2,669 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
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
Quasi-periodic oscillations discovered in the new X-ray pulsar XTE J1858+034
We report the discovery of low frequency quasi-periodic oscillations centered
at 0.11 Hz in the newly discovered 221 s X-ray pulsar XTE J1858+034. Among
about 30 known transient X-ray pulsars this is the sixth source in which QPOs
have been observed. If the QPOs are produced because of inhomogeneities in the
accretion disk at the magnetospheric boundary, the low frequency of the QPOs
indicate a large magnetosphere for this pulsar. Both the Keplerian frequency
model and the beat frequency model are applicable for production of QPOs in
this source. The QPOs and regular pulsations are found to be stronger at higher
energy which favours the beat frequency model. The magnetic field of the pulsar
is calculated as a function of its distance. The energy spectrum is found to be
very hard, consisting of two components, a cut-off power law and an iron
fluorescence line.Comment: 6 pages including 4 figures. To appear in Astronomy and Astrophysic
Quasi-periodic oscillations discovered in the new X-ray pulsar XTE J1858+034
We report the discovery of low frequency quasi-periodic oscillations centered at 0.11 Hz in the newly discovered 221 s X-ray pulsar XTE J1858+034. Among about 30 known transient X-ray pulsars this is the sixth source in which QPOs have been observed. If the QPOs are produced because of inhomogeneities in the accretion disk at the magnetospheric boundary, the low frequency of the QPOs indicate a large magnetosphere for this pulsar. Both the Keplerian frequency model and the beat frequency model are applicable for production of QPOs in this source. The QPOs and regular pulsations are found to be stronger at higher energy which favours the beat frequency model. The magnetic field of the pulsar is calculated as a function of its distance. The energy spectrum is found to be very hard, consisting of two components, a cut-off power law and an iron fluorescence line
Detection of a Series of X-ray Dips Associated with a Radio Flare in GRS 1915+105
We report the detection of a series of X-ray dips in the Galactic black hole
candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with
the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on
board the Indian satellite IRS-P3. The observations were made after the source
made a transition from a steady low-hard state to a chaotic state which
occuered within a few hours. Dips of about 20-160 seconds duration are observed
on most of the days. The X-ray emission outside the dips shows a QPO at ~ 4 Hz
which has characteristics similar to the ubiquitous 0.5 - 10 Hz QPO seen during
the low-hard state of the source. During the onset of dips this QPO is absent
and also the energy spectrum is soft and the variability is low compared to the
non-dip periods. These features gradually re-appear as the dip recovers. The
onset of the occurrence of a large number of such dips followed the start of a
huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as
the cause for mass ejection due to the evacuation of matter from an accretion
disk around the black hole. We propose that a super-position of a large number
of such dip events produces a huge radio jet in GRS 1915+105.Comment: 18 pages, 7 figures, Accepted for publication in Ap
Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts
We report ambipolar transport in bottom gold contact, pentacene field-effect transistors (FETs) fabricated by spin-coating and thermally converting its precursor on a benzocyclobutene/SiO2 gate dielectric with chemically modified source and drain electrodes. A wide range of aliphatic and aromatic self-assembled thiolate monolayers were used to derivatize the electrodes and all enhanced electron and hole currents, yet did not affect the observable thin film morphology. Hole and electron mobilities of 0.1–0.5 and 0.05–0.1 cm2 / V s are achieved, though the threshold for electron transport was \u3e80 V. These ambipolar FETs are used to demonstrate inverters with gains of up to 94
On the origin of the various types of radio emission in GRS 1915+105
We investigate the association between the radio ``plateau'' states and the
large superluminal flares in GRS 1915+105 and propose a qualitative scenario to
explain this association. We identify several candidate superluminal flare
events from available monitoring data on this source and analyze the
contemporaneous RXTE pointed observations. We detect a strong correlation
between the average X-ray flux during the ``plateau'' state and the total
energy emitted in radio during the subsequent radio flare. We find that the
sequence of events is similar for all large radio flares with a fast rise and
exponential decay morphology. Based on these results, we propose a qualitative
scenario in which the separating ejecta during the superluminal flares are
observed due to the interaction of the matter blob ejected during the X-ray
soft dips, with the steady jet already established during the ``plateau''
state. This picture can explain all types of radio emission observed from this
source in terms of its X-ray emission characteristics.Comment: Corrected typo in the author names, contents unchanged, accepted in
Ap
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