42 research outputs found
Chandra's X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion
KQ Vel is a peculiar A0p star with a strong surface magnetic field of about
7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (2015)
detected a binary companion of uncertain nature, and suggested it could be a
neutron star or a black hole. In this Letter we analyze X-ray data obtained by
the Chandra telescope to ascertain information about the stellar magnetic field
and interaction between the star and its companion. We confirm previous X-ray
detection of KQ Vel with a relatively large X-ray luminosity of 2\times 10^{30}
erg/s. X-ray spectra suggest the presence of hot gas at > 20MK and, possibly,
of a non-thermal component. X-ray light curves are variable, but better quality
data are needed to determine periodicity if any. We interpret X-ray spectra as
a combination of two components: the non-thermal emission arising from the
aurora on the A0p star and the hot thermal plasma filling the extended shell
surrounding the "propelling" neutron star. We explore various alternatives, but
a hybrid model involving the stellar magnetosphere along with a hot shell
around the propelling neutron star seems most plausible. We speculate that KQ
Vel was originally a triple system, and the Ap star is a merger product. We
conclude that KQ Vel is an intermediate-mass binary consisting of a strongly
magnetic main sequence star and a neutron star.Comment: accepted to A&AL. 8 pages including the appendix where the formalism
describing the X-ray emission from a hot shell around a propelling neutron
star in settling accretion regime is presente
On the Weak-Wind Problem in Massive Stars: X-ray Spectra Reveal a Massive Hot Wind in \mu\ Columbae
\mu\ Columbae is a prototypical weak-wind O-star for which we have obtained a
high-resolution X-ray spectrum with the Chandra LETG/ACIS-S instrument and a
low resolution spectrum with Suzaku. This allows us, for the first time, to
investigate the role of X-rays on the wind structure in a bona fide weak-wind
system and to determine whether there actually is a massive, hot wind. The
X-ray emission measure indicates that the outflow is an order of magnitude
greater than that derived from UV lines and is commensurate with the nominal
wind-luminosity relationship for O-stars. Therefore, the ``weak-wind
problem''---identified from cool wind UV/optical spectra---is largely resolved
by accounting for the hot wind seen in X-rays. From X-ray line profiles,
Doppler shifts, and relative strengths, we find that this weak-wind star is
typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically
confined plasma---the spectrum is soft and lines are broadened; Suzaku spectra
confirm the lack of emission above 2 keV. Nor do the relative line shifts and
widths suggest any wind decoupling by ions. The He-like triplets indicate that
the bulk of the X-ray emission is formed rather close to the star, within 5
stellar radii. Our results challenge the idea that some OB stars are
``weak-wind'' stars that deviate from the standard wind-luminosity
relationship. The wind is not weak, but it is hot and its bulk is only
detectable in X-rays.Comment: Accepted for publication in ApJ Letter
A new type of X-ray pulsar
X-ray emission from stars much more massive than the Sun was discovered only
35 years ago. Such stars drive fast stellar winds where shocks can develop, and
it is commonly assumed that the X-rays emerge from the shock-heated plasma.
Many massive stars additionally pulsate. However, hitherto it was neither
theoretically predicted nor observed that these pulsations would affect their
X-ray emission. Here we report the discovery of pulsating X-rays from the
massive B-type star Xi1 Canis Majoris. This star is a variable of beta Cephei
type and has a strong magnetic field. Our observations with the XMM-Newton
telescope reveal X-ray pulsations with the same period as the fundamental
stellar pulsation. This discovery challenges our understanding of stellar winds
from massive stars, their X-ray emission, and their magnetism.Comment: manuscript draft. The revised paper is published in Nature
Communication
X-rays observations of a super-Chandrasekhar object reveal an ONeMg and a CO white dwarf merger product embedded in a putative SN Iax remnant
The merger of two white dwarfs (WD) is a natural outcome from the evolution
of many binary stars. Recently, a WD merger product, IRAS 00500+6713, was
identified. IRAS 00500+6713 consists of a central star embedded in a circular
nebula. The analysis of the optical spectrum of the central star revealed that
it is hot, hydrogen and helium free, and drives an extremely fast wind with a
record breaking speed. The nebula is visible in infrared and in the [O III]
line images. No nebula spectroscopy was obtained prior to our observations.
