613 research outputs found
Stream-field interactions in the magnetic accretor AO Piscium
UV spectra of the magnetic accretor AO Psc show absorption features for half
the binary orbit. The absorption is unlike the wind-formed features often seen
in similar stars. Instead, we attribute it to a fraction of the stream that
overflows the impact with the accretion disk. Rapid velocity variations can be
explained by changes in the trajectory of the stream depending on the
orientation of the white-dwarf's magnetic field. Hence we are directly
observing the interaction of an accretion stream with a rotating field. We
compare this behavior to that seen in other intermediate polars and in SW Sex
stars.Comment: Accepted for ApJ; 6 page
On the accretion mode of the intermediate polar V1025 Centauri
The long white-dwarf spin periods in the magnetic cataclysmic variables EX
Hya and V1025 Cen imply that if the systems possess accretion discs then they
cannot be in equilibrium. It has been suggested that instead they are discless
accretors in which the spin-up torques resulting from accretion are balanced by
the ejection of part of the accretion flow back towards the secondary. We
present phase-resolved spectroscopy of V1025 Cen aimed at deducing the nature
of the accretion flow, and compare this with simulations of a discless
accretor. We find that both the conventional disc-fed model and the
discless-accretor model have strengths and weaknesses, and that further work is
needed before we can decide which applies to V1025 Cen.Comment: 9 pages, 8 figures, To appear in MNRAS, includes low-res figures to
reduce siz
The accretion flow in the discless intermediate polar V2400 Ophiuchi
RXTE observations confirm that the X-ray lightcurve of V2400 Oph is pulsed at
the beat cycle, as expected in a discless intermediate polar. There are no
X-ray modulations at the orbital or spin cycles, but optical line profiles vary
with all three cycles. We construct a model for line-profile variations in a
discless accretor, based on the idea that the accretion stream flips from one
magnetic pole to the other, and show that this accounts for the observed
behaviour over the spin and beat cycles. The minimal variability over the
orbital cycle implies that 1) V2400 Oph is at an inclination of only ~10 deg,
and 2) much of the accretion flow is not in a coherent stream, but is circling
the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss
the light this sheds on disc formation in intermediate polars.Comment: 10 pages, 12 figures, To appear in MNRAS, includes low-res figures to
reduce siz
An alternative model of the magnetic cataclysmic variable V1432 Aquilae (=RX J1940.1-1025)
V1432 Aql is currently considered to be an asynchronous AM Her type system,
with an orbital period of 12116.3 s and a spin period of 12150 s. I present an
alternative model in which V1432 Aql is an intermediate polar with disk
overflow or diskless accretion geometry, with a spin period near 4040 s. I
argue that published data are insufficient to distinguish between the two
models; instead, I provide a series of predictions of the two models that can
be tested against future observations.Comment: 10 pages LaTeX including 3 Postscript Figures, to be published in Ap
X-ray Variability of the Magnetic Cataclysmic Variable V1432 Aql and the Seyfert Galaxy NGC 6814
V1432 Aquilae (=RX J1940.2-1025) is the X-ray bright, eclipsing magnetic
cataclysmic variable ~37' away from the Seyfert galaxy, NGC 6814. Due to a 0.3%
difference between the orbital (12116.3 s) and the spin (12150 s) periods, the
accretion geometry changes over the ~50 day beat period. Here we report the
results of an RXTE campaign to observe the eclipse 25 times, as well as of
archival observations with ASCA and BeppoSAX. Having confirmed that the eclipse
is indeed caused by the secondary, we use the eclipse timings and profiles to
map the accretion geometry as a function of the beat phase. We find that the
accretion region is compact, and that it moves relative to the center of white
dwarf on the beat period. The amplitude of this movement suggest a low-mass
white dwarf, in contrast to the high mass previously estimated from its X-ray
spectrum. The size of the X-ray emission region appears to be larger than in
other eclipsing magnetic CVs. We also report on the RXTE data as well as the
long-term behavior of NGC 6814, indicating flux variability by a factor of at
least 10 on time scales of years.Comment: 44 pages including 16 figures; ApJ, in pres
Compton Scattering of Fe K alpha Lines in Magnetic Cataclysmic Variables
Compton scattering of X-rays in the bulk flow of the accretion column in
magnetic cataclysmic variables (mCVs) can significantly shift photon energies.
