Astron. Astrophys. 353, 124-128 (2000)

2. ROSAT observations

Cl0024+17 was observed with the ROSAT HRI in January 1994, July 1994, July 1995, and June to July 1996 with a total effective exposure time of 116.5 ksec. Fig. 1 shows the ROSAT HRI image of the cluster in the form of a contour plot. The image was divided by the exposure map, background subtracted, and corrected for vignetting effects. The image has been smoothed with a variable Gaussian filter, with a filter sigma varying from arcsec for the brightest to arcsec for the fainter regions, in order to provide a large dynamical range for the display of structural features. There are several point sources discussed in detail by Soucail et al. (1999).

Fig. 1. ROSAT HRI image of Cl0024+17. The image is background subtracted and vignetting corrected and has been smoothed with a variable Gaussian filter. See text for details.

Significant diffuse emission is detected from the cluster source out to a radius of about 1.5 arcmin ( Mpc). The total source count rate within a radius of 2 arcmin is cts s^-1 where the count rate of the closest point source with cts s^-1 has been subtracted. Assuming a temperature of 3.6 keV (this assumption is justified in Sect. 3) and considering the measured galactic hydrogen column density, cm^-2 (Dickey & Lockman 1990), this corresponds to a flux of erg s^{- 1} cm^-2 and a rest frame X-ray luminosity of erg s^-1 in the ROSAT band (0.1-2.4 keV). These results are very insensitive to the assumed temperature, would we have adopted a temperature of 7 keV for example the derived X-ray luminosity would be erg s^-1. These values are consistent with the X-ray data quoted in Smail et al. (1998).

We have determined an azimuthally averaged surface brightness profile for the HRI observation of Cl0024+17. A fit of a -model (e.g. Cavaliere & Fusco-Femiano 1976, Jones & Forman 1984) of the form

to the data is found to provide a good description of the surface brightness profile. Note, however, that the fit is restricted to the inner Mpc where we see significant X-ray emission. First we fitted the model directly to the photon data binned in concentric rings. Alternatively we took the smoothing effect of the HRI point spread function (PSF) into account by performing a 2-dimensional convolution of the -models with the HRI on-axis PSF (David et al. 1995a) before fitting to the observational data. The cluster has a surprisingly small core radius - only kpc in physical scale. In this case accounting for the PSF has a significant effect. The fitting results are summarized in Table 1 and the best fitting model is shown in Fig. 2 along with the observed data. In the fits the parameters for the core radius and are correlated and therefore have large individual uncertainties. For a 68% uncertainty level we find the following constraints for the two parameters: and .

Fig. 2. Surface brightness profile for the HRI image of Cl0024+17. The photon statistical errors are given as vertical error bars. The solid line shows the best fitting unconvolved -model and the dashed line shows the convolved, actually fitted profile.

Table 1. Results of the -model fits to the surface brightness profiles of the ROSAT HRI observations

As can be seen in Fig. 1, the cluster has a slight elongation in a northeast-southwest direction. To further quantify the cluster shape we have fitted a global elliptical model with one global slope parameter, . The best fitting ellipse has an orientation with a position angle of 41.5 degrees measured counter-clockwise from the north with a major axis core radius of 15" and a minor axis core radius of 13" (yielding an ellipticity of %). The slope parameter, , is well consistent with the fit of the spherically symmetric model. Note that in this analysis the profile was not deconvolved. The good agreement with the spherical model allows us to base the further analysis on the spherical model, as the effect of the ellipticity will almost average out as shown in Neumann & Böhringer (1996).

The residual image obtained by subtracting the elliptical model from the observed cluster image (smoothed with a Gaussian of ) is shown in Fig. 3. Two significant features can be noted in this image: i) there is a residual peak of the central maximum just north of the center of the elliptical model and ii) there is some more faint emission in the south than there is emission in the north (in addition to the possible faint point source which is located in the southern cluster area). Both features are easily explained as the result of a displacement of the central maximum with respect to the center of symmetry of the overall cluster. It results in an imperfect subtraction of the central maximum and the offset maximum shifts the center of the fitted ellipse slightly north with respect to the large-scale cluster center leaving residual emission in the southern part. The only significant trace of cluster substructure that can be observed in the X-ray image of Cl0024+17 is this center shift of the cluster core by about 12" ( kpc) to the north approximately in the direction of the position angle of the ellipse model. The two residuals, the small peak at the north of the center of symmetry and the southern extension are about features. The disturbance is therefore not very large, as far as the X-ray emission can be traced ( Mpc). The bright residual feature at the upper right of Fig. 3 is a point source not related to the cluster ICM (source S1 identified in Soucail et al. 1999).

Fig. 3. Residuals of the X-ray image of Cl0024+17 after subtracting an elliptical model from the observed cluster image. The model is overplotted in the form of contour lines. The maximum north of the cluster core and some emission in the south are the only significant residual emission regions in the cluster area. The scale of the image is arcmin.

It is also important to note that the analysis of the X-ray surface brightness profile together with an assumed gas temperature of 3.6-8 keV yields a central cooling time of the gas of about 7-8 Gyr. This is probably larger than the age of the cluster at the given redshift. Thus there was not enough time to develop a steady state cooling flow and we should expect a very marginal effect due to cooling of the gas in the cluster center.

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999
helpdesk.link@springer.de