33,466 research outputs found
Origin of the X-ray Quasi-Periodic Oscillations and Identification of a Transient Ultraluminous X-Ray Source in M82
The starburst galaxy M82 contains two ultraluminous X-ray sources (ULXs),
CXOM82 J095550.2+694047 (=X41.4+60) and CXOM82 J095551.1+694045 (=X42.3+59),
which are unresolved by XMM-Newton. We revisited the two XMM-Newton
observations of M82 and analyzed the surface brightness profiles using the
known Chandra source positions. We show that the quasi-periodic oscillations
(QPOs) detected with XMM-Newton originate from X41.4+60, the brightest X-ray
source in M82. Correcting for the contributions of the unresolved sources, the
QPO at a frequency of 55.8+/-1.3 mHz on 2001 May 06 had a fractional rms
amplitude of 32%, and the QPO at 112.9+/-1.3 mHz on 2004 April 21 had an
amplitude of 21%. The QPO frequency may possibly be correlated with the source
flux, similar to the type C QPOs in XTE 1550-564 and GRS 1915+105, but at
luminosities two orders of magnitude higher. X42.3+59, the second brightest
source in M82, displayed a strikingly high flux of 1.4E-11 ergs/cm^2/s in the
2-10 keV band on 2001 May 6. A seven-year light curve of X42.3+59 shows extreme
variability over a factor of 1000; the source is not detected in several
Chandra observations. This transient behavior suggests accretion from an
unstable disk. If the companion star is massive, as might be expected in the
young stellar environment, then the compact object would likely be an IMBH.Comment: 9 pages, 6 figures, submitted to ApJ on May 08, 200
The Semiclassical Limit for and Gauge Theory on the Torus
We prove that for and quantum gauge theory on a torus,
holonomy expectation values with respect to the Yang-Mills measure d\mu_T(\o)
=N_T^{-1}e^{-S_{YM}(\o)/T}[{\cal D}\o] converge, as , to
integrals with respect to a symplectic volume measure on the moduli
space of flat connections on the bundle. These moduli spaces and the symplectic
structures are described explicitly.Comment: 18 page
Insulin resistance induced by antiretroviral drugs: Current understanding of molecular mechanisms
The increase in incidence of HIV infection continues to be a major public health problem across the world, but more especially in sub-Saharan Africa. Treatment with highly active antiretroviral therapy (HAART) has improved the prognosis of patients with AIDS, but it has also increased the incidence of various metabolic disorders, in particular insulin resistance accompanied by dyslipidaemia, hyperglycaemia and lipodystrophy. This is often accompanied by frank type 2 diabetes and increased mortality from cardiovascular disease. It is important to understand the mechanistic basis for these side-effects as the incidence of these is likely to increase as the rollout of antiretroviral drugs continues
The connection between the 15 GHz radio and gamma-ray emission in blazars
Since mid-2007 we have carried out a dedicated long-term monitoring programme
at 15 GHz using the Owens Valley Radio Observatory 40 meter telescope. One of
the main goals of this programme is to study the relation between the radio and
gamma-ray emission in blazars and to use it as a tool to locate the site of
high energy emission. Using this large sample of objects we are able to
characterize the radio variability, and study the significance of correlations
between the radio and gamma-ray bands. We find that the radio variability of
many sources can be described using a simple power law power spectral density,
and that when taking into account the red-noise characteristics of the light
curves, cases with significant correlation are rare. We note that while
significant correlations are found in few individual objects, radio variations
are most often delayed with respect to the gamma-ray variations. This suggests
that the gamma-ray emission originates upstream of the radio emission. Because
strong flares in most known gamma-ray-loud blazars are infrequent, longer light
curves are required to settle the issue of the strength of radio-gamma
cross-correlations and establish confidently possible delays between the two.
For this reason continuous multiwavelength monitoring over a longer time period
is essential for statistical tests of jet emission models.Comment: To appear in the Proceedings of the IAU Symposium No. 313:
"Extragalactic jets from every angle," Galapagos, Ecuador, 15-19 September
2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska (Eds.),
Cambridge University Pres
Omega-3 Fatty Acids Improve Recovery, whereas Omega-6 Fatty Acids Worsen Outcome, after Spinal Cord Injury in the Adult Rat
Spinal cord injury (SCI) is a cause of major neurological disability, and no satisfactory treatment is currently available. Evidence suggests that polyunsaturated fatty acids (PUFAs) could target some of the pathological mechanisms that underlie damage after SCI. We examined the effects of treatment with PUFAs after lateral spinal cord hemisection in the rat. The ω-3 PUFAs α-linolenic acid and docosahexaenoic acid (DHA) injected 30 min after injury induced significantly improved locomotor performance and neuroprotection, including decreased lesion size and apoptosis and increased neuronal and oligodendrocyte survival. Evidence showing a decrease in RNA/DNA oxidation suggests that the neuroprotective effect of ω-3 PUFAs involved a significant antioxidant function. In contrast, animals treated with arachidonic acid, an ω-6 PUFA, had a significantly worse outcome than controls. We confirmed the neuroprotective effect of ω-3 PUFAs by examining the effects of DHA treatment after spinal cord compression injury. Results indicated that DHA administered 30 min after spinal cord compression not only greatly increased survival of neurons but also resulted in significantly better locomotor performance for up to 6 weeks after injury.
This report shows a striking difference in efficacy between the effects of treatment with ω-3 and ω-6 PUFAs on the outcome of SCI, with ω-3 PUFAs being neuroprotective and ω-6 PUFAs having a damaging effect. Given the proven clinical safety of ω-3 PUFAs, our observations show that these PUFAs have significant therapeutic potential in SCI. In contrast, the use of preparations enriched in ω-6 PUFAs after injury could worsen outcome after SCI
The Accretion Flows and Evolution of Magnetic Cataclysmic Variables
We have used a model of magnetic accretion to investigate the accretion flows
of magnetic cataclysmic variables. Numerical simulations demonstrate that four
types of flow are possible: discs, streams, rings and propellers. The
fundamental observable determining the accretion flow, for a given mass ratio,
is the spin-to-orbital period ratio of the system. If IPs are accreting at
their equilibrium spin rates, then for a mass ratio of 0.5, those with
Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be
stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. The spin to
orbital period ratio at which the systems transition between these flow types
increases as the mass ratio of the stellar components decreases.
For the first time we present evolutionary tracks of mCVs which allow
investigation of how their accretion flow changes with time. As systems evolve
to shorter orbital periods and smaller mass ratios, in order to maintain spin
equilibrium, their spin-to-orbital period ratio will generally increase. As a
result, the relative occurrence of ring-like flows will increase, and the
occurrence of disc-like flows will decrease, at short orbital periods. The
growing number of systems observed at high spin-to-orbital period ratios with
orbital periods below 2h, and the observational evidence for ring-like
accretion in EX Hya, are fully consistent with this picture.Comment: Accepted for publication in ApJ. 6 figures - included here at low
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