Clusters of galaxies: the soft X-ray excess and Sunyaev-

Zel’dovich effect

Richard LieuJonathan Mittaz

University of Alabama, Huntsville

Shuang-Nan Zhang National TsingHua University, Beijing

XMM-Newton has definitely confirmed the cluster soft excess

Fit with a single temperature acrossthe whole 0.3-7 keV band showssignificant residuals indicating acluster soft excess

Fit to the hot ICM (1-7 keV) showingthe cluster soft excess at energiesbelow 1 keV. The excess is seen at > 20% level above the hot ICM model

45.12 =νχ

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 ABOVETHE RANGE OF THIS PLOT!

DO YOU BELIEVE IN ZERO GALACTIC COLUMN?

Central soft excess for AS1101 and A3112 ROSAT/XMMjoint fits for region from any background subtraction issues

A3112 4 - 6 arcminuteAS1101 4 - 6 arcminute

Outer soft excess for AS1101 and A3112

Central soft excess (no background issues) for A1795 andComa

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 tailof the virialised intracluster gas at X-ray temperatures was made bythe EUVE mission in 1995

Coma Cluster in the EUV

Coma Cluster 6’ – 9’ ROSAT and EUVE DS

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.

ROSAT PSPCEUVE

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

⎟⎟⎠

⎞⎜⎜⎝

⎛=→= −−

w

hhwhotwarm T

TcmnnPP )10/( 33

If we take 3210,10 −−>= cmnTT wwh

Radiative cooling time is important

yrscmnKT w1335.069 )10/()10/(106 −−−×=τ

For 326 10,10~ −−> cmnKT ww years8106×<τ

WHAT SUSTAINS THE WARM GAS AGAINST SUCH RAPID RADIATIVECOOLING?

Thermal (mekal) model kT K610≈

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 ofthe 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 togetherwith the position of the OVII line at the redshift of Coma and of the Galaxy. Theexpected absorption from the CSE cannot be seen

Same as previous slide but now all observations have been added. Again, thereis no line at the expected redshift of Coma

One of the recent claims regarding the soft excess is thedetection of OVII line emission

Kaastra et al. (2003)

AS1101 (2’-5’) with ICM model (fitted from 2-7 keV) and backgrounds

Intrinsic background

Kaastra sky average background

SOFT EXCESSREMAINS ROBUST(after subtractingthe higher background)

Isothermal modelkT = 3.08 keVA = 0.194 solar

However, the importance of good background subtraction cannot be overstated – depending on what assumptions youtake the potential background can vary by a lot.

AS1101 10’-13’ background spectrum. OVII+OVIII linesconsistent with Galactic emission and not associated with the cluster redshift (z=0.058)

OVII+OVIII lines positioned at the cluster redshift in AS1101 background

For AS1101 there is also little evidence for redshifted OVII line emission

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 OVIIline 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 beof thermal origin.•Inside a cluster’s core the thermal model is very hard toimplement. If the origin is outlying filaments seen inprojection, the required column density will be enormous.if intracluster warm gas – problem with cooling time.

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. Tormenastro-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}

Very recent simulations of clusters seem to find CSE in outerregions of merging clusters.

Non-thermal interpretation of the cluster soft excess

Hwang, C.-Y., 1997, Science, 278, 191Ensslin, T.A. & Biermann, P.L., 1998, A&A, 330, 20Sarazin, 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 NEEDTO ACCOUNT FOR THE SOFT EXCESS BRIGHTNESS?

NB. Center can be e+/e- pairs, but outside has to be CR’sfrom supernova events.

A1795: single temperature fit (2-7 keV) for two annuli

Background 100x below clusterBackground 10x below cluster

kT = 4.88 +/- 0.08 keV A = 0.43 +/- 0.02 solar

kT = 6.05 +/- 0.15 keV A = 0.27 +/- 0.03 solar

Non-thermal interpretation of the Cluster Soft Excess

Abell 1795Region Power-law

Luminosity

(0.2-1leV)

( )

Photon Index

Hot Gas kT

(keV)

Relativistic

Electron

Energy

(total ergs)

Relativistic

Electron pressure

( )

Gas Pressure

( )

Gas density

( )

0’-1’

2’-5’

