TY  - JOUR
A1  - Bozzo, Enrico
A1  - Ferrigno, Carlo
A1  - Oskinova, Lida
A1  - Ducci, Lorenzo
T1  - Accretion of a clumped wind from a red supergiant donor on to a magnetar is suggested by the analysis of the XMM-Newton and NuSTAR observations of the X-ray binary 3A 1954+319
JF  - Monthly notices of the Royal Astronomical Society
N2  - 3A 1954+319 has been classified for a long time as a symbiotic X-ray binary, hosting a slowly rotating neutron star and an aged M red giant. Recently, this classification has been revised thanks to the discovery that the donor star is an M supergiant. This makes 3A 1954+319 a rare type of high-mass X-ray binary consisting of a neutron star and a red supergiant donor. In this paper, we analyse two archival and still unpublished XMM-Newton and NuSTAR observations of the source. We perform a detailed hardness ratio-resolved spectral analysis to search for spectral variability that could help investigating the structures of the inhomogeneous M supergiant wind from which the neutron star is accreting. We discuss our results in the context of wind-fed supergiant X-ray binaries and show that the newest findings on 3A 1954+319 reinforce the hypothesis that the neutron star in this system is endowed with a magnetar-like magnetic field strength (greater than or similar to 10(14) G).
KW  - accretion
KW  - stars: massive
KW  - stars: neutron
KW  - X-rays: binaries
KW  - X-rays: individual: 3A 1954+319
KW  - X-rays: stars
KW  - accretion discs
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab3688
SN  - 0035-8711
SN  - 1365-2966
VL  - 510
IS  - 3
SP  - 4645
EP  - 4653
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Bozzo, Enrico
A1  - Romano, Patrizia
A1  - Ferrigno, Carlo
A1  - Oskinova, Lida
T1  - The symbiotic X-ray binaries Sct X-1, 4U 1700+24, and IGR J17329-2731
JF  - Monthly notices of the Royal Astronomical Society
N2  - Symbiotic X-ray binaries are systems hosting a neutron star accreting form the wind of a late-type companion. These are rare objects and so far only a handful of them are known. One of the most puzzling aspects of the symbiotic X-ray binaries is the possibility that they contain strongly magnetized neutron stars. These are expected to be evolutionary much younger compared to their evolved companions and could thus be formed through the (yet poorly known) accretion induced collapse of a white dwarf. In this paper, we perform a broad-band X-ray and soft gamma-ray spectroscopy of two known symbiotic binaries, Sct X-1 and 4U 1700+24, looking for the presence of cyclotron scattering features that could confirm the presence of strongly magnetized NSs. We exploited available Chandra, Swift, and NuSTAR data. We find no evidence of cyclotron resonant scattering features (CRSFs) in the case of Sct X-1 but in the case of 4U 1700+24 we suggest the presence of a possible CRSF at similar to 16 keV and its first harmonic at similar to 31 keV, although we could not exclude alternative spectral models for the broad-band fit. If confirmed by future observations, 4U 1700+24 could be the second symbiotic X-ray binary with a highly magnetized accretor. We also report about our long-term monitoring of the last discovered symbiotic X-ray binary IGR J17329-2731 performed with Swift/XRT. The monitoring revealed that, as predicted, in 2017 this object became a persistent and variable source, showing X-ray flares lasting for a few days and intriguing obscuration events that are interpreted in the context of clumpy wind accretion.
KW  - accretion
KW  - accretion discs
KW  - stars: massive
KW  - stars: neutron
KW  - X-rays: binaries
KW  - X-rays: individual: SctX-1
KW  - X-rays: individual: 4U1700+24;
KW  - X-rays: stars
KW  - X-rays: individual: IGRJ17329-2731
Y1  - 2022
U6  - https://doi.org/10.1093/mnras/stac907
SN  - 0035-8711
SN  - 1365-2966
VL  - 513
IS  - 1
SP  - 42
EP  - 54
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Ilić Petković, Nikoleta
A1  - Poppenhäger, Katja
A1  - Hosseini, Seyede Marzieh
T1  - Tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems
JF  - Monthly notices of the Royal Astronomical Society
N2  - Tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. In cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star's rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. Observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. Here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. We also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation-activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction that would be expected to affect the stellar activity, but not necessarily the rotation. We conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence.
