The Observed Thermal Structure of Cool ICM Cores

New observations with XMM-Newton and Chandra lead to a revision of this scenario. XMM-Newton is now providing unprecedented detailed spectroscopic diagnostics of the central regions of clusters and new insights into the cooling flow picture. One set of observations, obtained with the XMM Reflection Grating Spectrometer (RGS), shows for several cooling core regions spectral signatures of different temperature phases ranging approximately from the hot virial temperature of the cluster to a lower limiting temperature, Tlow, which is still significantly above the "drop out" temperature where the gas would cease to emit significant X-ray radiation. That is, the clearly observable spectroscopic features of the lower temperature gas expected for a cooling flow model are not observed. Figure 23.14 shows for example the case of the massive, cooling flow cluster A1835 with a bulk temperature of about 8.2keV, where no lines or features for temperature phases below 2.7keV were observed as found by Peterson et al. [112]. Systematic studies of a sample of cooling flow clusters, e.g., by Kaastra [82] support these results.

The other set of relevant XMM-Newton observations are obtained with the energy sensitive imaging devices pn and MOS. Even though the spectral resolution is less for these instruments than for the RGS, they can very well be used to

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Rest Wavelength (A)

Fig. 23.14 XMM-Newton RGS spectra of the cooling core region of the massive cluster A1835 [112]. The observed spectrum is fit by several models: an isothermal model with a temperature of 8.2 keV (red), a classical cooling flow model with a hot temperature phase of 8.2 keV and the cooling flow component (blue), and a similar cooling flow model with a forced lower temperature cut-off at 2.7 keV (green). The inset shows clearly several lines by the blue marked cooling flow model, which are not observed in the cluster spectrum

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Rest Wavelength (A)

Fig. 23.14 XMM-Newton RGS spectra of the cooling core region of the massive cluster A1835 [112]. The observed spectrum is fit by several models: an isothermal model with a temperature of 8.2 keV (red), a classical cooling flow model with a hot temperature phase of 8.2 keV and the cooling flow component (blue), and a similar cooling flow model with a forced lower temperature cut-off at 2.7 keV (green). The inset shows clearly several lines by the blue marked cooling flow model, which are not observed in the cluster spectrum

channel energy (keV)

channel energy (keV)

Fig. 23.15 Left: The Fe L-line complex in X-ray spectra as a function of the plasma temperature for a metallicity value of 0.7 solar. The simulations show the appearance of the spectra as seen with the XMM pn. The emission measure was kept fixed when the temperature was varied. Middle: XMM pn-spectrum of the central region of the M87 X-ray halo in the radial range R = 1 — 2arcmin. The spectrum has been fitted with a cooling flow model with a best fitting mass deposition rate of 0.96 M0 yr—1 and a fixed absorption column density of 1.8 • 1020 cm—2, the galactic value, and a low temperature cut-off at 0.01 keV. Right: Same observed spectrum as in the middle panel fitted by a cooling flow spectrum artificially constraint to emission from the narrow temperature interval 1.44-2.0keV with a mass deposition rate of 2.4 M0 yr—1 [15]

energy [keV]

channel energy (keV)

channel energy (keV)

Fig. 23.15 Left: The Fe L-line complex in X-ray spectra as a function of the plasma temperature for a metallicity value of 0.7 solar. The simulations show the appearance of the spectra as seen with the XMM pn. The emission measure was kept fixed when the temperature was varied. Middle: XMM pn-spectrum of the central region of the M87 X-ray halo in the radial range R = 1 — 2arcmin. The spectrum has been fitted with a cooling flow model with a best fitting mass deposition rate of 0.96 M0 yr—1 and a fixed absorption column density of 1.8 • 1020 cm—2, the galactic value, and a low temperature cut-off at 0.01 keV. Right: Same observed spectrum as in the middle panel fitted by a cooling flow spectrum artificially constraint to emission from the narrow temperature interval 1.44-2.0keV with a mass deposition rate of 2.4 M0 yr—1 [15]

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detect temperature sensitive spectral features with high accuracy due to the good photon statistics. They provide in addition spatially resolved spectroscopic information across the entire cooling core region. Spectral fitting of the X-ray emission in the cooling flow clusters, notably M87 in the Virgo cluster, yielded two pieces of information which are incompatible with the classical cooling flow model. They showed that single temperature models provide a better representation of the data than cooling flow models [12] and they confirm the lack of low temperature components in the central ICM [15,94]. Figure 23.15 nicely illustrates the result for the most nearby and best to study cooling core in M87. For the relevant temperature range from a few tenth of a keV to about 3 keV, the complex of iron L-shell lines provides a superb ICM thermometer as illustrated in Fig. 23.15, left. The figure shows simulated X-ray spectra for the XMMpninstrument in the spectral region around the Fe L-shell lines for various temperatures from 0.4 to 2.0 keV and 0.7 solar metallicity. There is a very obvious shift in the location of the peak of the blend of iron L-shell lines, caused by an increasing degree of ionization of Fe with increasing temperature.

For a cooling flow with a broad range of temperatures (as explained above) one expects a composite of several of the relatively narrow line blend features, resulting in a quite broad peak. The right panels of Fig. 23.15b shows for example the depro-jected spectrum of the M87 halo plasma for the radial range 1-2 arcmin (outside the excess emission region at the inner radio lobes) and a fit of a cooling flow model with a mass deposition rate slightly less than 1M0 yr^1, as approximately expected for this radial range of the cooling core from the analysis of the surface brightness profile, e.g., [94]. There is obviously no good agreement between the observations and the model, while a fit of a plasma with a narrow range of temperatures from 1.44 to 2 keV provides an excellent fit. Even though attempts were made to save the classical cooling flow model (e.g. [54]) it is generally accepted that the lack of evidence of cooler temperature phases in the ICM implies no massive cooling in the cool core regions of galaxy clusters.

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