170 research outputs found
X-ray time variability across the atoll source states of 4U 1636--53
We have studied the rapid X-ray time variability in 149 pointed observations
with the \textit{Rossi X-ray Timing Explorer} (RXTE)'s Proportional Counter
Array of the atoll source 4U~1636--53 in the banana state and, for the first
time with RXTE, in the island state. We compare the frequencies of the
variability components of 4U~1636--53 with those in other atoll and Z-sources
and find that 4U~1636--53 follows the universal scheme of correlations
previously found for other atoll sources at (sometimes much) lower
luminosities. Our results on the hectohertz QPO suggest that the mechanism that
sets its frequency differs from that for the other components, while the
amplitude setting mechanism is common. A previously proposed interpretation of
the narrow low-frequency QPO frequencies in different sources in terms of
harmonic mode switching is not supported by our data, nor by some previous data
on other sources and the frequency range that this QPO covers is found not to
be related to spin, angular momentum or luminosity.Comment: 16 pages, 13 figures, accepted for publication in Ap
Hysteresis in spectral state transitions - a challenge for theoretical modeling
Many low-mass X-ray binaries show both hard and soft spectral states. For
several sources the transitions between these states have been observed, mostly
from the soft to the hard state during a luminosity decrease. In a few cases
also the transition from the hard to the soft state was observed, coincident
with an increase of the luminosity. Surprisingly this luminosity was not the
same as the one during a following change back to the hard state. The values
differed by a factor of about 3 to 5. We present a model for this hysteresis in
the light curves of low-mass X-ray binaries (sources with neutron stars or
black holes). We show that the different amount of Compton cooling or heating
acting on the accretion disk corona at the time of the transition causes this
switch in the accretion mode at different mass accretion rates and therefore
different luminosities. The inner disk during the soft state provides a certain
amount of Compton cooling which is either not present or much less if the inner
region is filled with a hot advection-dominated accretion flow (ADAF) that
radiates a hard spectrum.Comment: 8 pages, 2 figures, accepted for publication in Astron. Astrophy
The rotational broadening and the mass of the donor star of GRS 1915+105
The binary parameters of the microquasar GRS 1915+105 have been determined by
the detection of Doppler-shifted 12CO and 13CO lines in its K-band spectrum
(Greiner et al., 2001, Nature, 414, 522). Here, we present further analysis of
the same K-band VLT spectra and we derive a rotational broadening of the donor
star of V sin i=26+-3 km/s from the 12CO/13CO lines. Assuming that the K-type
star is tidally locked to the black hole and is filling its Roche-lobe surface,
then the implied mass ratio is q = M_d/M_x = 0.058+-0.033. This result,
combined with (P, K, i)=(33.5 d, 140 km/s, 66 deg) gives a more refined mass
estimate for the black hole, , than previously
estimated, using an inclination of i=66+-2 deg (Fender et al. 1999) as derived
from the orientation of the radio jets and a more accurate distance. The mass
for the early K-type giant star is , consistent with
a more evolved stripped-giant donor star in GRS 1915+105 than, for example, the
donor star of the prototype black-hole X-ray transient, V404 Cyg which has the
longest binary period after GRS 1915+105.Comment: 4 pages, 1 figure, A&A Lette
Short-Term Radio-X-ray Correlations of Cygnus X-1
We analyze simultaneous radio-X-ray data of Cygnus X-1 from the Ryle
telescope (RT) and RXTE over more than 4 a. We show that apparent correlations
on short time scales in the lightcurves of Cyg X-1 are probably the
coincidental outcome of white noise statistics.Comment: 1 page, 1 figure. Proceedings of the IAU Colloquium 194 "Compact
Binaries in the Galaxy and Beyond", to be published in: Revista Mexicana de
Astronomia y Astrofisica, Conference Serie
RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs
(Abridged) We report on ten weeks of RXTE observations of the X-ray transient
XTE J1701-462. Comparisons with other sources suggest it had all the
characteristics of the neutron star Z sources (the brightest persistent neutron
star LMXBs). These include Z tracks in X-ray color diagrams and typical
variability components detected in the power spectra. XTE J1701-462 is the
first transient Z source and provides unique insights into mass accretion rate
(Mdot) and luminosity dependencies in neutron star LMXBs. As its overall
luminosity decreased, we observed a switch between two types of Z-source
behavior, with most of the branches of the Z-track changing their shape and/or
orientation. We interpret this switch as an extreme case of the longterm
changes seen in the persistent Z sources and suggest that these result from
changes in Mdot. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1,
and GX 340+0) might be more luminous (> 50%) than the Sco-like Z sources (Sco
X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior
of kHz QPOs, which involves a prompt as well as a filtered response to changes
in Mdot, we propose that changes in Mdot can explain both movement along the Z
track and changes in the shape of the Z track. We discuss some consequences of
this and consider the possibility that the branches of the Z will evolve into
the branches observed in the X-ray color diagrams of the less luminous atoll
sources, but not in a way that was previously suggested.Comment: Accepted for publication in ApJ, 11 page
The complex behaviour of the microquasar GRS 1915+105 in the rho class observed with BeppoSAX. I: Timing analysis
GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For
about 80% of the time, the source was in the variability class ,
characterised by a series of recurrent bursts. We describe the results of the
timing analysis performed on the MECS (1.6--10 keV) and PDS (15--100 keV) data.
