3,770 research outputs found
Detecting Blackholes and Volcanoes in Directed Networks
In this paper, we formulate a novel problem for finding blackhole and volcano
patterns in a large directed graph. Specifically, a blackhole pattern is a
group which is made of a set of nodes in a way such that there are only inlinks
to this group from the rest nodes in the graph. In contrast, a volcano pattern
is a group which only has outlinks to the rest nodes in the graph. Both
patterns can be observed in real world. For instance, in a trading network, a
blackhole pattern may represent a group of traders who are manipulating the
market. In the paper, we first prove that the blackhole mining problem is a
dual problem of finding volcanoes. Therefore, we focus on finding the blackhole
patterns. Along this line, we design two pruning schemes to guide the blackhole
finding process. In the first pruning scheme, we strategically prune the search
space based on a set of pattern-size-independent pruning rules and develop an
iBlackhole algorithm. The second pruning scheme follows a divide-and-conquer
strategy to further exploit the pruning results from the first pruning scheme.
Indeed, a target directed graphs can be divided into several disconnected
subgraphs by the first pruning scheme, and thus the blackhole finding can be
conducted in each disconnected subgraph rather than in a large graph. Based on
these two pruning schemes, we also develop an iBlackhole-DC algorithm. Finally,
experimental results on real-world data show that the iBlackhole-DC algorithm
can be several orders of magnitude faster than the iBlackhole algorithm, which
has a huge computational advantage over a brute-force method.Comment: 18 page
High redshift supermassive blackholes: accretion through cold flows
We use zoom-in techniques to re-simulate three high-redshift (z > 5.5) halos
which host 10^9 solar mass blackholes from the ~ Gpc volume, MassiveBlack
cosmological hydrodynamic simulation. We examine a number of factors
potentially affecting supermassive blackhole growth at high redshift in
cosmological simulations. These include numerical resolution, feedback
prescriptions and formulation of smoothed particle hydrodynamics. We find that
varying the size of the region over which feedback energy is deposited
directly, either for fixed number of neighbours or fixed volume makes very
little difference to the accretion history of blackholes. Changing mass
resolution by factors of up to 64 also does not change the blackhole growth
history significantly. We find that switching from the density-entropy
formulation to the pressure-entropy formulation of smoothed particle
hydrodynamics slightly increases the accretion rate onto blackholes. In general
numerical details appear to have small effects on the main fueling mechanism
for blackholes at these high redshifts. We examine the fashion by which this
occurs, finding that the insensitivity to simulation technique seems to be a
hallmark of the cold flow feeding picture of these high-z supermassive
blackholes. We show that the gas that participates in critical accretion
phases, in these massive objects at z > 6~7 is in all cases colder, denser, and
forms more coherent streams than the average gas in the halo. This is also
mostly the case when the blackhole accretion is feedback regulated (z < 6),
however the distinction is less prominent. For our resimulated halos, cold
flows appear to be a viable mechanism for forming the most massive blackholes
in the early universe, occurring naturally in LambdaCDM models of structure
formation. Not requiring fine tuning of numerical parameters, they seem to be
physically inevitable in these objects.Comment: 15 pages, 12 figure
The Extreme Nuclear Environments of Sgr A* and Arp 220
The dense ISM which is the fuel for both nuclear starbursts is believed to be accreted to the nucleus by stellar bars and galactic interactions. In this contribution, I summarize the observational results for two galactic nuclei at the extreme ends of starburst/AGN activity − our own Galactic nucleus with SgrA* and the ULIRG Arp 220. I discuss theoretical considerations for the properties of the ISM − its density and scale height, whether it is likely to clump into gravitational bound GMCs − and the self-regulation of SB and AGN fueling due to radiation pressure support of the ISM. The latter yields an Eddington-like limit on the activity for both SB and AGN, corresponding to approximately 500 L_ʘ/M_ʘ for optically thick regions in which the radiation has been degraded to the NIR
Instabilities near the QCD phase transition in the holographic models
The paper discusses phenomena close to the critical QCD temperature, using
the holographic model. One issue studied is the overcooled high-T phase, in
which we calculate quasi normal sound modes. We do not find instabilities
associated with other first order phase transitions, but nevertheless observe
drastic changes in sound propagation/dissipation. The rest of the paper
considers a cluster of the high-T phase in the UV in coexistence with the low-T
phase, in a simplified ansatz in which the wall separating them is positioned
only in the holographic coordinate. This allows to find the force on the wall
and classical motion of the cluster. When classical motion is forbidden, we
evaluate tunneling probability through the remaining barrier.Comment: 11 pages, 7 figure
