4,539 research outputs found
The Transition State in a Noisy Environment
Transition State Theory overestimates reaction rates in solution because
conventional dividing surfaces between reagents and products are crossed many
times by the same reactive trajectory. We describe a recipe for constructing a
time-dependent dividing surface free of such recrossings in the presence of
noise. The no-recrossing limit of Transition State Theory thus becomes
generally available for the description of reactions in a fluctuating
environment
The Structure Of The Accretion Disk In The ADC Source 4U 1822-371
The low-mass X-ray binary (LMXB) 4U 1822-371 has an accretion disk corona (ADC) that scatters X-ray photons from the inner disk and neutron star out of the line of sight. It has a high orbital inclination and the secondary star eclipses the disk and ADC. We have obtained new time-resolved UV spectrograms and V- and I-band photometry of 4U 1822-371. The large quadratic term in our new optical eclipse ephemeris confirms that the system has an extremely high rate of mass transfer and mass accretion. The C IV lambda lambda = 1548 - 1550 angstrom emission line has a half width of similar to 4400 km/s, indicating a strong, high velocity wind is being driven off the accretion disk. Near the disk the wind is optically thick in UV, V, and J and the eclipse analysis shows that in V and J the optically thick wind extends nearly to the outer edge of the disk. The ADC must also extend vertically to a height equal to approximately half the disk radius.Astronom
Fast Photometry of Quiescent Soft X-ray Transients with the Gemini-South Acquisition Camera
We present a compilation of high time-resolution photometric observations of
quiescent soft X-ray transients obtained with the acquisition camera of
Gemini-South. A0620-00 was observed with a short cycle time and high precision.
Superimposed on the ellipsoidal modulation we find several prominent flares
together with weaker continual variability. The flares seen sample shorter
timescale than those reported in previous observations, with rise times as low
as 30s or less; most flares show unresolved peaks. The power density spectrum
(PDS) of A0620-00 appears to exhibit band-limited noise closely resembling the
X-ray PDS of black hole candidates in their low states, but with the
low-frequency break at a lower frequency. X-ray Nova Mus 1991 shows much larger
amplitude flares than A0620-00 and if a break is present it is at a lower
frequency. X-ray Nova Vel 1993 shows very little flaring and is, like A0620-00,
dominated by the ellipsoidal modulation. We discuss the possible origins for
the flares. They are clearly associated with the accretion flow rather than an
active companion, but whether they originate in the outer disc, or are driven
by events in the inner region is not yet resolved. The similarities of the PDS
to those of low/hard state sources would support the latter interpretation, and
the low break frequency is as would be expected if this frequency approximately
scales with the size of an inner evaporated region. We also report the
discovery of a new variable star only 14arcsec from XN Mus 1991. This appears
to be a W UMa star, with an orbital period of about 6hrs.Comment: 11 pages, accepted for publication in MNRA
Langevin Equation for the Rayleigh model with finite-ranged interactions
Both linear and nonlinear Langevin equations are derived directly from the
Liouville equation for an exactly solvable model consisting of a Brownian
particle of mass interacting with ideal gas molecules of mass via a
quadratic repulsive potential. Explicit microscopic expressions for all kinetic
coefficients appearing in these equations are presented. It is shown that the
range of applicability of the Langevin equation, as well as statistical
properties of random force, may depend not only on the mass ratio but
also by the parameter , involving the average number of molecules in
the interaction zone around the particle. For the case of a short-ranged
potential, when , analysis of the Langevin equations yields previously
obtained results for a hard-wall potential in which only binary collisions are
considered. For the finite-ranged potential, when multiple collisions are
important (), the model describes nontrivial dynamics on time scales
that are on the order of the collision time, a regime that is usually beyond
the scope of more phenomenological models.Comment: 21 pages, 1 figure. To appear in Phys. Rev.
ULTRACAM observations of the black hole X-ray transient XTE J1118+480 in quiescence
We present high time-resolution multicolour observations of the quiescent
soft X-ray transient XTE J1118+480 obtained with ULTRACAM. Superimposed on the
double-humped continuum g' and i'-band lightcurves are rapid flare events which
typically last a few minutes. The power density spectrum of the lightcurves can
be described by a broken power-law model with a break frequency at ~2 mHz or a
power-law model plus a broad quasi-periodic oscillation (QPO) at ~2 mHz. In the
context of the cellular-automaton we estimate the size of the quiescent
advection-dominated flow (ADAF) region to be ~10^4 Schwarzschild radii, similar
to that observed in other quiescent black hole X-ray transients, suggesting the
same underlying physics. The similarites between the low/hard and quiescent
state PDS suggest a similar origin for the optical and X-ray variability, most
likely from regions at/near the ADAF.Comment: 9 pages, 6 figures, accepted by MNRA
1/f spectrum and memory function analysis of solvation dynamics in a room-temperature ionic liquid
To understand the non-exponential relaxation associated with solvation
dynamics in the ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate,
we study power spectra of the fluctuating Franck-Condon energy gap of a
diatomic probe solute via molecular dynamics simulations. Results show 1/f
dependence in a wide frequency range over 2 to 3 decades, indicating
distributed relaxation times. We analyze the memory function and solvation time
in the framework of the generalized Langevin equation using a simple model
description for the power spectrum. It is found that the crossover frequency
toward the white noise plateau is directly related to the time scale for the
memory function and thus the solvation time. Specifically, the low crossover
frequency observed in the ionic liquid leads to a slowly-decaying tail in its
memory function and long solvation time. By contrast, acetonitrile
characterized by a high crossover frequency and (near) absence of 1/f behavior
in its power spectra shows fast relaxation of the memory function and
single-exponential decay of solvation dynamics in the long-time regime.Comment: 10 pages, 4 figure
SS433:the microquasar link with ULXs?
