2,080 research outputs found
Langevin Dynamics simulations of a 2-dimensional colloidal crystal under confinement and shear
Langevin Dynamics simulations are used to study the effect of shear on a
two-dimensional colloidal crystal confined by structured parallel walls. When
walls are sheared very slowly, only two or three crystalline layers next to the
walls move along with them, while the inner layers of the crystal are only
slightly tilted. At higher shear velocities, this inner part of the crystal
breaks into several pieces with different orientations. The velocity profile
across the slit is reminiscent of shear-banding in flowing soft materials,
where liquid and solid regions coexist; the difference, however, is that in the
latter case the solid regions are glassy while here they are crystalline. At
even higher shear velocities, the effect of the shearing becomes smaller again.
Also the effective temperature near the walls (deduced from the velocity
distributions of the particles) decreases again when the wall velocity gets
very large. When the walls are placed closer together, thereby introducing a
misfit, a structure containing a soliton staircase arises in simulations
without shear. Introducing shear increases the disorder in these systems until
no solitons are visible any more. Instead, similar structures like in the case
without misfit result. At high shear rates, configurations where the
incommensurability of the crystalline structure is compensated by the creation
of holes become relevant
Short term X-ray rms variability of Cyg X-1
A linear dependence of the amplitude of broadband noise variability on flux
for GBHC and AGN has been recently shown by Uttley & McHardy (2001). We present
the long term evolution of this rms-flux-relation for Cyg X-1 as monitored from
1998-2002 with RXTE. We confirm the linear relationship in the hard state and
analyze the evolution of the correlation for the period of 1996-2002. In the
intermediate and the soft state, we find considerable deviations from the
otherwise linear relationship. A possible explanation for the rms-flux-relation
is a superposition of local mass accretion rate variations.Comment: 3 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1
Because of their inherently high flux allowing the detection of clear
signals, black hole X-ray binaries are interesting candidates for polarization
studies, even if no polarization signals have been observed from them before.
Such measurements would provide further detailed insight into these sources'
emission mechanisms. We measured the polarization of the gamma-ray emission
from the black hole binary system Cygnus X-1 with the INTEGRAL/IBIS telescope.
Spectral modeling of the data reveals two emission mechanisms: The 250-400 keV
data are consistent with emission dominated by Compton scattering on thermal
electrons and are weakly polarized. The second spectral component seen in the
400keV-2MeV band is by contrast strongly polarized, revealing that the MeV
emission is probably related to the jet first detected in the radio band.Comment: 11 pages, 3 figures, to be published in Science in April 22nd 2011,
available on Science Express Web site (March 24th edition
New insights into ultraluminous X-ray sources from deep XMM-Newton observations
The controversy over whether ultraluminous X-ray sources (ULXs) contain a new
intermediate-mass class of black holes (IMBHs) remains unresolved. We present
new analyses of the deepest XMM-Newton observations of ULXs that address their
underlying nature. We examine both empirical and physical modelling of the
X-ray spectra of a sample of thirteen of the highest quality ULX datasets, and
find that there are anomalies in modelling ULXs as accreting IMBHs with
properties simply scaled-up from Galactic black holes. Most notably, spectral
curvature above 2 keV in several sources implies the presence of an
optically-thick, cool corona. We also present a new analysis of a 100 ks
observation of Holmberg II X-1, in which a rigorous analysis of the temporal
data limits the mass of its black hole to no more than 100 solar masses. We
argue that a combination of these results points towards many (though not
necessarily all) ULXs containing black holes that are at most a few 10s of
solar mass in size.Comment: 5 pages, 2 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934
We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a twoweek follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of
the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA = 00h 29m 03.08s, and Dec =+59â—¦ 34 19.2 (0.6 error), compatible with the optical and radio ones.
We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (∼1 keV, interpreted as a hot spot on
the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ∼1 keV thermal component, while we detected a colder thermal component (∼0.4 keV) that we interpret as the
emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would
suggest the presence of an expanding hot corona with high outflow velocities
Variability in high-mass X-ray binaries
Strongly magnetized, accreting neutron stars show periodic and aperiodic
variability over a wide range of time scales. By obtaining spectral and timing
information on these different time scales, we can have a closer look into the
physics of accretion close to the neutron star and the properties of the
accreted material. One of the most prominent time scales is the strong
pulsation, i.e., the rotation period of the neutron star itself. Over one
rotation, our view of the accretion column and the X-ray producing region
changes significantly. This allows us to sample different physical conditions
within the column but at the same time requires that we have
viewing-angle-resolved models to properly describe them. In wind-fed high-mass
X-ray binaries, the main source of aperiodic variability is the clumpy stellar
wind, which leads to changes in the accretion rate (i.e., luminosity) as well
as absorption column. This variability allows us to study the behavior of the
accretion column as a function of luminosity, as well as to investigate the
structure and physical properties of the wind, which we can compare to winds in
isolated stars.Comment: 6 pages, 4 figures, accepted for publication in Astronomische
Nachrichten (proceedings of the XMM-Newton Workshop 2019
Spectral state dependence of the 0.4-2 MeV polarized emission in Cygnus X-1 seen with INTEGRAL/IBIS, and links with the AMI radio data
Polarization of the >~400 keV hard tail of the microquasar Cygnus X-1 has
been independently reported by INTEGRAL/IBIS, and INTEGRAL/SPI and interpreted
as emission from a compact jet. These conclusions were, however, based on the
accumulation of all INTEGRAL data regardless of the spectral state. We utilize
additional INTEGRAL exposure accumulated until December 2012, and include the
AMI/Ryle (15 GHz) radio data in our study. We separate the observations into
hard, soft, and intermediate/transitional states and detect radio emission from
a compact jet in hard and intermediate states, but not in the soft. The 10-400
keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with
thermal Comptonization and reflection components. We detect a hard tail in the
0.4-2 MeV range for the hard state only. We extract the state dependent
polarigrams of Cyg X-1, which all are compatible to no or undetectable level of
polarization except in 400-2000 keV range in the hard state where the
polarization fraction is 7532 % and the polarization angle 40.0 +-14 deg.
