383 research outputs found
Discovery of an X-ray pulsar in the low-mass X-ray binary 2A 1822-371
We report the discovery of 0.59 s X-ray pulsations from the low-mass X-ray
binary, 5.57 hr dipping and eclipsing ADC source 2A 1822-371. Pulse arrival
time analysis indicates a circular orbit with e < 0.03 (95% confidence) and an
asini for the neutron star of 1.006(5) lightseconds, implying a mass function
of (2.03+-0.03) x 10^-2 M_sun. The barycentric pulse period was 0.59325(2) s in
1996.270 and 0.59308615(5) s in 1998.205, indicating an average spin up with
P_dot/P = (-1.52+-0.02) x 10^-4 yr^-1. For a magnetic field strength of ~1--5 x
10^12 G as derived from the X-ray spectrum the implied intrinsic X-ray
luminosity is ~2-4 x 10^37 erg s^-1. The pulse amplitude is low, but increases
steeply as a function of energy from a sinusoidal amplitude of 0.25% in 2-5.4
keV to ~3% above 20 keV. We discuss the constraints on the masses of the
companion star and the fact that several aspects of the energy spectrum are in
qualitative accordance with that of a strongly magnetised neutron star.Comment: 6 pages long, including 4 figures, uses emulateapj5, accepted for
ApJL, Replaced figure
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
Outbursts of EX Hydrae Revisited
We present optical spectroscopy of EX Hya during its 1991 outburst. This
outburst is characterised by strong irradiation of the front face of the
secondary star by the white dwarf, an overflowing stream which is seen strongly
in HeII and by a dip in the light curves, which extends from 0.1-0.6 in the
binary and spin phases. Strong irradiation of the accretion curtain and that of
the inner regions of the disc led to strong emission of HeII and to the
suppression of the Hg and Hb emission.
Disc overflow was observed in quiescence in earlier studies, where the
overflow stream material was modulated at high velocities close to 1000 km/s.
In outburst, the overflowing material is modulated at even higher velocities
(~1500 km/s). These are streaming velocities down the field lines close to the
white dwarf. Evidence for material collecting near the outer edge of the disc
and corotating with the accretion curtain was observed. In decline, this
material and the accretion curtain obscured almost all the emission near binary
phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse
minimum. This has provided additional evidence for an extended accretion
curtain, and for the corotation of material with the accretion curtain at the
outer edge of the disc. From these observations we suggest that a mechanism
similar to that of Spruit & Taam, where outbursts result due to the storage and
release of matter outside the magnetosphere, triggers the outbursts of EX Hya.
This is followed by the irradiation of the secondary star due to accretion
induced radiation.Comment: 12 pages, 14 figures, 1 table. Figures 6, 7, 8 and 11 at low
resolution. Paper accepted by the Monthly Notices of the Royal Astronomical
Societ
Why do some intermediate polars show soft X-ray emission? A survey of XMM-Newton spectra
We make a systematic analysis of the XMM-Newton X-ray spectra of intermediate
polars (IPs) and find that, contrary to the traditional picture, most show a
soft blackbody component. We compare the results with those from AM Her stars
and deduce that the blackbody emission arises from reprocessing of hard X-rays,
rather than from the blobby accretion sometimes seen in AM Hers. Whether an IP
shows a blackbody component appears to depend primarily on geometric factors: a
blackbody is not seen in those that have accretion footprints that are always
obscured by accretion curtains or are only visible when foreshortened on the
white-dwarf limb. Thus we argue against previous suggestions that the blackbody
emission characterises a separate sub-group of IPs which are more akin to AM
Hers, and develop a unified picture of the blackbody emission in these stars.Comment: 9 pages, 6 figures. Accepted for publication in Ap
The phase 0.5 absorption in SW Sextantis-type cataclysmic variables
The SW Sextantis stars are a group of cataclysmic variables with distinctive
observational characteristics, including absorption features in the emission
line cores at phases 0.2-0.6. Hellier and Robinson have proposed that these
features are caused by the accretion stream flowing over the accretion disk.