SS433 is the prototype microquasar in the Galaxy and may even be analogous to
the ULX sources if the jets' kinetic energy is taken into account. However, in
spite of 20 years of study, our constraints on the nature of the binary system
are extremely limited as a result of the difficulty of locating spectral
features that can reveal the nature and motion of the mass donor. Newly
acquired, high resolution blue spectra taken when the (precessing) disc is
edge-on suggest that the binary is close to a common-envelope phase, and hence
providing kinematic constraints is extremely difficult. Nevertheless, we do
find evidence for a massive donor, as expected for the inferred very high mass
transfer rate, and we compare SS433's properties with those of Cyg X-3.Comment: 4 pages, 3 figures, to appear in "Compact binaries in the Galaxy and
beyond
Multiwavelength Observations of Swift J1753.5-0127
We present contemporaneous X-ray, ultraviolet, optical and near-infrared
observations of the black hole binary system, Swift J1753.5-0127, acquired in
2012 October. The UV observations, obtained with the Cosmic Origins
Spectrograph on the Hubble Space Telescope, are the first UV spectra of this
system. The dereddened UV spectrum is characterized by a smooth, blue continuum
and broad emission lines of CIV and HeII. The system was stable in the UV to
<10% during our observations. We estimated the interstellar reddening by
fitting the 2175 A absorption feature and fit the interstellar absorption
profile of Ly to directly measure the neutral hydrogen column density
along the line of sight. By comparing the UV continuum flux to steady-state
thin accretion disk models, we determined upper limits on the distance to the
system as a function of black hole mass. The continuum is well fit with disk
models dominated by viscous heating rather than irradiation. The broadband
spectral energy distribution shows the system has declined at all wavelengths
since previous broadband observations in 2005 and 2007. If we assume that the
UV emission is dominated by the accretion disk the inner radius of the disk
must be truncated at radii above the ISCO to be consistent with the X-ray flux,
requiring significant mass loss from outflows and/or energy loss via advection
into the black hole to maintain energy balance.Comment: To appear in the Ap
Simultaneous multiwavelength observations of V404 Cygni during its 2015 June outburst decay strengthen the case for an extremely energetic jet-base
We present results of multiband optical photometry of the black hole X-ray
binary system V404 Cygni obtained using Wheaton College Observatory's 0.3m
telescope, along with strictly simultaneous INTEGRAL and Swift observations
during 2015 June 25.15--26.33 UT, and 2015 June 27.10--27.34 UT. These
observations were made during the 2015 June outburst of the source when it was
going through an epoch of violent activity in all wavelengths ranging from
radio to -rays. The multiwavelength variability timescale favors a
compact emission region, most likely originating in a jet outflow, for both
observing epochs presented in this work. The simultaneous INTEGRAL/Imager on
Board the Integral Satellite (IBIS) 20--40 keV light curve obtained during the
June 27 observing run correlates very strongly with the optical light curve,
with no detectable delay between the optical bands as well as between the
optical and hard X-rays. The average slope of the dereddened spectral energy
distribution was roughly flat between the - and -bands during the June
27 run, even though the optical and X-ray flux varied by 25 during
the run, ruling out an irradiation origin for the optical and suggesting that
the optically thick to optically thin jet synchrotron break during the
observations was at a frequency larger than that of -band, which is quite
extreme for X-ray binaries. These observations suggest that the optical
emission originated very close to the base of the jet. A strong H
emission line, probably originating in a quasi-spherical nebula around the
source, also contributes significantly in the -band. Our data, in
conjunction with contemporaneous data at other wavelengths presented by other
groups, strongly suggest that the jet-base was extremely compact and energetic
during this phase of the outburst.Comment: 15 pages, 2 tables, 5 figures. Accepted for publication in Ap
Radio sources in the Chandra Galactic Bulge Survey
We discuss radio sources in the Chandra Galactic Bulge Survey region. By cross-matching the X-ray sources in this field with the NRAO VLA Sky Survey archival data, we find 12 candidate matches. We present a classification scheme for radio/X-ray matches in surveys taken in or near the Galactic plane, taking into account other multiwavelength data. We show that none of the matches found here is likely to be due to coronal activity from normal stars because the radio to X-ray flux ratios are systematically too high. We show that one of the source could be a radio pulsar, and that one could be a planetary nebula, but that the bulk of the sources are likely to be background active galactic nuclei (AGN), with many confirmed through a variety of approaches. Several of the AGN are bright enough in the near-infrared (and presumably in the optical) to use as probes of the interstellar medium in the inner Galaxy
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