The Cluster soft excess in the XMM-Newton era

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The Cluster soft excess in the XMM-Newton era Powered By Docstoc
					Clusters of galaxies: the soft X-
   ray excess and Sunyaev-
        Zel’dovich effect
                 Richard Lieu
                Jonathan Mittaz
       University of Alabama, Huntsville
                 Shuang-Nan Zhang
        National TsingHua University, Beijing
XMM-Newton has definitely confirmed the cluster soft excess



                          2  1.45




  Fit with a single temperature across   Fit to the hot ICM (1-7 keV) showing
  the whole 0.3-7 keV band shows         the cluster soft excess at energies
  significant residuals indicating a     below 1 keV. The excess is seen at
  cluster soft excess                    > 20% level above the hot ICM model
Fit to PKS2155-304 to demonstrate the systematic errors in extrapolating to lower
energies from a 1-7 keV fit. Note the maximum residuals are at the 8% level, much
less than residuals seen showing the presence of a cluster soft excess
XMM Calibration uncertainties for PM/MOS ~ 5%
Soft excess is above the calibrational uncertainties

     Taken from “EPIC status of calibration and data analysis” Kirsch et al.
     XMM-SOC-CAL-TN-0018
Note for A3112, cluster soft excess is not a background effect
            Bregman et al., 2003, ApJ, 597, 399




BUT THE SOFT EXCESS WITNESSED BY XMM-NEWTON IS ABOVE
THE RANGE OF THIS PLOT!
DO YOU BELIEVE IN ZERO GALACTIC COLUMN?
 Central soft excess for AS1101 and A3112 ROSAT/XMM
joint fits for region from any background subtraction issues
      Outer soft excess for AS1101 and A3112

AS1101 4 - 6 arcminute           A3112 4 - 6 arcminute
Central soft excess (no background issues) for A1795 and
                          Coma
The discovery of cluster soft excess as extra photon emission in the
0.2 – 0.5 keV range above the level expected from the low energy tail
of the virialised intracluster gas at X-ray temperatures was made by
the EUVE mission in 1995




                                             Coma Cluster in the
                                             EUV
          Coma Cluster 6’ – 9’ ROSAT and EUVE DS

                                     ROSAT PSPC
                  EUVE




Solid line is the expected emission spectrum of the hot ICM at
kT = 8.7 +/- 0.4 keV and A = 0.3 solar, as measured by ASCA.
Best 3 Temperature model for the soft excess of Coma’s 6’-9’ region




          IS THIS A PHYSICALLY SENSIBLE MODEL?
   Physical constraints on the model
For intracluster origin of the WHIM
                                               Th 
           Pwarm    Phot  nw  (nh / 10 cm ) 
                                              3
                                              T 
                                                     3

                                               w
If we take Th  10Tw , nw  10 2 cm 3

Radiative cooling time is important

        6  10 9 (T / 10 6 K )0.5 ( nw / 10 3 cm 3 ) 1 yrs
For Tw ~ 10 6 K , nw  10 2 cm 3      6  10 8 years

WHAT SUSTAINS THE WARM GAS AGAINST SUCH RAPID RADIATIVE
COOLING?
Thermal (mekal) model kT  106 K
Giant ¼ keV Halo centered at Coma (as detailed by the ROSAT
sky survey)
ROSAT/PSPC Radial surface brightness of Coma
(Bonamente, Joy, & Lieu, 2003, ApJ, 585, 722)




         ¼ keV                       ¾ keV
        ROSAT/PSPC data of the Coma cluster (50’-70’ annulus)

 X-ray thermal model (kT~8keV)             Fitting the excess with a 2nd
                                                     component

                                                  Hot ICM + power-law
                                                  Hot ICM + warm component




Unlike the cluster core, strong soft excess at the outskirts of Coma.
Statistically the thermal model is preferred (to a power law).

