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

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4. MS 2053.7-0449

MS 2053.7-0449 is part of the EMSS sample of high-z clusters of galaxies ([FORMULA]) serendipitously discovered in the fields of the Einstein IPC. The X-ray source is located at [FORMULA] [FORMULA] and [FORMULA] and the unabsorbed IPC flux, corrected for the effect of the finite EMSS detection cell is [FORMULA] erg cm- 2 s-1 (Henry et al. 1992; Gioia & Luppino 1994). The redshift of [FORMULA] is based on 5 concordant galaxy redshifts (Fabricant, private communication).

Given its high luminosity, [FORMULA] erg s-1, MS 2053.7-0449 was part of the CCD imaging survey for gravitational lensing carried out with the telescopes at Mauna Kea by Luppino et al. (1999). Luppino & Gioia (1992) first reported the presence of a large arc (arc-length [FORMULA]) fragmented into two distinct clumps at a radius of [FORMULA] from the optically dominant cluster galaxy.

A weak lensing study (Clowe 1998) shows that MS 2053.7-0449 is not among the most massive z[FORMULA] 0.55 clusters of the EMSS. Clowe et al. (1999) report a mass value, from the weak lensing, of (2.3 [FORMULA][FORMULA] [FORMULA] within 0.5 h[FORMULA] Mpc. The mass profile is well fit by a "universal" CDM profile (Navarro et al. 1996) with parameters r200=590 h[FORMULA] kpc and c[FORMULA]2 assuming a background galaxy redshift zbg=1.5. MS 2053.7-0449 is also well fit by an isothermal sphere model with a velocity dispersion of [FORMULA] km s-1 for zbg=1.5, indicating that the cluster is close to virialization. Kelson et al. (1997) find that the fundamental plane relation of galaxies in MS 2053.7-0449 is very similar to that of Coma, suggesting that the structure of the early-type galaxies has changed little since [FORMULA].

The MECS [FORMULA] keV X-ray image of the field is shown in Fig. 5; the peak of the X-ray emission from the cluster is centered at [FORMULA] [FORMULA] and [FORMULA], which is consistent with the celestial position of the cluster within the SAX positional errors. The total net counts from MECS (LECS) are [FORMULA] ([FORMULA]) and represent about 36% (47%) of the total (source + background) gross counts in the source region.

[FIGURE] Fig. 5. MECS image in the [FORMULA] keV band of the MS 2053.7-0449 field ([FORMULA]). The raw data have been smoothed with a two-dimensional gaussian filter with [FORMULA]. Contours are at 3, 5, 7, 10 and 20 sigma above the background. The white circle represents the counts extraction region for MS 2053.7.7-0449 (radius[FORMULA]).

Table 3 reports the results of the spectral analysis. The source spectrum was fitted using a Raymond-Smith spectral model modified by galactic absorption ([FORMULA] cm-2, Dickey & Lockman 1990) along the line of sight at the cluster position. From the present data no constraints can be set on the abundance which was thus fixed to 0.3 solar. The cluster gas is best fit by a rest frame temperature [FORMULA] (68% confidence interval). The unfolded spectrum, the folded spectrum and the ratio between the data and best fit model are displayed in Fig. 6.

[FIGURE] Fig. 6. MS 2053-0449: Unfolded spectrum, folded spectrum and the ratio between data and best fit model. In the last two panels we have indicated the LECS (MECS) data as open (filled) squares.

The ratio of the model normalizations for LECS and MECS is about 1.1, which is slightly in excess of the value expected from the known differences in the absolute calibration of the two instruments. However, given the present statistics we consider the fit acceptable. We note that using only the MECS data we obtain for the temperature [FORMULA] (68% confidence interval), which is consistent with (and even more constrained than) that obtained by the combined (LECS + MECS) data set.

Finally, we note that ASCA (GIS + SIS) did measure a temperature of [FORMULA] (68% confidence level) for MS 2053.7-0449 (Henry, 1999); given the large uncertainties involved (both from BeppoSAX and ASCA) the two results are obviously consistent. On the other hand they both suggest a temperature less than about 13-14 keV.

Using the best fit model parameters reported in Table 3, we derive an unabsorbed ([FORMULA]) keV flux of [FORMULA] erg cm-2 s- 1 (LECS normalization) or [FORMULA] erg cm- 2 s-1 (MECS normalization). The measured flux is lower, but consistent within the errors, than that derived from Einstein IPC data (see Table 1). Henry (1999) finds from ASCA data a flux [FORMULA] erg cm-2 s- 1 in perfect agreement with the EMSS.

The derived unabsorbed [FORMULA] keV flux (computed assuming a mean value between the LECS and MECS normalizations) 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.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999