14 research outputs found
The hard X-ray perspective on the soft X-ray excess
The X-ray spectra of many active galactic nuclei (AGN) exhibit a `soft
excess' below 1keV, whose physical origin remains unclear. Diverse models have
been suggested to account for it, including ionised reflection of X-rays from
the inner part of the accretion disc, ionised winds/absorbers, and
Comptonisation. The ionised reflection model suggests a natural link between
the prominence of the soft excess and the Compton reflection hump strength
above 10keV, but it has not been clear what hard X-ray signatures, if any, are
expected from the other soft X-ray candidate models. Additionally, it has not
been possible up until recently to obtain high-quality simultaneous
measurements of both soft and hard X-ray emission necessary to distinguish
these models, but upcoming joint XMM-NuSTAR programmes provide precisely this
opportunity. In this paper, we present an extensive analysis of simulations of
XMM+NuSTAR observations, using two candidate soft excess models as inputs, to
determine whether such campaigns can disambiguate between them by using hard
and soft X-ray observations in tandem. The simulated spectra are fit with the
simplest "observer's model" of a black body and neutral reflection to
characterise the strength of the soft and hard excesses. A plot of the strength
of the hard excess against the soft excess strength provides a diagnostic plot
which allows the soft excess production mechanism to be determined in
individual sources and samples using current state-of-the-art and next
generation hard X-ray enabled observatories. This approach can be
straightforwardly extended to other candidate models for the soft excess.Comment: 12 pages, 11 figures, accepted for publication in ApJ. Added
reference
DDoS defense by offense
This article presents the design, implementation, analysis, and experimental evaluation of speak-up, a defense against application-level distributed denial-of-service (DDoS), in which attackers cripple a server by sending legitimate-looking requests that consume computational resources (e.g., CPU cycles, disk). With speak-up, a victimized server encourages all clients, resources permitting, to automatically send higher volumes of traffic. We suppose that attackers are already using most of their upload bandwidth so cannot react to the encouragement. Good clients, however, have spare upload bandwidth so can react to the encouragement with drastically higher volumes of traffic. The intended outcome of this traffic inflation is that the good clients crowd out the bad ones, thereby capturing a much larger fraction of the server's resources than before. We experiment under various conditions and find that speak-up causes the server to spend resources on a group of clients in rough proportion to their aggregate upload bandwidths, which is the intended result.National Science Foundation (U.S.) (NSF grant CNS-0225660)National Science Foundation (U.S.) (NSF grant CNS-0520241)United States. Dept. of Defense (National Security Science and Engineering Faculty Fellowship
A study on mountain front recharge by using integrated techniques in the hard rock aquifers of southern India
Mountain front recharge (MFR) is the contribution of mountains to recharge the aquifers in the adjacent basins. The estimation of MFR is essential to obtain a detailed investigation of recharge of the groundwater at the mountain front. This study summarises the current understanding of recharge processes by comparing daily groundwater fluctuation to daily rainfall and identifies the recharge rates. The recharge rates vary with time due to difference in water table depth and travel time. Thus to understand the MFR along the foothills of Courtallam, a total of 14 surface water, rainwater and groundwater samples were collected and measured for stable isotopes. The isotopic data were used to investigate the recharge process and to identify the elevations to recharge. The study findings also suggest that predominantly rainfall along the foothills contributes recharge to the riparian zone (basin block region), whereas foothill regions receive recharge from rainfall over mountain block
Thigh-length compression stockings and DVT after stroke
Controversy exists as to whether neoadjuvant chemotherapy improves survival in patients with invasive bladder cancer, despite randomised controlled trials of more than 3000 patients. We undertook a systematic review and meta-analysis to assess the effect of such treatment on survival in patients with this disease