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Astron. Astrophys. 353, 498-506 (2000)

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3. RXJ 0152.7-1357

RXJ 0152.7-1357 was discovered in the RDCS which has constructed an X-ray selected, flux-limited sample of clusters of galaxies via a serendipitous search for extended X-ray sources in deep pointed PSPC observations. A wavelet-based technique was employed to characterize low-surface brightness sources and select cluster candidates down to the flux limit [FORMULA] of [FORMULA] erg cm- 2 s-1, over [FORMULA] deg2 (Rosati et al., 1995). Optical follow-up imaging and spectroscopy have confirmed to date [FORMULA] clusters and groups, which span a large range in redshift [0.05-1.26] and X-ray luminosity [[FORMULA] erg s-1], RXJ 0152.7-1357 being the most luminous of the sample.

RXJ 0152.7-1357 was discovered in the ROSAT PSPC field rp600005n00 pointed at the nearby galaxy NGC 720. The cluster was detected at 13.6´ off-axis ([FORMULA] [FORMULA] and [FORMULA]). At this position the wavelet algorithm clearly reveals an extended X-ray source with an extent [FORMULA] above the local PSF of the ROSAT PSPC instrument and with a double core (the multi-scale wavelet analysis preserves source substructure information) with intensity peaks separated by [FORMULA] arcmin ([FORMULA] Mpc at the cluster redshift of [FORMULA]). This same cluster has also independently been identified in the WARPS survey (Ebeling et al. 1999) and recently reported also in Romer et al. (1999).

Integrating the ROSAT PSPC flux over a circle of 3 arcmin radius, corresponding to [FORMULA] Mpc at [FORMULA], we obtain [FORMULA] net counts 1 in the 0.5-2.0 keV energy band, with 6.2/arcmin2 background counts. With an effective exposure time of 19904 sec, a galactic HI column density along the line of sight of [FORMULA] (Dickey & Lockman 1990) and using a Raymond-Smith spectral model (Raymond & Smith 1977) with kT [FORMULA] 6 keV, we obtain an unabsorbed flux of [FORMULA] erg cm- 2 s-1. The rest frame X-ray luminosity within an aperture of [FORMULA] Mpc is [FORMULA] erg s-1.

Imaging and spectroscopic follow-up observations of RXJ1052.7-1357 were conducted with the EFOSC1 spectrograph at the ESO 3.6m in November 1996. With a long slit exposure of 2 hours it was possible to secure redshifts for 3 galaxies with [FORMULA] belonging to the main northern clump. Additional multiband imaging and spectroscopy have been carried out at Keck with the Low Resolution Imaging Spectrograph (LRIS, Oke et al., 1995) and are presented and discussed elsewhere (Squires et al. in preparation), along with a weak lensing analysis of the field. We only note here that the new spectroscopic data confirm the redshift of [FORMULA] also for the southern clump.

In Fig. 1, we show the R-band image (obtained with Keck-LRIS; 4000 s integration time) with the PSPC X-ray contours overlaid. The cluster galaxies distribution shows two main clumps and correlates very well with the double-peaked X-ray morphology.

[FIGURE] Fig. 1. R band image of RXJ1052.7-1357 with overlaid ROSAT PSPC X-ray contours at 3 (dashed line), 5, 10, 20 and 30 [FORMULA] above the background. The image was obtained with Keck-LRIS with 4000 s integration time (Squires et al. in preparation). The right and top axes give the physical linear scale at [FORMULA]. The center coordinates are [FORMULA] [FORMULA]; [FORMULA]; north is up, east to the left.

The Beppo SAX - MECS X-ray image of the field in the [FORMULA] keV band is shown in Fig. 2. The source, centered at [FORMULA] [FORMULA] and [FORMULA], is consistent with the celestial position of the cluster within the SAX positional errors. This source is marginally resolved at the spatial resolution of the Beppo SAX - MECS.

