6,631 research outputs found
A Lensed Arc in the Low Redshift Cluster Abell 2124
We report the discovery of an arc-like object 27" from the center of the cD
galaxy in the redshift cluster A2124. Observations with the Keck II
telescope reveal that the object is a background galaxy at ,
apparently lensed into an arc of length \sim 8 \farcs5 and total R magnitude
. The width of the arc is resolved; we estimate it to be
0\farcs6 after correcting for seeing. A lens model of the A2124 core mass
distribution consistent with the cluster galaxy velocity dispersion reproduces
the observed arc geometry and indicates a magnification factor \gta 9. With
this magnification, the strength of the [OII] \lambda 3727 line implies a
star-formation rate of SFR \sim 0.4 h^{-2}\msun yr^{-1}$. A2124 thus appears to
be the lowest redshift cluster known to exhibit strong lensing of a distant
background galaxy.Comment: 6 pages using emulateapj.sty; 4 Postscript figures; Figure 4 uses
color. Accepted for publication, but ApJ Letters' new policy of counting data
images makes the manuscript too long; will appear in main journal. This final
version has minor correction
On the macroion virial contribution to the osmotic pressure in charge-stabilized colloidal suspensions
Our interest goes to the different virial contributions to the equation of
state of charged colloidal suspensions. Neglect of surface effects in the
computation of the colloidal virial term leads to spurious and paradoxical
results. This pitfall is one of the several facets of the danger of a naive
implementation of the so called One Component Model, where the micro-ionic
degrees of freedom are integrated out to only keep in the description the
mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation
of wall induced forces dissolves the paradox brought forth in the naive
approach, provides a consistent description, and confirms that for salt-free
systems, the colloidal contribution to the pressure is dominated by the
micro-ionic one. Much emphasis is put on the no salt case but the situation
with added electrolyte is also discussed
The black hole candidate MAXI J1659-152: spectral and timing analysis during its 2010 outburst
We present a comprehensive spectral-timing study of the black hole candidate
MAXI J1659-152 during its 2010 outburst. We analysed 65 RXTE observations taken
along this period and computed the fundamental diagrams commonly used to study
black hole transients. We fitted power density and energy spectra and studied
the evolution of the spectral and timing parameters along the outburst. We
discuss the evolution of the variability observed at different energy bands on
the basis of the relative contribution of the disc and hard components to the
energy spectrum of the source. We conclude that hard emission accounts for the
observed fast variability, it being strongly quenched when type-B oscillations
are observed. We find that both disc and hard emission are responsible for
local count-rate peaks until the system reaches the soft state. From that
point, the peaks are only observed in the hard component, whereas the thermal
component drops monotonically probably following the accretion rate decrease.
We have also computed time-lags between soft and hard X-ray variability
confirming that lags are larger during the hard-to-soft transition than during
the hard state.Comment: Accepted for publication in MNRAS. 12 pages, 8 figures, 2 table
The X-ray Properties of Low-Frequency Quasi-Periodic Oscillations from GRS 1915+105 up to 120 keV
We present a study of the properties of strong 0.8-3.0 Hz quasi-periodic
oscillations (QPOs) that occurred during 1997 RXTE observations of the
microquasar GRS 1915+105 in the low-hard state. The high count rates allow us
to track individual QPO peaks, and we exploit this to develop a QPO folding
technique. In contrast to previous QPO studies with RXTE, we emphasize the high
energy QPO properties and report the detection of a QPO in the 60-124 keV
energy band. Our technique allows us, for the first time, to measure the phase
of the QPO harmonics relative to the fundamental. Variation in this phase
difference leads to changes in the shape of the QPO profile with energy and
over time. The strength of the QPO fundamental increases up to 19 keV, but the
data do not suggest that the strength continues to increase above this energy.
In some cases, the QPO amplitudes in the 30-60 keV and 60-124 keV energy bands
are significantly less than in the 13-19 keV and 19-29 keV energy bands. We
also use our technique to measure the phase lag of the QPO fundamental and
harmonics. In the case where negative phase lags are detected for the
fundamental, positive phase lags are detected for the first harmonic.Comment: Submitted to ApJ, Refereed, 9 page
Relations between x-ray timing features and spectral parameters of galactic black hole x-ray binaries
We present a study of correlations between spectral and timing parameters for a sample of black hole X-ray binary candidates. Data are taken from GX
339-4, H 1743-322, and XTE J1650-500, as the Rossi X-ray Timing Explorer
(RXTE) observed complete outbursts of these sources. In our study we investigate outbursts that happened before the end of 2009 to make use of the high-energy coverage of the HEXTE detector and select observations that
show a certain type of quasi-periodic oscillations (type-C QPOs). The spectral parameters are derived using the empirical convolution model simpl to model the Comptonized component of the emission together with a disc blackbody for the emission of the accretion disc. Additional spectral features, namely a reflection component, a high-energy cut-off, and excess emission at 6.4 keV, are taken into account. Our investigations confirm the known positive
correlation between photon index and centroid frequency of the QPOs and reveal an anti-correlation between the fraction of up-scattered photons and the QPO frequency. We show that both correlations behave as expected in the âsombreroâ
geometry. Furthermore, we find that during outburst decay the correlation between photon index and QPO frequency follow a general track, independent of individual outbursts
The faint 2011 outburst of the black hole X-ray binary candidate MAXI J1543-564
We report on a spectral-timing analysis of the black hole X-ray binary
candidate MAXI J1543- 564 during its 2011 outburst. All 99 pointed observations
of this outburst obtained with the Rossi X-ray Timing Explorer (RXTE) were
included in our study. We computed the fundamental diagrams commonly used to
study black hole transients, and fitted power density and energy spectra to
study the spectral and timing parameters along the outburst. The determination
of timing parameters and hence of exact transitions between different states
was hampered by the rather low count rate at which his outburst was observed.
