J. Nevalainen

The Cluster M-T Relation from Temperature Profiles Observed with ASCA and ROSAT

We calibrate the galaxy cluster mass-temperature (M-T) relation using the temperature profiles of intracluster gas observed with ASCA (for hot clusters) and ROSAT (for cool groups). Our sample consists of apparently relaxed clusters for which the total masses are derived, assuming hydrostatic equilibrium. The sample provides data on cluster X-ray emission-weighted cooling-flow-corrected temperatures and total masses up to r1000. The resulting M-T scaling in the 1-10 keV temperature range is M1000 = (1.23 ± 0.20) 1015 h M☉ (Tz = 0/10 keV)1.79±0.14 with 90% confidence errors, or significantly (99.99% confidence) steeper than the self-similar relation M ∝ T3/2. For any given temperature, our measured mass values are significantly smaller compared to the simulation results of Evrard, Metzler, & Navarro that are frequently used for mass-temperature scaling. The higher temperature subsample (kT ≥ 4 keV) is consistent with M ∝ T3/2, allowing the possibility that the self-similar scaling breaks down at low temperatures, perhaps due to heating by supernovae that is more important for low-temperature groups and galaxies, as suggested by earlier works.

XMM-Newton EPIC Background Modeling for Extended Sources

We use XMM-Newton blank-sky and closed-cover background data to explore background subtraction methods for large extended sources filling the EPIC field of view, such as nearby galaxy clusters, for which local background estimation is difficult. In particular, we investigate the uncertainties of the background modeling in the 0.8-7.0 keV band that affect cluster analyses. To model the background, we have constructed composite data sets from the blank-sky observations and compared them to the individual blank-sky observations to evaluate the modeling error. Our results apply to data obtained with thin and medium optical filters and in full frame and extended full frame modes. As expected, the modeling uncertainty is determined by how the EPIC background flares are filtered. We find that to keep this uncertainty tolerable, one has to use a much more restrictive filter than that commonly applied. In particular, because flares have highly variable spectra, not all of them are identified by filtering the E > 10 keV light curve. We tried using the outer part of the EPIC field of view for monitoring the background in a softer band (1-5 keV). We find that one needs to discard the time periods when either the hard-band or the soft-band rate exceeds the nominal value by more than 20% in order to limit the 90% CL background uncertainty to between ±5% at E = 4-7 keV and ±20% at E = 0.8-1 keV, for both MOS and PN. This compares to a 10%-30% respective PN uncertainty when only the hard-band light curve is used for filtering, and to a 15%-45% PN uncertainty when applying the commonly used 2-3 σ filtering method. Adding such a soft-band filter on average results in only a 5%-10% reduction of the useful exposure time. We illustrate our method on the nearby cluster A1795. The above background uncertainties convert into systematic temperature uncertainties between about ±1% at r = 3-4 and ±20%-25% (about ±1 keV for A1795) at r = 10-15. For comparison, the commonly applied 2-3 σ clipping of the hard-band light curve misses a significant number of flares, rendering the temperatures beyond r = 10 unconstrained. Thus, the background uncertainties do not prohibit the EPIC temperature profile analysis of low-brightness regions, such as outer regions of galaxy clusters, provided a conservative flare filtering such as the double-filtering method with ±20% limits is used.

Nonthermal Hard X-Ray Emission in Galaxy Clusters Observed with the BeppoSAX PDS

We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20-80 keV energy band. We estimate the nonthermal hard X-ray (HXR) cluster emission by modeling the thermal contribution from the cluster gas and the nonthermal contamination from the unobscured active galactic nuclei (AGNs) in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGNs, the resulting nonthermal component is detected at a 2 σ level in ~50% of the nonsignificantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus, and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of nonthermal origin, whereas merger clusters do, with a 20-80 keV luminosity of ~1043-1044 h ergs s-1. The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR emission with a photon index of 2.8 in the 12-115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGNs, which have harder spectra and a centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters, which has been proposed as a possible origin of the nonthermal hard X-ray emission models. Assuming that the cosmic microwave background photons experience inverse Compton scattering from the merger-accelerated relativistic electrons and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of μ = 3.8-5.0. In presence of cluster magnetic fields this relativistic electron population produces synchrotron emission with a spectral index of 1.4-2.1, consistent with radio halo observations of merger clusters. Thus both hard X-ray and radio observations of merger clusters are consistent with the inverse Compton model. The observed slope of the HXR emission is also consistent with that predicted by the nonthermal bremsstrahlung, which thus cannot be ruled out by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming a centrally concentrated distribution of HXR emission, the data require a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron inverse Compton model.We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20 -- 80 keV energy band. The non-thermal hard X-ray cluster emission (HXR) is detected at a 2 sigma level in 50% of the non-significantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of non-thermal origin, whereas merger clusters do, with a 20-80 keV luminosity of 10^(43-44) erg/s. The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR with a photon index of 2.8+0.3-0.4 in the 12-115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGN, which have harder spectra and centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters. Assuming that the Cosmic Microwave Background photons experience Inverse Compton scattering from the merger-accelerated relativistic electrons, and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of mu = 3.8-5.0. The observed slope of HXR is also consistent with that predicted by the non-thermal bremsstrahlung, which thus cannot be ruled by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming centrally concentrated distribution of HXR, the data requires a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron Inverse Compton model.