KW  - planet-star interactions
KW  - stars: activity
KW  - binaries: general
KW  - stars:
KW  - evolution
KW  - planets and satellites: general
KW  - X-rays: stars
Y1  - 2022
U6  - https://doi.org/10.1093/mnras/stac861
SN  - 0035-8711
SN  - 1365-2966
VL  - 513
IS  - 3
SP  - 4380
EP  - 4404
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Poppenhäger, Katja
T1  - Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances
JF  - Monthly notices of the Royal Astronomical Society
N2  - Transit observations in the helium triplet around 10 830 Angstrom are a successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionization and recombination of helium in the exoplanetary atmosphere. This ionization is caused by stellar photons at extreme ultraviolet (EUV) wavelengths; however, no currently active telescopes can observe this part of the stellar spectrum. The relevant part of the stellar EUV spectrum consists of individual emission lines, many of them being formed by iron at coronal temperatures. The stellar iron abundance in the corona is often observed to be depleted for high-activity low-mass stars due to the first ionization potential (FIP) effect. I show that stars with high versus low coronal iron abundances follow different scaling laws that tie together their X-ray emission and the narrow-band EUV flux that causes helium ionization. I also show that the stellar iron to oxygen abundance ratio in the corona can be measured reasonably well from X-ray CCD spectra, yielding similar results to high-resolution X-ray observations. Taking coronal iron abundance into account, the currently observed large scatter in the relationship of EUV irradiation with exoplanetary helium transit depths can be reduced, improving the target selection criteria for exoplanet transmission spectroscopy. In particular, previously puzzling non-detections of helium for Neptunic exoplanets are now in line with expectations from the revised scaling laws.
KW  - planets and satellites: atmospheres
KW  - stars: abundances
KW  - stars: coronae
KW  - stars: late-type
KW  - ultraviolet: stars
KW  - X-rays: stars
Y1  - 2022
U6  - https://doi.org/10.1093/mnras/stac507
SN  - 0035-8711
SN  - 1365-2966
VL  - 512
IS  - 2
SP  - 1751
EP  - 1764
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Foster, Mary Grace
A1  - Poppenhäger, Katja
A1  - Ilić Petković, Nikoleta
A1  - Schwope, Axel
T1  - Exoplanet X-ray irradiation and evaporation rates with eROSITA
JF  - Astronomy and astrophysics : an international weekly journal
N2  - High-energy irradiation is a driver for atmospheric evaporation and mass loss in exoplanets. This work is based on data from eROSITA, the soft X-ray instrument on board the Spectrum Roentgen Gamma mission, as well as on archival data from other missions. We aim to characterise the high-energy environment of known exoplanets and estimate their mass-loss rates. We use X-ray source catalogues from eROSITA, XMM-Newton, Chandra, and ROSAT to derive X-ray luminosities of exoplanet host stars in the 0.2–2 keV energy band with an underlying coronal, that is, optically thin thermal spectrum. We present a catalogue of stellar X-ray and EUV luminosities, exoplanetary X-ray and EUV irradiation fluxes, and estimated mass-loss rates for a total of 287 exoplanets, 96 of which are characterised for the first time based on new eROSITA detections. We identify 14 first-time X-ray detections of transiting exoplanets that are subject to irradiation levels known to cause observable evaporation signatures in other exoplanets. This makes them suitable targets for follow-up observations.
KW  - stars: coronae
KW  - stars: activity
KW  - planet-star interactions
KW  - planets and
KW  - satellites: atmospheres
KW  - X-rays: stars
Y1  - 2022
U6  - https://doi.org/10.1051/0004-6361/202141097
SN  - 0004-6361
SN  - 1432-0746
VL  - 661
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Poppenhäger, Katja
A1  - Ketzer, Laura
A1  - Mallonn, Matthias
T1  - X-ray irradiation and evaporation of the four young planets around V1298 Tau
JF  - Monthly notices of the Royal Astronomical Society
N2  - Planets around young stars are thought to undergo atmospheric evaporation due to the high magnetic activity of the host stars. Here we report on X-ray observations of V1298 Tau, a young star with four transiting exoplanets. We use X-ray observations of the host star with Chandra and ROSAT to measure the current high-energy irradiation level of the planets and employ a model for the stellar activity evolution together with exoplanetary mass-loss to estimate the possible evolution of the planets. We find that V1298 Tau is X-ray bright with log L-X [erg s(-1)] = 30.1 and has a mean coronal temperature of approximate to 9 MK. This places the star amongst the more X-ray luminous ones at this stellar age. We estimate the radiation-driven mass-loss of the exoplanets and find that it depends sensitively on the possible evolutionary spin-down tracks of the star as well as on the current planetary densities. Assuming the planets are of low density due to their youth, we find that the innermost two planets can lose significant parts of their gaseous envelopes and could be evaporated down to their rocky cores depending on the stellar spin evolution. However, if the planets are heavier and follow the mass-radius relation of older planets, then even in the highest XUV irradiation scenario none of the planets is expected to cross the radius gap into the rocky regime until the system reaches an age of 5 Gyr.
