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Ni ABUNDANCE IN THE CORE OF THE PERSEUS CLUSTER: AN ANSWER TO THE SIGNIFICANCE OF RESONANT SCATTERING AND SNIa ENRICHMENT. Fabio Gastaldello (CNR-IASF, University of Milan-Bicocca) & Silvano Molendi (CNR-IASF). OUTLINE. RESONANT SCATTERING
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Fabio Gastaldello (CNR-IASF, University of Milan-Bicocca)
Silvano Molendi (CNR-IASF)
THE QUESTION: RS OR INCREASED Ni ABUNDANCE IN THE CORE OF THE PERSEUS CLUSTER ?
THE XMM OBSERVATION OF PERSEUS AND HIS ANSWER TO THE QUESTION
CONCLUSIONS AND SUMMARY
Absorption of a line photon followed by immediate reemission in an other direction.
Although for densities and temperatures typical of clusters the gas is optically thin to Thomson scattering for the continuum, it can be optically thin in the resonance X-ray lines (in particular in the denser core) and in particular in the Fe He emission line at 6.7 keV (the most important emission line !) (Gilfanov, Sunyaev, Churazov 1987)
No longer optically thin, the surface brightness is distort and you underestimate abundances in the core of the cluster.
You can measure it by making the ratio of a supposed optically thick (Fe He ) and optically thin (Fe He at 7.90 keV) lines and see if there are deviation for the prediction of the plasma code.
If there is resonant scattering, you underestimate abundance in the prominent line at 6.7 keV you cannot reproduce the line at 7.9 keV and you see residuals in the fit
MECS spectrum of the core (inner 8) of the Perseus cluster (from Molendi et al, 1998)
Correcting the apparent abundances
Using only He
Assuming optically thin emission
Molendi et al. 1998
But the excess could be due also to overabundance of Ni respect to solar ratios, according to the experimental evidence of a central enhancement of SNIa ejecta (and Ni is produced only by SNIa) in cD clusters (Dupke & Arnaud 2001). The Ni He is at 7.80 keV and you cannot distinguish it from the Fe He ß at 7.90 keV with the MECS resolution
MOS 1 image of the core of Perseus
PN light curve 10-13 keV
XMM has for the first time the combination of resolutionand effective area at high energies to give an unambiguous answer. The Perseus cluster was observed for 53 ks MOS and 25 ks PN but the observation was badly affected by high background. But we consider all the observation exploiting the brightness of Perseus and modelling the soft proton background. We concentrate on hard bands ( 3 keV) which are the one of interest to determine abundances of Fe and Ni
We model the soft protons wich contaminate the spectra using in first approximation a power law as a background model (the model is not convolved via the effective area of the instrument)
The temperature profile obtained by our best fit model (mekal + pow/b in the 3-13.5 keV) is in good agreement with the temperature obtained by the ratio of He to H (at 6.97 keV) Fe line. It is also in agreement with the SAX-MECS temperature profile.
Ni He 7.80 keV
Fe He 7.90 keV
EPIC-PN spectra 1’-2’ bin
As we can see leaving the abundance of Ni free, the excess is due to this element and not to an anomalosly high Fe He line. There is no resonant scattering .
If you fit the XMM spectra with a MEKAL model you find again an excess …
The Fe and Ni abundance profiles obtained by our best fit model (mekal + pow/b in the 3-13.5 keV) show a gradient. They are also in good agreement with SAX-MECS determination, when we leave the hypotesis of resonant scattering. Further tests with 2T models in the broad band 0.5-10 keV (the plasma is not consistent with being isothermal) give similar results.
It is becoming progressively clearer that resonant scattering effects must be small and confined on small inner scalesIMPLICATIONS OF NO RS
AGN activity ? (caution: see Sakelliou et al. 2002)