1,740 research outputs found
Magnetic anisotropy of the spin ice compound Dy2Ti2O7
We report magnetization and ac susceptibility of single crystals of the spin
ice compound Dy2Ti2O7. Saturated moments at 1.8 K along the charasteristic axes
[100] and [110] agree with the expected values for an effective ferromagnetic
nearest-neighbor Ising pyrochlore with local anisotropy, where each
magnetic moment is constrained to obey the `ice-rule'. At high enough magnetic
fields along the [111] axis, the saturated moment exhibits a beaking of the
ice-rule; it agrees with the value expected for a three-in one-out spin
configuration. Assuming the realistic magnetic interaction between Dy ions
given by the dipolar spin ice model, we completely reproduce the results at 2 K
by Monte Carlo calculations. However, down to at least 60 mK, we have not found
any experimental evidence of the long-range magnetic ordering predicted by this
model to occur at around 180 mK. Instead, we confirm the spin freezing of the
system below 0.5 K.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
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 12' (or kpc;
km sMpc) in radius of the cluster center. The
azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a
central excess above an isothermal 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 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
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
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 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 20 km/s.Comment: 6 pages, 3 figures, accepted to the Astrophysical Journal Letter
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 -band. From the polarimetric observation carried
out with the 8.2-m Subaru telescope, we determine the degree of linear
polarization 3.4\%, or 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
Active Galaxies and Cluster Gas
Two lines of evidence indicate that active galaxies, principally radio
galaxies, have heated the diffuse hot gas in clusters. The first is the general
need for additional heating to explain the steepness of the X-ray
luminosity--temperature relation in clusters, the second is to solve the
cooling flow problem in cluster cores. The inner core of many clusters is
radiating energy as X-rays on a timescale much shorter than its likely age.
Although the temperature in this region drops by a factor of about 3 from that
of the surrounding gas, little evidence is found for gas much cooler than that.
Some form of heating appears to be taking place, probably by energy transported
outward from the central accreting black hole or radio source. How that energy
heats the gas depends on poorly understood transport properties (conductivity
and viscosity) of the intracluster medium. Viscous heating is discussed as a
possibility. Such heating processes have consequences for the truncation of the
luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So
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