KW  - planets and satellites: atmospheres
KW  - planet-star interactions
KW  - stars: activity
KW  - stars: individual: V1298 Tau
KW  - X-rays: stars
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa1462
SN  - 0035-8711
SN  - 1365-2966
VL  - 500
IS  - 4
SP  - 4560
EP  - 4572
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lida
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Ingallinera, A.
A1  - Leone, Francesco
A1  - Phillips, N. M.
A1  - Agliozzo, Claudia
A1  - Todt, Helge Tobias
A1  - Cerrigone, L.
T1  - A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.
KW  - stars: chemically peculiar
KW  - stars: early-type
KW  - stars: individual: HR 5907
KW  - stars: magnetic field
KW  - radio continuum: stars
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty244
SN  - 0035-8711
SN  - 1365-2966
VL  - 476
IS  - 1
SP  - 562
EP  - 579
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Robrade, Jan
A1  - Oskinova, Lida
A1  - Schmitt, J. H. M. M.
A1  - Leto, Paolo
A1  - Trigilio, C.
T1  - Outstanding X-ray emission from the stellar radio pulsar CU Virginis
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Among the intermediate-mass magnetic chemically peculiar (MCP) stars, CU Vir is one of the most intriguing objects. Its 100% circularly polarized beams of radio emission sweep the Earth as the star rotates, thereby making this strongly magnetic star the prototype of a class of nondegenerate stellar radio pulsars. While CU Vir is well studied in radio, its high-energy properties are not known. Yet, X-ray emission is expected from stellar magnetospheres and confined stellar winds. Aims. Using X-ray data we aim to test CU Vir for intrinsic X-ray emission and investigate mechanisms responsible for its generation. Methods. We present X-ray observations performed with XMM-Newton and Chandra and study obtained X-ray images, light curves, and spectra. Basic X-ray properties are derived from spectral modelling and are compared with model predictions. In this context we investigate potential thermal and nonthermal X-ray emission scenarios. Results. We detect an X-ray source at the position of CU Vir. With LX approximate to 3 x 10(28) erg s(-1) it is moderately X-ray bright, but the spectrum is extremely hard compared to other Ap stars. Spectral modelling requires multi-component models with predominant hot plasma at temperatures of about T-X = 25MK or, alternatively, a nonthermal spectral component. Both types of model provide a virtually equivalent description of the X-ray spectra. The Chandra observation was performed six years later than those by XMM-Newton, yet the source has similar X-ray flux and spectrum, suggesting a steady and persistent X-ray emission. This is further confirmed by the X-ray light curves that show only mild X-ray variability. Conclusions. CU Vir is also an exceptional star at X-ray energies. To explain its full X-ray properties, a generating mechanism beyond standard explanations, like the presence of a low-mass companion or magnetically confined wind-shocks, is required. Magnetospheric activity might be present or, as proposed for fast-rotating strongly magnetic Bp stars, the X-ray emission of CU Vir is predominantly auroral in nature.
KW  - individual: CU Vir
KW  - stars: activity
KW  - stars: chemically peculiar
KW  - stars: magnetic field
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833492
SN  - 1432-0746
VL  - 619
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Massa, Derck
A1  - Oskinova, Lida
A1  - Prinja, Raman
A1  - Ignace, Richard
T1  - Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant xi Per
BT  - the Connection between X-Rays and Large-scale Wind Structure
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7. III(n) ((f)) star xi Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of xi Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N IV lambda 1718 and Si IV lambda 1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si IV lambda 1402, N IV lambda 1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180 degrees, on the surface of the star. We note that the presence and persistence of two spots separated by 180 degrees suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in xi Per.
KW  - stars: activity
KW  - stars: early-type
KW  - stars: winds, outflows
KW  - ultraviolet: stars
KW  - X-rays: stars
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab0283
SN  - 0004-637X
SN  - 1538-4357
VL  - 873
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Gomez-Moran, Ada Nebot
A1  - Oskinova, Lida
T1  - The X-ray catalog of spectroscopically identified Galactic O stars Investigating the dependence of X-ray luminosity on stellar and wind parameters
JF  - Astronomy and astrophysics : an international weekly journal
N2  - The X-ray emission of O-type stars was first discovered in the early days of the Einstein satellite. Since then many different surveys have confirmed that the ratio of X-ray to bolometric luminosity in O-type stars is roughly constant, but there is a paucity of studies that account for detailed information on spectral and wind properties of O-stars. Recently a significant sample of O stars within our Galaxy was spectroscopically identified and presented in the Galactic O-Star Spectroscopic Survey (GOSS). At the same time, a large high-fidelity catalog of X-ray sources detected by the XMM-Newton X-ray telescope was released. Here we present the X-ray catalog of O stars with known spectral types and investigate the dependence of their X-ray properties on spectral type as well as stellar and wind parameters. We find that, among the GOSS sample, 127 O-stars have a unique XMM-Newton source counterpart and a Gaia data release 2 (DR2) association. Terminal velocities are known for a subsample of 35 of these stars. We confirm that the X-ray luminosities of dwarf and giant O stars correlate with their bolometric luminosity. For the subsample of O stars with measure terminal velocities we find that the X-ray luminosities of dwarf and giant O stars also correlate with wind parameters. However, we find that these correlations break down for supergiant stars. Moreover, we show that supergiant stars are systematically harder in X-rays compared to giant and dwarf O-type stars. We find that the X-ray luminosity depends on spectral type, but seems to be independent of whether the stars are single or in a binary system. Finally, we show that the distribution of log(L-X/L-bol) in our sample stars is non-Gaussian, with the peak of the distribution at log(L-X/L-bol) approximate to -6.6.
KW  - stars: massive
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833453
SN  - 1432-0746
VL  - 620
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lida
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Ingallinera, A.
A1  - Todt, Helge Tobias
A1  - Leone, F.
T1  - The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae
JF  - Monthly notices of the Royal Astronomical Society
N2  - In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars.
KW  - stars: chemically peculiar
KW  - stars: early-type
KW  - stars: individual: HR 7355
KW  - stars: magnetic field
KW  - radio continuum: stars
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.1093/mnras/stx267
SN  - 0035-8711
SN  - 1365-2966
VL  - 467
SP  - 2820
EP  - 2833
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Pillitteri, Ignazio
A1  - Wolk, Scott J.
A1  - Reale, Fabio
A1  - Oskinova, Lida
T1  - The early B-type star Rho Ophiuchi A is an X-ray lighthouse
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We present the results of a 140 ks XMM-Newton observation of the B2 star rho Oph A. The star has exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to that which we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is caused by an increase of the plasma emission measure, while the second increase of rate is a major flare with temperatures in excess of 60 MK (kT similar to 5 keV). From the analysis of its rise, we infer a magnetic field of >= 300 G and a size of the flaring region of similar to 1.4-1.9 x 10(11) cm, which corresponds to similar to 25%-30% of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of >= 2.5 yr, and suggests an overall large scale dipolar magnetic field that produces an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by its young age, and the tight orbit as in RS Cvn systems. In both cases rho Oph would constitute an extreme system that is worthy of further investigation.
KW  - stars: activity
KW  - stars: individual: Rho Ophiuchi
KW  - stars: early-type
KW  - stars: magnetic field
KW  - starspots
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201630070
SN  - 1432-0746
VL  - 602
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Huenemoerder, D. P.
A1  - Hamann, Wolf-Rainer
A1  - Shenar, Tomer
A1  - Sander, Andreas Alexander Christoph
A1  - Ignace, R.
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
T1  - On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions.
KW  - stars: individual (Cyg OB2 12)
KW  - stars: massive
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - supergiants
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.3847/1538-4357/aa7e79
SN  - 0004-637X
SN  - 1538-4357
VL  - 845
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Bozzo, Enrico
A1  - Oskinova, Lida
A1  - Lobel, A.
A1  - Hamann, Wolf-Rainer
T1  - The super-orbital modulation of supergiant high-mass X-ray binaries
JF  - Astronomy and astrophysics : an international weekly journal
N2  - The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.
KW  - X-rays: stars
KW  - X-rays: binaries
KW  - gamma rays: stars
KW  - stars: massive
KW  - stars: neutron
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201731930
SN  - 1432-0746
VL  - 606
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - De Becker, M.
A1  - del Valle, Maria Victoria
A1  - Romero, G. E.
A1  - Peri, C. S.
A1  - Benaglia, P.
T1  - X- ray study of bow shocks in runaway stars
JF  - Monthly notices of the Royal Astronomical Society
N2  - Massive runaway stars produce bow shocks through the interaction of their winds with the interstellar medium, with the prospect for particle acceleration by the shocks. These objects are consequently candidates for non-thermal emission. Our aim is to investigate the X-ray emission from these sources. We observed with XMM-Newton a sample of five bow shock runaways, which constitutes a significant improvement of the sample of bow shock runaways studied in X-rays so far. A careful analysis of the data did not reveal any X-ray emission related to the bow shocks. However, X-ray emission from the stars is detected, in agreement with the expected thermal emission from stellar winds. On the basis of background measurements we derive conservative upper limits between 0.3 and 10 keV on the bow shocks emission. Using a simple radiation model, these limits together with radio upper limits allow us to constrain some of the main physical quantities involved in the non-thermal emission processes, such as the magnetic field strength and the amount of incident infrared photons. The reasons likely responsible for the non-detection of non-thermal radiation are discussed. Finally, using energy budget arguments, we investigate the detectability of inverse Compton X-rays in a more extended sample of catalogued runaway star bow shocks. From our analysis we conclude that a clear identification of non-thermal X-rays from massive runaway bow shocks requires one order of magnitude (or higher) sensitivity improvement with respect to present observatories.
KW  - acceleration of particles
KW  - radiation mechanisms: non-thermal
KW  - stars: earlytype
KW  - X-rays: stars
Y1  - 2017
U6  - https://doi.org/10.1093/mnras/stx1826
SN  - 0035-8711
SN  - 1365-2966
VL  - 471
SP  - 4452
EP  - 4464
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Sandin, C.
A1  - Steffen, M.
A1  - Schoenberner, D.
A1  - Rühling, Ute
T1  - Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma
JF  - Frontiers in psychology
N2  - Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium-and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected.
KW  - conduction
KW  - hydrodynamics
KW  - planetary nebulae: general
KW  - stars: AGB and post-AGB
KW  - stars: Wolf-Rayet
KW  - X-rays: stars
Y1  - 2016
U6  - https://doi.org/10.1051/0004-6361/201527357
SN  - 1432-0746
VL  - 586
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Hamaguchi, K.
A1  - Oskinova, Lida
A1  - Russell, C. M. P.
A1  - Petre, R.
A1  - Enoto, T.
A1  - Morihana, K.
A1  - Ishida, M.
T1  - DISCOVERY OF RAPIDLY MOVING PARTIAL X-RAY ABSORBERS WITHIN GAMMA CASSIOPEIAE
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - detected six rapid X-ray spectral hardening events called "softness dips" in a similar to 100 ks observation in 2011. All the softness dip events show symmetric softness-ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either similar to 40% or similar to 70% partial covering absorption to kT similar to 12 keV plasma emission by matter with a neutral hydrogen column density of similar to(2-8) x 10(21) cm(-2), while the spectrum outside these dips is almost free of absorption. This result suggests the presence of two distinct X-ray-emitting spots in the.. Cas system, perhaps on a white dwarf (WD) companion with dipole mass accretion. The partial covering absorbers may be blobs in the Be stellar wind, the Be disk, or rotating around the WD companion. Weak correlations of the softness ratios to the hard X-ray flux suggest the presence of stable plasmas at kT similar to 0.9 and 5 keV, which may originate from the Be or WD winds. The formation of a Be star and WD binary system requires mass transfer between two stars; gamma Cas may have experienced such activity in the past.
