4,119 research outputs found
The peculiar extinction law of SN2014J measured with The Hubble Space Telescope
The wavelength-dependence of the extinction of Type Ia SN2014J in the nearby
galaxy M82 has been measured using UV to near-IR photometry obtained with the
Hubble Space Telescope, the Nordic Optical Telescope, and the Mount Abu
Infrared Telescope. This is the first time that the reddening of a SN Ia is
characterized over the full wavelength range of - microns. A
total-to-selective extinction, , is ruled out with high
significance. The best fit at maximum using a Galactic type extinction law
yields . The observed reddening of SN2014J is also compatible
with a power-law extinction, as expected from multiple scattering of light, with
. After correction for differences in reddening, SN2014J appears
to be very similar to SN2011fe over the 14 broad-band filter light curves used
in our study.Comment: Accepted for publication in ApJ
Evidence for a quantum phase transition in the electron-doped cuprate Pr2-xCexCuO4+d from Hall and resistivity measurements
The doping and temperature dependence of the Hall coefficient, RH, and
ab-plane resistivity in the normal state down to 350mK is reported for oriented
films of the electron-doped high-Tc superconductor Pr2-xCexCuO4+d. The doping
dependence of b (r=r0+AT^b) and R_sub_H (at 350 mK) suggest a quantum phase
transition at a critical doping near x=0.165.Comment: 11 pages 4 figures Phys. Rev. Lett. 92, 167001 (2004
An Unusual X-ray Burst from the Globular Cluster M28
We report the discovery of an unusual X-ray burst from the direction of the
Globular Cluster M28 using data acquired with the ASCA Observatory. The burst
was recorded by all four ASCA telescopes and displays a fast (~ 70 ms) rise
followed by an exponential decay (t = 7.5 s) and a steady afterglow which lasts
between 800 - 3250 s. The image of the burst is consistent with an ASCA point
source and is centered on quiescent X-ray emission from the core of M28. The
burst temporal profile is similar to Type-I bursts emitted by accreting neutron
stars of low mass X-ray binaries (LMXB). We argue that the burst arises from an
LMXB that is located in the core of M28. The burst is unique in two ways: it is
intrinsically sub-luminous, ~ 0.02 L_Edd and more importantly, originates from
a source whose quiescent luminosity is fainter than that of the known cluster
bursters by three orders of magnitude. We suggest that this burst is from a
highly magnetized neutron star accreting at a low rate. These accreting systems
may account for the mysterious low luminosity X-ray sources in globular
clusters.Comment: 13 pages with 3 eps figures, LATEX, aastex, psfig. To appear in the
Astrophysical Journal Letter
Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
(ABBREVIATED) We consider the structure of neutron star magnetospheres
threaded by large-scale electrical currents, and the effect of resonant Compton
scattering by the charge carriers (both electrons and ions) on the emergent
X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs,
these currents are maintained by magnetic stresses acting deep inside the star.
We construct self-similar, force-free equilibria of the current-carrying
magnetosphere with a power-law dependence of magnetic field on radius, B ~
r^(-2-p), and show that a large-scale twist softens the radial dependence to p
< 1. The spindown torque acting on the star is thereby increased in comparison
with a vacuum dipole. We comment on the strength of the surface magnetic field
in the SGR and AXP sources, and the implications of this model for the narrow
measured distribution of spin periods. A magnetosphere with a strong twist,
B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant
cyclotron scattering, independent of frequency (radius), surface magnetic field
strength, or charge/mass ratio of the scattering charge. When electrons and
ions supply the current, the stellar surface is also heated by the impacting
charges at a rate comparable to the observed X-ray output of the SGR and AXP
sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux
at the ion and electron cyclotron resonances will significantly modify the
emerging pulse profile and, through the Doppler effect, generate a non-thermal
tail to the X-ray spectrum. The sudden change in the pulse profile of SGR
1900+14 after the 27 August 1998 giant flare is related to an enhanced optical
depth to electron cyclotron scattering, resulting from a sudden twist imparted
to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.
Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys
We report the results of ab initio molecular dynamics simulations of liquid
Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600
K, just above the melting point of GaAs. The liquid is predicted to be metallic
at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity
maximum near x = 0.5, consistent with the Faber-Ziman expression. The
electronic density of states is finite at the Fermi energy for all
concentrations; there is, however, a significant pseudogap especially in the
As-rich samples. The Ga-rich density of states more closely resembles that of a
free-electron metal. The partial structure factors show only a weak indication
of chemical short-range order. There is also some residue of the covalent
bonding found in the solid, which shows up in the bond-angle distribution
functions of the liquid state. Finally, the atomic diffusion coefficients at
1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in
Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in
Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.
Self-organizing maps: a tool to ascertain taxonomic relatedness based on features derived from 16S rDNA sequence
Exploitation of microbial wealth, of which almost 95% or more is still unexplored, is a growing need. The taxonomic placements of a new isolate based on phenotypic characteristics are now being supported by information preserved in the 16S rRNA gene. However, the analysis of 16S rDNA sequences retrieved from metagenome, by the available bioinformatics tools, is subject to limitations. In this study, the occurrences of nucleotide features in 16S rDNA sequences have been used to ascertain the taxonomic placement of organisms. The tetra- and penta-nucleotide features were extracted from the training data set of the 16S rDNA sequence, and was subjected to an artificial neural network (ANN) based tool known as self-organizing map (SOM), which helped in visualization of unsupervised classification. For selection of significant features, principal component analysis (PCA) or curvilinear component analysis (CCA) was applied. The SOM along with these techniques could discriminate the sample sequences with more than 90% accuracy, highlighting the relevance of features. To ascertain the confidence level in the developed classification approach, the test data set was specifically evaluated for Thiobacillus, with Acidiphilium, Paracocus and Starkeya, which are taxonomically reassigned. The evaluation proved the excellent generalization capability of the developed tool. The topology of genera in SOM supported the conventional chemo-biochemical classification reported in the Bergey manual
The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73
We report the discovery of pulsed X-ray emission from the compact source 1E
1841-045, using data obtained with the Advanced Satellite for Cosmology and
Astrophysics. The X-ray source is located in the center of the small-diameter
supernova remnant (SNR) Kes 73 and is very likely to be the compact
stellar-remnant of the supernova which formed Kes 73. The X-rays are pulsed
with a period of ~ 11.8 s, and a sinusoidal modulation of roughly 30 %. We
interpret this modulation to be the rotation period of an embedded neutron
star, and as such would be the longest spin period for an isolated neutron star
to-date. This is especially remarkable since the surrounding SNR is very young,
at ~ 2000 yr old. We suggest that the observed characteristics of this object
are best understood within the framework of a neutron star with an enormous
dipolar magnetic field, B ~ 8x10^14 G
- …