8,949 research outputs found
Calibration of the galaxy cluster M_500-Y_X relation with XMM-Newton
The quantity Y_ X, the product of the X-ray temperature T_ X and gas mass M_
g, has recently been proposed as a robust low-scatter mass indicator for galaxy
clusters. Using precise measurements from XMM-Newton data of a sample of 10
relaxed nearby clusters, spanning a Y_ X range of 10^13 -10^15 M_sun keV, we
investigate the M_500-Y_ X relation. The M_500 - Y_ X data exhibit a power law
relation with slope alpha=0.548 \pm 0.027, close to the self-similar value
(3/5) and independent of the mass range considered. However, the normalisation
is \sim 20% below the prediction from numerical simulations including cooling
and galaxy feedback. We discuss two effects that could contribute to the
normalisation offset: an underestimate of the true mass due to the HE
assumption used in X-ray mass estimates, and an underestimate of the hot gas
mass fraction in the simulations. A comparison of the functional form and
scatter of the relations between various observables and the mass suggest that
Y_ X may indeed be a better mass proxy than T_ X or M_g,500.Comment: 4 pages, 2 figures, accepted for publication in A&
Final state interactions in two-particle interferometry
We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".Comment: 12 pages RevTeX, 2 ps-figures included, submitted to Phys. Rev.
The hot gas content of fossil galaxy clusters
We investigate the properties of the hot gas in four fossil galaxy systems
detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey.
XMM-Newton observations reveal overall temperatures of kT ~ 5-6 keV and yield
hydrostatic masses M500,HE > 3.5 x 10e14 Msun, confirming their nature as bona
fide massive clusters. We measure the thermodynamic properties of the hot gas
in X-rays (out to beyond R500 in three cases) and derive their individual
pressure profiles out to R ~ 2.5 R500 with the SZ data. We combine the X-ray
and SZ data to measure hydrostatic mass profiles and to examine the hot gas
content and its radial distribution. The average Navarro-Frenk-White (NFW)
concentration parameter, c500 = 3.2 +/- 0.4, is the same as that of relaxed
`normal' clusters. The gas mass fraction profiles exhibit striking variation in
the inner regions, but converge to approximately the cosmic baryon fraction
(corrected for depletion) at R500. Beyond R500 the gas mass fraction profiles
again diverge, which we interpret as being due to a difference in gas clumping
and/or a breakdown of hydrostatic equilibrium in the external regions. Overall
our observations point to considerable radial variation in the hot gas content
and in the gas clumping and/or hydrostatic equilibrium properties in these
fossil clusters, at odds with the interpretation of their being old, evolved
and undisturbed. At least some fossil objects appear to be dynamically young.Comment: 4 pages, 2 figures. Accepted for publication in A&
Early evolution of electron cyclotron driven current during suppression of tearing modes in a circular tokamak
When electron cyclotron (EC) driven current is first applied to the inside of
a magnetic island, the current spreads throughout the island and after a short
period achieves a steady level. Using a two equation fluid model for the EC
current that allows us to examine this early evolution in detail, we analyze
high-resolution simulations of a 2/1 classical tearing mode in a low-beta large
aspect-ratio circular tokamak. These simulations use a nonlinear 3D reduced-MHD
fluid model and the JOREK code. During the initial period where the EC driven
current grows and spreads throughout the magnetic island, it is not a function
of the magnetic flux. However, once it has reached a steady-state, it should be
a flux function. We demonstrate numerically that if sufficiently resolved
toroidally, the steady-state EC driven current becomes approximately a flux
function. We discuss the physics of this early period of EC evolution and its
impact on the size of the magnetic island.Comment: 12 pages, 7 figure
A system of relational syllogistic incorporating full Boolean reasoning
We present a system of relational syllogistic, based on classical
propositional logic, having primitives of the following form:
Some A are R-related to some B;
Some A are R-related to all B;
All A are R-related to some B;
All A are R-related to all B.
Such primitives formalize sentences from natural language like `All students
read some textbooks'. Here A and B denote arbitrary sets (of objects), and R
denotes an arbitrary binary relation between objects. The language of the logic
contains only variables denoting sets, determining the class of set terms, and
variables denoting binary relations between objects, determining the class of
relational terms. Both classes of terms are closed under the standard Boolean
operations. The set of relational terms is also closed under taking the
converse of a relation. The results of the paper are the completeness theorem
with respect to the intended semantics and the computational complexity of the
satisfiability problem.Comment: Available at
http://link.springer.com/article/10.1007/s10849-012-9165-
The structural and scaling properties of nearby galaxy clusters - II. The M-T relation
Using a sample of ten nearby (z< 0.15), relaxed galaxy clusters in the
temperature range [2-9] keV, we have investigated the scaling relation between
the mass at various density contrasts (delta=2500,1000,500,200) and the cluster
temperature. The masses are derived from NFW-type model fits to mass profiles,
obtained under the hydrostatic assumption using precise measurements, with XMM,
at least down to delta=1000. The logarithmic slope of the M-T relation is well
constrained and is the same at all delta, reflecting the self-similarity of the
mass profiles. At delta=500, the slope of the relation for the sub-sample of
hot clusters (kT>3.5 keV) is consistent with the standard self-similar
expectation: alpha= 1.49\pm0.15. The relation steepens when the whole sample is
considered: alpha=1.71\pm0.09. The normalisation of the relation is discrepant
(by ~ 30%), at all density contrasts, with the prediction from purely
gravitation based models. Models that take into account radiative cooling and
galaxy feedback are generally in better agreement with our data. We argue that
remaining discrepancies, in particular at low delta, are more likely due to
problems with models of the ICM thermal structure rather than to an incorrect
estimate of the mass from X-ray data.Comment: 11 pages, 4 figures, A&A in press, updated to match the accepted
version (19/08/2005). Minor text clarifications, more detailed analysis of
the M-T relations in Sect 3.2, corrected typo in Table 1 (redshift) and on
cluster markers in Fig 1 legen
Structural and scaling properties of galaxy clusters: probing the physics of structure formation
We present XMM-Newton studies of the total mass, gas density, temperature and
entropy profiles in nearby hot and cool clusters, together with follow-up
observations of distant clusters from the SHARC Survey. The observed structural
and scaling properties are compared with the predictions of the self-similar
model of cluster formation. These data indicate that clusters do form a
self-similar population down to low mass and up to high redshift, and give
support to the standard picture of structure formation for the dark matter
component. However, deviations from the standard scaling laws confirm that the
specific physics of the gas component is still insufficiently understood.Comment: 9 pages, 8 figures; to be published in Memorie della Societa'
Astronomica Italiana, the Proceedings of the EPIC Consortium (held on Oct
14-16, 2003 in Palermo
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