64 research outputs found
Dynamic Processes in Be Star Atmospheres.
This report describes a joint X ray/ultraviolet/ground based study of the abnormal Be star lambda Eri which has previously shown evidence of X ray flaring from Rosat observations in 1991. The 1991 flare event caught the astronomical hot star community by surprise because x ray flares have not been observed from other single B-type stars, before or since. Both optical (H-alpha) and UV/Voyager observations provide evidence for transient heating events near the surface of lambda Eri
A precessing accretion disc in the intermediate polar XY Ari?
XY Ari is the only intermediate polar to show deep X-ray eclipses of its
white dwarf. Previously published observations with Ginga and Chandra have also
revealed a broad X-ray orbital modulation, roughly antiphased with the eclipse,
and presumed to be due to absorption in an extended structure near the edge of
an accretion disc. The X-ray pulse profile is generally seen to be
double-peaked, although a single-peaked pulse was seen by RXTE during an
outburst in 1996.We intended to investigate the cause of the broad orbital
modulation in XY Ari to better understand the accretion flow in this system and
other intermediate polars. We observed XY Ari with RXTE and analysed previously
unpublished archival observations of the system made with ASCA and XMM-Newton.
These observations comprise six separate visits and span about ten years. The
various X-ray observations show that the broad orbital modulation varies in
phase and significance, then ultimately disappears entirely in the last few
years. In addition, the X-ray pulse profile shows variations in depth and
shape, and in the recent RXTE observations displays no evidence for changes in
hardness ratio. The observed changes indicates that both the pulse profile and
the orbital modulation are solely due to geometrical effects at the time of the
RXTE observations, rather than phase-dependent variations in photoelectric
absorption as seen previously. We suggest that this is evidence for a
precessing, tilted accretion disc in this system. The precession of the disc
moves structures out of our line of sight both at its outer edge (changing the
orbital modulation) and at its inner edge where the accretion curtains are
anchored (changing the pulse profile).Comment: Accepted for publication in Astronomy & Astrophysic
A dwarf nova in the globular cluster M13
Dwarf novae in globular clusters seem to be rare with only 13 detections in
the 157 known Galactic globular clusters. We report the identification of a new
dwarf nova in M13, the 14th dwarf nova identified in a globular cluster to
date. Using the 2m Faulkes Telescope North, we conducted a search for stars in
M13 that show variability over a year (2005-2006) on timescales of days and
months. This led to the detection of one dwarf nova showing several outbursts.
A Chandra X-ray source is coincident with this dwarf nova and shows both a
spectrum and variability consistent with that expected from a dwarf nova, thus
supporting the identification. We searched for a counterpart in Hubble Space
Telescope ACS/WFC archived images and found at least 11 candidates, of which we
could characterize only the 7 brightest, including one with a 3 sigma H-alpha
excess and a faint blue star. The detection of one dwarf nova when more could
have been expected likely indicates that our knowledge of the global Galactic
population of cataclysmic variables is too limited. The proportion of dwarf
novae may be lower than found in catalogs, or they may have a much smaller duty
cycle in general as proposed by some population synthesis models and recent
observations in the field.Comment: Accepted for publication in ApJ, 12 pages, 12 figures, 5 tables (v2
contains corrections of the proofs
57 second oscillations in Nova Centauri 1986 (V842 Cen)
High speed photometry in 2008 shows that the light curve of V842 Cen
possesses a coherent modulation at 56.825 s, with sidebands at 56.598 s and
57.054 s. These have appeared since this nova remnant was observed in 2000 and
2002. We deduce that the dominant signal is the rotation period of the white
dwarf primary and the sidebands are caused by reprocessing from a surface
moving with an orbital period of 3.94 h. Thus V842 Cen is an intermediate polar
(IP) of the DQ Herculis subclass, is the fastest rotating white dwarf among the
IPs and is the third fastest known in a cataclysmic variable. As in other IPs
we see no dwarf nova oscillations, but there are often quasi-periodic
oscillations in the range 350 - 1500 s. There is a strong brightness modulation
with a period of 3.78 h, which we attribute to negative superhumps, and there
is an even stronger signal at 2.886 h which is of unknown origin but is
probably a further example of that seen in GW Lib and some other systems. We
used the Swift satellite to observe V842 Cen in the ultra-violet and in X-rays,
although no periodic modulation was detected in the short observations. The
X-ray luminosity of this object appears to be much lower than that of other IPs
in which the accretion region is directly visible.Comment: 6 pages, 10 figures, accepted for publication in MNRA
Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars
The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary
stars is quite soft. Several symbiotic WDs, however, produce strong X-ray
emission at energies greater than ~20 keV. The Swift BAT instrument has
detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT
Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected
X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining
data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT
Cru, T CrB, and CD -57 3057 are well described by emission from a
single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9
keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright
soft component. For all 4 systems, the spectra suffer high levels of absorption
from material that both fully and partially covers the source of hard X-rays.
