24 research outputs found
Multi-Overlap Simulations for Transitions between Reference Configurations
We introduce a new procedure to construct weight factors, which flatten the
probability density of the overlap with respect to some pre-defined reference
configuration. This allows one to overcome free energy barriers in the overlap
variable. Subsequently, we generalize the approach to deal with the overlaps
with respect to two reference configurations so that transitions between them
are induced. We illustrate our approach by simulations of the brainpeptide
Met-enkephalin with the ECEPP/2 energy function using the global-energy-minimum
and the second lowest-energy states as reference configurations. The free
energy is obtained as functions of the dihedral and the root-mean-square
distances from these two configurations. The latter allows one to identify the
transition state and to estimate its associated free energy barrier.Comment: 12 pages, (RevTeX), 14 figures, Phys. Rev. E, submitte
Metropolis simulations of Met-Enkephalin with solvent-accessible area parameterizations
We investigate the solvent-accessible area method by means of Metropolis
simulations of the brain peptide Met-Enkephalin at 300. For the energy
function ECEPP/2 nine atomic solvation parameter (ASP) sets are studied. The
simulations are compared with one another, with simulations with a distance
dependent electrostatic permittivity , and with vacuum
simulations (). Parallel tempering and the biased Metropolis
techniques RM are employed and their performance is evaluated. The measured
observables include energy and dihedral probability densities (pds), integrated
autocorrelation times, and acceptance rates. Two of the ASP sets turn out to be
unsuitable for these simulations. For all other systems selected configurations
are minimized in search of the global energy minima, which are found for the
vacuum and the system, but for none of the ASP models. Other
observables show a remarkable dependence on the ASPs. In particular, we find
three ASP sets for which the autocorrelations at 300K are considerably
smaller than for vacuum simulations.Comment: 10 pages and 8 figure
SN 1987A : Tracing the flux decline and spectral evolution through a comparison of SRG/eROSITA and XMM-Newton observations
SN 1987A is the closest observed supernova in the last four centuries and provides a unique opportunity to witness the birth and evolution of a supernova remnant. The source has been monitored by XMM Newton EPIC-pn from 2007--2020. SRG/eROSITA also observed the source during its commissioning phase and the first light in Sept. and Oct. 2019. We investigated the spectral and flux evolution of SN 1987A in X-rays over the last fourteen years up to Nov. 2020 using XMM-Newton and eROSITA observations. We performed a detailed spectral analysis using a three-component plane-parallel shock model and analysed and modelled the EPIC-pn monitoring and eROSITA observations in a consistent manner. This paper reports a complete and most up to date flux evolution of SN 1987A in the soft (0.5-2 keV) and hard (3-10 keV) X-ray band. The flux in the soft band flattened around 9424 d and then displayed a turnover between 10142-10493 d after which it showed a continued decline. Around the same time, a break in the hard-band flux time evolution slope was detected. This implies that the blast wave has now passed beyond the dense structures of the equatorial ring and is expanding further into more tenuous circumstellar medium. The temporal evolution of the normalizations of the three shock components match well the results of hydrodynamical simulations predicting a blue supergiant progenitor scenario. The trend at recent epochs indicate that the emission caused by the forward shock after leaving the equatorial ring and by the reverse shock in the ejecta is becoming more dominant now. The elemental abundances in the hot plasma component are significantly higher than those in the `cooler' one, indicating its origin from the reverse shock propagating into the ejecta
Highlights in X-ray astronomy Symposium proceedings
The following topics were dealt with: X-ray binaries, pulsars, neutron stars, stellar magnetic fields, cyclotron lines, luminous supersoft X- ray sources, microquasars, quasars, accretion disks, Galactic Centre molecular clouds, X-ray spectrometers, X-ray telescopes, black holes, AGN, ULIRGs, warm absorbers, ultrasoft narrow line Seyferts, BL Lac objects, radio galaxies, merging galaxies, interacting galaxies, blue compact dwarf galaxies, surveys, luminosity functions, large scale structures, galaxy clusters, cosmology, X-ray background, LMC, SMC, high-velocity clouds, supernovae, supernova remnants, gamma-ray burst afterglow, stellar coronae, T Tauri stars, extremely young stellar objects, stellar clusters, cataclysmic variables, magnetic stars, cometsSIGLEAvailable from TIB Hannover: RA 234:ET(272) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman
