2,360 research outputs found
Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state
Hard X-ray spectra of black hole binaries in the low/hard state are well
modeled by thermal Comptonization of soft seed photons by a corona-type region
with \thinspace{\thinspace}keV and optical depth around 1.
Previous spectral studies of 1E{\thinspace}1740.72942, including both the
soft and the hard X-ray bands, were always limited by gaps in the spectra or by
a combination of observations with imaging and non-imaging instruments. In this
study, we have used three rare nearly-simultaneous observations of
1E{\thinspace}1740.71942 by both XMM-Newton and INTEGRAL satellites to
combine spectra from four different imaging instruments with no data gaps, and
we successfully applied the Comptonization scenario to explain the broadband
X-ray spectra of this source in the low/hard state. For two of the three
observations, our analysis also shows that, models including Compton reflection
can adequately fit the data, in agreement with previous reports. We show that
the observations can also be modeled by a more detailed Comptonization scheme.
Furthermore, we find the presence of an iron K-edge absorption feature in one
occasion, which confirms what had been previously observed by Suzaku. Our
broadband analysis of this limited sample shows a rich spectral variability in
1E{\thinspace}1740.72942 at the low/hard state, and we address the possible
causes of these variations. More simultaneous soft/hard X-ray observations of
this system and other black-hole binaries would be very helpful in constraining
the Comptonization scenario and shedding more light on the physics of these
systems.Comment: 6 pages, two figures, accepted for publication in A&
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
Two ~35 day clocks in Her X-1: evidence for neutron star free precession
We present evidence for the existence of two ~35 day clocks in the Her X-1/HZ
Her binary system. ~35 day modulations are observed 1) in the Turn-On cycles
with two on- and two off-states, and 2) in the changing shape of the pulse
profiles which re-appears regularly. The two ways of counting the 35 day cycles
are generally in synchronization. This synchronization did apparently break
down temporarily during the long Anomalous Low (AL3) which Her X-1 experienced
in 1999/2000, in the sense that there must have been one extra Turn-On cycle.
Our working hypothesis is that there are two clocks in the system, both with a
period of about ~35 days: precession of the accretion disk (the less stable
"Turn-On clock") and free precession of the neutron star (the more stable
"Pulse profile clock"). We suggest that free precession of the neutron star is
the master clock, and that the precession of the accretion disk is basically
synchronized to that of the neutron star through a feed-back mechanism in the
binary system. However, the Turn-On clock can slip against its master when the
accretion disk has a very low inclination, as is observed to be the case during
AL3. We take the apparent correlation between the histories of the Turn-Ons, of
the Anomalous Lows and of the pulse period evolution, with a 5 yr
quasi-periodicity, as evidence for strong physical interaction and feed-back
between the major components in the system. We speculate that the 5 yr (10 yr)
period is either due to a corresponding activity cycle of HZ Her or a natural
ringing period of the physical system of coupled components. The question
whether free precession really exists in neutron stars is of great importance
for the understanding of matter with supra-nuclear density.Comment: 6 pages, 5 figures, accepted for publication by A&
Swift/BAT measurements of the cyclotron line energy decay in the accreting neutron star Her X-1: indication of an evolution of the magnetic field?
Context: The magnetic field is a crucial ingredient of neutron stars. It
governs the physics of accretion and of the resulting high-energy emission in
accreting pulsars. Studies of the cyclotron resonant scattering features
(CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit
direct measuremets of the field strength. Aims: From an analysis of a number of
pointed observations with different instruments, the energy of CRSF, Ecyc, has
recently been found to decay in Her X-1, which is one of the best-studied
accreting pulsars. We present our analysis of a homogeneous and almost
uninterrupted monitoring of the line energy with Swift/BAT. Methods: We
analyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. The
data were used to measure the CRSF energy averaged over several months.
Results: The analysis confirms the long-term decay of the line energy. The
downward trend is highly significant and consistent with the trend measured
with the pointed observations: dEcyc/dt ~-0.3 keV per year. Conclusions: The
decay of Ecyc either indicates a local evolution of the magnetic field
structure in the polar regions of the neutron star or a geometrical
displacement of the line-forming region due to long-term changes in the
structure of the X-ray emitting region. The shortness of the observed timescale
of the decay, -Ecyc/(dEcyc/dt) ~ 100 yr, suggests that trend reversals and/or
jumps of the line energy might be observed in the future.Comment: Accepted for publication in Astronomy&Astrophysic
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
Different kinds of long-term variability from Cygnus X-1
We present a study of the long-term variability of Cyg X-1 using data from
the RXTE/ASM and the RXTE/PCA during the time between the two soft states of
1996 and 2001/2002. This period has been characterized by many short ASM
flaring episodes which we have identified as "failed state transitions". The
150 d period which has been seen before and shortly after the 1996 soft state
is not obviously present in the ASM rate during most of this time. Applying
selection criteria from our pointed RXTE/PCA observations to exclude the
flaring episodes we show that the 150 d period can indeed still be
significantly detected in the hard state. Furthermore, while the ~420 d
timescale associated with the flaring is reduced in the selected hard state
count rate, it is still pronounced in the temporal evolution of the
corresponding hardness ratios. The Ryle radio flux is also consistent with the
150 d period being present but distorted during this time.Comment: 4 pages, 6 figures, to appear in Proceedings of "X-ray Timing 2003:
Rossi and Beyond", ed. P. Kaaret, F.K. Lamb, & J.H. Swan
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
Variable pulse profiles of Her X-1 repeating with the same irregular 35d clock as the turn-ons
The accreting X-ray pulsar Her X-1 shows two types of long-term variations,
both with periods of ~35 days: 1) Turn-on cycles, a modulation of the flux},
with a ten-day long Main-On and a five-day long Short-On, separated by two
Off-states, and 2) a systematic variation in the shape of the 1.24 s pulse
profile. While there is general consensus that the flux modulation is due to
variable shading of the X-ray emitting regions on the surface of the neutron
star by the precessing accretion disk, the physical reason for the variation in
the pulse profiles has remained controversial. Following the suggestion that
free precession of the neutron star may be responsible for the variation in the
pulse profiles, we developed a physical model of strong feedback interaction
between the neutron star and the accretion disk in order to explain the
seemingly identical values for the periods of the two types of variations,
which were found to be in basic synchronization. In a deep analysis of pulse
profiles observed by several different satellites over the last three decades
we now find that the clock behind the pulse profile variations shows exactly
the same erratic behavior as the turn-on clock, even on short time scales (a
few 35 d cycles), suggesting that there may in fact be only one 35 d clock in
the system. If this is true, it raises serious questions with respect to the
idea of free precession of the neutron star, namely how the neutron star can
change its precessional period every few years by up to 2.5% and how the
feedback can be so strong, such that these changes can be transmitted to the
accretion disk on rather short time scales.Comment: 9 pages, 13 figures, accepted by Astronomy & Astrophysics. arXiv
admin note: substantial text overlap with arXiv:1110.671
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