3/ cmergs

3/ cmergs 3cm

4310

42107.9

sergs /

30.2

81.2

0.5

3.6

60102.1 601018.1

12108 14107

101048.1 11107.1

21085.1 3107.1

In the center 0’-1’ region, the central galaxy may quite easily supply cosmic rays of total electronenergy of a few ergs. As mentioned before, the ratio of proton to electron pressure in theCR 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 onthe best fit adundance of 0.32 solar for the 2’-5’ region, the amount of iron in the gas is SN’s in the past. Assuming each SN outputs ergs of CRs, one estimates ergs,mostlyin protons. Within the < 3Gyrs of loss time against inverse-Compton scattering these protons produce ergs of secondary electrons: NOT ENOUGH

6010

sunM9107.5 10107.5

50103~ 61107.1)'5'2( CRE

58108

Leading annihilation channels

Rate

Cosmological relic χdensity

Cross-section χ=0.3, h=0.5)

χχ annihilation in galaxy clusters

Secondary electrons with Ee Mχ are produced in situ

[Colafrancesco & Mele 2001, ApJ, 562, 24; Colafrancesco 2004, A&A, 422, L23 ]

EUV/soft X-ray ICS emission is produced by the secondary electrons - created by χχ annihilation -which scatter the CMB photons (Colafrancesco 2004)

The EUV/soft X-ray excess in Coma is best fitted by a neutralino with:

scmV A /104 326

GeVM 30χ

quite independent of the χ model.

The EUV/soft X-ray excess provides the bound

121327 )(10330

hscm

VGeVM A

χχ

121327 )(103

30

hscm

VGeVM A

χχ

To Observer

Sunyaev-Zel’dovich Effect:Compton up scattering of CMBphotons by cluster hot electrons

Basics of the Sunyaev-Zeldovich Effect CMB

41

)1()(2 x

x

eTeCMB e

exndl

cm

kT

T

T

The electron density of the hot gas is obtained by fitting ROSAT X-raysurface brightness profiles with the 2 parameter isothermal -modelignoring the central cooling flow

)(XI

2

132

20

23

2

0 1)(1)(

CX

ee nI

r

rnrn

The decrement in TCMB is then given by

2

32

1

2

3

2

)(

10

2

1

2

32

338.38)0(where1)0()(

xj

Mpc

r

keV

kT

cm

nSZ

CSZSZ

CKTTT

with 41

)1()(

x

x

e

exxj

Is the discrepancy due to an error in the -model in representing the X-ray plasma propertiesat 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 . Suppose atthe model is completely overestimating

c5rr = c5rr >)0(TSZ

drdrTc c

c

c r r

rr

r

rSZ

23

51

23

5

01

22

)0(

−∞

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+ ⎥⎦

⎤⎢⎣⎡+⎥⎦

⎤⎢⎣⎡=

Now 2nd integral

ccrc

rrdrr

rI

c 25

4

13

5 313

52 =−

=⎥⎦

⎤⎢⎣

⎡≈

−−∞

for a typical value of 2/3=

First integral

)5arctan(arctan1

1

5

0

2

2 cc

c

r

C

rr

rrdr

r

rI

c =⎟⎟⎠

⎞⎜⎜⎝

⎛=

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+=

−

The ratio only. Thus the central SZE is well predicted by using X-ray data%12II 12 ≅

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 .

The product 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

2≤

kTne

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

If this causes our discrepancy it must explain distributed over 0.5 degree.

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

by the solid angle where

with degrees. This gives a factor of 10 discrepancy since we get

at GHz (Q Band)

In the W band it is even worse since the spectra goes as where givinga shortfall of at least two orders of magnitude

1228 HzWm10 −−−

K105 5−×≈CMBTδ

CMBTδ

22 /2 cTk CMBνδπ

πδ 4/Ω

2πδ =

5.0=

1227 HzWm10 −−− 30=ναν −~F 2≥α

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 of the clusters, because what WMAP should measure in these directions is dominated by the SZE at

0>0=

For instance at (typically ) and c5 = '10≈

5/)0()'10( SZSZ TT ≈=

WMAP is expected to find a to 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 is non-negotiable. The bulk of the discrepancy between WMAP observed andPredicted SZE profiles cannot be explained by blaming the X-ray data

)'10(T )0(T

)0( = T