We present Monte Carlo simulations based on a nonlinear algorithm demonstrating
the effects of Compton scattering on the H-like, He-like and neutral Fe K alpha
lines produced in the post-shock region of the accretion column. The peak line
emissivities of the photons in the post-shock flow are taken into consideration
and frequency shifts due to Doppler effects are also included. We find that
line profiles are most distorted by Compton scattering effects in strongly
magnetized mCVs with a low white dwarf mass and high mass accretion rate and
which are viewed at an oblique angle with respect to the accretion column. The
resulting line profiles are most sensitive to the inclination angle. We have
also explored the effects of modifying the accretion column width and using a
realistic emissivity profile. We find that these do not have a significant
overall effect on the resulting line profiles. A comparison of our simulated
line spectra with high resolution Chandra/HETGS observations of the mCV GK Per
indicates that a wing feature redward of the 6.4 keV line may result from
Compton recoil near the base of the accretion column.Comment: Accepted for publication in MNRAS, 10 pages with 8 figure
Constraints on Thermal Emission Models of Anomalous X-ray Pulsars
Thermal emission from the surface of an ultramagnetic neutron star is
believed to contribute significantly to the soft X-ray flux of the Anomalous
X-ray Pulsars. We compare the detailed predictions of models of the surface
emission from a magnetar to the observed properties of AXPs. In particular, we
focus on the combination of their luminosities and energy-dependent pulsed
fractions. We use the results of recent calculations for strongly magnetized
atmospheres to obtain the angle- and energy-dependence of the surface emission.
We include in our calculations the effects of general relativistic photon
transport and interstellar extinction. We find that the combination of the
large pulsed fractions and the high luminosities of AXPs cannot be accounted
for by surface emission from a magnetar with two antipodal hot regions or a
temperature distribution characteristic of a magnetic dipole. This result is
robust for reasonable neutron star radii, for the range of magnetic field
strengths inferred from the observed spin down rates, and for surface
temperatures consistent with the spectral properties of AXPs. Models with a
single hot emitting region can reproduce the observations, provided that the
distance to one of the sources is ~30% less than the current best estimate, and
allowing for systematic uncertainties in the spectral fit of a second source.
Finally, the thermal emission models with antipodal emission geometry predict a
characteristic strong increase of the pulsed fraction with photon energy, which
is apparently inconsistent with the current data. The energy-dependence of the
pulsed fraction in the models with one hot region shows a wider range of
behavior and can be consistent with the existing data. Upcoming high-resolution
observations with Chandra and XMM-Newton will provide a conclusive test.Comment: 25 preprint pages, 7 color figures, ApJ, in pres
WASP-43b: The closest-orbiting hot Jupiter
We report the discovery of WASP-43b, a hot Jupiter transiting a K7V star
every 0.81 d. At 0.6-Msun the host star has the lowest mass of any star hosting
a hot Jupiter. It also shows a 15.6-d rotation period. The planet has a mass of
1.8 Mjup, a radius of 0.9 Rjup, and with a semi-major axis of only 0.014 AU has
the smallest orbital distance of any known hot Jupiter. The discovery of such a
planet around a K7V star shows that planets with apparently short remaining
lifetimes owing to tidal decay of the orbit are also found around stars with
deep convection zones.Comment: 4 page
WASP-44b, WASP-45b and WASP-46b: three short-period, transiting extrasolar planets
We report the discovery of three extrasolar planets that transit their
moderately bright (Vmag = 12-13) host stars. WASP-44b is a 0.89-MJup planet in
a 2.42-day orbit around a G8V star. WASP-45b is a 1.03-MJup planet which passes
in front of the limb of its K2V host star every 3.13 days. Weak Ca II H+K
emission seen in the spectra of WASP-45 suggests the star is chromospherically
active. WASP-46b is a 2.10-MJup planet in a 1.43-day orbit around a G6V star.
Rotational modulation of the light curves of WASP-46 and weak Ca II H+K
emission in its spectra show the star to be photospherically and
chromospherically active.
We imposed circular orbits in our analyses as the radial velocity data are
consistent with (near-)circular orbits, as could be expected from both
empirical and tidal-theory perspectives for such short-period, Jupiter-mass
planets. We discuss the impact of fitting for eccentric orbits for such planets
when not supported by the data. The derived planetary and stellar radii depend
on the fitted eccentricity and these parameters inform intense theoretical
efforts concerning tidal circularisation and heating, bulk planetary
composition and the observed systematic errors in planetary and stellar radii.
As such, we recommend exercising caution in fitting the orbits of short period,
Jupiter-mass planets with an eccentric model when there is no evidence of
non-circularity.Comment: 12 pages, 8 figures, 6 tables. As accepted for publication in MNRA
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