229 research outputs found
Evidence of two unique variability classes from IGR J17091-3624
IGR J17091-3624 is the second black hole X-ray binary after GRS 1915+105,
which showed large and distinct variabilities. The study of these variability
classes can be useful to understand the accretion-ejection mechanisms of
accreting black holes, and hence to probe the strong gravity regime. We report
the discovery of two new variability classes (C1 and C2) from IGR J17091-3624
from the 2011 outburst Rossi X-ray Timing Explorer data. These unique classes
will be useful to have complete details about the source, and to learn new
aspects about variabilities. For examples, the C1 class shows that the
intensity and period of oscillations, energy spectrum and power spectrum can
clearly evolve in tens of seconds. Moreover, in such a small time scale,
soft-lag becomes hard-lag. The C2 class shows that the variability and the
nonvariability can occur at similar energy spectrum, and a soft state is not
required for variability to happen.Comment: 5 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Letter
Comparison of time/phase lags in the hard state and plateau state of GRS 1915+105
We investigate the complex behavior of energy- and frequency-dependent
time/phase lags in the plateau state and the radio-quiet hard state of GRS
1915+105. In our timing analysis, we find that when the source is faint in the
radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft
photons lag hard photons), whereas QPOs in the radio-bright plateau state are
found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO
frequency is strongly anti-correlated with the QPO frequency, changing sign at
2.2 Hz. However, the phase lag at the frequency of the first harmonic is
positive and nearly independent of frequency at at ~0.172 rad, regardless of
the radio emission. The lag-energy dependence at the first harmonic is also
independent of radio flux. However, the lags at the QPO frequency are negative
at all energies during the radio-quiet state, but lags at the QPO frequency
during the plateau state are positive at all energies and show a
'reflection-type' evolution of the lag-energy spectra with respect to the
radio-quiet state. The lag-energy dependence is roughly logarithmic, but there
is some evidence for a break around 4-6 keV. Finally, the Fourier
frequency-dependent phase lag spectra are fairly flat during the plateau state,
but increase from negative to positive during the radio-quiet state. We discuss
the implications of our results in the light of some generic models.Comment: 9 pages, 7 figures, accepted for publication in Ap
Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR J17091-3624 and their implications
We report a comprehensive study on spectral and timing properties of hard
X-ray dips uniquely observed in some so-called variability classes of the
micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by
a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12
simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31
observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are
observed in different types of variability classes, and we find that a
dichotomy is observed between classes with abrupt transitions vs those with
smoother evolution. For example, both energy-lag spectra and frequency-lag
spectra of hard X-ray dips in classes with abrupt transitions and shorter dip
intervals show hard-lag (hard photons lag soft photons), while both lag spectra
during hard dips in classes with smoother evolution and longer dip intervals
show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also
show that timing and spectral properties of hard X-ray dips observed in light
curves of IGR J17091-3624 during its 2011 outburst are consistent with the
properties of the abrupt transitions in GRS 1915+105 rather than smooth
evolution. A global correlation between the X-ray intensity cycle time and hard
dip time is observed for both abrupt and smooth transition which may be due to
two distinct physical processes whose time-scales are eventually correlated. We
discuss implications of our results in the light of some generic models.Comment: 17 pages, 5 figures, accepted for publication in the Astrophysical
Journa
X-ray spectral state evolution in IGR J17091-3624 and comparison of its heartbeat oscillation properties with those of GRS 1915+105
In this work, we study the X-ray timing and spectral evolution of the
transient low mass X-ray binary IGR J17091-3624 during first 66 days of its
2011 outburst. We present results obtained from observations with two
instruments, Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array
(PCA) and SWIFT/X-ray telescope (XRT), between 09 February, 2011 and 15 April,
2011. Using quasi-periodic oscillation classifications, power density spectrum
characteristics, time-lag behavior and energy spectral properties, we determine
source states and their transitions at different times of the outburst. During
the first part of the evolution, the source followed trends usually observed
from transient black hole X-ray binaries (BHXBs). Interestingly, a gradual
transition is observed in IGR J17091-3624 from the low-variability SIMS,
commonly seen in BHXBs, to a high-variability state with regular, repetitive
and structured pulsations, seen only from GRS 1915+105 (also known as 'rho'
class variability/'heartbeat' oscillations). We study the time evolution of
characteristic time-scale, quality factor and rms amplitude of heartbeat
oscillations in IGR J17091-3624. We also present a detailed comparison of the
timing and spectral properties of heartbeat oscillations and their evolution in
IGR J17091-3624 and GRS 1915+105.Comment: 23 pages, 10 figures, 3 tables, accepted for publication in the Ap
Transient Analysis of Primary Feed Pump Trip for 700 MWe IPHWR
700 MWe Indian Pressurized Heavy Water Reactor (IPHWR) is a horizontal channel type reactor with two loops of Primary Heat Transport (PHTS) system. Three (two operating and one stand by) main boiler feed water pumps (BFP) supply feed water to Steam Generators (SGs). In the event of one of the running BFP trip, standby comes on line on auto. Transient analysis for this event is performed using in- house computer code ATMIKA.T .The transient has been initiated by tripping one of the pumps.