Here we report the first deep X-ray imaging spectroscopic observations of IRAS
00500+6713. Both the central star and the nebula are detected in X-rays,
heralding the WD merger products as a new distinct type of strong X-ray
sources. Low-resolution X-ray spectra reveal large neon, magnesium, silicon,
and sulfur enrichment of the central star and the nebula. We conclude that IRAS
00500+6713 resulted from a merger of an ONe and a CO WD, which supports earlier
suggestion for a super-Chandrasekhar mass of this object. X-ray analysis
indicates that the merger was associated with an episode of carbon burning and
possibly accompanied by a SN Iax. In X-rays, we observe the point source
associated with the merger product while the surrounding diffuse nebula is a
supernova remnant. IRAS 00500+6713 will likely terminate its evolution with
another peculiar Type I supernova, where the final core collapse to a neutron
star might be induced by electron captures.Comment: accepted by A&A Letters, 9 pages including appendi
On the contribution of ULXs to stellar feedback: an intermediate mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO 338-4
X-ray radiation from accreting compact objects is an important part of
stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous
starburst during the last 40 Myr and contains some of the most massive super
star clusters in the nearby Universe. Given its starburst age and its
star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures
of neutron stars and black holes, hence providing an excellent laboratory for
feedback studies. We compared X-ray images and spectra obtained by XMM-Newton
and Chandra telescopes with integral field spectroscopic VLT MUSE observations
in the optical to constrain the nature of strong X-ray emitters. X-ray
observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The
brightest among them, ESO 338~X-1, has X-ray luminosity in excess of 10^{40}
erg/s. We speculate that ESO 338-4 is powered by accretion on an
intermediate-mass (~300Msun) black hole. We show that X-ray radiation from ULXs
and hot superbubbles strongly contributes to HeII ionization and general
stellar feedback in this template starburst galaxy.Comment: A&A, in pres
On the binary nature of massive blue hypergiants: high-resolution X-ray spectroscopy suggests that Cyg OB2 12 is a colliding wind binary
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 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 de-populating the metastable upper levels of the
forbidden lines of the He-like ions SiXIV and MgXII. 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-rays 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 companion.Comment: accepted to Ap
Coupling hydrodynamics with comoving frame radiative transfer: II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
CONTEXT: Vela X-1, a prototypical high mass X-ray binary (HMXB), hosts a
neutron star (NS) in a close orbit around an early-B supergiant donor star.
Accretion of the donor star's wind onto the NS powers its strong X-ray
luminosity. To understand the physics of HMXBs, detailed knowledge about the
donor star winds is required. AIMS: To gain a realistic picture of the donor
star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model
describing the wind stratification while properly reproducing the observed
donor spectrum. To investigate how X-ray illumination affects the stellar wind,
we calculated additional models for different X-ray luminosity regimes.
METHODS: We use the recently updated version of the PoWR code to consistently
solve the hydrodynamic equation together with the statistical equations and the
radiative transfer. RESULTS: The wind flow in Vela X-1 is driven by ions from
various elements with Fe III and S III leading in the outer wind. The
model-predicted mass-loss rate is in line with earlier empirical studies. The
mass-loss rate is almost unaffected by the presence of the accreting NS in the
wind. The terminal wind velocity is confirmed at km/s.
On the other hand, the wind velocity in the inner region where the NS is
located is only km/s, which is not expected on the basis of a
standard -velocity law. In models with an enhanced level of X-rays, the
velocity field in the outer wind can be altered. If the X-ray flux is too high,
the acceleration breaks down because the ionization increases. CONCLUSIONS:
Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model
reveals a low wind speed at the NS location, and it provides quantitative
information on wind driving in this important HMXB.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
X-ray Observations of Bow Shocks around Runaway O Stars. The Case of ζ Oph and BD+43°3654
Non-thermal radiation has been predicted within bow shocks around runaway stars by recent theoretical works. We present X-ray observations toward the runaway stars ζ Oph by Chandra and Suzaku and of BD+43°3654 by XMM-Newton to search for the presence of non-thermal X-ray emission. We found no evidence of non-thermal emission spatially coincident with the bow shocks; nonetheless, diffuse emission was detected in the vicinity of ζ Oph. After a careful analysis of its spectral characteristics, we conclude that this emission has a thermal nature with a plasma temperature of T ≈ 2 × 106 K. The cometary shape of this emission seems to be in line with recent predictions of radiation-hydrodynamic models of runaway stars. The case of BD+43°3654 is puzzling, as non-thermal emission has been reported in a previous work for this source