4,126 research outputs found
Echo-Mapping of Swift J1753.5-0127
We present two epochs of coordinated X-ray-optical timing observations of the
black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first
epoch in July occurred at outburst peak. Two consecutive nights of observations
using the McDonald Observatory Argos camera with the Rossi X-ray Timing
Explorer show a consistent correlation with an immediate response and an
extended tail lasting ~5s. The properties of the variability and the
correlation are consistent with thermal reprocessing in an accretion disk. The
shortness of the lag suggests a short orbital period consistent with that
recently claimed. The second epoch in August used the VLT FORS2 HIT mode again
in conjunction with RXTE. Again a repeatable correlation is seen between two
independent subsets of the data. In this case, though, the cross-correlation
function has an unusual structure comprising a dip followed by a double-peak.
We suggest that this may be equivalent to the dip plus single peak structure
seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to
synchrotron emission; a similar structure was seen during later activity of
Swift J1753.5-0127 by Durant et al. (2008).Comment: 7 pages, accepted for publication in Monthly Notices of the Royal
Astronomical Societ
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
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
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
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
Discovery of 15-second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst
We report the discovery of 15-s oscillations in ultraviolet observations of
WZ Sge obtained with the Hubble Space Telescope approximately one month after
the peak of the 2001 outburst. This is the earliest detection of oscillations
in WZ Sge following an outburst and the first time that a signal near 15 s has
been seen to be dominant. The oscillations are quite strong (amplitude about
5%), but not particularly coherent. In one instance, the oscillation period
changed by 0.7 s between successive observations separated by less than 1 hour.
We have also found evidence for weaker signals with periods near 6.5 s in some
of our data. We discuss the implications of our results for the models that
have been proposed to account for the 28-s oscillations seen in quiescence. If
the periods of the 15-s oscillations can be identified with the periods of
revolution of material rotating about the white dwarf, the mass of the white
dwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-s
signals is M_WD > 1.03 M_sun.Comment: accepted for publication in ApJ Letters; 13 pages, 4 postscript
figures; new version corrects a few typos and matches version that will
appear in ApJ
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
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