35 research outputs found
Dust absorption and scattering in the silicon K-edge
The composition and properties of interstellar silicate dust are not well
understood. In X-rays, interstellar dust can be studied in detail by making use
of the fine structure features in the Si K-edge. The features in the Si K-edge
offer a range of possibilities to study silicon-bearing dust, such as
investigating the crystallinity, abundance, and the chemical composition along
a given line of sight. We present newly acquired laboratory measurements of the
silicon K-edge of several silicate-compounds that complement our measurements
from our earlier pilot study. The resulting dust extinction profiles serve as
templates for the interstellar extinction that we observe. The extinction
profiles were used to model the interstellar dust in the dense environments of
the Galaxy. The laboratory measurements, taken at the Soleil synchrotron
facility in Paris, were adapted for astrophysical data analysis and implemented
in the SPEX spectral fitting program. The models were used to fit the spectra
of nine low-mass X-ray binaries located in the Galactic center neighborhood in
order to determine the dust properties along those lines of sight. Most lines
of sight can be fit well by amorphous olivine. We also established upper limits
on the amount of crystalline material that the modeling allows. We obtained
values of the total silicon abundance, silicon dust abundance, and depletion
along each of the sightlines. We find a possible gradient of
dex/kpc for the total silicon abundance versus the Galactocentric distance. We
do not find a relation between the depletion and the extinction along the line
of sight.Comment: 18 pages, 16 figures. Accepted for publication in Astronomy and
Astrophysic
Investigating the interstellar dust through the Fe K-edge
The chemical and physical properties of interstellar dust in the densest
regions of the Galaxy are still not well understood. X-rays provide a powerful
probe since they can penetrate gas and dust over a wide range of column
densities (up to ). The interaction (scattering and
absorption) with the medium imprints spectral signatures that reflect the
individual atoms which constitute the gas, molecule, or solid. In this work we
investigate the ability of high resolution X-ray spectroscopy to probe the
properties of cosmic grains containing iron. Although iron is heavily depleted
into interstellar dust, the nature of the Fe-bearing grains is still largely
uncertain. In our analysis we use iron K-edge synchrotron data of minerals
likely present in the ISM dust taken at the European Synchrotron Radiation
Facility. We explore the prospects of determining the chemical composition and
the size of astrophysical dust in the Galactic centre and in molecular clouds
with future X-ray missions. The energy resolution and the effective area of the
present X-ray telescopes are not sufficient to detect and study the Fe K-edge,
even for bright X-ray sources. From the analysis of the extinction cross
sections of our dust models implemented in the spectral fitting program SPEX,
the Fe K-edge is promising for investigating both the chemistry and the size
distribution of the interstellar dust. We find that the chemical composition
regulates the X-ray absorption fine structures in the post edge region, whereas
the scattering feature in the pre-edge is sensitive to the mean grain size.
Finally, we note that the Fe K-edge is insensitive to other dust properties,
such as the porosity and the geometry of the dust.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Optical and X-ray Rest-frame Light Curves of the BAT6 sample
We present the rest-frame light curves in the optical and X-ray bands of an
unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the
BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest
level of completeness in redshift ( 95%), allowing us to compute the
rest-frame X-ray and optical light curves for 55 and 47 objects, respectively.
We compute the X-ray and optical luminosities accounting for any possible
source of absorption (Galactic and intrinsic) that could affect the observed
fluxes in these two bands. We compare the behaviour observed in the X-ray and
in the optical bands to assess the relative contribution of the emission during
the prompt and afterglow phases. We unarguably demonstrate that the GRBs
rest-frame optical luminosity distribution is not bimodal, being rather
clustered around the mean value Log(L) = 29.9 0.8 when estimated at
a rest frame time of 12 hr. This is in contrast with what found in previous
works and confirms that the GRB population has an intrinsic unimodal luminosity
distribution. For more than 70% of the events the rest-frame light curves in
the X-ray and optical bands have a different evolution, indicating distinct
emitting regions and/or mechanisms. The X-ray light curves normalised to the
GRB isotropic energy (E), provide evidence for X-ray emission still
powered by the prompt emission until late times ( hours after the burst
event). On the other hand, the same test performed for the E-normalised optical light curves shows that the optical emission is a
better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table
X-ray extinction from interstellar dust: Prospects of observing carbon, sulfur and other trace elements
We present a study on the prospects of observing carbon, sulfur, and other
lower abundance elements (namely Al, Ca, Ti and Ni) present in the interstellar
medium using future X-ray instruments. We focus in particular on the detection
and characterization of interstellar dust along the lines of sight. We compare
the simulated data with different sets of dust aggregates, either obtained from
past literature or measured by us using the SOLEIL-LUCIA synchrotron beamline.
