6,692 research outputs found
Cessation of X-ray Pulsation of GX 1+4
We report results from our weekly monitoring campaign on the X-ray pulsar GX
1+4 with the {\em Rossi X-ray Timing Explorer} satellite. The spin-down trend
of GX 1+4 was continuing, with the pulsar being at its longest period ever
measured (about 138.7 s). At the late stage of the campaign, the source entered
an extended faint state, when its X-ray (2-60 keV) flux decreased significantly
to an average level of . It was
highly variable in the faint state; the flux dropped to as low as . In several observations during this
period, the X-ray pulsation became undetectable. We can, therefore, conclude
conservatively that the pulsed fraction, which is normally 70%
(peak-to-peak), must have decreased drastically in those cases. This is very
similar to what was observed of GX 1+4 in 1996 when it became similarly faint
in X-ray. In fact, the flux at which the cessation of X-ray pulsation first
occurred is nearly the same as it was in 1996. We suggest that we have, once
again, observed the propeller effect in GX 1+4, a phenomenon that is predicted
by theoretical models of accreting X-ray pulsars.Comment: 13 pages, 9 figures (available at
http://www.physics.purdue.edu/~cui/ftp/cuifigs.tar.gz). To appear in Ap
Cognitive Control Signals in Posterior Cingulate Cortex
Efficiently shifting between tasks is a central function of cognitive control. The role of the default network – a constellation of areas with high baseline activity that declines during task performance – in cognitive control remains poorly understood. We hypothesized that task switching demands cognitive control to shift the balance of processing toward the external world, and therefore predicted that switching between the two tasks would require suppression of activity of neurons within the posterior cingulate cortex (CGp). To test this idea, we recorded the activity of single neurons in CGp, a central node in the default network, in monkeys performing two interleaved tasks. As predicted, we found that basal levels of neuronal activity were reduced following a switch from one task to another and gradually returned to pre-switch baseline on subsequent trials. We failed to observe these effects in lateral intraparietal cortex, part of the dorsal fronto-parietal cortical attention network directly connected to CGp. These findings indicate that suppression of neuronal activity in CGp facilitates cognitive control, and suggest that activity in the default network reflects processes that directly compete with control processes elsewhere in the brain
Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the \ion{He}{1} recombination line ratios,
designated (\ion{He}{1}), are presented for 48 planetary nebulae
(PNe). We study the effect that temperature fluctuations inside nebulae have on
the (\ion{He}{1}) value. We show that a comparison between (\ion{He}{1}) and the electron temperature derived from the Balmer jump of
the \ion{H}{1} recombination spectrum, designated (\ion{H}{1}),
provides an opportunity to discriminate between the paradigms of a chemically
homogeneous plasma with temperature and density variations, and a two-abundance
nebular model with hydrogen-deficient material embedded in diffuse gas of a
``normal'' chemical composition (i.e. solar), as the possible causes of
the dichotomy between the abundances that are deduced from collisionally
excited lines to those deduced from recombination lines. We find that (\ion{He}{1}) values are significantly lower than (\ion{H}{1})
values, with an average difference of (\ion{H}{1})-(\ion{He}{1}) K. The result is consistent with the expectation of
the two-abundance nebular model but is opposite to the prediction of the
scenarios of temperature fluctuations and/or density inhomogeneities. From the
observed difference between (\ion{He}{1}) and (\ion{H}{1}), we estimate that the filling factor ofhydrogen-deficient
components has a typical value of . In spite of its small mass, the
existence of hydrogen-deficient inclusions may potentially have a profound
effect in enhancing the intensities of \ion{He}{1} recombination lines and
thereby lead to apparently overestimated helium abundances for PNe.Comment: 27 pages, 7 figures, accepted for publication in MNRA
What you know can influence what you are going to know (especially for older adults)
Stimuli related to an individual's knowledge/experience are often more memorable than abstract stimuli, particularly for older adults. This has been found when material that is congruent with knowledge is contrasted with material that is incongruent with knowledge, but there is little research on a possible graded effect of congruency. The present study manipulated the degree of congruency of study material with participants’ knowledge. Young and older participants associated two famous names to nonfamous faces, where the similarity between the nonfamous faces and the real famous individuals varied. These associations were incrementally easier to remember as the name-face combinations became more congruent with prior knowledge, demonstrating a graded congruency effect, as opposed to an effect based simply on the presence or absence of associations to prior knowledge. Older adults tended to show greater susceptibility to the effect than young adults, with a significant age difference for extreme stimuli, in line with previous literature showing that schematic support in memory tasks particularly benefits older adults
Revealing evolved massive stars with Spitzer
Massive evolved stars loss a large fraction of their mass via copious stellar
wind or instant outbursts and during certain evolutionary phases they can be
identified via the presence of their circumstellar nebulae. In this paper, we
present the results of search for compact nebulae (reminiscent of circumstellar
nebulae around evolved massive stars) using archival 24 m data obtained
with the Multiband Imaging Photometer for Spitzer. We discovered 115 nebulae,
most of which bear a striking resemblance to the circumstellar nebulae
associated with Luminous Blue Variables (LBVs) and late WN-type (WNL)
Wolf-Rayet (WR) stars in the Milky Way and the Large Magellanic Cloud (LMC). We
interpret this similarity as an indication that the central stars of detected
nebulae are either LBVs or related evolved massive stars. Our interpretation is
supported by follow-up spectroscopy of two dozens of these central stars, most
of which turns out to be either candidate LBVs (cLBVs), blue supergiants or WNL
stars. We expect that the forthcoming spectroscopy of the remaining objects
from our list, accompanied by the spectrophotometric monitoring of the already