XMM-Newton observation of 4U 1820-30: Broad band spectrum and the contribution of the cold interstellar medium
We present the analysis of the bright X-ray binary 4U 1820-30, based mainly
on XMM-Newton-RGS data, but using complementary data from XMM-Epic, Integral,
and Chandra-HETG, to investigate different aspects of the source. The broad
band continuum is well fitted by a classical combination of black body and
Comptonized emission. The continuum shape and the high flux of the source
(L/L_Edd\sim0.16) are consistent with a "high state" of the source. We do not
find significant evidence of iron emission at energies >=6.4 keV. The soft
X-ray spectrum contain a number of absorption features. Here we focus on the
cold-mildly ionized gas. The neutral gas column density is N_H\sim1.63x10^21
cm^-2. The detailed study of the oxygen and iron edge reveals that those
elements are depleted, defined here as the ratio between dust and the total ISM
cold phase, by a factor 0.20\pm0.02 and 0.87\pm0.14, respectively. Using the
available dust models, the best fit points to a major contribution of Mg-rich
silicates, with metallic iron inclusion. Although we find that a large fraction
of Fe is in dust form, the fit shows that Fe-rich silicates are disfavored. The
measured Mg:Fe ratio is 2.0\pm0.3. Interestingly, this modeling may point to a
well studied dust constituent (GEMS), sometimes proposed as a silicate
constituent in our Galaxy. Oxygen and iron are found to be slightly over- and
under-abundant, respectively (1.23 and 0.85 times the solar value) along this
line of sight. We also report the detection of two absorption lines,
tentatively identified as part of an outflow of mildly ionized gas
(\xi\sim-0.5) at a velocity of \sim1200 km/s.Comment: 13 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
The XMM-Newton serendipitous survey. X: The second source catalogue from overlapping XMM-Newton observations and its long-term variable content
International audienceContext. The XMM-Newton Survey Science Centre Consortium (SSC) develops software in close collaboration with the Science Operations Centre to perform a pipeline analysis of all XMM-Newton observations. In celebration of the twentieth anniversary of the XMM-Newton launch, the SSC has compiled the fourth generation of serendipitous source catalogues, 4XMM.Aims. The catalogue described here, 4XMM-DR9s, explores sky areas that were observed more than once by XMM-Newton. These observations are bundled in groups referred to as stacks. Stacking leads to a higher sensitivity, resulting in newly discovered sources and better constrained source parameters, and unveils long-term brightness variations.Methods. The 4XMM-DR9s catalogue was constructed from simultaneous source detection on overlapping observations. As a novel feature, positional rectification was applied beforehand. Observations with all filters and suitable camera settings were included. Exposures with a high background were discarded. The high-background thresholds were determined through a statistical analysis of all exposures in each instrument configuration. The X-ray background maps used in source detection were modelled via an adaptive smoothing procedure with newly determined parameters. Source fluxes were derived for all contributing observations, irrespective of whether the source would be detectable in an individual observation.Results. The new catalogue lists the X-ray sources detected in 1329 stacks with 6604 contributing observations over repeatedly covered 300 square degrees in the sky. Most stacks are composed of two observations, the largest one comprises 352 observations. We find 288 191 sources of which 218 283 were observed several times. The number of observations of a source ranges from 1 to 40. Auxiliary products, like X-ray full-band and false-colour images, long-term X-ray light curves, and optical finding charts, are published as well.Conclusions. 4XMM-DR9s contains new detections and is considered a prime resource to explore long-term variability of X-ray sources discovered by XMM-Newton. Regular incremental releases, including new public observations, are planned.Key words: catalogs / astronomical databases: miscellaneous / surveys / X-rays: general⋆ The catalogue is available in FITS format via the SSC web pages at http://xmmssc.irap.omp.eu and https://xmmssc.aip.de and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/641/A137, XSA https://www.cosmos.esa.int/web/xmm-newton/xsa, and HEASARC https://heasarc.gsfc.nasa.gov/W3Browse/xmm-newton/xmmstack.html data services.⋆⋆ Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA