1,127 research outputs found
X-ray Evidence for Spectroscopic Diversity of Type Ia Supernovae: XMM observation of the elemental abundance pattern in M87
We present the results of a detailed element abundance study of hot gas in
M87, observed by XMM-Newton. We choose two radial bins, 1'-3' and 8'-16'
(8'-14' for EMOS; hereafter the central and the outer zones), where the
temperature is almost constant, to carry out the detailed abundance
measurements of O, Ne, Mg, Si, S, Ar, Ca, Fe and Ni using EPIC-PN (EPN) and
-MOS (EMOS) data. First, we find that the element abundance pattern in the
central compared to the outer zone in M87 is characterized by SN Ia enrichment
of a high (roughly solar) ratio of Si-group elements (Si, S, Ar, Ca) to Fe,
implying that Si burning in SN Ia is highly incomplete. In nucleosynthesis
modeling this is associated with either a lower density of the
deflagration-detonation transition and/or lower C/O and/or lower central
ignition density and observationally detected as optically subluminous SNe Ia
in early-type galaxies. Second, we find that SN Ia enrichment has a
systematically lower ratio of the Si-group elements to Fe by 0.2 dex in the
outer zone associated with the ICM of the Virgo cluster. We find that such a
ratio and even lower values by another 0.1 dex are a characteristic of the ICM
in many clusters using observed Si:S:Fe ratios as found with ASCA. Third, the
Ni/Fe ratio in the central zone of M87 is 1.5+/-0.3 solar (meteoritic), while
values around 3 times solar are reported for other clusters. In modeling of SN
Ia, this implies a reduced influence of fast deflagration SN Ia models in the
chemical enrichment of M87's ISM. Thus, to describe the SN Ia metal enrichment
in clusters, both deflagration as well as delayed detonation scenarios are
required, supporting a similar conclusion, derived from optical studies on SNe
Ia. Abridged.Comment: 11 pages, A&A, in pres
Recent X-ray Observations and the Evolution of Hot Gas in Elliptical Galaxies: Evidence for Circumgalactic Gas
X-ray emitting gaseous halos, such as that in elliptical galaxies like NGC
4472, cannot have been produced solely from gas expelled from galactic stars.
In traditional models for the evolution of hot interstellar gas (cooling flows)
in ellipticals, the galaxies are assumed to have been cleared of gas by
SNII-driven winds at some early time then gas is subsequently replenished by
mass loss from an evolving population of old stars. To test this, we accurately
determine the stellar and dark halo mass of NGC 4472 using hydrostatic
equilibrium, then solve the standard time-dependent cooling flow equations to
recover the observed hot gas temperature and density distributions when evolved
to the present time. This procedure fails: the computed gas density gradient is
too steep, the total gas mass is too low, and the gas temperatures are much too
low. All variants on this basic procedure also fail: increasing the SNIa rate,
using the mass dropout assumption, arbitrarily adjusting uncertain
coefficients, etc. However, agreement is achieved if the galaxy is supplied
with additional, spatially-extended hot gas early in its evolution. This old
``circumgalactic'' gas can be retained to the present time and may be related
to cosmological ``secondary infall''.Comment: 15 pages in two-column AASTEX LaTeX including 1 table and 8 figures;
abstract corrected in replacement; accepted by Astrophysical Journa
X-ray Spectroscopy of the Core of the Perseus Cluster with Suzaku: Elemental Abundances in the Intracluster Medium
The results from Suzaku observations of the central region of the Perseus
cluster are presented. Deep exposures with the X-ray Imaging Spectrometer
provide high quality X-ray spectra from the intracluster medium. X-ray lines
from helium-like Cr and Mn have been detected significantly for the first time
in clusters. In addition, elemental abundances of Ne, Mg, Si, S, Ar, Ca, Fe,
and Ni are accurately measured within 10' (or 220 kpc) from the cluster center.
The relative abundance ratios are found to be within a range of 0.8-1.5 times
the solar value. These abundance ratios are compared with previous
measurements, those in extremely metal-poor stars in the Galaxy, and
theoretical models.Comment: 10 pages, 3 figures, accepted for ApJ
Robustness of Learning That Is Based on Covariance-Driven Synaptic Plasticity
It is widely believed that learning is due, at least in part, to long-lasting modifications of the strengths of synapses in the brain. Theoretical studies have shown that a family of synaptic plasticity rules, in which synaptic changes are driven by covariance, is particularly useful for many forms of learning, including associative memory, gradient estimation, and operant conditioning. Covariance-based plasticity is inherently sensitive. Even a slight mistuning of the parameters of a covariance-based plasticity rule is likely to result in substantial changes in synaptic efficacies. Therefore, the biological relevance of covariance-based plasticity models is questionable. Here, we study the effects of mistuning parameters of the plasticity rule in a decision making model in which synaptic plasticity is driven by the covariance of reward and neural activity. An exact covariance plasticity rule yields Herrnstein's matching law. We show that although the effect of slight mistuning of the plasticity rule on the synaptic efficacies is large, the behavioral effect is small. Thus, matching behavior is robust to mistuning of the parameters of the covariance-based plasticity rule. Furthermore, the mistuned covariance rule results in undermatching, which is consistent with experimentally observed behavior. These results substantiate the hypothesis that approximate covariance-based synaptic plasticity underlies operant conditioning. However, we show that the mistuning of the mean subtraction makes behavior sensitive to the mistuning of the properties of the decision making network. Thus, there is a tradeoff between the robustness of matching behavior to changes in the plasticity rule and its robustness to changes in the properties of the decision making network
KeV Warm Dark Matter and Composite Neutrinos
Elementary keV sterile Dirac neutrinos can be a natural ingredient of the
composite neutrino scenario. For a certain class of composite neutrino
theories, these sterile neutrinos naturally have the appropriate mixing angles
to be resonantly produced warm dark matter (WDM). Alternatively, we show these
sterile neutrinos can be WDM produced by an entropy-diluted thermal freeze-out,
with the necessary entropy production arising not from an out-of-equilibrium
decay, but rather from the confinement of the composite neutrino sector,
provided there is sufficient supercooling.Comment: 12 pages, 2 figures, published versio
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