1. CLUSTER SOFT EXCESS: NEW FACES OF AN OLD ENIGMA Richard Lieu University of Alabama, Huntsville Jonathan Mittaz Max Bonamente

2. 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 virialized intracluster gas at X-ray temperatures was made by the EUVE mission in 1995 Coma Cluster in the EUV

3. 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

4. XMM-Newton has definitively confirmed the cluster soft excess Fit with a single temperature across the whole 0.3-7 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

5. 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

6. Note for A3112, cluster soft excess is not a background effect

7. One of the recent claims regarding the CSE is the detection of OVII line emission Kaastra et al. (2003)

8. 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 = 0.194 solar However, the importance of good background subtraction cannot be overstated

9. 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 And little evidence for redshifted OVII line emission

10. 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

11. Best thermal candidate is emission from the WHIM (Warm Hot Intergalactic Medium) Gas betweenfrom a cosmological simulation (from R. Cen’s homepage) The CSE and thermal models

12. There is emission from the WHIM but it is very weak Maximum emission for Cells with T < 1keV Emission from a typical sightline Simply not enough material to give the observed cluster soft excess (Mittaz et al. 2004)

13. 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

14. 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 ]

15. 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

16. 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

18. 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

19. as measured by ASCA

20. Virial speed of mass in the cluster is given by Virial theorem For a rich cluster like Coma, A proton moving in this potential has kinetic energy

21. 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

22. XMM-Newton fit to Coma 0’-5’ region Fit across the whole XMM-Newton band with 1 temperature mode Limiting the fit to Coma between 2 – 7 keV yields a more consistent fit This temperature is inconsistent with other estimates e.g. ASCA Since the temperature of the full band fit is low – suggests the presence of soft excess emission < 2keV. If we model this with a power-law This model has a temperature consistent with the 2-7keV/ASCA fit F-Test indicates a significant improvement at > 99.99% over single temperature fit.

23. XMM-Newton spectra of the Coma Cluster 0’-5’ EPIC PN SpectrumEPIC 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 between 2.0 and 7.0 keV IS THE SOFT EXCESS AT A LEVEL ABOVE THE KNOWN SYSTEMATIC UNCERTAINTIES IN THE CALIBRATION OF XMM?

24. 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

25. Could the cluster soft excess be due to an incorrect subtraction of the background?

26. BUT THE BACKGROUND IS ~30 TIMES BELOW COMA COULD THE SOFT EXCESS BE DUE TO AN INCORRECT ESTIMATION OF THE FOREGROUND GALACTIC HI ABSORPTION ? Coma Cluster Average sky background at high latitudes

27. Bregman et al., 2003, ApJ, 597, 399 BUT THE SOFT EXCESS WITNESSED BY XMM-NEWTON IS ABOVE THE RANGE OF THIS PLOT!

28. DO YOU BELIEVE IN ZERO GALACTIC COLUMN?

29. Best 3 Temperature model for the soft excess of Coma’s 6’-9’ region IS THIS A PHYSICALLY SENSIBLE MODEL?

30. 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?

31. Thermal (mekal) model kT

32. Giant ¼ keV Halo centered at Coma (as detailed by the ROSAT sky survey)

33. ROSAT/PSPC Radial surface brightness of Coma (Bonamente, Joy, & Lieu, 2003, ApJ, 585, 722) ¼ keV ¾ keV

34. 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.

35. Coma cluster 0.5 – 2 keV with XMM-Newton pointings

36. 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 model for the outer soft excess?

37. 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 BEWARE of constraints specific to this scenario. For example the emission measure EM is given by Which is fixed by observations (i.e. lower n implies higher L). For a given value of EM, the line-of-sight COLUMN DENSITY of warm filaments is which can be observationally constrained!

38. 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) Coupled with the EM of the soft excess which is non-negotiable this means 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

39. Arguments against thermal origin of the soft excess 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?

40. Cen & Ostriker, 1999, ApJ, 514, 1

41. Comparing emission from the WHIM with XMM-Newton observations Emission weighted temperature map of one projection of the simulation showing the simulated Cluster at the top right hand corner (Mittaz et al. 2004)

42. 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)

43. Arguments against thermal origin of the soft excess 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!

44. Thermal line detections from Kaastra et al., 2003, A&A, 397, 445 ¼ ROSAT all sky survey image around AS1101

45. Arguments against thermal origin of the soft excess 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!

46. CLUSTER SOFT EXCESS DIAGNOSTICS The soft excess outside clusters’ cores is now open to question, because the effect is real. Inside a cluster’s core the thermal model is even more disqualified. If the origin is outlying filaments seen in projection, the required column density will be enormous. if intracluster warm gas – problem with cooling time. THERE IS, HOWEVER, AN ALTOGETHER DIFFERENT APPROACH.

47. A1795 Goodness of Fit 0 – 1’ region (MOS1 + MOS2) Null probability Broad band (0.4-7keV) fit Limited (2-7keV) fit 2 – 5’ region (MOS1 + MOS2) Broad band (0.4-7keV) fit Null probability Limited (2-7keV) fit

48. A1795: single temperature fit (2-7 keV) for two annuli Background 100x below cluster Background 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

49. Comparison of backgrounds comparing Lumb data set (red) and Lumb + double subtraction background in the offset field 2’-5’

50. 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.

51. 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 CLUSTER SOFT EXCESS HAS BECOME AN OBSERVATIONAL REALITY, IT REMAINS A THEORETICAL ENIGMA.

52. 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. Background z = 0

53. It can also be demonstrated that the reduction in best-fit temperature when the 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 power law (to account for any possible hard tail) the resulting improvement is marginal Mekal + Power-law Mekal only F-Test gives likelihood of improvement at only 32%

54. Coma XMM-Newton MOS1+2 and PN fits to 0’-5’ region Single temperature Single temperature + Power-law kT=7.34 keVkT=8.79 keV Assuming a soft excess in Coma dramatically improves the fit

55. Coma in the Radio Coma has relativistic electrons!

56. Coma Hard tail : data from Fusco-Femiano

57. 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-thermal approach to cluster soft excess does not suffer from any obvious setbacks.

58. Detection of very extended EUV excess in A2199 (Lieu et al., 1999, ApJ, 527, L77

59. 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 Gyrs Initial pressure ratio all protons : soft excess emitting electrons ~ 200:1. After ~ 3 Gyrs this ratio > 1,000:1

60. 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 the required level of cosmic rays’ presence? This can be 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.

61. XMM-Newton fit to Coma 0’-5’ region Fit across the whole XMM-Newton band with 1 temperature mode Limiting the fit to Coma between 2 – 7 keV yields a more consistent fit This temperature is inconsistent with other estimates e.g. ASCA Since the temperature of the full band fit is low – suggests the presence of soft excess emission < 2keV. If we model this with a power-law This model has a temperature consistent with the 2-7keV/ASCA fit F-Test indicates a significant improvement at > 99.99% over single temperature fit.

62. 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.