The X-ray count rate from \grss showed an increasing trend with different
characteristics in the various energy bands. Fourier and wavelet analyses
detect a variation in the recurrence time of the bursts, from 45--50 s to about
75 s, which appear well correlated with the count rate. From the power
distribution of peaks in Fourier periodograms and wavelet spectra, we
distinguished between the {\it regular} and {\it irregular} variability modes
of the class, which are related to variations in the count rate in the
3--10 keV range. We identified two components in the burst structure: the slow
leading trail, and the pulse, superimposed on a rather stable level. We found
that the change in the recurrence time of the regular mode is caused by the
slow leading trails, while the duration of the pulse phase remains far more
stable. The evolution in the mean count rates shows that the time behaviour of
both the leading trail and the baseline level are very similar to those
observed in the 1.6--3 and 15--100 keV ranges, while that of the pulse follows
the peak number. These differences in the time behaviour and count rates at
different energies indicate that the process responsible for the pulses must
produce the strongest emission between 3 and 10 keV, while that associated with
both the leading trail and the baseline dominates at lower and higher energiesComment: Astronomy and Astrophysics, in pres
The SDO Education and Outreach (E/PO) Program: Changing Perceptions One Program at a Time
The Solar Dynamics Observatory (SDO) Education and Public Outreach (E/PO) program began as a series of discrete efforts implemented by each of the instrument teams and has evolved into a well-rounded program with a full suite of national and international programs. The SDO E/PO team has put forth much effort in the past few years to increase our cohesiveness by adopting common goals and increasing the amount of overlap between our programs. In this paper, we outline the context and overall philosophy for our combined programs, present a brief overview of all SDO E/PO programs along with more detailed highlight of a few key programs, followed by a review of our results up to date. Concluding is a summary of the successes, failures, and lessons learned that future missions can use as a guide, while further incorporating their own content to enhance the public's knowledge and appreciation of NASA?s science and technology as well as its benefit to society
Multiple relativistic outbursts of GRS 1915+105: radio emission and internal shocks
We present 5-GHz MERLIN radio images of the microquasar GRS 1915+105 during
two separate outbursts in 2001 March and 2001 July, following the evolution of
the jet components as they move outwards from the core of the system. Proper
motions constrain the intrinsic jet speed to be >0.57c, but the uncertainty in
the source distance prevents an accurate determination of the jet speed. No
deceleration is observed in the jet components out to an angular separation of
about 300mas. Linear polarisation is observed in the approaching jet component,
with a gradual rotation in position angle and a decreasing fractional
polarisation with time. Our data lend support to the internal shock model
whereby the jet velocity increases leading to internal shocks in the
pre-existing outflow before the jet switches off. The compact nuclear jet is
seen to re-establish itself within two days, and is visible as core emission at
all epochs. The energetics of the source are calculated for the possible range
of distances; a minimum power of 1-10 per cent of the Eddington luminosity is
required to launch the jet.Comment: 18 pages, 14 figures, accepted for publication in MNRAS. For
higher-resolution versions of Figures 3, 5, and 12, see
http://remote.science.uva.nl/~jmiller/grs1915/figures.htm
Fumonisin B1 Toxicity in Grower-Finisher Pigs: A Comparative Analysis of Genetically Engineered Bt Corn and non-Bt Corn by Using Quantitative Dietary Exposure Assessment Modeling
In this study, we investigate the long-term exposure (20 weeks) to fumonisin B1 (FB1) in grower-finisher pigs by conducting a quantitative exposure assessment (QEA). Our analytical approach involved both deterministic and semi-stochastic modeling for dietary comparative analyses of FB1 exposures originating from genetically engineered Bacillus thuringiensis (Bt)-corn, conventional non-Bt corn and distillerâs dried grains with solubles (DDGS) derived from Bt and/or non-Bt corn. Results from both deterministic and semi-stochastic demonstrated a distinct difference of FB1 toxicity in feed between Bt corn and non-Bt corn. Semi-stochastic results predicted the lowest FB1 exposure for Bt grain with a mean of 1.5 mg FB1/kg diet and the highest FB1 exposure for a diet consisting of non-Bt grain and non-Bt DDGS with a mean of 7.87 mg FB1/kg diet; the chronic toxicological incipient level of concern is 1.0 mg of FB1/kg of diet. Deterministic results closely mirrored but tended to slightly under predict the mean result for the semi-stochastic analysis. This novel comparative QEA model reveals that diet scenarios where the source of grain is derived from Bt corn presents less potential to induce FB1 toxicity than diets containing non-Bt corn
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