However, in a simple model the absorption occurred at all orbital phases, which
is contradicted by the data. I show that invoking a flared accretion disk
resolves this problem.Comment: 4 pages, 3 figures, LaTeX, To appear in PASP. Also at
http://www.astro.keele.ac.uk/~ch
Interaction of amino acids with glycyl-glycine transport in the mammalian intestine
In order to investigate a possible interaction between free amino acids and dipeptides during their mucosal uptake in man and monkey, perfusion studies in vivo and uptake studiesin vitro using labelled and non-labelled dipeptides and amino acids have been carried out. In contrast to the observations of other workers, inhibition of glycyl-glycine uptake was observed with free leucine and methioninc but not with glycine, proline, hydroxyproline or alanine. Leucine and methionine caused inhibition of cytosol glycyl-glycine hydrolase activity, while glycine had no effect. The dipeptide uptake and dipeptide hydrolysis by cytosol enzyme was competitively inhibited by leucine. Although brush border glycyl-glycine hydrolase was also inhibited by leucine, the inhibition was noncompetitive. These data indicate that a few free amino acids can interact with dipeptides during uptake. This interaction might occur either at the transport step or at the stage of intracellular dipeptide hydrolysis
Periodicities In The X-Ray Intensity Variations of TV Columbae: An Intermediate Polar
We present results from a temporal analysis of the longest and the most
sensitive X-ray observations of TV Columbae--an intermediate polar. The
observations were carried out with the RXTE PCA, ROSAT PSPC, and ASCA. Data
were analyzed using a 1-dimensional CLEAN and Bayesian algorithms. The presence
of a nearly sinusoidal modulation due to the spin of the white dwarf is seen
clearly in all the data, confirming the previous reports based on the EXOSAT
data. An improved period of 1909.7+/-2.5s is derived for the spin from the RXTE
data.The binary period of 5.5hr is detected unambiguously in X-rays for the
first time. Several side-bands due to the interaction of these periods are
observed in the power spectra, thereby suggesting contributions from both the
disk-fed and the stream-fed accretion for TV Col. The accretion disk could
perhaps be precessing as side-bands due to the influence of 4 day period on the
orbital period are seen. The presence of a significant power at certain
side-bands of the spin frequency indicates that the emission poles are
asymmetrically located. The strong power at the orbital side-bands seen in both
the RXTE and ROSAT data gives an indication for an absorption site fixed in the
orbital frame. Both the spin and the binary modulation are found to be
energy-dependent. Increased hardness ratio during a broad dip in the intensity
at binary phase of 0.75--1.0 confirms the presence of a strong attenuation due
to additional absorbers probably from an impact site of the accretion stream
with the disk or magnetosphere. Hardness ratio variations and the energy
dependent modulation depth during the spin modulation can be explained by
partially covered absorbers in the path of X-ray emission region in the
accretion stream.Comment: 34 pages, including 12 figures, Accepted for publication in
Astronomical Journal, scheduled for January 2004 issue (vol. 127
PX Andromedae: Superhumps and variable eclipse depth
Results of a photometric study of the SW Sex novalike PX And are presented.
The periodogram analysis of the observations obtained in October 2000 reveals
the presence of three signals with periods of 0.142, 4.8 and 0.207 days. The
first two periods are recognized as "negative superhumps" and the corresponding
retrograde precession period of the accretion disk. The origin of the third
periodic signal remains unknown. The observations in September-October 2001
point only to the presence of "negative superhumps" and possibly to the
precession period. The origin of the "negative superhumps" is discussed and two
possible mechanisms are suggested. All light curves show strong flickering
activity and power spectra with a typical red noise shape. PX And shows
eclipses with highly variable shape and depth. The analysis suggests that the
eclipse depth is modulated with the precession period and two possible
explanations of this phenomenon are discussed. An improved orbital ephemeris is
also determined: T_min[HJD]=49238.83662(14)+0.146352739(11)E.Comment: LaTeX, 8 pages, 9 EPS figures, final version to appear in A&A, uses
times font, few minor correction
Photometry of the SW Sex-type nova-like BH Lyncis in high state
Aims: We present a photometric study of the deeply eclipsing SW Sex-type
nova-like cataclysmic variable star BH Lyn
Methods: Time-resolved V-band CCD photometry was obtained for seven nights
between 1999 and 2004.
Results: We determined 11 new eclipse timings of BH Lyn and derived a refined
orbital ephemeris with an orbital period of 0.155875577(14) day. During the
observations, BH Lyn was in high-state with V~15.5 mag. The star presents ~1.5
mag deep eclipses with mean full-width at half-flux of 0.0683(+/-0.0054)P_orb.
The eclipse shape is highly variable, even changing form cycle to cycle. This
is most likely due to accretion disc surface brightness distribution
variations, most probably caused by strong flickering. Time-dependent accretion
disc self-occultation or variations of the hot spot(s) intensity are also
possible explanations. Negative superhumps with period of ~0.145 day are
detected in two long runs in 2000. A possible connection between SW Sex and
negative superhump phenomena through the presence of tilted accretion disc is
discussed, and a way to observationally test this is suggested
General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars
Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray
spectra and pulsations by means of radiation processes that occur on the
surfaces of neutron stars. For any such model, general relativistic deflection
of light severely suppresses the amplitude of the observed pulsations. We
calculate the expected pulsation amplitudes of AXPs according to various models
and compare the results with observations. We show that the high (<= 70%) pulse
amplitudes observed in some AXPs can be accounted for only if the surface
emission is localized (spot radius <40 degrees) and strongly beamed
(cos^n[theta'] with n>2, where theta' is the angle to the normal). These
constraints are incompatible with those cooling and magnetar models in which
the observed X-rays originate as thermal emission from the neutron-star
surface. Accretion models, on the other hand, are compatible with observations
for a wide range of parameters. Finally, definitive conclusions cannot be
reached on magnetospheric models, since their localization and beaming
properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa
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