cluster soft excess: new faces of an old enigma richard lieu university of alabama, huntsville...
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CLUSTER SOFT EXCESS: NEW FACES OF AN OLD ENIGMA
Richard Lieu University of Alabama, Huntsville
Jonathan Mittaz Max Bonamente
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 virialized 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
XMM-Newton has definitively 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
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
Note for A3112, cluster soft excess is not a background effect
One of the recent claims regarding the CSE is the detection ofOVII 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
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
And 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
As yet no strong evidence for redshifted line emission
Best thermal candidate is emission from the WHIM (Warm Hot Intergalactic Medium)
Gas between KTK 75 1010 from a cosmological simulation (from R. Cen’s homepage)
The CSE and thermal models
There is emission from the WHIM but it is very weak
Maximum emission forCells with T < 1keV
Emission from atypical sightline
Simply not enough material to give the observed cluster soft excess
(Mittaz et al. 2004)
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 density
Cross-section =0.3, h=0.5)
annihilation in galaxy clusters
Secondary electrons with Ee 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:
scmV A /104 326
GeVM 30
quite independent of the model.
The EUV/soft X-ray excess provides the bound
121327 )(10330
hscm
VGeVM A
121327 )(103
30
hscm
VGeVM A
CONCLUSION
•XMM-Newton Observations of Clusters in the 0.4-7 keV range were made. They confirm ‘beyond reasonable doubt’ the existence of a soft excess first noted by the EUVE and ROSAT teams. In particular the soft excess:
•is strong and in XMM extends to 1-2 keV energy, at a level beyond the uncertainties of the detector responses•is seen in regions where the cluster low energy flux is 10-100 times above background•may be explained as a Galactic absorption anomaly only by asserting that the HI column should actually be zero.•At the outskirts of clusters, the OVII line claimed by Kaastra Lieu et al. is due to Galactic foreground emission•Non-thermal models require an unrealistic shock acceleration mechanism• Signature of dark matter not excluded
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
3.0,4.07.8 AkeVkT as measured by ASCA
R
GMv
R
GMmmvVT 202
Virial speed of mass in the cluster is given by
Virial theorem
For a rich cluster like Coma,
skmvMpcRMM sun /10002,105 14
A proton moving in this potential has kinetic energy
kTkeVvmp 52
1 2
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
XMM-Newton fit to Coma 0’-5’ region
Fit across the whole XMM-Newton band with 1 temperature mode092.0086.0336.7
kT014.0013.0176.0
A
Limiting the fit to Coma between 2 – 7 keV yields a more consistent fit26.025.000.9
kT018.0018.0197.0
A
21.023.079.8
kT019.0024.0219.0
A
This temperature is inconsistent with other estimates e.g. ASCA
Since the temperature of the full band fit is low – suggests the presenceof soft excess emission < 2keV. If we model this with a power-law
27.013.022.2
This model has a temperature consistent with the 2-7keV/ASCA fitF-Test indicates a significant improvement at > 99.99% over singletemperature fit.
XMM-Newton spectra of the Coma Cluster 0’-5’
EPIC PN Spectrum EPIC MOS1 + MOS2 Spectra
(Nevalainen et al., 2003, ApJ, 584, 716)
Solid line is the best fit single temperature hot ICM emission model (kT = 9.6 keV, A = 0.22 solar) as obtained by fitting the XMM data between2.0 and 7.0 keV
IS THE SOFT EXCESS AT A LEVEL ABOVE THE KNOWN SYSTEMATICUNCERTAINTIES IN THE CALIBRATION OF XMM?
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
Could the cluster soft excess be due to an
incorrect subtraction of the background?
BUT THE BACKGROUND IS ~30 TIMES BELOW COMA
COULD THE SOFT EXCESS BE DUE TO AN INCORRECT ESTIMATIONOF THE FOREGROUND GALACTIC HI ABSORPTION ?
Coma Cluster
Average sky backgroundat high latitudes
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?
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
For 326 10,10~ cmnKT ww years8106
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)
What are the PROS and CONS if a thermal intercluster filament modelfor the outer soft excess?