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Common security issues and challenges in wireless sensor networks and IEEE 802.11 wireless mesh networks
Both Wireless Mesh Network (WMN) and Wireless Sensor Network (WSN) are multi-hop wireless networks. WMN is an emerging community based integrated broadband wireless network which ensures high bandwidth ubiquitous internet provision to users, while, WSN is application specific and ensures large scale real-time data processing in complex environment. Both these wireless networks have some common vulnerable features which may increase the chances of different sorts of security attacks. Wireless sensor nodes have computation, memory and power limitations, which do not allow for implementation of complex security mechanism. In this paper, we discuss the common limitations and vulnerable features of WMN and WSN, along with the associated security threats and possible countermeasures. We also propose security mechanisms keeping in view the architecture and limitations of both. This article will serve as a baseline guide for the new researchers who are concern with the security aspects of WMN and WSN
On the origin of the featureless soft X-ray excess emission from the Seyfert 1 galaxy ESO~198--G24
We present medium and high resolution X-ray spectral study of a Seyfert 1
galaxy ESO~198--G24 using a long (122 ks) XMM-Newton observation performed in
February 2006. The source has a prominent featureless soft X-ray excess below
2\kev. This makes the source well suited to investigate the origin of the
soft excess. Two physical models -- blurred reflection, and optically thick
thermal Comptonization in a warm plasma, describe the soft-excess equally well
resulting in similar fits in the 0.3-10\kev band. These models also yield
similar fits to the broad-band UV (Optical Monitor) and X-ray data. XMM-Newton
observations performed in 2000, 2001 and 2006 on this source show flux
variability. From 2001 to 2006, the UV flux increased by while the
2-10\kev X-ray flux as well as the soft-excess flux decreased by ~ 20. This
observation can be described in the blurred reflection scenario by a truncated
accretion disk whose inner-most radius had come closer to the blackhole. We
find that the best-fit inner radius of the accretion disk decreases from
R_{in}=4.93_{-1.10}^{+1.12}R_G to R_{in}<2.5R_G from 2001 to 2006. This leads
to an increase in the UV flux and compressing the corona, leading to reduction
of the powerlaw flux and therefore the soft-excess. The blurred reflection
model seems to better describe the soft-excess for this source.Comment: Accepted for publication in the MNRA
Security in Wireless Sensor Networks: Issues and Challenges
Wireless Sensor Network (WSN) is an emerging technology that shows great
promise for various futuristic applications both for mass public and military.
The sensing technology combined with processing power and wireless
communication makes it lucrative for being exploited in abundance in future.
The inclusion of wireless communication technology also incurs various types of
security threats. The intent of this paper is to investigate the security
related issues and challenges in wireless sensor networks. We identify the
security threats, review proposed security mechanisms for wireless sensor
networks. We also discuss the holistic view of security for ensuring layered
and robust security in wireless sensor networks.Comment: 6 page
First results from the IllustrisTNG simulations: the galaxy color bimodality
We introduce the first two simulations of the IllustrisTNG project, a next
generation of cosmological magnetohydrodynamical simulations, focusing on the
optical colors of galaxies. We explore TNG100, a rerun of the original
Illustris box, and TNG300, which includes 2x2500^3 resolution elements in a
volume twenty times larger. Here we present first results on the galaxy color
bimodality at low redshift. Accounting for the attenuation of stellar light by
dust, we compare the simulated (g-r) colors of 10^9 < M*/Msun < 10^12.5
galaxies to the observed distribution from the Sloan Digital Sky Survey (SDSS).
We find a striking improvement with respect to the original Illustris
simulation, as well as excellent quantitative agreement in comparison to the
observations, with a sharp transition in median color from blue to red at a
characteristic M* ~ 10^10.5 Msun. Investigating the build-up of the color-mass
plane and the formation of the red sequence, we demonstrate that the primary
driver of galaxy color transition in the TNG model is supermassive blackhole
feedback in its low-accretion state. Across the entire population we measure a
median color transition timescale dt_green of ~1.6 Gyr, a value which drops for
increasingly massive galaxies. We find signatures of the physical process of
quenching: at fixed stellar mass, redder galaxies have lower SFRs, gas
fractions, and gas metallicities; their stellar populations are also older and
their large-scale interstellar magnetic fields weaker than in bluer galaxies.
Finally, we measure the amount of stellar mass growth on the red sequence.
Galaxies with M* > 10^11 Msun which redden at z<1 accumulate on average ~25% of
their final z=0 mass post-reddening; at the same time, ~18% of such massive
galaxies acquire half or more of their final stellar mass while on the red
sequence.Comment: The IllustrisTNG project website is http://www.tng-project.or
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