3,876,285 research outputs found
The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer
We show that the 1996 outburst of the X-ray binary transient system GRO
J1655-40 can be explained by the standard dwarf-nova type disc instability,
followed by an episode of enhanced mass transfer from the secondary if the mass
transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit.
We argue that irradiation of the secondary during the onset of the outburst
driven by the thermal instability in the outer disc can increase the mass
transfer rate above the minimum value required for stable accretion. This will
then produce the period of near-constant X-ray emission seen in this system.
This scenario can also explain the observed anti-correlation between the
optical and X-ray fluxes. It is generally accepted that optical emission in
low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays.
There is also strong circumstantial evidence that in order for the outer disc
to see the irradiating flux, it must be warped. Depending on the warp
propagation mechanism, either a burst of mass from the secondary or viscous
decay are likely to decrease the degree of warping, thereby causing the
decrease in the observed optical flux while the X-ray flux remains constant or
even increases, exactly as observed in GRO J1655-40. Finally, the decrease of
the disc warping and, therefore, irradiation will cause the disc to become
unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres
Vorticity generation in large-scale structure caustics
A fundamental hypothesis for the interpretation of the measured large-scale
line-of-sight peculiar velocities of galaxies is that the large-scale cosmic
flows are irrotational. In order to assess the validity of this assumption, we
estimate, within the frame of the gravitational instability scenario, the
amount of vorticity generated after the first shell crossings in large-scale
caustics. In the Zel'dovich approximation the first emerging singularities form
sheet like structures. Here we compute the expectation profile of an initial
overdensity under the constraint that it goes through its first shell crossing
at the present time. We find that this profile corresponds to rather oblate
structures in Lagrangian space. Assuming the Zel'dovich approximation is still
adequate not only at the first stages of the evolution but also slightly after
the first shell crossing, we calculate the size and shape of those caustics and
their vorticity content as a function of time and for different cosmologies.
The average vorticity created in these caustics is small: of the order of one
(in units of the Hubble constant). To illustrate this point we compute the
contribution of such caustics to the probability distribution function of the
filtered vorticity at large scales. We find that this contribution that this
yields a negligible contribution at the 10 to 15 Mpc scales. It becomes
significant only at the scales of 3 to 4 Mpc, that is, slightly above
the galaxy cluster scales.Comment: 25 pages 16 figures; accepted for publication by A&A vol 342 (1999
X-ray observations and mass determinations in the cluster of galaxies Cl0024+17
We present a detailed analysis of the mass distribution in the rich and
distant cluster of galaxies Cl0024+17. X-ray data come from both a deep
ROSAT/HRI image of the field (Bohringer et al. 1999) and ASCA spectral data.
Using a wide field CCD image of the cluster, we optically identify all the
faint X-ray sources, whose counts are compatible with deep X-ray number counts.
In addition we marginally detect the X-ray counter-part of the gravitational
shear perturbation detected by Bonnet et al. (1994) at a 2.5 level. A
careful spectral analysis of ASCA data is also presented. In particular, we
extract a low resolution spectrum of the cluster free from the contamination by
a nearby point source located 1.2 arcmin from the center. The X-ray temperature
deduced from this analysis is keV at the 90%
confidence level. The comparison between the mass derived from a standard X-ray
analysis and from other methods such as the Virial Theorem or the gravitational
lensing effect lead to a mass discrepancy of a factor 1.5 to 3. We discuss all
the possible sources of uncertainties in each method of mass determination and
give some indications on the way to reduce them. A complementary study of
optical data is in progress and may solve the X-ray/optical discrepancy through
a better understanding of the dynamics of the cluster.Comment: Revised version, accepted in Astronomy and Astrophysics (Main
Journal). Few changes in the discussio
Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255
Results of a careful analysis of the absorption systems with zabs = zem seen
toward the bright, z_em ~ 3.91, gravitationally lensed quasar APM 08279+5255
are presented. Two of the narrow-line systems, at z_abs = 3.8931 and z_abs =
3.9135, show absorptions from singly ionized species with weak or no NV and O V
absorptions at the same redshift. Absorption due to fine structure transitions
of C II and Si II (excitation energies corresponding to, respectively,
156m and 34m) are detected at z_abs = 3.8931. Excitation by IR
radiation is favored as the column density ratios are consistent with the shape
of APM 08279+5255 IR spectrum. The low-ionization state of the system favors a
picture where the cloud is closer to the IR source than to the UV source,
supporting the idea that the extension of the IR source is larger than ~ 200
pc. The absence of fine structure lines at z_abs = 3.9135 suggests that the gas