If emission is caused by warm intercluster filaments:Can avoid the need for pressure balance with the hot intracluster gas so the warm gas density can be 3210 cmnw
BEWARE of constraints specific to this scenario. For example the emission measure EM is given by
LAnEM 2
Which is fixed by observations (i.e. lower n implies higher L). For a given valueof EM, the line-of-sight COLUMN DENSITY of warm filaments is
nnL
1~ which can be observationally constrained!
XMM-Newton PN Spectrum of X-Comae (z=0.091 28 arcminutes from Coma center). Shown is a continuum model together with OVII 21.6A line with an equivalent width of 28eV
This can be used to set a limit on the absorbing column (Nicastro et al. 1999)222108.1 cmNH
Coupled with the EM of the soft excess which is non-negotiable this means34105.5 cmn
For a warm gas appropriate for the cluster halo (A=0.1, kT = 0.2 Bonamente et al. 2003,Finoguenov et al. 2003) this gives a possible lifetime of or yrsw
91012.2 18.0z
Arguments against thermal origin of the softexcess at cluster outskirts
•Minimum required warm gas column density is contradicted by absorption line measurements of quasar spectra
•Is the outer soft excess really associated with the Warm Hot Intergalactic Medium?
Cen & Ostriker, 1999, ApJ, 514, 1
Comparing emission from the WHIM with XMM-Newton observations
Emission weighted temperature map of one projection of the simulation showing the simulatedCluster at the top right hand corner (Mittaz et al. 2004)
Simulated ‘Coma’ X-ray spectrum from Cen’s datacube for a typical sightline in region 5’-13’ assuming the redshift of Coma
No cluster soft excess seen
(Mittaz et al. 2004)
Arguments against thermal origin of the softexcess at cluster outskirts
•Minimum required warm gas column density is contradicted by absorption line measurements of quasar spectra•Is the outer soft excess really associated with the Warm Hot Intergalactic Medium? •Are the OVII emission lines found on top the soft excess spectra at the outskirts of some cluster real?
NO!
Thermal line detections from Kaastra et al., 2003, A&A, 397, 445
¼ ROSAT all sky survey image around AS1101
Arguments against thermal origin of the softexcess at cluster outskirts
•Minimum required warm gas column density is contradicted by absorption line measurements of quasar spectra.•Is the outer soft excess really associated with the Warm Hot Intergalactic Medium? NO!
•Are the OVII emission lines found on top of the soft excess spectra at the outskirts of some clusters real? NO!
CLUSTER SOFT EXCESS DIAGNOSTICS
•The soft excess outside clusters’ cores is now open toquestion, because the effect is real.•Inside a cluster’s core the thermal model is even moredisqualified. If the origin is outlying filaments seen inprojection, the required column density will be enormous.if intracluster warm gas – problem with cooling time.
THERE IS, HOWEVER, AN ALTOGETHER DIFFERENTAPPROACH.
A1795 Goodness of Fit0 – 1’ region (MOS1 + MOS2)
027.0199.4 kT 789,316.12
Null probability9102.5
Broad band (0.4-7keV) fit
Limited (2-7keV) fit
12.001.5 kT
2 – 5’ region (MOS1 + MOS2)
Broad band (0.4-7keV) fit
05.052.5 kT 835,095.12
Null probability21094.2
Limited (2-7keV) fit
21.026.6 kT
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
Comparison of backgrounds comparing Lumb data set (red) and Lumb + double subtraction background in the offset field 2’-5’
CONCLUSION
•XMM-Newton Observations of Clusters in the 0.4-7 keV range were made. They confirm ‘beyond reasonable doubt’ the existence of a soft excess first noted by the EUVE and ROSAT teams. In particular the soft excess:
•is strong and in XMM extends to 1-2 keV energy, at a level beyond the uncertainties of the detector responses•is seen in regions where the cluster low energy flux is 10-100 times above background•may be explained as a Galactic absorption anomaly only by asserting that the HI column should actually be zero.