Extinction by interstellar grains induces modulations of a given photolelectric
edge, which can be in principle traced back to the chemistry of the absorbing
grains. We simulated data of instruments with characteristics of resolution and
sensitivity of the current Athena, XRISM and Arcus concepts. In the relatively
near future, the depletion and abundances of the elements under study will be
determined with confidence. In the case of carbon and sulfur, the
characterization of the chemistry of the absorbing dust will be also
determined, depending on the dominant compound. For aluminum and calcium,
despite the large depletion in the interstellar medium and the prominent dust
absorption, in many cases the edge feature may not be changing significantly
with the change of chemistry in the Al or Ca bearing compounds. The
exinction signature of large grains may be detected and modeled, allowing a
test on different grain size distributions for these elements. The low cosmic
abundance of Ti and Ni will not allow us a detailed study of the edge features.Comment: 12 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
Interstellar oxygen along the line of sight of Cygnus X-2
Interstellar dust permeates our Galaxy and plays an important role in many
physical processes in the diffuse and dense regions of the interstellar medium.
High-resolution X-ray spectroscopy, coupled with modelling based on laboratory
dust measurements, provides a unique probe to investigate the interstellar dust
properties along our line of sight towards Galactic X-ray sources. Here, we
focus on the oxygen content of the interstellar medium through its absorption
features in the X-ray spectra. To model the dust features, we perform a
laboratory experiment using the electron microscope facility located at the
University of Cadiz in Spain, where we acquire new laboratory data in the
oxygen K-edge. We study 18 dust samples of silicates and oxides with different
chemical compositions. The laboratory measurements are adopted for our
astronomical data analysis. We carry out a case study on the X-ray spectrum of
the bright low-mass X-ray binary Cygnus X-2, observed by XMM-Newton. We
determine different temperature phases of the ISM, and parameterize oxygen in
both gas (neutral and ionised) and dust form. We find Solar abundances of
oxygen along the line of sight towards the source. Due to both the relatively
low depletion of oxygen into dust form and the shape of the oxygen cross
section profiles, it is challenging to determine the precise chemistry of
interstellar dust. However, silicates provide an acceptable fit. Finally, we
discuss the systematic discrepancies in the atomic (gaseous phase) data of the
oxygen edge spectral region using different X-ray atomic databases, and also
consider future prospects for studying the ISM with the Arcus concept mission.Comment: Accepted for publication in A&A, 15 pages, 11 figure
The Long Stare at Hercules X-1. I. Emission Lines from the Outer Disk, the Magnetosphere Boundary, and the Accretion Curtain
© 2022. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, https://creativecommons.org/licenses/by/4.0/Hercules X-1 is a nearly edge-on accreting X-ray pulsar with a warped accretion disk, precessing with a period of about 35 days. The disk precession allows for unique and changing sightlines toward the X-ray source. To investigate the accretion flow at a variety of sightlines, we obtained a large observational campaign on Her X-1 with XMM-Newton (380 ks exposure) and Chandra (50 ks exposure) for a significant fraction of a single disk precession cycle, resulting in one of the best data sets taken to date on a neutron star X-ray binary. Here we present the spectral analysis of the high state high-resolution grating and CCD data sets, including the extensive archival data available for this famous system. The observations reveal a complex Fe K region structure, with three emission line components of different velocity widths. Similarly, the high-resolution soft X-ray spectra reveal a number of emission lines of various widths. We correct for the uncertain gain of the European Photon Imaging Camera pn Timing mode spectra, and track the evolution of these spectral components with Her X-1 precession phase and observed luminosity. We find evidence for three groups of emission lines, the first of which originates in the outer accretion disk (105 R G from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (103 R G). The last group is too broad to arise in the magnetically truncated disk and instead must originate very close to the neutron star surface, likely from X-ray reflection from the accretion curtain (∼102 R G).Peer reviewe
AGN STORM 2. IV. Swift X-ray and ultraviolet/optical monitoring of Mrk 817
The AGN STORM 2 campaign is a large, multiwavelength reverberation mapping
project designed to trace out the structure of Mrk 817 from the inner accretion
disk to the broad emission line region and out to the dusty torus. As part of
this campaign, Swift performed daily monitoring of Mrk 817 for approximately 15
months, obtaining observations in X-rays and six UV/optical filters. The X-ray
monitoring shows that Mrk 817 was in a significantly fainter state than in
previous observations, with only a brief flare where it reached prior flux
levels. The X-ray spectrum is heavily obscured. The UV/optical light curves
show significant variability throughout the campaign and are well correlated
with one another, but uncorrelated with the X-rays. Combining the Swift
UV/optical light curves with Hubble UV continuum light curves, we measure
interband continuum lags, , that increase with increasing
wavelength roughly following , the
dependence expected for a geometrically thin, optically thick, centrally
illuminated disk. Modeling of the light curves reveals a period at the
beginning of the campaign where the response of the continuum is suppressed
compared to later in the light curve - the light curves are not simple shifted
and scaled versions of each other. The interval of suppressed response
corresponds to a period of high UV line and X-ray absorption, and reduced
emission line variability amplitudes. We suggest that this indicates a
significant contribution to the continuum from the broad line region gas that
sees an absorbed ionizing continuum.Comment: 20 pages, 13 figures, 3 tables, accepted for publication in Ap