Abell 754: A Non-Head On Collision of Subclusters

We have analyzed spatially resolved spectra of A754 obtained with ASCA. The combination of spectral and imaging capabilities of ASCA has set unprecedented constraints on the hydrodynamical effects of a cluster merger. We find significant gas temperature variations over the cluster face, indicating shock heating of the atmosphere during the merger. The hottest region, >12 keV, is located in the region of the Northwest galaxy clump though the entire region along the cluster axis appears to be hotter than the mean cluster temperature (~9 keV). The cool, ~5 keV gas originally found with the HEAO1-A2 experiment, resides in the exterior of the cluster atmosphere and in plume of gas we identify with a stripped cool atmosphere of the infalling subcluster. We have also attempted to reconstruct an iron abundance map of this merging system. Though poorly constrained, no significant deviations of abundance from the mean value are apparent in the individual regions. A754 is the only cluster so far which shows the significant temperature pattern expected in a subcluster merger, in both the ROSAT (Henry & Briel 1995) and ASCA data, providing the first possibility to compare it with theoretical predictions. The observed temperature and surface brightness maps suggest that the two colliding subunits have missed each other by about 1 Mpc, and are now moving perpendicular to the cluster axis in the image plane (as, e.g., in the simulations by Evrard etal 1996).Comment: Latex, 10 pages, 3 figures incl. color plate, uses aaspp4.sty, flushrt.sty and pstricks.sty. Submitted to ApJ Letter

A Differential X-Ray Gunn-Peterson Test Using a Giant Cluster Filament

Using CCD detectors onboard the forthcoming X-ray observatories Chandra and XMM, it is possible to devise a measurement of the absolute density of heavy elements in the hypothetical warm gas filling intercluster space. This gas may be the largest reservoir of baryonic matter in the Universe, but even its existence has not been proven observationally at low redshifts. The proposed measurement would make use of a unique filament of galaxy clusters spanning over 700 Mpc (0.1<z<0.2) along the line of sight in a small area of the sky in Aquarius. The surface density of Abell clusters there is more than 6 times the sky average. It is likely that the intercluster matter column density is enhanced by a similar factor, making its detection feasible under certain optimistic assumptions about its density and elemental abundances. One can compare photoabsorption depth, mostly in the partially ionized oxygen edges, in the spectra of clusters at different distances along the filament, looking for a systematic increase of depth with the distance. The absorption can be measured by the same detector and through the same Galactic column, hence the differential test. A CCD moderate energy resolution (about 100 eV) is adequate for detecting an absorption edge at a known redshift.Comment: Latex, 4 pages, 3 figures, uses emulateapj.sty. ApJ Letters in pres

Heating of the Intracluster Gas in the Triangulum Australis Cluster

ASCA and ROSAT X-ray data are used to obtain two-dimensional maps of the gas temperature, pressure and specific entropy in the Triangulum Australis cluster of galaxies. We find that this hot (T_e=10.3+-0.8 keV) system probably has a temperature peak (T_e>12 keV) at the cluster core, which approximately corresponds to the adiabatic relation. An underdense gas sector, found in the ROSAT cluster image eastward of the core, has a higher temperature than average at that radius. At this higher temperature, the gas pressure in this region is equal to that of the rest of the cluster at the same radius, but the specific entropy of this gas is significantly higher (although the temperature difference itself is only marginally significant). We speculate that the existence of this region of underdense high-entropy gas, as well as the adiabatic central temperature peak, indicate recent or ongoing heating of the intergalactic medium in this cluster. The most probable source of such heating is a subcluster merger, for which the hydrodynamic simulations predict a qualitatively similar temperature structure. We point out that entropy maps can provide a physically meaningful way of diagnosing merging clusters and comparing the predictions of merger simulations to the data.Comment: Text revised to give more technical details and clarify discussion. Accepted for ApJ Letters. Latex, 5 pages incl. figures, macros include

Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 keV band with Chandra

We present the absolute measurement of the unresolved 0.5-8 keV cosmic X-ray background (CXB) in the Chandra Deep Fields (CDFs) North and South, the longest observations with Chandra (2 Ms and 1 Ms, respectively). We measure the unresolved CXB intensity by extracting spectra of the sky, removing all point and extended sources detected in the CDF. To model and subtract the instrumental background, we use observations obtained with ACIS in stowed position, not exposed to the sky. The unresolved signal in the 0.5-1 keV band is dominated by diffuse Galactic and local thermal-like emission. In the 1-8 keV band, the unresolved spectrum is adequately described by a power law with a photon index 1.5. We find unresolved CXB intensities of (1.04+/-0.14)x10^-12 ergs cm^-2 s^-1 deg^-2 for the 1-2 keV band and (3.4+/-1.7)x10^-12 ergs cm^-2 s^-1 deg^-2 for the 2-8 keV band. Our detected unresolved intensities in these bands significantly exceed the expected flux from sources below the CDF detection limits, if one extrapolates the logN/logS curve to zero flux. Thus these background intensities imply either a genuine diffuse component, or a steepening of the logN/logS curve at low fluxes, most significantly for energies <2 keV. Adding the unresolved intensity to the total contribution from sources detected in these fields and wider-field surveys, we obtain a total intensity of the extragalactic CXB of (4.6+/-0.3)x10^-12 ergs cm^-2 s^-1 deg^-2 for 1-2 keV and (1.7+/-0.2)x10^-11 ergs cm^-2 s^-1 deg^-2 for 2-8 keV. These totals correspond to a CXB power law normalization (for photon index 1.4) of 10.9 photons cm^-2 s^-1 keV^-1 sr^-1 at 1 keV. This corresponds to resolved fracations of 77+/-3% and 80+/-8% for 1-2 and 2-8 keV, respectively.Comment: 23 emulateapj pages, accepted for publication in ApJ. Minor revisions, most notably a new summary of the error analysi

Physics of the Merging Clusters Cygnus A, A3667, and A2065

We present ASCA gas temperature maps of the nearby merging galaxy clusters Cygnus A, A3667, and A2065. Cygnus A appears to have a particularly simple merger geometry that allows an estimate of the subcluster collision velocity from the observed temperature variations. We estimate it to be ~2000 km/s. Interestingly, this is similar to the free-fall velocity that the two Cygnus A subclusters should have achieved at the observed separation, suggesting that merger has been effective in dissipating the kinetic energy of gas halos into thermal energy, without channeling its major fraction elsewhere (e.g., into turbulence). In A3667, we may be observing a spatial lag between the shock front seen in the X-ray image and the corresponding rise of the electron temperature. A lag of the order of hundreds of kiloparsecs is possible due to the combination of thermal conduction and a finite electron-ion equilibration time. Forthcoming better spatial resolution data will allow a direct measurement of these phenomena using such lags. A2065 has gas density peaks coincident with two central galaxies. A merger with the collision velocity estimated from the temperature map should have swept away such peaks if the subcluster total mass distributions had flat cores in the centers. The fact that the peaks have survived (or quickly reemerged) suggests that the gravitational potential also is strongly peaked. Finally, the observed specific entropy variations in A3667 and Cygnus A indicate that energy injection from a single major merger may be of the order of the full thermal energy of the gas. We hope that these order of magnitude estimates will encourage further work on hydrodynamic simulations, as well as more quantitative representation of the simulation results.Comment: Corrected the Cyg-A figure (errors shown were 1-sigma not 90%); text unchanged. ApJ in press. Latex, 5 pages, 3 figures (2 color), uses emulateapj.st

X-Ray Spectral Properties of the Cluster Abell 2029

We have analyzed ASCA and ROSAT PSPC spectra and images of the galaxy cluster Abell 2029. The ASCA spectra of the cluster indicate that the gas temperature declines with radius. The PSPC image shows that the cluster is very regular and smooth. Also, there is no significant evidence for any irregularities in the temperature distribution in the cluster, as would be produced by a subcluster merger. These results suggest that A2029 is a relaxed cluster, and that the gas is in hydrostatic equilibrium. We use the assumption of equilibrium to determine the gravitational mass of the cluster as a function of radius. At a radius of 16' (1.92 h_50^-1 Mpc), the gravitational mass is M_tot = (9.42+-4.22) x 10^14 h_50^-1 Msun, while the mass of gas is M_gas = (2.52+-0.77) x 10^14 h_50^-5/2 Msun. The gas fraction is found to increase with radius; within a spherical radius of 16', the fraction is (M_gas/M_tot) = (0.26+-0.14) h_50^-3/2. The iron abundance in the gas is found to be (0.40+-0.04) of solar. There is no significant evidence for any variation in the abundance with position in the cluster. The global X-ray spectra, central X-ray spectra, and ROSAT surface brightness all require a cooling flow at the cluster center. The global X-ray spectrum implies that the total cooling rate is 363^+79_-96 h_50^-2 Msun/yr. The global X-ray spectra are consistent with the Galactic value for the soft X-ray absorption toward the cluster. The PSPC spectra of the central regions of the cluster are inconsistent with the large value of foreground excess absorption found by White et al. (1991) based on the Einstein SSS spectrum. The upper limit on excess foreground absorption is 7.3 x 10^19 cm^-2. However, the spectra do not rule a significant amount of intrinsic absorbing gas located within the cooling flow region.Comment: ApJ, in press, 16 pages including 13 figures, formatted with emulateapj Latex styl

