Abstract
Hercules X-1 is a nearly edge-on X-ray binary with a warped, precessing accretion disk, which manifests through a 35-day cycle of alternating High and Low flux states. This disk precession ...introduces a changing line of sight toward the X-ray source, through an ionized accretion disk wind. The sightline variation allows us to uniquely determine how the wind properties vary with height above the disk. All the previous wind measurements were made in the brighter Main High state of Her X-1. Here, we analyze the only Chandra observation during the fainter “Short” High state and significantly detect blueshifted ionized absorption. We find a column density of
2.0
−
0.6
+
1.1
×
10
22
cm
−2
, an ionization parameter
log
(
ξ
/erg cm s
−1
) =
3.41
−
0.12
+
0.15
, and an outflow velocity of 380 ± 40 km s
−1
. The properties of the outflow measured during the Short High state are in good agreement with those measured at equivalent precession phases during the Main High state. We conclude that we are sampling the same wind structure, seen during both the Main and Short High states, which is precessing alongside the warped accretion disk every 35 days. Finally, the high spectral resolution of Chandra gratings above 1 keV in this observation enabled us to measure the abundances of certain elements in the outflow. We find Mg/O
=
1.5
−
0.4
+
0.5
, Si/O =1.5 ± 0.4, and S/O
=
3.0
−
1.1
+
1.2
, whereas in our previous study of Her X-1 with XMM-Newton, we found an overabundance of N, Ne, and Fe compared with O. These peculiar abundance ratios were likely introduced by the pollution of the donor by the supernova that created Her X-1.
Context. 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 1024 cm-2). The interaction (scattering and absorption) with the medium imprints spectral signatures that reflect the individual atoms which constitute the gas, molecule, or solid. Aims. 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. Methods. 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. Results. 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.
ABSTRACT
The narrow-line Seyfert 1 galaxy $\rm I\,\,Zwicky\, 1$ shows a unique and complex system of ionized gas in outflow, which consists of an ultra-fast wind and a two-component warm absorber. In ...the last two decades, XMM–Newton monitored the source multiple times enabling the study of the long-term variability of the various outflows. Plasma in photoionization equilibrium with the ionizing source responds and varies accordingly to any change of the ionizing luminosity. However, detailed modelling of the past Reflection Grating Spectrometer (RGS) data has shown no correlation between the plasma ionization state and the ionizing continuum, revealing a complex long-term variability of the multiphase warm absorber. Here, we present a new observation of $\rm I\,\,Zwicky\, 1$ by XMM–Newton taken in early 2020 characterized by a lower X-ray flux state. The soft X-ray spectrum from the RGS reveals the two components of the warm absorber with log ξ ∼ −1.0 and log ξ ∼ 1.7. Comparing our results with the previous observations, the ionization state of the two absorbing gas components is continuously changing, following the same unpredictable behaviour. The new results strengthen the scenario in which the ionization state of the warm absorber is driven by the density of the gas rather than the ionizing luminosity. In particular, the presence of a radiation driven, inhomogeneous clumpy outflow may explain both the variability in ionization throughout the years and the line-locked N v system observed in the ultraviolet band. Finally, the EPIC-pn spectrum reveals an ultra-fast wind with an outflow velocity of ∼0.26c and ionization parameter of log ξ ∼ 3.8.
Aims. We present an analysis of the Hubble Space Telescope Cosmic Origins Spectrograph spectrum of I Zw 1 aiming to probe the absorbing medium associated with the active galactic nucleus (AGN). ...Methods. We fitted the emission spectrum and performed spectral analysis of the identified absorption features to derive the corresponding ionic column densities and covering fractions of the associated outflows. We employed photoionisation modelling to constrain the total column density and the ionisation parameter of four detected kinematic components. By investigating the implications of the results together with the observed kinematic properties of both emission and absorption features, we derived constraints on the structure and geometry of the absorbing medium in the AGN environment. Results. We find and characterise absorption line systems from outflowing ionised gas in four distinct kinematic components, located at −60, −280, −1950, and −2900 km s −1 with respect to the source rest frame. While the two slower outflows are consistent with a full covering of the underlying radiation source, the well-constrained doublet line ratios of the faster two, higher column density, outflows suggest partial covering, with a covering fraction of C f ∼ 0.4. The faster outflows show also line-locking in the N V doublet, a signature of acceleration via line absorption. This makes I Zw 1 possibly the closest object that shows evidence for hosting line-driven winds. The observed −1950 km s −1 absorption is likely due to the same gas as an X-ray warm absorber. Furthermore, the behaviour in UV and X-ray bands implies that this outflow has a clumpy structure. We find that the highly asymmetric broad emission lines in I Zw 1, indicative of a collimated, outflowing broad line region, are covered by the absorbing gas. Finally, the strongest UV–X-ray absorber may be connected to some of the blueshifted line emission, indicative of a more spatially extended structure of this ionised medium.
Context.
The dense Galactic environment is a large reservoir of interstellar dust. Therefore, this region represents a perfect laboratory to study the properties of cosmic dust grains. X-rays are the ...most direct way to detect the interaction of light with dust present in these dense environments.
Aims.
The interaction between the radiation and the interstellar matter imprints specific absorption features on the X-ray spectrum. We study them with the aim of defining the chemical composition, the crystallinity, and structure of the dust grains that populate the inner regions of the Galaxy.