discovered cLBVs, will further increase the known population of Galactic LBVs,
which in turn would have profound consequences for better understanding the LBV
phenomenon and its role in the transition between hydrogen burning O stars and
helium burning WR stars. We also report the detection of an arc-like structure
attached to the cLBV HD326823 and an arc associated with the LBV R99 (HD269445)
in the LMC.Comment: 9 pages, 10 figures, accepted to MNRA
Measurements of Ice Shelf Water beneath the front of the Ross Ice Shelf using gliders
Measurements made by an underwater glider deployed near the Ross Ice Shelf were used to identify the presence of Ice Shelf Water (ISW), which is defined as seawater with its potential temperature lower than its surface freezing point temperature. Properties logged by the glider included in situ temperature, electrical conductivity, pressure, GPS location at surfacings and time. For most of the first 30 recorded dives of its deployment, evidence suggests the glider was prevented from surfacing due to being under the ice shelf. For dives under the ice shelf, farthest from the ice shelf front, ISW layers of varying thicknesses and depth locations were observed; between 2 m thick (centred at 231 m depth) to >93 m thick (centred at >360 m). For dives under the ice shelf, close to the ice shelf front, either no ISW was observed or ISW layers were centred at shallower depths (116–127 m). Thicker ISW layers (e.g. up to 250 m thickness centred at 421 m) were observed for some glider dives in open water in front of the Ross Ice Shelf. No in situ supercooling (water colder than the pressure-dependent freezing point temperature) was observed
WR bubbles and HeII emission
We present the very first high quality images of the HeII 4686 emission in
three high excitation nebulae of the Magellanic Clouds. A fourth high
excitation nebula, situated around the WR star BAT99-2, was analysed in a
previous letter. Using VLT FORS data, we investigate the morphology of the ring
nebulae around the early-type WN stars BAT99-49 & AB7. We derive the total HeII
fluxes for each object and compare them with the most recent theoretical WR
models. Using Halpha, [OIII] and HeI 5876 images along with long-slit
spectroscopy, we investigate the physical properties of these ring nebulae and
find only moderate chemical enrichment. We also surveyed seven other LMC WR
stars but we failed to detect any HeII emission but note that the nebula around
BAT99-11 shows a N/O ratio and an oxygen abundance slightly lower than the LMC
values, while the nebula around BAT99-134 presents moderate chemical enrichment
similar to the one seen near BAT99-2, 49 and AB7. The third high excitation
nebula presented in this paper, N44C, does not harbor stars hotter than mid-O
main sequence stars. It was suggested to be a fossil X-ray nebula ionized but
our observations of N44C reveal no substantial changes in the excitation
compared to previous results reported in the literature.Comment: 13 pages, 8 figures (7 in jpg), accepted by A&A, also available from
http://vela.astro.ulg.ac.be/Preprints/P81/index.htm
Constraints on CDM cosmology from galaxy power spectrum, CMB and SNIa evolution
We examine the constraints that can be obtained on standard cold dark matter
models from the most currently used data set: CMB anisotropies, type Ia
supernovae and the SDSS luminous red galaxies. We also examine how these
constraints are widened when the equation of state parameter and the
curvature parameter are left as free parameters. For the
CDM model, our 'vanilla' model, cosmological parameters are tightly
constrained and consistent with current estimates from various methods. When
the dark energy parameter is free we find that the constraints remain
mostly unchanged, i.e. changes are smaller than the 1 sigma uncertainties.
Similarly, relaxing the assumption of a flat universe leads to nearly identical
constraints on the dark energy density parameter of the universe
, baryon density of the universe , the optical
depth , the index of the power spectrum of primordial fluctuations ,
with most one sigma uncertainties better than 5%. More significant changes
appear on other parameters: while preferred values are almost unchanged,
uncertainties for the physical dark matter density , Hubble
constant and are typically twice as large. We found that
different methodological approaches on large scale structure estimates lead to
appreciable differences in preferred values and uncertainty widths. We also
found that possible evolution in SNIa intrinsic luminosity does not alter these
constraints by much, except for , for which the uncertainty is twice as
large. At the same time, this possible evolution is severely constrained. We
conclude that systematic uncertainties for some estimated quantities are
similar or larger than statistical ones.Comment: Revised version, 9 pages, 8 figures, accepted for publication in A&
MN112: a new Galactic candidate Luminous Blue Variable
We report the discovery of a new Galactic candidate Luminous Blue Variable
(cLBV) via detection of an infrared circular nebula and follow-up spectroscopy
of its central star. The nebula, MN112, is one of many dozens of circular
nebulae detected at m in the {\it Spitzer Space Telescope} archival
data, whose morphology is similar to that of nebulae associated with known
(c)LBVs and related evolved massive stars. Specifically, the core-halo
morphology of MN112 bears a striking resemblance to the circumstellar nebula
associated with the Galactic cLBV GAL 079.29+00.46, which suggests that both
nebulae might have a similar origin and that the central star of MN112 is a
LBV. The spectroscopy of the central star showed that its spectrum is almost
identical to that of the bona fide LBV P Cygni, which also supports the LBV
classification of the object. To further constrain the nature of MN112, we
searched for signatures of possible high-amplitude (\ga 1 mag) photometric
variability of the central star using archival and newly obtained photometric
data covering a 45 year period. We found that the B magnitude of the star was
constant ( 17.10.3 mag) over this period, while in the I band the
star brightened by mag during the last 17 years. Although the
non-detection of large photometric variability leads us to use the prefix
`candidate' in the classification of MN112, we remind that the long-term
photometric stability is not unusual for genuine LBVs and that the brightness
of P Cygni remains relatively stable during the last three centuries.Comment: 6 pages, 3 figures, accepted to MNRA
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