338 research outputs found
The origin of seed photons for Comptonization in the black hole binary Swift J1753.5-0127
Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data set
to study accretion flows. Various investigations of this system and of other
black holes have not, however, led to an agreement on the accretion flow
geometry or on the seed photon source for Comptonization during different
stages of X-ray outbursts. We place constraints on these accretion flow
properties by studying long-term spectral variations of this source. Methods.
We performed phenomenological and self-consistent broad band spectral modeling
of Swift J1753.5-0127 using quasi-simultaneous archived data from
INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and MAXI/GSC instruments.
Results. We identify a critical flux limit, F \sim 1.5 \times 10^{-8}
erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are
markedly different above and below this value. Above the limit, during the
outburst peak, the hot medium seems to intercept roughly 50 percent of the disk
emission. Below it, in the outburst tail, the contribution of the disk photons
reduces significantly and the entire spectrum from the optical to X-rays can be
produced by a synchrotron-self-Compton mechanism. The long-term variations in
the hard X-ray spectra are caused by erratic changes of the electron
temperatures in the hot medium. Thermal Comptonization models indicate
unreasonably low hot medium optical depths during the short incursions into the
soft state after 2010, suggesting that non-thermal electrons produce the
Comptonized tail in this state. The soft X-ray excess, likely produced by the
accretion disk, shows peculiarly stable temperatures for over an order of
magnitude changes in flux. Conclusions. The long-term spectral trends of SWIFT
J1753.5-0127 are likely set by variations of the truncation radius and a
formation of a hot, quasi-spherical inner flow in the vicinity of the black
hole. (abridged)Comment: 16 pages, 8 figures, published in A&
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs)
in low-mass X-ray binary systems can be used to constrain NS masses and radii.
Most previous work of this type has set these constraints using Planck function
fits as a proxy: both the models and the data are fit with diluted blackbody
functions to yield normalizations and temperatures which are then compared
against each other. Here, for the first time, we fit atmosphere models of X-ray
bursting NSs directly to the observed spectra. We present a hierarchical
Bayesian fitting framework that uses state-of-the-art X-ray bursting NS
atmosphere models with realistic opacities and relativistic exact Compton
scattering kernels as a model for the surface emission. We test our approach
against synthetic data, and find that for data that are well-described by our
model we can obtain robust radius, mass, distance, and composition
measurements. We then apply our technique to Rossi X-ray Timing Explorer
observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to
all five bursts shows that the theoretical atmosphere models describe the data
well but there are still some unmodeled features in the spectrum corresponding
to a relative error of 1-5% of the energy flux. After marginalizing over this
intrinsic scatter, we find that at 68% credibility the circumferential radius
of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is
M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass
fraction is X < 0.09.Comment: 15 pages, 11 figures, submitted to A&
Diffractive shaping of excimer-laser beams for pulsed laser deposition
We present a beam-shaping system for a pulsed laser deposition setup. This system is based on two diffractive beam-splitter gratings and is able to produce 2*2-mm2 flat-top distributions of UV light with a fluence of 3 J/cm2 on the target some 30 cm behind the system. We have applied the setup to deposit ferromagnetic Ni-Mn-Ga films
The origin of seed photons for Comptonization in the black hole binary Swift J1753.5-0127
Aims. The black hole binary Swift J1753.5-0127 is providing a unique data set to study accretion flows. Various investigations of this system and of other black holes have not, however, led to an agreement on the accretion flow geometry or on the seed photon source for Comptonization during different stages of X-ray outbursts. We place constraints on these accretion flow properties by studying long-term spectral variations of this source.Methods. We performed phenomenological and self-consistent broad band spectral modeling of Swift J1753.5-0127 using quasi-simultaneous archived data from INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE, and MAXI/GSC instruments.Results. We identify a critical flux limit, F similar to 1.5 x 10(-8) erg cm(-2) s(-1), and show that the spectral properties of Swift J1753.5-0127 are markedly different above and below this value. Above the limit, during the outburst peak, the hot medium seems to intercept roughly 50 percent of the disk emission. Below it, in the outburst tail, the contribution of the disk photons reduces significantly and the entire spectrum from the optical to X-rays can be produced by a synchrotron-self-Compton mechanism. The long-term variations in the hard X-ray spectra are caused by erratic changes of the electron temperatures in the hot medium. Thermal Comptonization models indicate unreasonably low hot medium optical depths during the short incursions into the soft state after 2010, suggesting that non-thermal electrons produce the Comptonized tail in this state. The soft X-ray excess, likely produced by the accretion disk, shows peculiarly stable temperatures for over an order of magnitude changes in flux.Conclusions. The long-term spectral trends of Swift J1753.5-0127 are likely set by variations of the truncation radius and a formation of a hot, quasi-spherical inner flow in the vicinity of the black hole. In the late outburst stages, at fluxes below the critical limit, the source of seed photons for Comptonization is not the thermal disk, but more likely they are produced by non-thermal synchrotron emission within the hot flow near the black hole. The stability of the soft excess temperature is, however, not consistent with this picture and further investigations are needed to understand its behavior
Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070
Aims. The optical emission of black hole transients increases by several
magnitudes during the X-ray outbursts. Whether the extra light arises from the
X-ray heated outer disc, from the inner hot accretion flow, or from the jet is
currently debated. Optical polarisation measurements are able to distinguish
the relative contributions of these components. Methods. We present the results
of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070
during the period of March-April 2018. Results. We detect small, 0.7%,
but statistically significant polarisation, part of which is of interstellar
origin. Depending on the interstellar polarisation estimate, the intrinsic
polarisation degree of the source is between 0.3% and 0.7%, and the
polarisation position angle is between . We show that the
polarisation increases after MJD 58222 (2018 April 14). The change is of the
order of 0.1% and is most pronounced in the R band. The change of the source
Stokes parameters occurs simultaneously with the drop of the observed V-band
flux and a slow softening of the X-ray spectrum. The Stokes vectors of
intrinsic polarisation before and after the drop are parallel, at least in the
V and R filters. Conclusions. We suggest that the increased polarisation is due
to the decreasing contribution of the non-polarized component, which we
associate with the the hot flow or jet emission. The low polarisation can
result from the tangled geometry of the magnetic field or from the Faraday
rotation in the dense, ionised, and magnetised medium close to the black hole.
