1,110 research outputs found
Joint spectral-timing modelling of the hard lags in GX 339-4: constraints on reflection models
The X-ray variations of hard state black hole X-ray binaries above 2 keV show
'hard lags', in that the variations at harder energies follow variations at
softer energies, with a time-lag \tau depending on frequency \nu approximately
as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain
this time delay, including fluctuations propagating through an accretion flow,
spectral variations during coronal flares, Comptonisation in the extended hot
corona or a jet, or time-delays due to large-scale reflection from the
accretion disc. In principle these models can be used to predict the shape of
the energy spectrum as well as the frequency-dependence of the time-lags,
through the construction of energy-dependent response functions which map the
emission as a function of time-delay in the system. Here we use this approach
to test a simple reflection model for the frequency-dependent lags seen in the
hard state of GX 339-4, by simultaneously fitting the model to the
frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and
2009. Our model cannot simultaneously fit both the lag and spectral data, since
the relatively large lags require an extremely flared disc which subtends a
large solid angle to the continuum at large radii, in disagreement with the
observed Fe K\alpha emission. Therefore, we consider it more likely that the
lags > 2 keV are caused by propagation effects in the accretion flow, possibly
related to the accretion disc fluctuations which have been observed previously.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
A Complexity-Brightness Correlation in Gamma Ray Bursts
We observe strong correlations between the temporal properties of gamma ray
bursts (GRBs) and their apparent peak brightness. The strongest effect (with a
significance level of 10^{-6}) is the difference between the brightness
distributions of simple bursts (dominated by a single smooth pulse) and complex
bursts (consisting of overlapping pulses). The latter has a break at a peak
flux of 1.5 ph/cm^2/s, while the distribution of simple bursts is smooth down
to the BATSE threshold. We also observe brightness dependent variations in the
shape of the average peak aligned time profile (ATP) of GRBs. The decaying
slope of the ATP shows time dilation when comparing bright and dim bursts while
the rising slope hardly changes. Both slopes of the ATP are deformed for weak
bursts as compared to strong bursts. The interpretation of these effects is
simple: a complex burst where a number of independent pulses overlap in time
appears intrinsically stronger than a simple burst. Then the BATSE sample of
complex bursts covers larger redshifts where some cosmological factor causes
the break in the peak brightness distribution. This break could correspond to
the peak in the star formation rate that was recently shown to occur at a
redshift of z~1.5.Comment: 13 pages; 11 figures; replaced with the published versio
Radiation mechanisms and geometry of Cygnus X-1 in the soft state
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition
from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in
1996 May and June. The spectra consist of a dominant soft component below ~2
keV and a power-law-like continuum extending to at least ~800 keV. We interpret
them as emission from an optically-thick, cold accretion disc and from an
optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total
available power is dissipated in the corona. We model the soft component by
multi-colour blackbody disc emission taking into account the torque-free
inner-boundary condition. If the disc extends down to the minimum stable orbit,
the ASCA/RXTE data yield the most probable black hole mass of about 10 solar
masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft
state in the upper part of the stable, gas-pressure dominated, accretion-disc
solution branch. The spectrum of the corona is well modelled by repeated
Compton scattering of seed photons from the disc off electrons with a hybrid,
thermal/non-thermal distribution. The electron distribution can be
characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50
keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The
compactness of the corona is between 2 and 7, and a presence of a significant
population of electron-positron pairs is ruled out. We find strong signatures
of Compton reflection from a cold and ionized medium, presumably an accretion
disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum
is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted
to MNRA
Searching for X-ray sources in nearby late-type galaxies with low star formation rates
Late type non-starburst galaxies have been shown to contain X-ray emitting
objects, some being ultraluminous X-ray sources. We report on XMM-Newton
observations of 11 nearby, late-type galaxies previously observed with the
Hubble Space Telescope (HST) in order to find such objects. We found 18 X-ray
sources in or near the optical extent of the galaxies, most being point-like.