Methods.
We investigated the magnesium and the silicon K-edges detected in the
Chandra
/HETG spectra of eight bright X-ray binaries, distributed in the neighbourhood of the Galactic centre. We modelled the two spectral features using accurate extinction cross-sections of silicates, which we measured at the synchrotron facility Soleil, France.
Results.
Near the Galactic centre, magnesium and silicon show abundances similar to the solar ones and they are highly depleted from the gas phase (
δ
Mg
> 0.90 and
δ
Si
> 0.96). We find that amorphous olivine with a composition of MgFeSiO
4
is the most representative compound along all lines of sight according to our fits. The contribution of Mg-rich silicates and quartz is low (less than 10%). On average we observe a percentage of crystalline dust equal to 11%. For the extragalactic source LMC X-1, we find a preference for forsterite, a magnesium-rich olivine. Along this line of sight we also observe an under-abundance of silicon
A
Si
∕
A
LMC
= 0.5 ± 0.2.
Context. Studying absorption and scattering of X-ray radiation by interstellar dust grains allows us to access the physical and chemical properties of cosmic grains even in the densest regions of the ...Galaxy. Aims. We aim at characterising the dust silicate population which presents clear absorption features in the energy band covered by the Chandra X-ray Observatory. Through these absorption features, in principle, it is possible to infer the size distribution, composition, and structure of silicate in the interstellar medium. In particular, in this work we investigate magnesium and silicon K-edges. Methods. We built X-ray extinction models for 15 dust candidates using newly acquired synchrotron measurements. These models were adapted for astrophysical analysis and implemented in the SPEX spectral fitting program. We used the models to reproduce the dust absorption features observed in the spectrum of the bright low mass X-ray binary GX 3+1, which is used as a background source. Results. With the simultaneous analysis of the two edges we test two different size distributions of dust: one corresponding to the standard Mathis-Rumpl-Nordsieck model and one considering larger grains ( n(a) ∝ ai−3.5 $n(a) \propto a_i^{-3.5}$ n(a)∝ a i −3.5 with 0.005 μm < a1 < 0.25 μm and 0.05 μm < a2 < 0.5 μm, respectively, with a the grain size). These distributions may be representative of the complex Galactic region towards this source. We find that up to 70% of dust is constituted by amorphous olivine. We discuss the crystallinity of the cosmic dust found along this line of sight. Both magnesium and silicon are highly depleted into dust (δZ = 0.89 and 0.94, respectively), while their total abundance does not depart from solar values.
Context.
High-ionisation lines in the soft X-ray band are generally associated to either interstellar hot gas along the line of sight or to photoionised gas intrinsic to the source. In the low-mass ...X-ray binary 4U 1820-30, the nature of these lines is not well understood.
Aims.
We aim to characterise the ionised gas present along the line of sight towards 4U 1820-30 producing the X-ray absorption lines of Mg
XI
, Ne
IX
, Fe
XVII
, O
VII
, and O
VIII
.
Methods.
We analysed all the observations available for this source in the
XMM-Newton
and
Chandra
archives that were taken with the RGS, HETG, and LETG spectrometers. We accurately examined the high-resolution grating spectra using a standard X-ray analysis procedure based on the
C
-statistic and using Bayesian parameter inference. We tested two physical models which describe a plasma in either collisional ionisation or photoionisation equilibrium. We adopted the Bayesian model comparison to statistically compare the different combinations of models used for the analysis.
Results.
We find that the lines are consistent with hot gas in the interstellar medium rather than the intrinsic gas of the X-ray binary. Our best-fit model reveals the presence of a collisionally ionised plasma with a temperature of
T
= (1.98 ± 0.05) × 10
6
K. The photoionisation model fails to fit the Fe
XVII
line (which is detected with a significance of 6.5
σ
) because of the low column density predicted by the model. Moreover, the low inclination of the binary system is likely the reason for the non-detection of ionised gas intrinsic to the source.
Abstract
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 (10
5
R
G
from the neutron star). The second line group plausibly originates at the boundary between the inner disk and the pulsar magnetosphere (10
3
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 (∼10
2
R
G
).
Interstellar dust permeates our Galaxy and plays an important role in many physical processes in the diffuse and dense regions of the interstellar medium (ISM). High-resolution X-ray spectroscopy, ...coupled with modelling based on laboratory dust measurements, provides a unique probe for investigating the interstellar dust properties along our line of sight towards Galactic X-ray sources. Here, we focus on the oxygen content of the ISM 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 parameterise 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 as well as consider future prospects for studying the ISM with the Arcus concept mission.
Aims. We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of the Swift long gamma-ray bursts (GRBs), namely, the BAT6 sample. Methods. 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, which accounte for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. Results. We compare the behaviour observed in the X-ray to that in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that rest-frame optical luminosity distribution of the GRBs is not bimodal and is clustered around the mean value Log(LR) = 29.9 ± 0.8 when estimated at a rest-frame time of 12 h. This is in contrast to what is 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, which are normalised to the GRB isotropic energy (Eiso), provide evidence for X-ray emission that is still powered by the prompt emission until late times (~hours after the burst event). On the other hand, the same test performed for the Eiso-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.
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