13,519 research outputs found
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&
X-ray properties of the microquasar GRS 1915+105 during a variability class transition
We present a detailed X-ray study of the microquasar GRS 1915+105 during a
variability class transition observed in 2000 June with the PPCs of the Indian
X-ray Astronomy Experiment. We supplement this observation with data from the
RXTE archives. The source made a transition from a steady low-hard state to a
regular oscillatory behaviour in the light curve known as bursts or class `rho'
(Belloni et al. 2000) between 2000 May 11 and 17 and reverted back to the
low-hard state on 2000 June 27. A gradual change in the burst recurrence time
from about 75 s to about 40 s was observed which then increased to about 120 s
during the ~ 40 days of class `rho'. The regular bursts disappeared from the
X-ray light curves and the class transition was observed to occur within 1.5
hours on 2000 June 27 with the PPCs. A correlation is found between the
observed QPO frequency at 5-8 Hz in the quiescent phase and the average X-ray
intensity of the source during the class `rho'. We notice a strong similarity
between the properties of the source during the class `rho' and those during
the oscillatory phase of the observations of class `alpha'. From the timing and
spectral analysis, it is found that the observed properties of the source over
tens of days during the class `rho' are identical to those over a time scale of
a few hundreds of seconds in the class `alpha'. Examining the light curves from
the beginning of the RXTE/PCA and RXTE/ASM observations, it is found that the
change of state from radio-quiet low-hard state to high state occurs through
the X-ray classes `rho' and `alpha' which appear together during the state
transition. It is further inferred that the source switches from low-hard state
to the class `rho' through the intermediate class `alpha'.Comment: 10 pages with 9 figures, LaTex. To be appeared in MNRA
Thermal Decomposition Mechanism of Silver Nitrobenzoates
The infra-red spectra of silver salts of 3-mono-, 3,5-di-and 2,4,6-tri-nitro-benzoic acids have been recorded at elevated temperatures in a fabricated high temperature cell. The thermogravimetry i.e. TG analysis has also been carried out. The results of this study suggest that decomposition of these salts is a two stage process i.e. decarboxylation followed by explosion
Cosmic ray intensity distribution perpendicular to solar equatorial plane at 1 A.U. during 1978-83
The distribution of cosmic ray intensity perpendicular to solar equatorial plane, was investigated by using its yearly variation with respect to the heliolatitudinal position of the Earth, for the two intervals 1978 to 1980 and 1981 to 1983. The monthly mean values of two high latitude stations along with the solar and geomagnetic indices are used to derive the cosmic ray intensity distribution free from the changes due to variation in solar activity. The correction is found to be significant only during the interval 1976 to 1980. The results indicate a significant linear increase in cosmic ray intensity from north to south of solar equator, contrary to that observed during 1973 to 1975. No symmetrical gradients are found during the period of study, in agreement with earlier results
Isolated unstable Weibel modes in unmagnetized plasmas with tunable asymmetry
In this letter, an initially unmagnetized pair plasma with asymmetric
velocity distributions is investigated where any unstable Weibel mode must be
isolated, with discrete values for the growth rates and the unstable
wavenumbers. For both a non-relativistic distribution with thermal spread and a
high-relativistic two-stream distribution it is shown that isolated modes are
excited and that, as the asymmetry tends to zero, the growth rate remains
finite, as long as the distribution function is not precisely symmetric.Comment: Comments: references adde
Different types of X-ray bursts from GRS 1915+105 and their origin
We report the X-ray observations of the Galactic X-ray transient source GRS
1915+105 with the PPCs of the Indian X-ray Astronomy Experiment(IXAE) onboard
the Indian satellite IRS-P3 during 1997 June - August, which have revealed the
presence of four types of intense X-ray bursts. All the observed bursts have a
slow exponential rise, a sharp linear decay, and they can broadly be put in two
classes: irregular and quasi-regular bursts in one class, and regular bursts in
another class. The regular bursts are found to have two distinct time scales
and they persist over extended durations. There is a strong correlation between
the preceding quiescent time and the burst duration for the quasi-regular and
irregular bursts. No such correlation is found for the regular bursts. The
ratio of average flux during the burst time to the average flux during the
quiescent phase is high and variable for the quasi- regular and irregular
bursts while it is low and constant for the regular bursts. We suggest that the
peculiar bursts that we have seen are charact- eristic of the change of state
of the source. The source can switch back and forth between the low-hard state
and the high-soft state near critical accretion rates in a very short time
scale. A test of the model is presented using the publicly available 13-60 keV
RXTE/PCA data for irregular and regular bursts concurrent with our
observations.Comment: 13 pages, 12 figures, Accepted in APJ, emulateapj style use
Evolving temporal association rules with genetic algorithms
A novel framework for mining temporal association rules by discovering itemsets with a genetic algorithm is introduced. Metaheuristics have been applied to association rule mining, we show the efficacy of extending this to another variant - temporal association rule mining. Our framework is an enhancement to existing temporal association rule mining methods as it employs a genetic algorithm to simultaneously search the rule space and temporal space. A methodology for validating the ability of the proposed framework isolates target temporal itemsets in synthetic datasets. The Iterative Rule Learning method successfully discovers these targets in datasets with varying levels of difficulty
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