Suppression of Magnetic Order by Pressure in BaFe2As2

We performed the dc resistivity and the ZF 75As-NMR measurement of BaFe2As2 under high pressure. The T-P phase diagram of BaFe2As2 determined from resistivity anomalies and the ZF 75As-NMR clearly revealed that the SDW anomaly is quite robust against P.Comment: 2 pages, 2 figure

The Galaxy Cluster Luminosity-Temperature Relationship and Iron Abundances - A Measure of Formation History ?

Both the X-ray luminosity-temperature (L-T) relationship and the iron abundance distribution of galaxy clusters show intrinsic dispersion. Using a large set of galaxy clusters with measured iron abundances we find a correlation between abundance and the relative deviation of a cluster from the mean L-T relationship. We argue that these observations can be explained by taking into account the range of cluster formation epochs expected within a hierarchical universe. The known relationship of cooling flow mass deposition rate to luminosity and temperature is also consistent with this explanation. From the observed cluster population we estimate that the oldest clusters formed at z>~2. We propose that the iron abundance of a galaxy cluster can provide a parameterization of its age and dynamical history.Comment: 13 pages Latex, 2 figures, postscript. Accepted for publication in ApJ Letter

X-ray Measurements of the Gravitational Potential Profile in the Central Region of the Abell 1060 Cluster of Galaxies

X-ray spectral and imaging data from ASCA and ROSAT were used to measure the total mass profile in the central region of Abell 1060, a nearby and relatively poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the spatial response of the X-ray telescope, show an isothermal distribution of the intra-cluster medium (ICM) within at least $\sim$ 12' (or $160h_{70}^{-1}$ kpc; $H_0 = 70 h_{70}$ km s$^{-1}$Mpc$^{-1}$) in radius of the cluster center. The azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a central excess above an isothermal $\beta$ model. The ring-sorted ASCA GIS spectra and the radial surface brightness distribution from the ROSAT PSPC were simultaneously utilized to constrain the gravitational potential profile. Some analytic models of the total mass density profile were examined. The ICM density profile was also specified by analytic forms. The ICM temperature distribution was constrained to satisfy the hydrostatic equilibrium, and to be consistent with the data. Then, the total mass distribution was found to be described better by the universal dark halo profile proposed by Navarro, Frenk, and White (1996;1997) than by a King-type model with a flat density core. A profile with a central cusp together with a logarithmic radial slope of $\sim 1.5$ was also consistent with the data. Discussions are made concerning the estimated dark matter distribution around the cluster center.Comment: 32 pages. Accepted: ApJ 2000, 35 pages, Title was correcte

Chandra observtaion of A2256 - a cluster at the early stage of merging

We present here \chandra observations of the rich cluster of galaxies A2256. In addition to the known cool subcluster, a new structure was resolved 2$'$ east of the peak of the main cluster. Its position is roughtly at the center of a low-brightness radio halo. Spectral analysis shows that the "shoulder" has high iron abundance ($\sim$ 1). We suggest that this structure is either another merging component or an internal structure of the main cluster. The X-ray redshifts of several regions were measured. The results agree with the optical ones and suggest that the main cluster, the subcluster and the "shoulder" are physically associated and interacting. The subcluster has low temperature ($\sim$ 4.5 keV) and high iron abundance ($\sim$ 0.6) in the central 150 kpc. The \chandra image shows a relatively sharp brightness gradient at the south of the subcluster peak running south-south-east (SSE). A temperature jump was found across the edge, with higher temperature ahead of the edge in the low density region. This phenomenon is qualitatively similar to the "cold fronts" found in A2142 and A3667. If the "shoulder" is ignored, the temperature map resembles those simulations at the early stage of merging while the subcluster approached the main cluster from somewhere west. This fact and the observed edge, in combination with the clear iron abundance contrast between the center of the subcluster ($\sim$ 0.6) and the main cluster ($\sim$ 0.2), all imply that the ongoing merger is still at the early stage. At least three member galaxies, including a radio head-tail galaxy, were found to have corresponding X-ray emission.Comment: The revised version. The shown abstract is shrunk. Accepted by ApJ. If it is possible, please try to look at the high-resolution version is http://cfa160.harvard.edu/~sunm/a2256.tar.g