An upper limit on the 0.4-2 MeV soft state polarization fraction is 70%. Due to
the short exposure, we obtain no meaningful constraint for the intermediate
state. The likely detection of a >400 keV polarized tail in the hard state,
together with the simultaneous presence of a radio jet, reinforce the notion of
a compact jet origin of the 400 keV emission.Comment: 13 pages, 5 figures, accepted for publication in Ap
Short-period X-ray oscillations in super-soft novae and persistent SSS
Transient short-period <100s oscillations have been found in the X-ray light
curves of three novae during their SSS phase and in one persistent SSS. We
pursue an observational approach to determine possible driving mechanisms and
relations to fundamental system parameters such as the white dwarf mass.
We performed a systematic search for short-period oscillations in all
available XMM-Newton and Chandra X-ray light curves of persistent SSS and novae
during their SSS phase. To study time evolution, we divided each light curve
into short time segments and computed power spectra. We then constructed
dynamic power spectra from which we identified transient periodic signals even
when only present for a short time. From all time segments of each system, we
computed fractions of time when periodic signals were detected.
In addition to the previously known systems with short-period oscillations,
RS Oph (35s), KT Eri (35s), V339 Del (54s), and Cal 83 (67s), we found one
additional system, LMC 2009a (33s), and also confirm the 35s period from
Chandra data of KT Eri. The amplitudes of oscillations are of order <15% of the
respective count rates and vary without any clear dependence on the X-ray count
rate. The fractions of the time when the respective periods were detected at
2-sigma significance (duty cycle) are 11.3%, 38.8%, 16.9%, 49.2%, and 18.7% for
LMC 2009a, RS Oph, KT Eri, V339 Del, and Cal 83, respectively. The respective
highest duty cycles found in a single observation are 38.1%, 74.5%, 61.4%,
67.8%, and 61.8%.Comment: accepted for publication in A&
Unveiling the nature of IGR J17177-3656 with X-ray, NIR and Radio observations
We report on the first broad-band (1-200 keV) simultaneous Chandra-INTEGRAL
observations of the recently discovered hard X-ray transient IGR J17177-3656
that took place on 2011, March 22, about two weeks after the source discovery.
The source had an average absorbed 1-200 keV flux of about 8x10^(-10) erg
cm^(-2) s^(-1). We extracted a precise X-ray position of IGR J17177-3656, RA=17
17 42.62, DEC= -36 56 04.5 (90% uncertainty of 0.6"). We also report Swift,
near infrared and quasi simultaneous radio follow-up observations. With the
multi-wavelength information at hand, we propose IGR J17177-3656 is a low-mass
X-ray binary, seen at high inclination, probably hosting a black hole.Comment: 8 pages, 8 figures, accepted for publication in Ap
A Chandra observation of the millisecond X-ray pulsar IGR J17511-3057
IGR J17511-3057 is a low mass X-ray binary hosting a neutron star and is one
of the few accreting millisecond X-ray pulsars with X-ray bursts. We report on
a 20ksec Chandra grating observation of IGR J17511-3057, performed on 2009
September 22. We determine the most accurate X-ray position of IGR J17511-3057,
alpha(J2000) = 17h 51m 08.66s, delta(J2000) = -30deg 57' 41.0" (90% uncertainty
of 0.6"). During the observation, a ~54sec long type-I X-ray burst is detected.
The persistent (non-burst) emission has an absorbed 0.5-8keV luminosity of 1.7
x 10^36 erg/sec (at 6.9kpc) and can be well described by a thermal
Comptonization model of soft, ~0.6keV, seed photons up-scattered by a hot
corona. The type-I X-ray burst spectrum, with average luminosity over the 54sec
duration L(0.5-8keV)=1.6 x 10^37 erg/sec, can be well described by a blackbody
with kT_(bb)~1.6keV and R_(bb)~5km. While an evolution in temperature of the
blackbody can be appreciated throughout the burst (average peak
kT_(bb)=2.5(+0.8/-0.4)keV to tail kT_(bb)=1.3(+0.2/-0.1)keV), the relative
emitting surface shows no evolution. The overall persistent and type-I burst
properties observed during the Chandra observation are consistent with what was
previously reported during the 2009 outburst of IGR J17511-3057.Comment: 6 pages, 4 figures, accepted for publication in ApJ (2012-06-08
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