Quasi-hydrostatic intracluster gas under radiative cooling

Quasi-hydrostatic cooling of the intracluster gas is studied. In the quasi-hydrostatic model, work done by gravity on the inflow gas with dP \neq 0, where P is the gas pressure, is taken into account in the thermal balance. The gas flows in from the outer part so as to compensate the pressure loss of the gas undergoing radiative cooling, but the mass flow is so moderate and smooth that the gas is considered to be quasi-hydrostatic. The temperature of the cooling gas decreases toward the cluster center, but, unlike cooling flows with dP = 0, approaches a constant temperature of \sim 1/3 the temperature of the non-cooling ambient gas. This does not mean that gravitational work cancels out radiative cooling, but means that the temperature of the cooling gas appears to approach a constant value toward the cluster center if the gas maintains the quasi-hydrostatic balance. We discuss the mass flow in quasi-hydrostatic cooling, and compare it with the standard isobaric cooling flow model. We also discuss the implication of \dot{M} for the standard cooling flow model.Comment: 5 pages, 1 figure, accepted for publication in A&

Suzaku Discovery of the Strong Radiative Recombination Continuum of Iron from the Supernova Remnant W49B

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant W49B obtained with the Suzaku satellite. The spectrum exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This bump cannot be explained by any combination of high-temperature plasmas in ionization equilibrium. We firmly conclude that this bump is caused by the strong radiative recombination continuum (RRC) of iron, detected for the first time in a supernova remnant. The electron temperature derived from the bremsstrahlung continuum shape and the slope of the RRC is 1.5 keV. On the other hand, the ionization temperature derived from the observed intensity ratios between the RRC and K-alpha lines of iron is 2.7 keV. These results indicate that the plasma is in a highly overionized state. Volume emission measures independently determined from the fluxes of the thermal and RRC components are consistent with each other, suggesting the same origin of these components.Comment: 5 pages,4 figures, accepted for publication in ApJ Lette

Discovery of Strong Radiative Recombination Continua from The Supernova Remnant IC 443 with Suzaku

We present the Suzaku spectroscopic study of the Galactic middle-aged supernova remnant (SNR) IC 443. The X-ray spectrum in the 1.75-6.0 keV band is described by an optically-thin thermal plasma with the electron temperature of 0.6 keV and several additional Lyman lines. We robustly detect, for the first time, strong radiative recombination continua (RRC) of H-like Si and S around at 2.7 and 3.5 keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like K-alpha line are 1.0 keV and 1.2 keV, respectively. We thus find firm evidence for an extremely-overionized (recombining) plasma. As the origin of the overionization, a thermal conduction scenario argued in previous work is not favored in our new results. We propose that the highly-ionized gas were made at the initial phase of the SNR evolution in dense regions around a massive progenitor, and the low electron temperature is due to a rapid cooling by an adiabatic expansion.Comment: 5 pages, 5 figures, accepted by ApJ Lette

L_X-T Relation and Related Properties of Galaxy Clusters

An observational approach is presented to constrain the global structure and evolution of the intracluster medium based on the ROSAT and ASCA distant cluster sample. From statistical analysis of the gas density profile and the connection to the LX-T relation under the beta-model, the scaled gas profile is nearly universal for the outer region and the LX(>0.2r500) is tightly related to the temperature through T^3 rather than T^2. On the other hand, a large density scatter exists in the core region and there is clearly a deviation from the self-similar scaling for clusters with a small core size. A direct link between the core size and the radiative cooling timescale suggest that t_cool is a parameter to control the gas structure and the appearance of small cores in regular clusters may be much connected with the thermal evolution. We derive the luminosity-ambient temperature (T') relation, assuming the universal temperature profile to find the dispersion around the relation significantly decreases: L_1keV is almost constant for a wide range of t_cool. We further examined the LX-Tbeta and LX-T'beta relations and showed a trend that merging clusters segregate from the regular clusters on the planes. A good correlation between t_cool and the X-ray morphology on the L_1keV-t_cool/t_age plane leads us to define three phases according to the different level of cooling, and draw a phenomenological picture: after a cluster collapses and t_cool falls below t_age, the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential, and the central density gradually becomes higher to evolve from an outer-core-dominant cluster to inner-core-dominant cluster.Comment: 39 pages, 37 figures. Accepted for publication in ApJ. Version with high-quality color figures at http://cosmic.riken.jp/ota/publications/index.htm