If associated with the corresponding galaxies, the source luminosities range
from erg s to erg s. We
found one ultraluminous X-ray source, which is in the galaxy IC 5052, and one
source coincident with the galaxy IC 4662 with a blackbody temperature of
keV that could be a quasi-soft source or a quiescent neutron
star X-ray binary in the Milky Way. One X-ray source, XMMU J205206.0691316,
is extended and coincident with a galaxy cluster visible on an HST image. The
X-ray spectrum of the cluster reveals a redshift of and a
temperature of 3.60.4 keV. The redshift was mainly determined by a cluster
of Fe XXIV lines between the observed energy range keV.Comment: 8 pages, to appear in MNRA
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
Efficient light-induced phase transitions in halogen-bonded liquid crystals
Here, we present a new family of light-responsive, fluorinated supramolecular liquid crystals (LCs) showing efficient and reversible light-induced LC-to-isotropic phase transitions. Our materials design is based on fluorinated azobenzenes, where the fluorination serves to strengthen the noncovalent interaction with bond-accepting stilbazole molecules, and increase the lifetime of the cis-form of the azobenzene units. The halogen-bonded LCs were characterized by means of X-ray diffraction, hot-stage polarized optical microscopy, and differential scanning calorimetry. Simultaneous analysis of light-induced changes in birefringence, absorption, and optical scattering allowed us to estimate that <4% of the mesogenic units in the cis-form suffices to trigger the full LC-to-isotropic phase transition. We also report a light-induced and reversible crystal-to-isotropic phase transition, which has not been previously observed in supramolecular complexes. In addition to fundamental understanding of light-responsive supramolecular complexes, we foresee this study to be important in the development of bistable photonic devices and supramolecular actuators
Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing
We describe the evolution of optical and X-ray temporal characteristics
during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127.
The optical/X-ray cross-correlation function demonstrates a single positive
correlation at the outburst peak, then it has multiple dips and peaks during
the decline stage, which are then replaced by the precognition dip plus peak
structure in the outburst tail. Power spectral densities and phase lags show a
complex evolution, revealing the presence of intrinsically connected optical
and X-ray quasi-periodic oscillations. For the first time, we quantitatively
explain the evolution of these timing properties during the entire outburst
within one model, the essence of which is the expansion of the hot accretion
flow towards the tail of the outburst. The pivoting of the spectrum produced by
synchrotron Comptonization in the hot flow is responsible for the appearance of
the anti-correlation with the X-rays and for the optical quasi-periodic
oscillations. Our model reproduces well the cross-correlation and phase lag
spectrum during the decline stage, which could not be understood with any model
proposed before.Comment: 13 pages, 11 figures, MNRAS submitte
INTEGRAL and RXTE observations of accreting millisecond pulsar IGR J00291+5934 in outburst
Simultaneous observations of the accretion-powered millisecond pulsar IGR
J00291+5934 by International Gamma-Ray Astrophysics Laboratory and Rossi X-ray
Timing Explorer during the 2004 December outburst are analysed. The average
spectrum is well described by thermal Comptonization with an electron
temperature of 50 keV and Thomson optical depth tau_T ~ 1 in a slab geometry.
The spectral shape is almost constant during the outburst. We detect a spin-up
of the pulsar with nudot=8.4x10E-13 Hz/s. The ISGRI data reveal the pulsation
of X-rays at a period of 1.67 milliseconds up to ~150 keV. The pulsed fraction
is shown to increase from 6 per cent at 6 keV to 12--20 per cent at 100 keV.
This is naturally explained by the action of the Doppler effect the
exponentially cutoff Comptonization spectrum from the hot spot. The nearly
sinusoidal pulses show soft lags with complex energy dependence, increasing up
to 7 keV, then decreasing to 15 keV, and seemingly saturating at higher
energies.Comment: 11 pages, 8 figures, 3 tables, accepted for publication on A&
Observations of Seyferts by OSSE and parameters of their X-ray/gamma-ray sources
We present a summary of spectra of Seyfert galaxies observed by the OSSE
detector aboard Compton Gamma Ray Observatory. We obtain average spectra of
Seyferts of type 1 and 2, and find they are well fitted by thermal
Comptonization. We present detailed parameter ranges for the plasma temperature
and the Compton parameter in the case of spherical and slab geometries. We find
both the average and individual OSSE spectra of Seyfert 2s are significantly
harder than those of Seyfert 1s, which difference can be due to anisotropy of
Compton reflection and/or Thomson-thick absorption.Comment: ApJ, 10 Nov. 2000, in press, 13 page
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