•At the cluster cores, a population of cosmic rays accelerated at the ‘central engine’ could account for the phenomena. The proton pressure of these CR’s attains equipartition with the hot ICM i.e. sufficient to choke a cooling flow.
•At the cluster outskirts the soft excess does not appear to be thermal in origin, because:
•requisite column density of warm gas is inconsistent with the spectrum of background quasars;•the excess is 100-1000 times brighter than the level predicted by WHIM models;•the OVII line claims by Kaastra Lieu et al. is due to Galactic foreground emission.
•On the other hand, non-thermal models also do not seem to be relevant here, because there is no obvious source of relativistic particles.
THUS, AFTER A FULL DECADE OF RESEARCH, WHILE CLUSTERSOFT EXCESS HAS BECOME AN OBSERVATIONAL REALITY, ITREMAINS A THEORETICAL ENIGMA.
Simulation of A1795 (2’-5’) hot ICM with a thermal soft excess in Astro-E. Model is a single temperature fit to the spectrum. Lines due to the soft excess component can be clearly seen. Exposure time is 40 ksec.
Backgroundz = 0
It can also be demonstrated that the reduction in best-fit temperature whenthe 0.4 – 2 keV band is included in the fitted procedure is NOT because A hard tail is present in the Coma spectra (thereby biaing upwards the temperature inferred from the 2-7 keV band). Reason is that when the 2-7 keV data are modeled with hot isothermal bremsstrahlung plus a powerlaw (to account for any possible hard tail) the resulting improvement ismarginal
740.0
1098,7.812
21.0
60.9
2
2
A
kT
761.0
1100,6.837
20.0
0.9
2
2
A
kTMekal + Power-law Mekal only
F-Test gives likelihood of improvement at only 32%
Coma XMM-Newton MOS1+2 and PN fits to 0’-5’ region
Single temperature Single temperature + Power-law
kT=7.34 keV kT=8.79 keV
Assuming a soft excess in Coma dramatically improves the fit
Coma in the Radio
Coma has relativistic electrons!
Coma Hard tail : data from Fusco-Femiano
Cosmic rays in equipartition with the hot ICM?Where do these particles come from?
Still, at least within the central 20’ radius of Coma the non-thermalapproach to cluster soft excess does not suffer from any obvioussetbacks.
Detection of very extended EUV excess in A2199 (Lieu et al., 1999,ApJ, 527, L77
Most of the CR pressure is in the protons
In above plots curve 1: 0 Gyrs, 2: 0.5 Gyrs, 3: 1.5 Gyrs 4: 3.0 Gyrs, 5: 4.5 GyrsInitial pressure ratio all protons : soft excess emitting electrons ~ 200:1.After ~ 3 Gyrs this ratio > 1,000:1
• At the core of a cluster, the central galaxy is an obvious source of cosmic rays and pairs. Normally they are transported to larger radii by Bohm diffusion – very slow.
• At the outskirts, one can only expect cosmic rays (i.e. no pairs) generated in-situ by supernova remnants within member galaxies of the same region.
Do we have enough SN’s to account for therequired level of cosmic rays’ presence? This canbe answered by looking at the iron content of the gas.
We will use the Newton data of Abell 1795 to test the non-thermal scenario.
XMM-Newton fit to Coma 0’-5’ region
Fit across the whole XMM-Newton band with 1 temperature mode092.0086.0336.7
kT014.0013.0176.0
A1733,2.17702
02.12
Limiting the fit to Coma between 2 – 7 keV yields a more consistent fit26.025.000.9
kT018.0018.0197.0
A1100,6.8372
761.02
21.023.079.8
kT019.0024.0219.0
A1731,2.14042
811.02
This temperature is inconsistent with other estimates e.g. ASCA
Since the temperature of the full band fit is low – suggests the presenceof soft excess emission < 2keV. If we model this with a power-law
27.013.022.2
This model has a temperature consistent with the 2-7keV/ASCA fitF-Test indicates a significant improvement at > 99.99% over singletemperature fit.
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.