the center of the circle to any location within 30" of the nominal cluster centre, thus allowing for possible inaccurate reconstruction of the satellite pointing. For an expected number of 10 photons, the Poisson probability of
detecting 5 or fewer photons is 7%. We thus take 10 photons as the
2- This implies that the flux of the transient in NGC 6440 dropped by a factor 250 or more between the BeppoSAX observation on Aug 26 and the ROSAT HRI observation on Sep 8. 2.2. Earlier ROSAT observations: standard analysisThe ROSAT data archive contains several hitherto unpublished observations of NGC 6440 made with the ROSAT HRI after the 1991 observation reported by Johnston et al. (1995). A list of all ROSAT observations is given in Table 1. We have analyzed each observation separately with the standard procedure, and detect the source in NGC 6440 in the 1993 observation and in the Sep 1994 observation, i.e. in the observations with the longer exposure times. In the shorter observations, we only obtain upper limits. From the observed number of 3 counts in a circle with 5" radius near the cluster center we derive an upper limit to the countrate of 8 counts for the central source for both the 1992 and the March 1994 observation. The three detections are compatible with a constant countrate, at a level below the derived upper limits. We therefore have no indication of variability between the ROSAT observations. On the other hand, a variation by a factor 2 in the countrate between the 1991 and the 1993 detections is well within the range allowed by the limited statistics. The only source, other than the central source, detected
significantly in any of the ROSAT HRI pointings, is a point source in
the 1993 observation, listed as X3 in Table 3. There is no bright
( For each of the two longest observations, the standard analysis indicates that the central source is extended, i.e. the distributions of the photons is not compatible with that of a single point source. No such indication is present in the 1991 observation, which has a very small number of detected counts. 2.3. Closer investigation of the central sourceTo establish the nature of the extension of the central source we first study the longest observation, obtained in 1993. A smoothed X-ray image of the central region of NGC 6440, shown in Fig. 1, suggests that two sources are present. They are too close to be separated by the standard analysis. We therefore implement a further analysis, based on the maximum-likelihood method (see e.g. Cash 1979, Mattox et al. 1996), as follows.
The probability at detector pixel i to obtain
The probability that the model describes the observations is given
by the product of the probabilities for all i in the region
considered: The last term in this equation doesn't depend on the assumed model,
and - in terms of selecting the best model - may be considered as a
constant. Thus maximizing Our further analysis of the 1993 ROSAT HRI observation is limited
to the central In performing the fits, we use the analytical result that the best
fit has a number of model photons equal to the number of detected
photons. Thus, in the model of a constant background, i.e. a constant
value of In fitting a constant plus one source, we distribute the source
counts around the source position according to the ROSAT HRI point
spread function at the center of the detector (David et al. 1995). We
then vary the background and source counts, and the source position,
to minimize Next, we fit a constant background plus two or three point sources.
In this fit, the parameters of the first source are allowed to vary;
we use a Downhill Simplex method as implemented by Press et al. (1992)
to minimize The results of this fitting procedure are summarized in
Table 2 ; the resulting source positions are shown in Fig. 1 and
listed in Table 3. A third source in the center is nominally
significant at the 3- Table 2. Results of further maximum likelihood analysis of the ROSAT HRI observations of two central sources in NGC 6440. For each observation we give the number of counts and the countrate for both sources X1 and X2, the significance of detection (in terms of Table 3. Positions of the SAX transient, of the three X-ray sources detected in the 1993 March ROSAT observation, of the center of NGC 6440 (Picard & Johnston 1995), and of optical variables discussed in the text. X1 and X2 are in the center of NGC 6440; X3 is not related to the cluster. The errors ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 2000 Online publication: July 13, 2000 ![]() |