Enhanced abundances in three large-diameter mixed-morphology supernova remnants

We present an X-ray study of three mixed-morphology supernova remnants (SNRs), HB 21, CTB 1 and HB 3, using archival ASCA and ROSAT data. These data are complemented by archival Chandra X-ray Observatory data for CTB 1 and XMM-Newton X-ray Observatory data for HB 3. The spectra from HB 21 and HB 3 are well-described with a single-temperature thermal plasma in ionization equilibrium, while a two-temperature thermal plasma is found in CTB 1. We found enhanced abundances in all three SNRs. The elemental abundance of Mg is clearly enhanced in CTB 1, while HB 21 has enhanced abundances of Si and S. The situation is not so clear in HB 3 -- the plasma in this SNR either has significantly enhanced abundances of O, Ne and Mg, or it has marginally enhanced abundances of Mg and under-abundant Fe. We discuss the plausibility of mixed-morphology SNR models for the three SNRs and the presence of enhanced abundances. We revise a list of MM SNRs and their properties, compare the three SNRs studied here with other members of this class, and discuss the presence of enhanced elemental abundances in MM SNRs. We also report the ASCA detection of a compact source in the southern part of HB 3. The source spectrum is consistent with a power law with a photon index of ~2.7, and an unabsorbed X-ray flux of ~10^{-12} erg/cm^2/s in the 0.5--10.0 keV band. The column density towards this source differs from that towards the SNR, and it is therefore unlikely they are related.Comment: 26 pages, 15 figures, revised version (minor changes), accepted for publication in ApJ (10 Aug 2006

ASCA Observations of the Supernova Remnant IC 443: Thermal Structure and Detection of Overionized Plasma

We present the results of X-ray spatial and spectral studies of the ``mixed-morphology'' supernova remnant IC 443 using ASCA. IC 443 has a center-filled image in X-ray band, contrasting with the shell-like appearance in radio and optical bands. The overall X-ray emission is thermal, not from a synchrotron nebula. ASCA observed IC 443 three times, covering the whole remnant. From the image analysis, we found that the softness-ratio map reveals a shell-like structure. At the same time, its spectra require two (1.0 keV and 0.2 keV) plasma components; the emission of the 0.2 keV plasma is stronger in the region near the shell than the center. These results can be explained by a simple model that IC 443 has a hot (1.0 keV) interior surrounded by a cool (0.2 keV) outer shell. From the emission measures, we infer that the 0.2 keV plasma is denser than the 1.0 keV plasma, suggesting pressure equilibrium between the two. In addition, we found that the ionization temperature of sulfur, obtained from H-like K$\alpha$ to He-like K$\alpha$ intensity ratio, is 1.5 keV, significantly higher than the gas temperature of 1.0 keV suggested from the continuum spectrum. The same can be concluded for silicon. Neither an additional, hotter plasma component nor a multi-temperature plasma successfully accounts for this ratio, and we conclude that the 1.0 keV plasma is overionized. This is the first time that overionized gas has been detected in a SNR. For the gas to become overionized in the absence of a photoionizing flux, it must cool faster than the ions recombine. Thermal conduction from the 1.0 keV plasma to the 0.2 keV one could cause the 1.0 keV plasma to become overionized, which is plausible within an old (3$\times10^4$ yr) SNR.Comment: 11 pages, 15 figures, 2 tables, accepted for publication in The Astrophysical Journa

Suzaku broad-band spectroscopy of RX J1347.5-1145: constraints on the extremely hot gas and non-thermal emission

We present the results from the analysis of long Suzaku observations of the most X-ray luminous galaxy cluster RX J1347.5-1145 at z=0.451. Aims: We study physical properties of the hot (~20 keV) gas clump in the south-east (SE) region discovered by the Sunyaev-Zel'dovich (SZ) effect observations, to understand the gas physics of a violent cluster merger. We also explore a signature of non-thermal emission using the hard X-ray data. Results: We find that the single-temperature model fails to reproduce the continuum emission and Fe-K lines measured by XIS simultaneously. The two-temperature model with a very hot component improves the fit, although the XIS data can only give a lower bound on its temperature. We detect the hard X-ray emission in the 12-40 keV band at the 7 sigma level; however, the significance becomes marginal when the systematic error in the background estimation is included. With the Suzaku + Chandra joint analysis, we determine the temperature of the SE excess component to be 25.3^{+6.1}_{-4.5} ^{+6.9}_{-9.5} keV (90% statistical and systematic errors), which is in an excellent agreement with the previous SZ + X-ray analysis. This is the first time that the X-ray spectroscopy alone gives a good measurement of the temperature of the hot component in the SE region, which is made possible by Suzaku's unprecedented sensitivity to the wide X-ray band. These results strongly indicate that the cluster has undergone a recent, violent merger. The spectral analysis shows that the SE component is consistent with being thermal. We find the 3 sigma upper limit on the non-thermal flux, F < 8e-12 erg s^{-1} cm^{-2} in the 12-60 keV band. Combining this limit with a recent discovery of the radio mini halo at 1.4 GHz, we find a lower limit on the strength of the intracluster magnetic field, B > 0.007 micro G.Comment: 15 pages, 13 figures. Accepted for publication in A&