The XRT data did not show any of the rapid, periodic variations that one would
expect if the X-ray emission were due to accretion onto a rotating, highly
magnetized WD. The X-rays were thus more likely from the accretion-disk
boundary layer around a massive, non-magnetic WD in each binary. The X-ray
emission from RT Cru varied on timescales of a few days. This variability is
consistent with being due to changes in the absorber that partially covers the
source, suggesting localized absorption from a clumpy medium moving into the
line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during
the 9 months of Swift observations, in a manner that was also consistent with
variable absorption.Comment: Accepted for publication in ApJ. 9 pages, 6 figure
A population of isolated hard X-ray sources near the supernova remnant Kes 69
Recent X-ray observations of the supernova remnant IC443 interacting with
molecular clouds have shown the presence of a new population of hard X-ray
sources related to the remnant itself, which has been interpreted in terms of
fast ejecta fragment propagating inside the dense environment. Prompted by
these studies, we have obtained a deep {\sl XMM-Newton} observation of the
supernova remnant (SNR) Kes 69, which also shows signs of shock-cloud
interaction. We report on the detection of 18 hard X-ray sources in the field
of Kes 69, a significant excess of the expected galactic source population in
the field, spatially correlated with CO emission from the cloud in the remnant
environment. The spectra of 3 of the 18 sources can be described as hard power
laws with photon index <2 plus line emission associated to K-shell transitions.
We discuss the two most promising scenarios for the interpretation of the
sources, namely fast ejecta fragments (as in IC443) and cataclysmic variables.
While most of the observational evidences are consistent with the former
interpretation, we cannot rule out the latter.Comment: 9 pages, 5 figures, A&A in pres
RXTE determination of the intermediate polar status of XSS J00564+4548, IGR J17195-4100, and XSS J12270-4859
Aims: We determine the nature of the intermediate polar candidates XSS
J00564+4548, IGR J17195-4100, and XSS J12270-4859.
Methods: Pointed RXTE observations searched for intermediate polar
characteristics in these candidate systems.
Results: XSS J00564+4548 exhibits a period of 465.68+-0.07 s, which we
interpret as the spin period, an energy dependent modulation depth, and a
spectrum that is fit by a 22 keV photoelectrically absorbed bremsstrahlung with
an iron line profile. IGR J17195-4100 shows several candidate periodicities and
a spectrum that is fit by a power law with an iron line. XSS J12270-4859
exhibits a candidate spin period of 859.57+-0.64 s and a spectrum that is fit
by a power law with no evidence of an iron line.