Two cases are postulated:
1: BFP Trip and Standby BFP available on auto 2: BFP Trip and Standby pump not available.
This paper provides timelines of following sequence of events which is important for operator’s action to maneuver the event, and the main findings of the study are:
Following the tripping of one BFP, feed flow reduces and SGs level start falling. As SGs level fall, feed control valves open up in level control mode and system resistance in feed water circuit reduces. If the standby pump comes on auto, the SGs level recovers with a slight dip in level. The feed flow increases and settles down to normal value. Subsequently all the parameters converge to steady state value. Reactor continues to operate at 100% FP.
In the event of main BFP trip without the availability of standby BFP, feed flow rate drops. SGs pressure rise slightly due to reduction in sub cooled feed flow and SGs level start to decrease. Reactor setback starts as SG level goes below set back limit. SG level continues to fall and reactor trips on SG Level very very low trip setting
Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30
Broad relativistic iron lines from neutron star X-ray binaries are important
probes of the inner accretion disk. The X-ray reflection features can be
weakened due to strong magnetic fields or very low iron abundances such as is
possible in X-ray binaries with low mass, first generation stars as companions.
Here we investigate the reality of the broad iron line detected earlier from
the neutron star low mass X-ray binary 4U~1820--30 with a degenerate helium
dwarf companion. We perform a comprehensive, systematic broadband spectral
study of the atoll source using \suzaku{} and simultaneous \nustar{} \&
\swift{} observations. We have used different continuum models involving
accretion disk emission, thermal blackbody and thermal Comptonization of either
disk or blackbody photons. The \suzaku{} data show positive and negative
residuals in the region of iron K band. These features are well described by
two absorption edges at 7.67\pm0.14\kev and 6.93\pm0.07\kev or partial
covering photoionized absorption or by blurred reflection. Though, the
simultaneous \swift{} and \nustar{} data do not clearly reveal the emission or
absorption features, the data are consistent with the presence of either
absorption or emission features. Thus, the absorption based models provide an
alternative to the broad iron line or reflection model. The absorption features
may arise in winds from the inner accretion disk. The broadband spectra appear
to disfavour continuum models in which the blackbody emission from the neutron
star surface provides the seed photons for thermal Comptonization. Our results
suggest emission from a thin accretion disk (kT_{disk} \sim 1\kev),
Comptonization of disk photons in a boundary layer most likely covering a large
fraction of the neutron star surface and innermost parts of the accretion disk,
and blackbody emission (kT_{bb} \sim 2\kev) from the polar regions.Comment: 12 pages, 8 figures, 5 tables. Accepted for publication in MNRA
Expression pattern of three-finger toxin and phospholipase A2 genes in the venom glands of two sea snakes, Lapemis curtus and Acalyptophis peronii: Comparison of evolution of these toxins in land snakes, sea kraits and sea snakes
10.1186/1471-2148-7-175BMC Evolutionary Biology7
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