The polarized optical emission is likely produced by the irradiated disc or by
scattering of its radiation in the optically thin outflow.Comment: 11 pages, 12 figures, A&A in pres
The repetitive structure of DNA clamps: An overlooked protein tandem repeat
Structured tandem repeats proteins (STRPs) are a specific kind of tandem repeat proteins characterized by a modular and repetitive three-dimensional structure arrangement. The majority of STRPs adopt solenoid structures, but with the increasing availability of experimental structures and high-quality predicted structural models, more STRP folds can be characterized. Here, we describe “Box repeats”, an overlooked STRP fold present in the DNA sliding clamp processivity factors, which has eluded classification although structural data has been available since the late 1990s. Each Box repeat is a β⍺βββ module of about 60 residues, which forms a class V “beads-on-a-string” type STRP. The number of repeats present in processivity factors is organism dependent. Monomers of PCNA proteins in both Archaea and Eukarya have 4 repeats, while the monomers of bacterial beta-sliding clamps have 6 repeats. This new repeat fold has been added to the RepeatsDB database, which now provides structural annotation for 66 Box repeat proteins belonging to different organisms, including viruses
Shift of fibril-forming ability of the designed alpha-helical coiled-coil peptides into the physiological pH region
Recently, we designed a short alpha-helical fibril-forming peptide (alphaFFP) that can form alpha-helical nanofibrils at acid pH. The non-physiological conditions of the fibril formation hamper biomedical application of alphaFFP. It was hypothesized that electrostatic repulsion between glutamic acid residues present at positions (g) of the alphaFFP coiled-coil sequence prevent the fibrillogenesis at neutral pH, while their protonation below pH 5.5 triggers axial growth of the fibril. To test this hypothesis, we synthesized alphaFFPs where all glutamic acid residues were substituted by glutamines or serines. The electron microscopy study confirmed that the modified alphaFFPs form nanofibrils in a wider range of pH (2.5-11). Circular dichroism spectroscopy, sedimentation, diffusion and differential scanning calorimetry showed that the fibrils are alpha-helical and have elongated and highly stable cooperative tertiary structures. This work leads to a better understanding of interactions that control the fibrillogenesis of the alphaFFPs and opens opportunities for their biomedical application
Detection of burning ashes from thermonuclear X-ray bursts
When neutron stars (NS) accrete gas from low-mass binary companions, explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium into heavier elements. The resulting thermonuclear (type-I) X-ray bursts produce energy spectra that are fit well with black bodies, but a significant number of burst observations show deviations from Planck spectra. Here we present our analysis of RXTE/ PCA observations of X-ray bursts from the NS low-mass X-ray binary HETE J1900.1-2455. We have discovered that the non-Planckian spectra are caused by photoionization edges. The anticorrelation between the strength of the edges and the colour temperature suggests that the edges are produced by the nuclear burning ashes that have been transported upwards by convection and become exposed at the photosphere. The atmosphere model fits show that occasionally the photosphere can consist entirely of metals, and that the peculiar changes in blackbody temperature and radius can be attributed to the emergence and disappearance of metals in the photosphere. As the metals are detected already in the Eddington-limited phase, it is possible that a radiatively driven wind ejects some of the burning ashes into the interstellar space.</p
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
© ESO, 2017. Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs) in low-mass X-ray binary systems can be used to constrain NS masses and radii. Most previous work of this type has set these constraints using Planck function fits as a proxy: The models and the data are both fit with diluted blackbody functions to yield normalizations and temperatures that are then compared with each other. For the first time, we here fit atmosphere models of X-ray bursting NSs directly to the observed spectra. We present a hierarchical Bayesian fitting framework that uses current X-ray bursting NS atmosphere models with realistic opacities and relativistic exact Compton scattering kernels as a model for the surface emission. We test our approach against synthetic data and find that for data that are well described by our model, we can obtain robust radius, mass, distance, and composition measurements. We then apply our technique to Rossi X-ray Timing Explorer observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to all five bursts shows that the theoretical atmosphere models describe the data well, but there are still some unmodeled features in the spectrum corresponding to a relative error of 1-5% of the energy flux. After marginalizing over this intrinsic scatter, we find that at 68% credibility, the circumferential radius of the NS in 4U 1702-429 is R = 12.4±0.4 km, the gravitational mass is M = 1.9±0.3 M ⊙ , the distance is 5.1 < D/ kpc < 6.2, and the hydrogen mass fraction is X < 0.09
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