The warm gas here may be part of the WHIM (e.g. Cen & Ostriker 1999)
not in physical contact with the hot ICM. XMM-Newton confirmation of
the Coma soft excess halo.
Coma cluster 0.5 – 2 keV with XMM-Newton pointings
XMM-Newton spectrum of the Coma 11 region
(Finoguenov, Briel & Henry, 2003, A&A, 410, 777)
RGS spectra of X-Comae which lies behind the Coma cluster. Shown are
the individual spectra from three separate observations of X-Comae together
with the position of the OVII line at the redshift of Coma and of the Galaxy. The
expected absorption from the CSE cannot be seen
Same as previous slide but now all observations have been added. Again, there
is no line at the expected redshift of Coma
One of the recent claims regarding the soft excess is the
detection of OVII line emission




                                                Kaastra et al. (2003)
However, the importance of good background subtraction
cannot be overstated – depending on what assumptions you
take the potential background can vary by a lot.
                                             Isothermal model
                                             kT = 3.08 keV
                                             A = 0.194 solar


                                              SOFT EXCESS
                                              REMAINS ROBUST
                                              (after subtracting
                                              the higher background)

                  Intrinsic background




            Kaastra sky average background




 AS1101 (2’-5’) with ICM model (fitted from 2-7 keV) and backgrounds
For AS1101 there is also little evidence for redshifted
OVII line emission
AS1101 10’-13’ background spectrum. OVII+OVIII lines
consistent with Galactic emission and not associated with the
cluster redshift (z=0.058)




  OVII+OVIII lines positioned at the cluster redshift in AS1101 background
Line fit to the OVII+OVIII complex with no constraint on the energy of
the line




 Line completely consistent with zero redshift i.e. Galactic origin
Suzaku observations of A2052 seem to show no need for a strong thermal OVII
line from a large scale soft WHIM component
The central Suzaku spectrum (1 arcminute) showing no OVII line
         CLUSTER SOFT EXCESS DIAGNOSTICS



•The soft excess outside clusters’ cores might still be
of thermal origin.
•Inside a cluster’s core the thermal model is very hard to
implement. If the origin is outlying filaments seen in
projection, the required column density will be enormous.
if intracluster warm gas – problem with cooling time.
 Very recent simulations of clusters seem to find CSE in outer
 regions of merging clusters.

Simulating the Soft X-ray excess in clusters of galaxies

L.-M. Cheng, S. Borgani, P. Tozzi, L. Tornatore, A. Diaferio, K. Dolag, X.-T. He, L. Moscardini, G. Murante, G. Tormen
astro-ph/0409707

The detection of excess of soft X-ray or Extreme Ultraviolet (EUV) radiation, above the thermal contribution from the hot
intracluster medium (ICM), has been a controversial subject ever since the initial discovery of this phenomenon. We use a
large--scale hydrodynamical simulation of a concordance LambdaCDM model, to investigate the possible thermal origin for
such an excess in a set of 20 simulated clusters having temperatures in the range 1--7 keV. Simulated clusters are analysed
by mimicking the observational procedure applied to ROSAT--PSPC data, which for the first time showed evidences for the
soft X-ray excess. For cluster--centric distances 0.4< R/R_{vir}< 0.7 we detect a significant excess in most of the simulated
clusters, whose relative amount changes from cluster to cluster and, for the same cluster, by changing the projection direction.
In about 30 per cent of the cases, the soft X-ray flux is measured to be at least 50 per cent larger than predicted by the one--
temperature plasma model. We find that this excess is generated in most cases within the cluster virialized regions. It is mainly
contributed by low--entropy and high--density gas associated with merging sub--halos, rather than to diffuse warm gas. Only in
a few cases the excess arises from fore/background groups observed in projection, while no evidence is found for a significant
contribution from gas lying within large--scale filaments. We compute the distribution of the relative soft excess, as a function
of the cluster--centric distance, and compare it with the observational result by Bonamente et al. (2003) for the Coma cluster.
Similar to observations, we find that the relative excess increases with the distance from the cluster center, with no significant
excess detected for R<0.4R_{vir}
Non-thermal interpretation of the cluster soft excess

      Hwang, C.-Y., 1997, Science, 278, 191
      Ensslin, T.A. & Biermann, P.L., 1998, A&A, 330, 20
      Sarazin, C.L. & Lieu, R., 1998, ApJ, 494, L177


 Proposed the origin of the cluster soft excess emission as due to
 inverse-Compton scattering between intracluster cosmic rays
 (relativistic electrons with Lorentz factors of a few hundred)
 and the cosmic microwave background

 HOW LARGE A COSMIC-RAY (CR) POPULATION DO WE NEED
 TO ACCOUNT FOR THE SOFT EXCESS BRIGHTNESS?

