101 research outputs found
Variability in high-mass X-ray binaries
Strongly magnetized, accreting neutron stars show periodic and aperiodic
variability over a wide range of time scales. By obtaining spectral and timing
information on these different time scales, we can have a closer look into the
physics of accretion close to the neutron star and the properties of the
accreted material. One of the most prominent time scales is the strong
pulsation, i.e., the rotation period of the neutron star itself. Over one
rotation, our view of the accretion column and the X-ray producing region
changes significantly. This allows us to sample different physical conditions
within the column but at the same time requires that we have
viewing-angle-resolved models to properly describe them. In wind-fed high-mass
X-ray binaries, the main source of aperiodic variability is the clumpy stellar
wind, which leads to changes in the accretion rate (i.e., luminosity) as well
as absorption column. This variability allows us to study the behavior of the
accretion column as a function of luminosity, as well as to investigate the
structure and physical properties of the wind, which we can compare to winds in
isolated stars.Comment: 6 pages, 4 figures, accepted for publication in Astronomische
Nachrichten (proceedings of the XMM-Newton Workshop 2019
Vela X-1 as a laboratory for accretion in High-Mass X-ray Binaries
Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s
accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib
supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a
prototype of wind-accreting HMXB and it has been used as a baseline in
different theoretical or modelling studies.
We discuss the observational properties of the system and the use of the
observational data as laboratory to test recent developments in modelling the
accretion process in High-Mass X-ray Binaries (e.g., Sander et al. 2018; El
Mellah et al. 2018), which range from detailed descriptions of the wind
acceleration to modelling of the structure of the flow of matter close to the
neutron star and its variations.Comment: 4 pages, 2 figures, proceedings of the 12th INTEGRAL conference
"INTEGRAL looks AHEAD to Multimessenger astronomy" in Geneva (Switzerland) on
11-15 February 201
Radiography in high mass X-ray binaries -- Micro-structure of the stellar wind through variability of the column density
In high mass X-ray binaries (HMXBs), an accreting compact object orbits a
high mass star which loses mass through a dense and inhomogeneous wind. Using
the compact object as an X-ray backlight, the time variability of the absorbing
column density in the wind can be exploited in order to shed light on the
micro-structure of the wind and obtain unbiased stellar mass loss rates for
high mass stars. We explore the impact of clumpiness on the variability of the
column density with a simplified wind model. In particular, we focus on the
standard deviation of the column density and the characteristic duration of
enhanced absorption episodes, and compare them with analytical predictions
based on the porosity length. We identified the favorable systems and orbital
phases to determine the wind micro-structure. The coherence time scale of the
column density is shown to be the self-crossing time of a clump in front of the
compact object. We provide a recipe to get accurate measurements of the size
and of the mass of the clumps, purely based on the observable time variability
of the column density. The coherence time scale grants direct access to the
size of the clumps while their mass can be deduced separately from the
amplitude of the variability. If it is due to unaccreted passing-by clumps, the
high column density variations in some HMXBs requires high mass clumps to
reproduce the observed peak-to-peak amplitude and coherence time scales. These
clump properties are hardly compatible with the ones derived from first
principles. Alternatively, other components could contribute to the variability
of the column density: larger orbital scale structures produced by a mechanism
still to be identified, or a dense environment in the immediate vicinity of the
accretor such as an accretion disk, an outflow or a spherical shell around the
magnetosphere of the accreting neutron star
Clumpy wind studies and the non-detection of cyclotron line in OAO 1657-415
Winds of massive stars are suspected to be inhomogeneous (or clumpy), which
biases the measures of their mass loss rates. In High Mass X-ray Binaries
(HMXBs), the compact object can be used as an orbiting X-ray point source to
probe the wind and constrain its clumpiness. We perform spectro-timing analysis
of the HMXB OAO 1657-415 with non-simultaneous NuSTAR and NICER observations.
We compute the hardness ratio from the energy-resolved light curves, and using
an adaptive rebinning technique, we thus select appropriate time segments to
search for rapid spectral variations on timescales of a few hundreds to
thousands of seconds. Column density and intensity of Iron K line were
strongly correlated, and the recorded spectral variations were consistent with
accretion from a clumpy wind. We also illustrate a novel framework to measure
clump sizes, masses in HMXBs more accurately based on absorption measurements
and orbital parameters of the source. We then discuss the limitations posed by
current X-ray spacecrafts in such measurements and present prospects with
future X-ray missions. We find that the source pulse profiles show a moderate
dependence on energy. We identify a previously undetected dip in the pulse
profile visible throughout the NuSTAR observation near spin phase 0.15 possibly
caused by intrinsic changes in accretion geometry close to the neutron star. We
do not find any evidence for the debated cyclotron line at 36\,keV in
the time-averaged or the phase-resolved spectra with NuSTAR.Comment: 32 pages, 11 figures in main text, 7 figures in Appendix, Accepted
for publication in Ap
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview:Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview: Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
(Sub)stellar companions shape the winds of evolved stars
Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe
ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
This overview paper presents atomium, a Large Programme in Cycle 6 with the Atacama Large Millimeter/submillimeter Array (ALMA). The goal of atomium is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. Seventeen oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (0.025-0.050), medium (0.13-0.24), and low (~1) angular resolution. The sensitivity per ~1.3 km s-1 channel was 1.5-5 mJy beam-1, and the line-free channels were used to image the millimetre wave continuum. Our primary molecules for studying the gas dynamics and dust formation are CO, SiO, AlO, AlOH, TiO, TiO2, and HCN; secondary molecules include SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described. In addition, we highlight one scientific result - the wind kinematics of the atomium sources. Our analysis suggests that the atomium sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion. Moreover, the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In fifteen sources, some molecular transitions other than 12CO v = 0 J = 2 - 1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. We propose that a binary interaction with a (sub)stellar companion may (partly) explain the non-monotonic behaviour of the projected velocity field. The atomium data hence provide a crucial benchmark for the wind dynamics of evolved stars in single and binary star models
The first mm detection of a neutron star high-mass X-ray binary
Neutron stars accreting from OB supergiants are often divided between persistently and transiently accreting systems, called supergiant X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs). This dichotomy in accretion behaviour is typically attributed to systematic differences in the massive stellar wind, binary orbit, or magnetic field configuration, but direct observational evidence for these hypotheses remains sparse. To investigate their stellar winds, we present the results of pilot 100-GHz observations of one SFXT and one SgXB with the Northern Extended Millimetre Array. The SFXT, IGR J18410-0535, is detected as a point source at 63.4 ± 9.6 μJy, while the SgXB, IGR J18410-0535 remains undetected. Radio observations of IGR J18410-0535 imply a flat or inverted low-frequency spectrum, arguing for wind emission and against non-thermal flaring. Due to the uncertain SFXT distance, however, the observations do not necessarily imply a difference between the wind properties of the SFXT and SgXB. We compare the mm constraints with other HMXBs and isolated OB supergiants, before considering how future mm campaigns can constrain HMXB wind properties by including X-ray measurements. Specifically, we discuss caveats and future steps to successfully measure wind mass-loss rates and velocities in HMXBs with coordinated mm, radio, and X-ray campaigns
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