The Lx-T Relation and Temperature Function for Nearby Clusters Revisited

The X-ray luminosity-temperature relation for nearby T=3.5-10 keV clusters is rederived using new ASCA temperatures and ROSAT luminosities. Both quantities are derived by directly excluding the cooling flow regions. This correction results in a greatly reduced scatter in the Lx-T relation; cooling flow clusters are similar to others outside the small cooling flow regions. For a fit of the form L_bol ~ T^alpha, we obtain alpha=2.64+-0.27 (90%) and a residual rms scatter in log L_bol of 0.10. The derived relation can be directly compared to theoretical predictions that do not include radiative cooling. It also provides an accurate reference point for future evolution searches and comparison to cooler clusters. The new temperatures together with a newly selected cluster sample are used to update the temperature function at z~0.05. The resulting function is generally higher and flatter than the previous estimates by Edge et al (1990) and Henry & Arnaud (1991, corrected). For a qualitative estimate of constraints that the new data place on the density fluctuation spectrum, we apply the Press-Schechter formalism for Omega=1 and 0.3. For Omega=1, assuming cluster isothermality, the temperature function implies sigma_8=0.55+-0.03, while taking into account the observed cluster temperature profiles, sigma_8=0.51+-0.03, consistent with the previously derived range. The dependence of sigma_8 on Omega differs from earlier findings, because of our treatment of n as a free parameter. We find n=-(2.0-2.3)+-0.3, somewhat steeper than derived from the earlier temperature function data, in agreement with the local slope of the galaxy fluctuation spectrum from the APM survey, and significantly steeper than the standard CDM prediction.Comment: Accepted for ApJ. Significand additions: Omega=0.3 model for the TF; a discussion of the TF derivation method; a comparison with recent Allen & Fabian L-T results. Latex, 8 pages, uses emulateapj.st

Hitomi (ASTRO-H) X-ray Astronomy Satellite

The Hitomi (ASTRO-H) mission is the sixth Japanese x-ray astronomy satellite developed by a large international collaboration, including Japan, USA, Canada, and Europe. The mission aimed to provide the highest energy resolution ever achieved at E  >  2  keV, using a microcalorimeter instrument, and to cover a wide energy range spanning four decades in energy from soft x-rays to gamma rays. After a successful launch on February 17, 2016, the spacecraft lost its function on March 26, 2016, but the commissioning phase for about a month provided valuable information on the onboard instruments and the spacecraft system, including astrophysical results obtained from first light observations. The paper describes the Hitomi (ASTRO-H) mission, its capabilities, the initial operation, and the instruments/spacecraft performances confirmed during the commissioning operations for about a month

Hitomi X-Ray Studies of Giant Radio Pulses from the Crab Pulsar

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2300 keV band and the Kashima NICT radio telescope in the 1.41.7 GHz band with a net exposure of about 2 ks on 2016 March 25, just before the loss of the Hitomi mission. The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1000 and 100 GRPs were simultaneously observed at the main pulse and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main pulse or inter-pulse phase. All variations are within the 2 fluctuations of the X-ray fluxes at the pulse peaks, and the 3 upper limits of variations of main pulse or inter-pulse GRPs are 22% or 80% of the peak flux in a 0.20 phase width, respectively, in the 2300 keV band. The values for main pulse or inter-pulse GRPs become 25% or 110%, respectively, when the phase width is restricted to the 0.03 phase. Among the upper limits from the Hitomi satellite, those in the 4.510 keV and 70300 keV bands are obtained for the first time, and those in other bands are consistent with previous reports. Numerically, the upper limits of the main pulse and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) 10(exp 11) erg cm(exp 2), respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere. Although the number of photon-emitting particles should temporarily increase to account for the brightening of the radio emission, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a >0.02% brightening of the pulse-peak flux under such conditions