6 research outputs found
Spectral variability of ultraluminous X-ray sources
We study spectral variability of 11 ultraluminous X-ray sources (ULX) using
archived XMM-Newton and Chandra observations. We use three models to describe
the observed spectra; a power-law, a multi-colour disk (MCD) and a combination
of these two models. We find that out of the 11 ULXs in our sample, 7 ULXs show
a correlation between the luminosity and the photon index Gamma (hereafter
L-Gamma correlation). Furthermore, out of the 7 ULXs that have the L-Gamma
correlation, 4 ULXs also show spectral pivoting in the observed energy band. We
also find that two ULXs show an L-Gamma anti-correlation. The spectra of 4 ULXs
in the sample can be adequately fitted with a MCD model. We compare these
sources to known black hole binaries (BHB) and find that they follow similar
paths in their luminosity-temperature (hereafter L-T) diagrams. Finally we show
that the 'soft excess' reported for many of these ULXs at 0.2 keV seem to
follow a trend L \propto T^{-4} when modeled with a power-law plus a 'cool' MCD
model. This is contrary to the expected L \propto T^4 relation that is expected
from theory and what is seen for many accreting BHBs.Comment: 9 pages, 3 figures; to appear in the proceedings of "Cool discs, hot
flows: The Varying Faces of Accreting Compact Objects", Funasdalen, Sweden,
2008, ed. M. Axelsson (New York: AIP
Oxygen Isotopic Compositions of Asteroidal Materials returned from Itokawa by the Hayabusa Mission
Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves hav
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IMP dehydrogenase-2 drives aberrant nucleolar activity and promotes tumorigenesis in glioblastoma
In many cancers, high proliferation rates correlate with elevation of rRNA and tRNA levels, and nucleolar hypertrophy. However, the underlying mechanisms linking increased nucleolar transcription and tumorigenesis are only minimally understood. Here we show that IMP dehydrogenase-2 (IMPDH2), the rate-limiting enzyme for de novo guanine nucleotide biosynthesis, is overexpressed in the highly lethal brain cancer glioblastoma. This leads to increased rRNA and tRNA synthesis, stabilization of the nucleolar GTP-binding protein nucleostemin, and enlarged, malformed nucleoli. Pharmacological or genetic inactivation of IMPDH2 in glioblastoma reverses these effects and inhibits cell proliferation, whereas untransformed glia cells are unaffected by similar IMPDH2 perturbations. Impairment of IMPDH2 activity triggers nucleolar stress and growth arrest of glioblastoma cells even in the absence of functional p53. Our results reveal that upregulation of IMPDH2 is a prerequisite for the occurance of aberrant nucleolar function and increased anabolic processes in glioblastoma, which constitutes a primary event in gliomagenesis