Conclusions: XSS J00564+4548 is confirmed to be an intermediate polar. IGR
J17195-4100 and XSS J12270-4859 both show some properties of intermediate
polars, but cannot be confirmed as definite members of the class here.Comment: 5 pages, 9 figures. Accepted for publication in A&
Swift follow-up observations of unclassified ASCA sources
We present Swift follow-up observations of a sample of 35 unclassified faint
X-ray sources drawn from the ASCA Galactic centre and plane surveys. Our short
pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of
~1E-13 erg cm-2 s-1, which translates into a luminosity of ~1E33 erg s-1 for an
assumed distance of D=8 kpc. The brightest source in our sample reaches a
maximum 0.3-10 keV luminosity of ~2E36 (D/8kpc)^2 erg s-1 during our
observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19
were not detected during our program. Since we are probing the faint end of the
ASCA source populations, we expect a large fraction of the non-detections to be
due to the Eddington bias. This is strengthened by the fact that we find the
observed XRT count rates to be predominantly lower than expected based on the
reported ASCA intensities. Investigation of the ASCA properties and long-term
X-ray variability leads us to conclude that the non-detections likely include
two spurious ASCA detections and three objects that are variable or transient
X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies
of ~2-4", which significantly improves upon their ASCA uncertainties of ~1'. We
use the X-ray spectra and variability to characterise these objects. Most
appear to be faint, persistent X-ray emitters that have highly absorbed
spectra. Based on their X-ray properties we identify three accreting compact
objects: one confirmed (AX J1754.2-2754) and one candidate (AX J1538.3-5541)
X-ray binary, and one possible magnetically accreting white dwarf (AX
J1620.1-5002). Furthermore, we use the improved positions of XRT-detected
sources to search for counterparts in simultaneously obtained Swift/UVOT images
and possible associations with cataloged sources at various wavelengths. This
reveals three possible main sequence stars amongst our sample.Comment: 14 pages, 7 figures, 7 tables. Accepted for publication in A&A
without revision. Updated a few reference
Metformin efficacy and safety for colorectal polyps: a double-blind randomized controlled trial
<p>Abstract</p> <p>Background</p> <p>Colorectal cancer is one of the major neoplasms and a leading cause of cancer death worldwide, and new preventive strategies are needed to lower the burden of this disease. Metformin, a biguanide, which is widely used for treating diabetes mellitus, has recently been suggestive to have a suppressive effect on tumorigenesis and cancer cell growth. In a previous study conducted in non-diabetic subjects, we showed that oral short-term low-dose metformin suppressed the development of colorectal aberrant crypt foci (ACF). ACF have been considered as a useful surrogate biomarker of CRC, although the biological significance of these lesions remains controversial. We devised a prospective randomized controlled trial to evaluate the chemopreventive effect of metformin against metachronous colorectal polyps and the safety of this drug in non-diabetic post-polypectomy patients.</p> <p>Methods/Design</p> <p>This study is a multi-center, double-blind, placebo-controlled, randomized controlled trial to be conducted in non-diabetic patients with a recent history of undergoing colorectal polypectomy. All adult patients visiting the Yokohama City University hospital or affiliated hospitals for polypectomy shall be recruited for the study. Eligible patients will then be allocated randomly into either one of two groups: the metformin group and the placebo group. Patients in the metformin group shall receive oral metformin at 250 mg per day, and those in the placebo group shall receive an oral placebo tablet. At the end of 1 year of administration of metformin/placebo, colonoscopy will be performed to evaluate the polyp formation.</p> <p>Discussion</p> <p>This is the first study proposed to explore the effect of metformin against colorectal polyp formation. Metformin activates AMPK, which inhibits the mammalian target of rapamycin (mTOR) pathway. The mTOR pathway plays an important role in the cellular protein translational machinery and cell proliferation. Patients with type 2 diabetes taking under treatment with metformin have been reported to be at a lower risk of cancer development than those not taking under treatment with metformin. We showed in a previous study that metformin suppressed the formation of human colorectal ACF. We therefore decided to conduct a study to determine whether metformin might suppress the formation of human colorectal polyps.</p> <p>Trial registration</p> <p>This trial has been registered in the University hospital Medical Information Network (UMIN) Clinical Trials Registry as <a href="http://www.clinicaltrials.gov/ct2/show/UMIN000006254">UMIN000006254</a></p
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