 NB. Center can be e+/e- pairs, but outside has to be CR’s
 from supernova events.
         A1795: single temperature fit (2-7 keV) for two annuli



                                    Background 10x below cluster
Background 100x below cluster




  kT = 4.88 +/- 0.08 keV               kT = 6.05 +/- 0.15 keV
   A = 0.43 +/- 0.02 solar              A = 0.27 +/- 0.03 solar
 Non-thermal interpretation of the Cluster Soft Excess
                                   Abell 1795
Region   Power-law      Photon     Hot Gas kT    Relativistic    Relativistic     Gas Pressure     Gas density
         Luminosity     Index      (keV)         Electron        Electron         ( ergs / cm3 )   ( cm3)
         (0.2-1leV)                              Energy          pressure
         ( ergs / s )                            (total ergs)    ( ergs / cm3 )
0’-1’       1043          2.30         5.0         1.2  1060      8  1012       1.48  1010    1.85  102
2’-5’     9.7  1042      2.81         6.3         1.18  1060     7  1014       1.7  1011     1.7  103


In the center 0’-1’ region, the central galaxy may quite easily supply cosmic rays of total electron
energy of a few  10 ergs. As mentioned before, the ratio of proton to electron pressure in the
                      60


CR population is a few x100. Thus the CR protons can obtain (or surpass) equipartition with the
gas

REASON FOR THE ABSENCE OF A COOLING FLOW?

In the outer parts, the CR’s have to come from supernovae within the member galaxies. Based on
the best fit adundance of 0.32 solar for the 2’-5’ region, the amount of iron in the gas is 5.7  10 9 M sun
  5.7  1010 SN’s in the past.
  Assuming each SN outputs ~ 3  10 ergs of CRs, one estimates ECR ( 2'5' )  1.7  10 ergs,mostly
                                      50                                                61


in protons.
  Within the < 3Gyrs of loss time against inverse-Compton scattering these protons produce
   8  1058 ergs of secondary electrons: NOT ENOUGH
            annihilation in galaxy
                  clusters
         [Colafrancesco & Mele 2001, ApJ, 562, 24; Colafrancesco 2004, A&A, 422, L23 ]


Rate


Cosmological relic  density

Cross-section       ( W=0.3, h=0.5)

Leading annihilation channels




       Secondary electrons with Ee  M are produced in
       situ
 EUV/soft X-ray ICS emission is produced by the
 secondary electrons - created by  annihilation -
 which scatter the CMB photons (Colafrancesco 2004)
The EUV/soft X-ray excess
in Coma is best fitted by a
neutralino with:
 M   30 GeV
 V  A  4 10 26 cm3 / s

quite independent of the 
model.


The EUV/soft X-ray excess
provides the bound

                                 V  A
M   30 GeV             27
                 3 10          cm 3 s 1 (W  h 2 ) 1
                                   To Observer
Sunyaev-Zel’dovich Effect:
Compton up scattering of CMB
photons by cluster hot electrons
                    Basics of the Sunyaev-Zeldovich Effect                                               CMB


   T ( )     kT                 x(e x  1) 
                    T  dl ne  x          4
                                  e 1
                   2
    TCMB      me c                              
 The electron density of the hot gas is obtained by fitting ROSAT X-ray
 surface brightness profiles I X ( ) with the 2 parameter isothermal  -model
 ignoring the central cooling flow

                             3                                               1
                          2  2                                      2 3   2
                r                                                                               
  ne (r )  n0 1   
                                   I X ( )  n 1  
                                                     2
                                                                
                                                                     
                                                      C
                                                     0
                 re 
                          
                                                                   
                                                                      
 The decrement in TCMB is then given by
                                   3 1
                       
TSZ ( )  TSZ (0) 1   
                                2  2 2
                                 
                                          where TSZ (0)  38.8K    (     n            rC   ( 
                                                                                    ( (   ( 
                                                                                     kT         j ( x)      3
                                                                                                              2
                                                                                                                  
                                                                                                                    1
                                                                                                                    2
                                                                                                              3
                       C 
                                                                           3
                                                                          10 cm3                2
                                 
                                                                                    keV   Mpc
                                                                                                               2
                                                                        x(e x  1)
                                                          with j ( x)             4
                                                                         e 1
                                                                           x
Is the discrepancy due to an error in the -model in representing the X-ray plasma properties
at cluster outskirts – regions where the ROSAT data do not constrain the model well?

