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 Fit with a single temperature across the whole keV band shows significant residuals indicating a cluster soft excess Fit to the hot ICM (1-7 keV) showing the cluster soft excess at energies below 1 keV. The excess is seen at > 20% level above the hot ICM model

Fit to PKS 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

A arcminuteAS arcminute Outer soft excess for AS1101 and A3112

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 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 PSPC EUVE

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 If we take Radiative cooling time is important For WHAT SUSTAINS THE WARM GAS AGAINST SUCH RAPID RADIATIVE COOLING?

Thermal (mekal) model kT

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 2 nd 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)

AS1101 (2’-5’) with ICM model (fitted from 2-7 keV) and backgrounds Intrinsic background Kaastra sky average background SOFT EXCESS REMAINS ROBUST (after subtracting the higher background) Isothermal model kT = 3.08 keV A = solar However, the importance of good background subtraction cannot be overstated – depending on what assumptions you take the potential background can vary by a lot.

AS ’-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 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 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.

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/ 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 outer regions of merging clusters.

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 100x below cluster Background 10x below cluster kT = / keV A = / solar kT = / keV A = / solar

Non-thermal interpretation of the Cluster Soft Excess Abell 1795 RegionPower-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’ In the center 0’-1’ region, the central galaxy may quite easily supply cosmic rays of total electron energy of a few ergs. As mentioned before, the ratio of proton to electron pressure in the 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 SN’s in the past. Assuming each SN outputs ergs of CRs, one estimates ergs,mostly in protons. Within the < 3Gyrs of loss time against inverse-Compton scattering these protons produce ergs of secondary electrons: NOT ENOUGH

Leading annihilation channels Rate Cosmological relic  density Cross-section   =0.3, h=0.5)  annihilation in galaxy clusters Secondary electrons with E e  M  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  annihilation - which scatter the CMB photons (Colafrancesco 2004) The EUV/soft X-ray excess in Coma is best fitted by a neutralino with: quite independent of the  model. The EUV/soft X-ray excess provides the bound

To Observer Sunyaev-Zel’dovich Effect: Compton up scattering of CMB photons by cluster hot electrons

Basics of the Sunyaev-Zeldovich Effect CMB The electron density of the hot gas is obtained by fitting ROSAT X-ray surface brightness profiles with the 2 parameter isothermal -model ignoring the central cooling flow The decrement in T CMB is then given by with

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. Suppose at the model is completely overestimating Now 2nd integral for a typical value of First integral The ratio 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. 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

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 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 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 For instance at (typically ) and 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 and Predicted SZE profiles cannot be explained by blaming the X-ray data