227,175 research outputs found
Radio emission from the high-mass X-ray binary BP Cru: first detection
BP Cru is a well known high-mass X-ray binary composed of a late B hypergiant
(Wray 977) and a neutron star, also observed as the X-ray pulsar GX 301-2. No
information about emission from BP Cru in other bands than X-rays and optical
has been reported to date in the literature, though massive X-ray binaries
containing black holes can have radio emission from a jet. In order to assess
the presence of a radio jet, we searched for radio emission towards BP Cru
using the Australia Compact Array Telescope during a survey for radio emission
from Be/X-ray transients. We probed the 41.5d orbit of BP Cru with the
Australia Telescope Compact Array not only close to periastron but also close
to apastron. BP Cru was clearly detected in our data on 4, possibly 6, of 12
occasions at 4.8 and 8.6 GHz. Our data suggest that the spectral index of the
radio emission is modulated either by the X-ray flux or the orbital phase of
the system. We propose that the radio emission of BP Cru probably arises from
two components: a persistent component, coming from the mass donor Wray 977,
and a periodic component connected to the accretion onto the neutron star,
possibly coming from a (weak and short lived) jet.Comment: 2 figures, accepted for publication in A+A letter
Climate Change: Scottish Implications Scoping Study
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The Physical Characteristics of the Small-Scale Interstellar Structure towards Mu Crucis
We present HST/GHRS echelle observations of multiple interstellar lines of
CI, MgI, CrII, and ZnII towards both stars in the mu Cru binary system. Despite
large differences in the profiles of the neutral species, no significant
variations between the stars are seen in the CrII and ZnII line profiles. In
particular, the ZnII absorption observed at -8.6 km/sec towards mu Cru is
constant despite greatly enhanced columns of the neutral species at this
velocity towards mu^1 Cru. An analysis of the fine-structure excitation of CI
in this cloud implies that the density is n_H < 250 cm^{-3}. From the lack of
variation in the (optical) NaI D2 line profiles towards mu^1 and mu^2 Cru in
spectra taken 21 months apart, we can place a lower limit to the size of the
structures of ~10 AU. These results are discussed in the context of recent
radio and optical studies of apparently pervasive high density small-scale
interstellar structure.Comment: 10 pages, 2 figures, to appear in the Astrophysical Journal (Letters
The Nature of the Hard-X-Ray Emitting Symbiotic Star RT Cru
We describe Chandra High-Energy Transmission Grating Spectrometer
observations of RT Cru, the first of a new sub-class of symbiotic stars that
appear to contain white dwarfs (WDs) capable of producing hard X-ray emission
out to greater than 50 keV. The production of such hard X-ray emission from the
objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg)
challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV
X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the
optically thin accretion-disk boundary layers of some dwarf novae. The
parameters of the spectral fit confirm that the compact accretor is a WD, and
they are consistent with the WD being massive. We detect rapid, stochastic
variability from the X-ray emission below 4 keV. The combination of flickering
variability and a cooling-flow spectrum indicates that RT Cru is likely powered
by accretion through a disk. Whereas the cataclysmic variable stars with the
hardest X-ray emission are typically magnetic accretors with X-ray flux
modulated at the WD spin period, we find that the X-ray emission from RT Cru is
not pulsed. RT Cru therefore shows no evidence for magnetically channeled
accretion, consistent with our interpretation that the Chandra spectrum arises
from an accretion-disk boundary layer.Comment: 3 figures, accepted for publication in Ap
BANANA IV: Two aligned stellar rotation axes in the young eccentric binary system EP Crucis: primordial orientation and tidal alignment
With observations of the EP Cru system, we continue our series of
measurements of spin-orbit angles in eclipsing binary star systems, the BANANA
project (Binaries Are Not Always Neatly Aligned). We find a close alignment
between the sky projections of the rotational and orbital angular momentum
vectors for both stars (beta_p = -1.8+-1.6 deg and |beta_s|<17 deg). We also
derive precise absolute dimensions and stellar ages for this system. The EP Cru
and DI Her systems provide an interesting comparison: they have similar stellar
types and orbital properties, but DI Her is younger and has major spin-orbit
misalignments, raising the question of whether EP Cru also had a large
misalignment at an earlier phase of evolution. We show that tidal dissipation
is an unlikely explanation for the good alignment observed today, because
realignment happens on the same timescale as spin-orbit synchronization, and
the stars in EP Cru are far from syncrhonization (they are spinning 9 times too
quickly). Therefore it seems that some binaries form with aligned axes, while
other superficially similar binaries are formed with misaligned axes.Comment: ApJ accepted, 10 pages, 7 figure
Chandra Spectroscopy Of The Hot Star β Crucis And The Discovery Of A Pre-Main-Sequence Companion
In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, beta Crucis (beta Cru; B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3 million K, and that the emission lines are resolved but quite narrow, with half widths of 150 km s(-1). The forbidden-to-intercombination line ratios of Ne IX and Mg XI indicate that the hot plasma is distributed in the wind, rather than confined near the photosphere. It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars, primarily because of the narrow lines, but also because of the high X-ray production efficiency. A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data, but not very well motivated theoretically. It is possible that magnetic channelling could explain the X-ray properties, although no field has been detected on beta Cru. We detected periodic variability in the hard (h nu \u3e 1 keV) X-rays, modulated on the known optical period of 4.58 h, which is the period of the primary beta Cephei pulsation mode for this star. We also have detected, for the first time, an apparent companion to beta Cru at a projected separation of 4 arcsec. This companion was likely never seen in optical images because of the presumed very high contrast between it and beta Cru in the optical. However, the brightness contrast in the X-ray is only 3:1, which is consistent with the companion being an X-ray active low-mass pre-main-sequence star. The companion\u27s X-ray spectrum is relatively hard and variable, as would be expected from a post-T Tauri star. The age of the beta Cru system (between 8 and 10 Myr) is consistent with this interpretation which, if correct, would add beta Cru to the roster of Lindroos binaries - B stars with low-mass pre-main-sequence companions
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