As we saw already, model is usually well guided by the data out to                            r  5rc . Suppose at r  5rc
the model is completely overestimating TSZ (0)
                                                         3                                  3 
                                          1   r                          1   r  
                                                   2            2                       2            2
                                   5 rc
                 TSZ (0)                     
                                            rc                  dr     r                     dr
                                   0                                  5 rc      c 


Now 2nd integral
                           3 
                 r
                                      513       4
       I2                     dr         rc     rc                        for a typical value of          2/3
                  c
             5 rc r                    3  1      25
First integral
                                       2 1
                 5 rc    r                            r
        I2            1  
                                  
                                          dr  rc arctan   rc arctan(5)
                                                          r 
                 0
                          rC
                                        
                                                          c
 The ratio I 2 I1  12 % only. Thus the central SZE is well predicted by using X-ray data
If the core region of clusters dominate the WMAP SZE profiles, could our
neglect of the cooling flow effect in these regions be the problem?

Typically in a CF the central surface brightness rises by ~ 10 times i.e.
is incresed by ~ 3 times, while kT drops by a factor of  2 .

The product ne kT which is the quantity of interest to SZE predictions then
rises slightly towards the core as compared with isothermal  -model
predictions. We use Abell 2029 as an example to illustrate this.


Thus CF effects cannot explain the factor of 6 SZE discrepancy of our
analysis
Could cluster radio sources be responsible? The Owens valley radio interferometry
survey (Bonamente et al. 2005) shows on average one ~1 mJy source at 30 GHz per
cluster. Given their sample is more distant than ours, this would scale to ~ 10mJy or

                                1028 Wm2 Hz 1

If this causes our discrepancy it must explain TCMB  5  10 K distributed over 0.5
                                                             5

degree.

We can convert TCMB to flux by multiplying the Rayleigh-Jeans sky flux

                             2 kTCMB 2 / c 2
by the solid angle W / 4 where

                              W   2
with   0.5 degrees. This gives a factor of 10 discrepancy since we get

               1027 Wm2 Hz 1 at   30 GHz (Q Band)
In the W band it is even worse since the spectra goes as   F ~   where   2 giving
a shortfall of at least two orders of magnitude
 The role of relativistic electrons
• Quenby & Lieu (2006) invoked a power-law population
  of intracluster relativistic electrons extending to TeV
  energies.
• In the 0.1-0.5 GeV range they inverse-Compton scatter
  the CMB to cause soft X-ray excess in clusters.
• At 50 GeV energies synchrotron radiation in micro-gauss
  field occurs at 40 GHz to reduce the SZE.
• The numbers can account for our observations without
  violating the EGRET gamma-ray flux upper limit.
• The relativistic electrons could be the product of
  extended intracluster Fermi acceleration, or neutralino
  annihilation and decay.
               Conclusion
• Cluster soft X-ray/EUV excess now a
  definite phenomenon. The outer excess is
  almost certainly of thermal origin.
• The inner excess is equally likely to be of
  non-thermal origin, the same population of
  relativistic electrons is responsible for the
  absence of the Sunyaev-Zel’dovich effect.
• The non-thermal population is made by
  Fermi acceleration, or neutralino decay.
Note also that the same conclusion must apply to the SZE (detection vs. prediction) anomaly at
the outer parts   0 of the clusters, because what WMAP should measure in these directions
is dominated by the SZE at   0


For instance at     5 c (typically   10' ) and



                      TSZ (  10 ' )  TSZ (0) / 5

WMAP is expected to find a T (10' ) to T (0) ratio much larger than 20%. The rest is simply
due to the central decrement being dispersed by the PSF.

Thus since we demonstrated using ROSAT data that the expected central decrement
T (  0) is non-negotiable. The bulk of the discrepancy between WMAP observed and
Predicted SZE profiles cannot be explained by blaming the X-ray data

				
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posted:9/29/2012
language:English
pages:66