ASCA PV observations of the Seyfert 2 galaxy NGC 4388: the obscured nucleus and its X-ray emission

We present results on the Seyfert 2 galaxy NGC4388 in the Virgo cluster observed with ASCA during its PV phase. The 0.5-10 keV X-ray spectrum consists of multiple components; (1) a continuum component heavily absorbed by a column density NH = 4E23 cm-2 above 3 keV; (2) a strong 6.4 keV line (EW = 500 eV); (3) a weak flat continuum between 1 and 3 keV; and (4) excess soft X-ray emission below 1 keV. The detection of strong absorption for the hard X-ray component is firm evidence for an obscured active nucleus in this Seyfert 2 galaxy. The absorption corrected X-ray luminosity is about 2E42 erg/s. This is the first time that the fluorescent iron-K line has been detected in this object. The flat spectrum in the intermediate energy range may be a scattered continuum from the central source. The soft X-ray emission below 1 keV can be thermal emission from a temperature kT = 0.5 keV, consistent with the spatially extended emission observed by ROSAT HRI. However, the low abundance (0.05 Zs) and high mass flow rate required for the thermal model and an iron-K line stronger than expected from the obscuring torus model are puzzling. An alternative consistent solution can be obtained if the central source was a hundred times more luminous over than a thousand years ago. All the X-ray emission below 3 keV is then scattered radiation.Comment: 9 pages, 5 Postscript figures, to be published in MNRA

Optical I-band Linear Polarimetry of the Magnetar 4U 0142+61 with Subaru

The magnetar 4U~0142+61 has been well studied at optical and infrared wavelengths and is known to have a complicated broad-band spectrum over the wavelength range. Here we report the result from our linear imaging polarimetry of the magnetar at optical $I$-band. From the polarimetric observation carried out with the 8.2-m Subaru telescope, we determine the degree of linear polarization $P=1.0\pm$3.4\%, or $P\leq$5.6\% (90\% confidence level). Considering models suggested for optical emission from magnetars, we discuss the implications of our result. The upper limit measurement indicates that different from radio pulsars, magnetars probably would not have strongly polarized optical emission if the emission arises from their magnetosphere as suggested.Comment: 5 pages, 1 figure, accepted for publication on Ap

Metallicity Gradients in the Intracluster Gas of Abell 496

Analysis of spatially resolved ASCA spectra of the intracluster gas in Abell 496 confirms there are mild metal abundance enhancements near the center, as previously found by White et al. (1994) in a joint analysis of Ginga LAC and Einstein SSS spectra. Simultaneous analysis of spectra from all ASCA instruments (SIS + GIS) shows that the iron abundance is 0.36 +- 0.03 solar 3-12' from the center of the cluster and rises ~50% to 0.53 +- 0.04 solar within the central 2'. The F-test shows that this abundance gradient is significant at the >99.99% level. Nickel and sulfur abundances are also centrally enhanced. We use a variety of elemental abundance ratios to assess the relative contribution of SN Ia and SN II to the metal enrichment of the intracluster gas. We find spatial gradients in several abundance ratios, indicating that the fraction of iron from SN Ia increases toward the cluster center, with SN Ia accounting for ~50% of the iron mass 3-12' from the center and ~70% within 2'. The increased proportion of SN Ia ejecta at the center is such that the central iron abundance enhancement can be attributed wholly to SN Ia; we find no significant gradient in SN II ejecta. These spatial gradients in the proportion of SN Ia/II ejecta imply that the dominant metal enrichment mechanism near the center is different than in the outer parts of the cluster. We show that the central abundance enhancement is unlikely to be due to ram pressure stripping of gas from cluster galaxies, or to secularly accumulated stellar mass loss within the central cD. We suggest that the additional SN Ia ejecta near the center is the vestige of a secondary SN Ia-driven wind from the cD (following a more energetic protogalactic SN II-driven wind phase), which was partially smothered in the cD due to its location at the cluster center.Comment: 25 pages AASTeX; 6 encapsulated PostScript figures; accepted for publication in ApJ. Replaced with revised versio