We detected two periods of exponential decay, one after the source was
brightest, which was interrupted by several flares, and another one during the
high/soft state. The detection of these decays allowed us to obtain an estimate
for the source distance of at least 8.5 kpc. This leaves two possible
explanations for the observed low count rate; either the source has a distance
similar to that of other black hole X-ray binary candidates and it is
intrinsically faint, or it has a similar luminosity, but is located more than
12 kpc away from us. Furthermore, in the high/soft state the source spectrum
appears to be completely disc dominated.Comment: 11 pages, 5 figures, accepted for publication in MNRA
On the two types of steady hard X-ray states of GRS 1915+105
Using the data of 5 years of RXTE observations we investigate the X-ray
spectral and timing properties of GRS 1915+105 during the hard steady states.
According to the results of our simultaneous X-ray spectral and timing analysis
the behavior the source during the hard steady states can be reduced to a
couple of major distinct types. i) Type I states: The dominant hard component
of the energy spectrum has characteristic quasi- exponential cut-off at 50-120
keV. The broad-band power density spectrum of the source shows significant high
frequency noise component with a cut-off at 60-80 Hz. ii) Type II states: The
hard spectral component has a break in its slope at ~12-20 keV. The high
frequency part of the power density spectrum fades quickly lacking significant
variability at frequencies higher than ~30 Hz. These two types of the X-ray
hard states are also clearly distinguished by their properties in the radio
band: while during the type I observations the source tends to be
'radio-quiet', the type II observations are characterized by high level of
radio flux ('plateau' radio states). In this work we demonstrate aforementioned
differences using the data of 12 representative hard steady state observations.
We conclude that the difference between these two types can be probably
explained in terms of different structure of the accretion flow in the
immediate vicinity of the compact object due to presence of relativistic
outflow of matter.Comment: 16 pages, including 3 figures, submitted to Astrophysical Journal
Letter
Broad-band X-ray spectral evolution of GX 339-4 during a state transition
We report on X-ray and soft gamma-ray observations of the black-hole
candidate GX 339-4 during its 2007 outburst, performed with the RXTE and
INTEGRAL satellites. The hardness-intensity diagram of all RXTE/PCA data
combined shows a q-shaped track similar to that observed in previous
outbursts.The evolution in the diagram suggested that a transition from
hard-intermediate state to soft-intermediate state occurred, simultaneously
with INTEGRAL observations performed in March. The transition is confirmed by
the timing analysis presented in this work, which reveals that a weak type-A
quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same
time, spectral analysis shows that the flux of the high-energy component shows
a significant decrease in its flux. However, we observe a delay (roughly one
day) between variations of the spectral parameters of the high-energy component
and changes in the flux and timing properties. The changes in the high-energy
component can be explained either in terms the high-energy cut-off or in terms
of a variations in the reflection component. We compare our results with those
from a similar transition during the 2004 outburst of GX 339-4.Comment: 8 pages, 6 figures, accepted for publication in MNRAS Main Journa
Strong Aperiodic X-ray Variability and Quasi-Periodic Oscillation in X-ray Nova XTE J1550-564
We report the discovery of strong aperiodic X-ray variability and
quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray Nova,
XTE J1550-564, and the evolution of the observed temporal properties during the
rise of the recent X-ray outburst. The power spectral analysis of the first
observation reveals strong aperiodic X-ray variability of the source (~28%), as
well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14%
over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO
with the amplitude ~9%. As the X-ray flux increases, the source tends to become
less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz,
over the flux (2-50 keV) range of 1.73-5.75 x 10^{-8} ergs cm^{-2} s^{-1}. The
amplitude of the fundamental component of the QPO varies little, while that of
the harmonic follows a decreasing trend. The fundamental component strengthens
toward high energies, while its harmonic weakens. Initially, the power spectrum
is roughly flat at low frequencies and turns into a power law at high
frequencies, with the QPO harmonic sitting roughly at the break. In later
observations, however, the high-frequency portion of the continuum can actually
be better described by a broken power law (as opposed to a simple power law).
This effect becomes more apparent at higher energies. The overall amplitude of
the continuum shows a similar energy dependence to that of the fundamental
component of the QPO. Strong rapid X-ray variability, as well as hard energy
spectrum, makes XTE J1550-564 a good black hole candidate. We compare its
temporal properties with those of other black hole candidates.Comment: 12 pages, including 5 figures. To appear in ApJ Letters, vol. 512
(1999
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