SOFT X-RAY EXCESS EMISSION IN THREE CLUSTERS OF GALAXIES OBSERVED WITH XMM-NEWTON

We present results on the spectroscopic analysis of XMM-Newton EPIC data of the central 0.5/h_50 Mpc regions of the clusters of galaxies Coma, A1795 and A3112. The temperature of the hot intracluster gas as determined by modeling the 2 - 7 keV PN and MOS data is consistent with that inferred from the FeXXV-FeXXVI line ratio. A significant warm emission component at a level above the systematic uncertainties is evident in the data and confirmed by ROSAT PSPC data for Coma and A1795. The non-thermal origin of the phenomenon cannot be ruled out at the current level of calibration accuracy, but the thermal model fits the data significantly better, with temperatures in the range of 0.6 -- 1.3 keV and electron densities of the order of 10^{-4} -- 10^{-3} cm^{-3}. In the outer parts of the clusters the properties of the warm component are marginally consistent with the results of recent cosmological simulations, which predict a large fraction of the current epochs bayons located in a warm-hot intergalactic medium (WHIM). However, the derived densities are too high in the cluster cores, compared to WHIM simulations, and thus more theoretical work is needed to fully understand the origin of the observed soft X-ray excess.We present results on the spectroscopic analysis of XMM-Newton EPIC data of the central 0.5 h 50 -1 Mpc regions of the clusters of galaxies Coma, A1795 and A3112. A significant warm emission component at a level above the systematic uncertainties is evident in the data and confirmed by ROSAT PSPC data for Coma and A1795. The non-thermal origin of the phenomenon cannot be ruled out at the current level of calibration accuracy, but the thermal model fits the data significantly better, with temperatures in the range of 0.6 – 1.3 keV and electron densities of the order of 10-4 – 10-3 cm-3. In the outer parts of the clusters the properties of the warm component are marginally consistent with the results of recent cosmological simulations, which predict a large fraction of the current epoch’s bayons located in a warm-hot intergalactic medium (WHIM). However, the derived densities are too high in the cluster cores, compared to WHIM simulations, and thus more theoretical work is needed to fully understand the origin of the observed soft X-ray excess.

The XMM-

The evolution with redshift of the temperature-luminosity relation of X-ray galaxy clusters is a key ingredient to break degeneracies in the interpretation of X-ray clusters redshift number counts. We therefore take advantage of the recent measurements of the temperature-luminosity relation of distant clusters observed with XMM-Newton and Chandra satellites to examine theoretical number counts expected for different available X-rays cluster samples, namely the RDCS, EMSS, SHARC, 160deg^2 and the MACS at redshift greater than 0.3. We derive these counts without any adjustment, using models previously normalized to the local temperature distribution function and to the high-z (z = 0.33) TDF. We find that these models having Omega_M in the range [0.85-1.] predict counts in remarkable agreement with the observed counts in the different samples. We illustrate that this conclusion is weakly sensitive to the various ingredients of the modeling. Therefore number counts provide a robust evidence of an evolving population. A realistic flat low density model (Omega_M = 0.3), normalized to the local abundance of clusters is found to overproduce cluster abundance at high redshift (above z = 0.5) by nearly an order of magnitude. This result is in conflict with the popular concordance model. The conflict could indicate a deviation from the expected scaling of the M-T relation with redshift.

\Omega

We constrain the total mass distribution in the cluster A3571, combining spatially resolved ASCA temperature data with ROSAT imaging data with the assumption that the cluster is in hydrostatic equilibrium. The total mass within r500 (1.7 h Mpc) is M500 = 7.8 ? 1014 h M? at 90% confidence, 1.1 times smaller than the isothermal estimate. The Navarro, Frenk, & White universal profile is a good description of the dark matter density distribution in A3571. The gas density profile is shallower than the dark matter profile, scaling as r-2.1 at large radii, leading to a monotonically increasing gas mass fraction with radius. Within r500 the gas mass fraction reaches a value of fgas = 0.19 h (90% confidence errors). Assuming that this value of fgas is a lower limit for the universal value of the baryon fraction, we estimate the 90% confidence upper limit of the cosmological matter density to be ?m < 0.4.

project - II. Cosmological implications from the high redshift L – T relation of X-ray clusters

The Galactic superluminal source GRS 1915+105 was found to experience a peculiar X-ray variability in a narrow count rate range (9300-12,100 counts s-1, 5 PCUs) of the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. This can be seen as a ring-shaped pattern in the two-color diagram of count rates, where the hard hardness F(13-40 keV)/F(2-13 keV) is plotted against the soft hardness F(5-13 keV)/F(2-5 keV). The system runs one cycle with periods ranging between 50 and 100 s for different observations, one rotation in the two-color diagram corresponding to the time between two contiguous maxima in the light curve. We model this behavior successfully with the help of a self-consistent two-phase thermal model in which seed photons from an optically thick classical disk are Comptonized in a hot spherical corona surrounding the inner disk (Poutanen & Svensson; Vilhu et al.; Nevalainen et al.). In the model, changes of two parameters regulate the paths in the two-color diagram: the blackbody temperature Tin of the inner disk and the Thomson optical depth multiplied by the electron temperature of the hot phase τTe. These parameters oscillate with time but with a phase shift between each other, causing the ring-shaped pattern. During the observation studied in more detail (20402-01-30-00), the inner disk radius varied with a 97 s period between 20 and 35 km with an anticorrelation between the coronal τTe and the mass accretion rate through the disk, possibly indicating a coupling between the disk and coronal accretion. During a typical cycle, the inner disk radius rapidly shrank and returned more slowly back to the original larger value. In the rings we may see phenomena close to the black hole horizon under near Eddington accretion rates.

X-Ray Total Mass Estimate for the Nearby Relaxed Cluster A3571

We study the gas mass fraction,

Two-Phase Modeling of the Rings in the RXTE Two-Color Diagram of GRS 1915+105

f_{rm gas},

XMM-Newton\Omega project: III. Gas mass fraction shape in high redshift clusters

behavior in