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

Relative Sizes of X-ray and Optical Images of Elliptical Galaxies; Correlation with X-ray Luminosity

Optical parameters of elliptical galaxies are tightly correlated, but their x-ray parameters vary widely. The x-ray luminosity L_x ranges over more than an order of magnitude for ellipticals having similar optical luminosity L_B. The source of this scatter has been elusive. We show here that the dispersion in L_x for fixed optical luminosity L_B correlates strongly with the dimensionless ratio of the sizes of the x-ray and optical images, r_ex/r_e. Specifically, we find that (L_x/L_B) is proportional to (r_{ex}/r_e)^{0.60 \pm 0.30}, a version of the correlation that is independent of distance. This correlation may be a natural result of mergings and tidal truncations that are expected during the formation and early evolution of ellipticals in groups of galaxies. The radial structure of x-ray images also varies: some are compact (e.g. NGC 4649, 7626, 5044), others diffuse (e.g. NGC 4636, 1399).Comment: 5 pages, 3 figures. Accepted for publication in Astrophysical Journal Letter

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

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

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

Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT

We report on a detailed investigation of the high-energy gamma-ray emission from NGC\,1275, a well-known radio galaxy hosted by a giant elliptical located at the center of the nearby Perseus cluster. With the increased photon statistics, the center of the gamma-ray emitting region is now measured to be separated by only 0.46' from the nucleus of NGC1275, well within the 95% confidence error circle with radius ~1.5'. Early Fermi-LAT observations revealed a significant decade-timescale brightening of NGC1275 at GeV photon energies, with a flux about seven times higher than the one implied by the upper limit from previous EGRET observations. With the accumulation of one-year of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral variations of this source on month timescales, as reported in this paper. The average >100 MeV gamma-ray spectrum of NGC1275 shows a possible deviation from a simple power-law shape, indicating a spectral cut-off around an observed photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X 10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest gamma-ray flaring event was observed in April--May 2009 and was accompanied by significant spectral variability above E > 1-2 GeV. The gamma-ray activity of NGC1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the gamma-ray source was detected at the very end of the observation, with the observed energy of E = 67.4GeV and an angular separation of about 2.4' from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed gamma-ray spectral evolution of NGC1275 in the context of gamma-ray blazar sources in general.Comment: 20 pages, 6 figures, accepted for publication in the Ap

Detecting the Gravitational Redshift of Cluster Gas

We examine the gravitational redshift of radiation emitted from within the potential of a cluster. Spectral lines from the intracluster medium (ICM) are redshifted in proportion to the emission-weighted mean potential along the line of sight, amounting to approximately 50 km/s at a radius of 100 kpc/h, for a cluster dispersion of 1200 km/s. We show that the relative redshifts of different ionization states of metals in the ICM provide a unique probe of the three-dimensional matter distribution. An examination of the reported peculiar velocities of cD galaxies in well studied Abell clusters reveals they are typically redshifted by an average of $\sim +200$ km/s. This can be achieved by gravity with the addition of a steep central potential associated with the cD galaxy. Note that in general gravitational redshifts cause a small overestimate of the recessional velocities of clusters by an average of $\sim$ 20 km/s.Comment: 6 pages, 3 figures, accepted to the Astrophysical Journal Letter

Orbital ordering transition in Ca2_2RuO4_4 observed with resonant x-ray diffraction

Resonant x-ray diffraction performed at the $\rm L_{II}$ and $\rm L_{III}$ absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca$_2$RuO$_4$ single crystals. A large resonant enhancement due to electric dipole $2p\to 4d$ transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at $\rm T_{N}=110$ K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru $t_{2g}$ electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca$_2$RuO$_4$.Comment: to appear in PR