Ni ABUNDANCE IN THE CORE Ni ABUNDANCE IN THE CORE OF THE PERSEUS CLUSTER: OF THE PERSEUS CLUSTER:
AN ANSWER TO THE AN ANSWER TO THE SIGNIFICANCE OF RESONANT SIGNIFICANCE OF RESONANT
SCATTERING AND SNIa SCATTERING AND SNIa ENRICHMENTENRICHMENT
Fabio Gastaldello (CNR-IASF, Fabio Gastaldello (CNR-IASF, University of Milan-Bicocca) University of Milan-Bicocca)
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Silvano Molendi (CNR-IASF)Silvano Molendi (CNR-IASF)
OUTLINEOUTLINE
•RESONANT SCATTERINGRESONANT SCATTERING•THE QUESTION: RS OR INCREASED Ni THE QUESTION: RS OR INCREASED Ni ABUNDANCE IN THE CORE OF THE ABUNDANCE IN THE CORE OF THE PERSEUS CLUSTER ?PERSEUS CLUSTER ?•THE XMM OBSERVATION OF PERSEUS THE XMM OBSERVATION OF PERSEUS AND HIS ANSWER TO THE QUESTIONAND HIS ANSWER TO THE QUESTION•CONCLUSIONS AND SUMMARYCONCLUSIONS AND SUMMARY
RESONANCE SCATTERINGRESONANCE SCATTERINGAbsorption 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.
RESONANT SCATTERING IN PERSEUS ?RESONANT SCATTERING IN PERSEUS ?
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)
RESONANT SCATTERING IN PERSEUS ?RESONANT SCATTERING IN PERSEUS ?
Assuming optically thin emission
Using only He
Correcting the apparent abundances
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
XMM OBSERVATION OF PERSEUS XMM OBSERVATION OF PERSEUS
XMM has for the first time the combination of resolution and 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
MOS 1 image of the core of Perseus
PN light curve 10-13 keV
OUR WORKING PROCEDURE ... OUR WORKING PROCEDURE ...
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)
IRON-K LINE TERMOMETER IRON-K LINE TERMOMETER
OUR WORKING PROCEDURE ... OUR WORKING PROCEDURE ...
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.
EPIC-PN spectra 1’-2’ bin
If you fit the XMM spectra with a MEKAL model you find again an excess …
RESONANT SCATTERING IN PERSEUS ?RESONANT SCATTERING IN PERSEUS ?
Fe He 7.90 keV
Ni He 7.80 keV
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 .
ABUNDANCE GRADIENTS IN Fe AND Ni ABUNDANCE GRADIENTS IN Fe AND Ni
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.
IMPLICATIONS OF NO RS IMPLICATIONS OF NO RS
•The absence of a clear evidence of resonant scattering strongly points towards the presence of significant turbolence (Gilfanov et al. 1987; Mathews et al. 2001)
AGN activity ? (caution: see Sakelliou et al. 2002)
•It is a paradox that the first clear example of resonant scattering, NGC 4636 (Xu et al. 2002) is the most striking example of chaotic gas kinematics in the optical (Caon et al. 2000)
It is becoming progressively clearer that resonant scattering effects must be small and confined on small inner scales
SUMMARY SUMMARY
•There is no need to invoke resonant scattering in the Fe He line in the Perseus cluster core the Fe abundance determination with optically thin emission models is reliable. It is now spectroscopically clear that the excess in the 8 keV line complex is due to Ni. Fe and Ni show a gradient, adding another piece of evidence to the scenario of a central enhancement of SNIa ejecta in cD clusters.
•Turbulence could be an explanation for the absence of resonant scattering in the best candidates as M87 and the core of Perseus, maybe induced by AGN activity.
