6,778 research outputs found
Defining dangerous: Report of the annex 1 workshop on article 2 of the climate convention. (HOT WD1)
The Effect of Variability on the Estimation of Quasar Black Hole Masses
We investigate the time-dependent variations of ultraviolet (UV) black hole
mass estimates of quasars in the Sloan Digital Sky Survey (SDSS). From SDSS
spectra of 615 high-redshift (1.69 < z < 4.75) quasars with spectra from two
epochs, we estimate black hole masses, using a single-epoch technique which
employs an additional, automated night-sky-line removal, and relies on UV
continuum luminosity and CIV (1549A) emission line dispersion. Mass estimates
show variations between epochs at about the 30% level for the sample as a
whole. We determine that, for our full sample, measurement error in the line
dispersion likely plays a larger role than the inherent variability, in terms
of contributing to variations in mass estimates between epochs. However, we use
the variations in quasars with r-band spectral signal-to-noise ratio greater
than 15 to estimate that the contribution to these variations from inherent
variability is roughly 20%. We conclude that these differences in black hole
mass estimates between epochs indicate variability is not a large contributer
to the current factor of two scatter between mass estimates derived from low-
and high-ionization emission lines.Comment: 76 pages, 15 figures, 2 (long) tables; Accepted for publication in
ApJ (November 10, 2007
Spectral Variability of Quasars in the Sloan Digital Sky Survey. II: The C IV Line
We examine the variability of the high-ionizaton C IV line in a sample of 105
quasars observed at multiple epochs by the Sloan Digital Sky Survey. We find a
strong correlation between the change in the C IV line flux and the change in
the line width, but no correlations between the change in flux and changes in
line center and skewness. The relation between line flux change and line width
change is consistent with a model in which a broad line base varies with
greater amplitude than the line core. The objects studied here are more
luminous and at higher redshift than those normally studied for variability,
ranging in redshift from 1.65 to 4.00 and in absolute r-band magnitude from
roughly -24 to -28. Using moment analysis line-fitting techniques, we measure
line fluxes, centers, widths and skewnesses for the C IV line at two epochs for
each object. The well-known Baldwin Effect is seen for these objects, with a
slope beta = -0.22. The sample has a median intrinsic Baldwin Effect slope of
beta = -0.85; the C IV lines in these high-luminosity quasars appear to be less
responsive to continuum variations than those in lower luminosity AGN.
Additionally, we find no evidence for variability of the well known blueshift
of the C IV line with respect to the low-ionization Mg II line in the highest
flux objects, indicating that this blueshift might be useful as a measure of
orientation.Comment: 52 pages, 14 figures, accepted for publication in Ap
Mayhem: Targeted Corruption of Register and Stack Variables
In the past decade, many vulnerabilities were discovered in
microarchitectures which yielded attack vectors and motivated the study of
countermeasures. Further, architectural and physical imperfections in DRAMs led
to the discovery of Rowhammer attacks which give an adversary power to
introduce bit flips in a victim's memory space. Numerous studies analyzed
Rowhammer and proposed techniques to prevent it altogether or to mitigate its
effects.
In this work, we push the boundary and show how Rowhammer can be further
exploited to inject faults into stack variables and even register values in a
victim's process. We achieve this by targeting the register value that is
stored in the process's stack, which subsequently is flushed out into the
memory, where it becomes vulnerable to Rowhammer. When the faulty value is
restored into the register, it will end up used in subsequent iterations. The
register value can be stored in the stack via latent function calls in the
source or by actively triggering signal handlers. We demonstrate the power of
the findings by applying the techniques to bypass SUDO and SSH authentication.
We further outline how MySQL and other cryptographic libraries can be targeted
with the new attack vector. There are a number of challenges this work
overcomes with extensive experimentation before coming together to yield an
end-to-end attack on an OpenSSL digital signature: achieving co-location with
stack and register variables, with synchronization provided via a blocking
window. We show that stack and registers are no longer safe from the Rowhammer
attack
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