KW  - blue stragglers
KW  - stars: emission-line, Be
KW  - stars: individual (gamma Cassiopeiae)
KW  - stars: winds, outflows
KW  - white dwarfs
KW  - X-rays: stars
Y1  - 2016
U6  - https://doi.org/10.3847/0004-637X/832/2/140
SN  - 0004-637X
SN  - 1538-4357
VL  - 832
SP  - 33
EP  - 49
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Shrader, C. R.
A1  - Hamaguchi, K.
A1  - Sturner, S. J.
A1  - Oskinova, Lida
A1  - Almeyda, T.
A1  - Petre, R.
T1  - Hifg-energy properties of the enigmatic be STAR gamma Cassiopeiae
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present the results of a broadband X-ray study of the enigmatic Be star Gamma Cassiopeiae (herein gamma Cas) based on observations made with both the Suzaku and INTEGRAL observatories.. Cas has long been recognized as the prototypical example of a small subclass of Be stars with moderately strong X-ray emission dominated by a hot thermal component in the 0.5-12 keV energy range (L-x approximate to 10(32)-10(33) erg s(-1)). This places them at the high end of the known luminosity distribution for stellar emission, but several orders of magnitude below typical accretion-powered Be X-ray binaries. The INTEGRAL observations spanned an eight-year baseline and represent the deepest measurement to date at energies above similar to 50 keV. We find that the INTEGRAL data are consistent within statistics to a constant intensity source above 20 keV, with emission extending up to similar to 100 keV, and that searches for all of the previously reported periodicities of the system at lower energies led to null results. We further find that our combined Suzaku and INTEGRAL spectrum, which we suggest is the most accurate broadband X-ray measurement of gamma Cas to date, is fitted extremely well with a thermal plasma emission model with a single absorption component. We found no compelling need for an additional non-thermal high-energy component. We discuss these results in the context of a currently favored models for gamma Cas and its analogs.
KW  - gamma rays: stars
KW  - stars: emission-line, Be
KW  - stars: individual (gamma Cassiopeiae)
KW  - white dwarfs
KW  - X-rays: binaries
KW  - X-rays: stars
Y1  - 2015
U6  - https://doi.org/10.1088/0004-637X/799/1/84
SN  - 0004-637X
SN  - 1538-4357
VL  - 799
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Hubrig, Swetlana
A1  - Schöller, Markus
A1  - Kholtygin, Alexander F.
A1  - Tsumura, Hiroki
A1  - Hoshino, Akio
A1  - Kitamoto, Shunji
A1  - Oskinova, Lida
A1  - Ignace, Richard
A1  - Todt, Helge Tobias
A1  - Ilyin, Ilya
T1  - New multiwavelength observations of the Of?p star CPD-28 degrees 2561
JF  - Monthly notices of the Royal Astronomical Society
N2  - A rather strong mean longitudinal magnetic field of the order of a few hundred gauss was detected a few years ago in the Of?p star CPD -28 degrees 2561 using FORS2 (FOcal Reducer low dispersion Spectrograph 2) low-resolution spectropolarimetric observations. In this work, we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 mounted at the 8-m Antu telescope of the Very Large Telescope (VLT). These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole B-d is larger than 1.15 kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD -28 degrees 2561 with that of another Of?p star, HD 148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log L-X/L-bol approximate to -5.7. The low-resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK. We associate the 24 MK plasma in CPD -28 degrees 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.
KW  - stars: atmospheres
KW  - stars: individual: CPD-28 degrees 2561
KW  - stars: magnetic field
KW  - stars: mass-loss
KW  - stars: variables: general
KW  - X-rays: stars
Y1  - 2015
U6  - https://doi.org/10.1093/mnras/stu2516
SN  - 0035-8711
SN  - 1365-2966
VL  - 447
IS  - 2
SP  - 1885
EP  - 1894
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lida
A1  - Todt, Helge Tobias
A1  - Huenemoerder, David P.
A1  - Hubrig, Swetlana
A1  - Ignace, Richard
A1  - Hamann, Wolf-Rainer
A1  - Balona, Luis
T1  - On X-ray pulsations in beta Cephei-type variables
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays.
 Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars.
 Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity.
 Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question.
KW  - stars: massive
KW  - stars: variables: general
KW  - X-rays: stars
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201525908
SN  - 0004-6361
SN  - 1432-0746
VL  - 577
PB  - EDP Sciences
CY  - Les Ulis
ER  -