30,279 research outputs found
Anti-correlated time lags in the Z source GX 5-1: Possible evidence for a truncated accretion disk
We investigate the nature of the inner accretion disk in the neutron star
source GX 5-1 by making a detailed study of time lags between X-rays of
different energies. Using the cross-correlation analysis, we found
anti-correlated hard and soft time lags of the order of a few tens to a few
hundred seconds and the corresponding intensity states were mostly the
horizontal branch (HB) and upper normal branch (NB). The model independent and
dependent spectral analysis showed that during these time lags the structure of
accretion disk significantly varied. Both eastern and western approaches were
used to unfold the X-ray continuum and systematic changes were observed in soft
and hard spectral components. These changes along with a systematic shift in
the frequency of quasi-periodic oscillations (QPOs) made it substantially
evident that the geometry of the accretion disk is truncated. Simultaneous
energy spectral and power density spectral study shows that the production of
the horizontal branch oscillations (HBOs) are closely related to the
Comptonizing region rather than the disk component in the accretion disk. We
found that as the HBO frequency decreases from the hard apex to upper HB, the
disk temperature increases along with an increase in the coronal temperature
which is in sharp contrast with the changes found in black hole binaries where
the decrease in QPO frequency is accompanied by a decrease in the disk
temperature and a simultaneous increase in the coronal temperature. We discuss
the results in the context of re-condensation of coronal material in the inner
region of the disk.Comment: 40 pages, 7 figures, accepted for publication in The Astrophysical
Journal Supplement (ApJS
A double peaked pulse profile observed in GX 1+4
The hard X-ray pulsar GX 1+4 was observed several times in the last few years
with a pair of balloon-borne Xenon filled Multi-cell Proportional Counters
(XMPC). In a balloon flight made on 22 March 1995, the source was detected in a
bright state, the average observed source count rate being per
detector. X-ray pulsations with a period of s were detected in
the source with a broad double peak pulse feature. When observed in December
1993 with the same instrument, the pulse profile of GX 1+4 showed a single
peak. This change in the pulse profile to a double pulse structure in about 15
months indicates either activation of the opposite pole of the neutron star if
the magnetic field is asymmetric or possibly a change in the beam pattern, from
a pencil beam to a fan beam. Assuming a fan beam configuration, the pulse
profile is used to find the inclinations of the magnetic axis and the viewing
axis with the spin axis. The derived angles support the GINGA observations of a
dip in the pulse profile which was resolved to have a local maximum in one of
the observations and was explained with resonance scattering of cyclotron line
energy photons by the accretion column (Makishima et al., \markcite{maki1988},
Dotani et al., \markcite{dotani1989}.). Compared to our previous observation of
the same source with the same telescope (Rao et al., \markcite{rao1994}) a
period change rate of is obtained which is the lowest rate
of change of period for this source since its discovery. Average pulse fraction
in the hard X-ray range is low (30%), consistent with its anti correlation with
luminosity as reported by us earlier (Rao et al., \markcite{rao1994}) and the
observed spectrum is very hard (power law photon index ).Comment: 10 pages, to appear in A&
Magnetic and electron transport properties of the rare-earth cobaltates, La0.7-xLnxCa0.3CoO3 (Ln = Pr, Nd, Gd and Dy) : A case of phase separation
Magnetic and electrical properties of four series of rare earth cobaltates of
the formula La0.7-xLnxCa0.3CoO3 with Ln = Pr, Nd, Gd and Dy have been
investigated. Compositions close to x = 0.0 contain large ferromagnetic
clusters or domains, and show Brillouin-like behaviour of the field-cooled DC
magnetization data with fairly high ferromagnetic Tc values, besides low
electrical resistivities with near-zero temperature coefficients. The
zero-field-cooled data generally show a non-monotonic behaviour with a peak at
a temperatures slightly lower than Tc. The near x = 0.0 compositions show a
prominent peak corresponding to the Tc in the AC-susceptibility data. The
ferromagnetic Tc varies linearly with x or the average radius of the A-site
cations, (rA). With increase in x or decrease in (rA), the magnetization value
at any given temperature decreases markedly and the AC-susceptibility
measurements show a prominent transition arising from small magnetic clusters
with some characteristics of a spin-glass. Electrical resistivity increases
with increase in x, showed a significant increase around a critical value of x
or (rA), at which composition the small clusters also begin to dominate. These
properties can be understood in terms of a phase separation scenario wherein
large magnetic clusters give way to smaller ones with increase in x, with both
types of clusters being present in certain compositions. The changes in
magnetic and electrical properties occur parallely since the large
ferromagnetic clusters are hole-rich and the small clusters are hole-poor.
Variable-range hopping seems to occur at low temperatures in these cobaltates.Comment: 23 pages including figure
Quenching of fluorescence of aromatic molecules by graphene due to electron transfer
Investigations on the fluorescence quenching of graphene have been carried
out with two organic donor molecules, pyrene butanaoic acid succinimidyl ester
(PyBS, I) and oligo(p-phenylenevinylene) methyl ester (OPV-ester, II).
Absorption and photoluminescence spectra of I and II recorded in mixture with
increasing the concentrations of graphene showed no change in the former, but
remarkable quenching of fluorescence. The property of graphene to quench
fluorescence of these aromatic molecules is shown to be associated with
photo-induced electron transfer, on the basis of fluorescence decay and
time-resolved transient absorption spectroscopic measurements.Comment: 18 pages, 6 figure
Estimation in a growth study with irregular measurement times
Between 1982 and 1988 a growth study was carried out at the Division of Pediatric Oncology of the University Hospital of Groningen. A special feature of the project was that sample sizes are small and that ages at entry may be very different. In addition the intended design was not fully complied with. This paper highlights some aspects of the statistical analysis which is based on (1) reference scores, (2) statistical procedures allowing for an irregular pattern of measurement times caused by missing data and shifted measurement times
A plausible mechanism for the evolution of helical forms in nanostructure growth
The observation of helices and coils in nano-tube/-fiber (NT/NF) syntheses is explained on the basis of the interactions between specific catalyst particles and the growing nanostructure. In addition to rationalizing nonlinear structure, the proposed model probes the interplay between thermodynamic quantities and predicts conditions for optimal growth. Experimental results on the effect of indium catalyst on affecting the coil pitch in NTs and NFs are presented
Transport in Luttinger Liquids
We give a brief introduction to Luttinger liquids and to the phenomena of
electronic transport or conductance in quantum wires. We explain why the
subject of transport in Luttinger liquids is relevant and fascinating and
review some important results on tunneling through barriers in a
one-dimensional quantum wire and the phenomena of persistent currents in
mesoscopic rings. We give a brief description of our own work on transport
through doubly-crossed Luttinger liquids and transport in the Schulz-Shastry
exactly solvable Luttinger-like model.Comment: Latex file, 15 pages, four eps figure
Developments in the production of soderberg paste for electrodes in ferro alloy furnaces
Using raw petroleum coke as raw-material, NML has devel-oped a dense carbon aggregate as a substitute for gas calcined anthracite. This dense carbon aggregate has been used for making soderberg paste, the properties of which have been studied and compared with a Norwegian and two indigenous pastes. The suitability of this material has been tested on a commercial scale in ferro alloy and pig iron furnaces.
Attempts also have been made to produce dense carbon aggregate by partial replacement of petroleum coke with low ash coal and the study of its properties have been described in this paper
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