responsible for this system is farther away from the IR source. Abundances are
~ 0.01 and 1 at z_abs = 3.913 and 3.8931 and aluminum could be
over-abundant with respect to silicon and carbon by at least a factor of two
and five. All this suggests that whereas the \zabs = 3.8931 system is probably
located within 200 pc from the QSO and ejected at a velocity larger than 1000
kms^{-1}, the \zabs = 3.9135 system is farther away and part of the
host-galaxy. (abridged)Comment: 15 pages with 15 figures (psfiles), To appear in A&
A direct view of the AGN powering IRAS12393+3520
We report the first direct X-ray evidence that an AGN is hidden in the center
of IRAS12393+3520. An ASCA observation of this target unveiled a bright (0.5-10
keV luminosity 3.9 x 10^42 erg/s) and variable source, with minimum observed
doubling/halving time scale comprised in the range 30-75 ks. A model composed
by a simple power-law, with photon index ~1.8 and an absorption edge, whose
threshold energy is consistent with K-shell photoionization of OVII, provides
an adequate fit of the spectrum. This suggests that we are observing the
emission from the nuclear region through a warm absorber of N_H a few
10^{21}/cm/cm. If it has internal dust with Galactic gas-to-dust ratio, it
could explain the lack of broad Hbeta emission, even in the episodic presence
of a broad Halpha emission line. Optical spectra obtained over several years
show indeed variations in the strength of this broad Halpha component. A
distribution of dusty, optically thick matter on spatial scales a few hundreds
parsec, which does not intercept the line of sight towards the nucleus, is
probably required to account simultaneously for the relative [OIII] luminosity
deficit in comparison to the X-rays. The high IR to X-ray luminosity ratio is
most likely due to intense star formation in the circumnuclear region.
IRAS12393+3520 might thus exhibit simultaneously nuclear activity and
remarkable star formation.Comment: 9 Latex pages, 8 figures, Accepted for publication in Astronomy &
Astrophysic
pH Protocol
The purpose of this resource is to measure the pH of water. Students use either a pH meter or pH paper to measure the pH. If using the pH meter, the meter needs to be calibrated with buffer solutions that have pH values of 4, 7, and 10. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school
Galaxy Modelling - II. Multi-Wavelength Faint Counts from a Semi-Analytic Model of Galaxy Formation
(Abridged) This paper predicts self-consistent faint galaxy counts from the
UV to the submm wavelength range. The STARDUST spectral energy distributions
described in Devriendt et al. (1999) are embedded within the explicit
cosmological framework of a simple semi-analytic model of galaxy formation and
evolution. We build a class of models which capture the luminosity budget of
the universe through faint galaxy counts and redshift distributions in the
whole wavelength range spanned by our spectra. In contrast with a rather stable
behaviour in the optical and even in the far-IR, the submm counts are
dramatically sensitive to variations in the cosmological parameters and changes
in the star formation history. Faint submm counts are more easily accommodated
within an open universe with a low value of , or a flat universe with
a non-zero cosmological constant. This study illustrates the implementation of
multi-wavelength spectra into a semi-analytic model. In spite of its
simplicity, it already provides fair fits of the current data of faint counts,
and a physically motivated way of interpolating and extrapolating these data to
other wavelengths and fainter flux levels.Comment: 13 pages, 10 figures, to appear in A&
Counting, Fanout, and the Complexity of Quantum ACC
We propose definitions of \QAC^0, the quantum analog of the classical class
\AC^0 of constant-depth circuits with AND and OR gates of arbitrary fan-in,
and \QACC[q], the analog of the class \ACC[q] where \Mod_q gates are also
allowed. We prove that parity or fanout allows us to construct quantum \MOD_q
gates in constant depth for any , so \QACC[2] = \QACC. More generally, we
show that for any , \MOD_q is equivalent to \MOD_p (up to constant
depth). This implies that \QAC^0 with unbounded fanout gates, denoted
\QACwf^0, is the same as \QACC[q] and \QACC for all . Since \ACC[p]
\ne \ACC[q] whenever and are distinct primes, \QACC[q] is strictly
more powerful than its classical counterpart, as is \QAC^0 when fanout is
allowed. This adds to the growing list of quantum complexity classes which are
provably more powerful than their classical counterparts.
We also develop techniques for proving upper bounds for \QACC^0 in terms of
related language classes. We define classes of languages \EQACC, \NQACC and
\BQACC_{\rats}. We define a notion of -planar \QACC operators and
show the appropriately restricted versions of \EQACC and \NQACC are
contained in \P/\poly. We also define a notion of -gate restricted
\QACC operators and show the appropriately restricted versions of \EQACC
and \NQACC are contained in \TC^0
pH Game
The purpose of this resource is to teach students about the acidity levels of liquids and other substances around their school so they understand what pH levels tell us about the environment. Students will create mixtures of water samples, soil samples, plants and other natural materials to better understand the importance of pH levels. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school
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