[FIGURE] Fig. 2. MECS image in the [FORMULA] keV band of the RXJ 0152.7-1357 field ([FORMULA]). The raw data have been smoothed with a gaussian filter with [FORMULA]. Contours are at 3, 5, 7, 10 and 20 sigma above the background. The white [FORMULA] radius circle represents the Beppo SAX counts extraction region for RXJ 0152.7-1357. The inset shows the ROSAT-PSPC cutout of the same central region in the [0.5-2.0] keV band, smoothed with a gaussian with [FORMULA]. The inset clearly shows the double core nature of RXJ 0152.7-1357.

The total net counts in the source extraction region are [FORMULA] for the LECS and [FORMULA] for the MECS. The net counts represent about 59% (39%) of the total (source + background) counts in the source region for LECS (MECS).

The source spectrum was fitted using a Raymond-Smith spectral model modified by galactic absorption ([FORMULA] cm-2) along the line of sight at the cluster position. The results are reported in Table 3. The cluster gas is best fitted by a rest frame temperature of [FORMULA] and metallicity of [FORMULA] (68% confidence interval). The ratio of the model normalizations from LECS and MECS is about 0.71, which is consistent with the known differences in the absolute calibration of the two instruments.


Table 3. Results of the Spectral Fit (LECS+MECS): Raymond-Smith Thermal Model.
Errors are the 68% confidence interval for one interesting parameter ([FORMULA]).
a) Normalization at 1 keV. This number is equal to [FORMULA], where D is the distance to the source in cm, [FORMULA] is the electron density in units of cm-3 and V is the volume filled by the X-ray emitting gas in cm3

The unfolded spectrum, the folded spectrum and the ratio between the data and best fit model are displayed in Fig. 3, while in Fig. 4 we report the two-parameter [FORMULA] contours (68.3%, 90% and 99% confidence levels) for the cluster metallicity (in solar units) and temperature (in keV). Both temperature and metal abundance are rather well constrained, which is partially due to the detection of the iron [FORMULA] complex at the redshifted energy of about 3.7 keV. It is worth noting that leaving the redshift, along with the metallicity and temperature, as free parameters we obtain best fit values ([FORMULA]) of [FORMULA], [FORMULA] and metallicity [FORMULA] (68% confidence interval). Finally, we have also tried a fit with the absorbing column density as a free parameter. We obtain best fit values ([FORMULA]) of [FORMULA], [FORMULA] and [FORMULA]; the best fit absorbing column density converges to zero with a [FORMULA] upper limit of [FORMULA] cm-2. These results confirm the robustness of our spectral fit.

[FIGURE] Fig. 3. RXJ 0152.7-1357: Unfolded spectrum, folded spectrum and the ratio between the data and best fit model. In the last two panels we have indicated the LECS (MECS) data as open (filled) squares. A prominent iron [FORMULA] complex at the redshifted energy of [FORMULA] keV is visible.

[FIGURE] Fig. 4. RXJ 0152.7-1357: Two dimensional [FORMULA] contours at 68.3%, 90% and 99% confidence levels ([FORMULA] = 2.3, 4.61 and 9.21) for the cluster abundance (solar units) and temperature (in keV).

Using the best fit model parameters reported in Table 3 and assuming a mean value between the LECS and MECS normalizations, we derive an unabsorbed (0.5-2.0) keV flux of [FORMULA] erg cm- 2 s-1. The measured flux is in good agreement with that derived from the ROSAT PSPC data (see Table 1).

The derived unabsorbed (2-10) keV flux is [FORMULA] erg cm-2 s- 1, corresponding to a luminosity in the cluster rest frame of [FORMULA] erg s-1, and to a bolometric luminosity of [FORMULA] erg s-1.

Given the uncertain dynamical state of RXJ0152.7-1357 and the difficulty in measuring its gas density profile, it is difficult to provide an accurate estimate of its mass 2.

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Online publication: December 17, 1999