Combining social network analysis and the NATO Approach Space to define agility. Topic 2: networks and networking

This paper takes the NATO SAS-050 Approach Space, a widely accepted model of command and control, and gives each of its primary axes a quantitative measure using social network analysis. This means that the actual point in the approach space adopted by real-life command and control organizations can be plotted along with the way in which that point varies over time and function. Part 1 of the paper presents the rationale behind this innovation and how it was subject to verification using theoretical data. Part 2 shows how the enhanced approach space was put to use in the context of a large scale military command post exercise. Agility is represented by the number of distinct areas in the approach space that the organization was able to occupy and there was a marked disparity between where the organization thought it should be and where it actually was, furthermore, agility varied across function. The humans in this particular scenario bestowed upon the organization the levels of agility that were observed, thus the findings are properly considered from a socio-technical perspective

The Detectability of AGN Cavities in Cooling-Flow Clusters

Chandra X-ray Observatory has revealed X-ray cavities in many nearby cooling flow clusters. The cavities trace feedback from the central active galactic nulceus (AGN) on the intracluster medium (ICM), an important ingredient in stabilizing cooling flows and in the process of galaxy formation and evolution. But, the prevalence and duty cycle of such AGN outbursts is not well understood. To this end, we study how the cooling is balanced by the cavity heating for a complete sample of clusters (the Brightest 55 clusters of galaxies, hereafter B55). In the B55, we found 33 cooling flow clusters, 20 of which have detected X-ray bubbles in their ICM. Among the remaining 13, all except Ophiuchus could have significant cavity power yet remain undetected in existing images. This implies that the duty cycle of AGN outbursts with significant heating potential in cooling flow clusters is at least 60 % and could approach 100 %, but deeper data is required to constrain this further.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters' Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric Wilcots; added annotation to the figur

Jet Interactions with the Hot Halos of Clusters and Galaxies

X-ray observations of cavities and shock fronts produced by jets streaming through hot halos have significantly advanced our understanding of the energetics and dynamics of extragalactic radio sources. Radio sources at the centers of clusters have dynamical ages between ten and several hundred million years. They liberate between 1E58-1E62 erg per outburst, which is enough energy to regulate cooling of hot halos from galaxies to the richest clusters. Jet power scales approximately with the radio synchrotron luminosity to the one half power. However, the synchrotron efficiency varies widely from nearly unity to one part in 10,000, such that relatively feeble radio source can have quasar-like mechanical power. The synchrotron ages of cluster radio sources are decoupled from their dynamical ages, which tend to be factors of several to orders of magnitude older. Magnetic fields and particles in the lobes tend to be out of equipartition. The lobes may be maintained by heavy particles (e.g., protons), low energy electrons, a hot, diffuse thermal gas, or possibly magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and cavities can be used to study the dynamics of extragalactic radio sources.Comment: 10 pages, 3 figures, invited review, "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, held in Girdwood, Alaska, U.S.A. 21-24 May, 2007, minor text changes; one added referenc

A Powerful AGN Outburst in RBS 797

Utilizing $\sim 50$ ks of Chandra X-ray Observatory imaging, we present an analysis of the intracluster medium (ICM) and cavity system in the galaxy cluster RBS 797. In addition to the two previously known cavities in the cluster core, the new and deeper X-ray image has revealed additional structure associated with the active galactic nucleus (AGN). The surface brightness decrements of the two cavities are unusually large, and are consistent with elongated cavities lying close to our line-of-sight. We estimate a total AGN outburst energy and mean jet power of $\approx 3 - 6 \times 10^{60}$ erg and $\approx 3 - 6 \times 10^{45}$ erg s$^{-1}$, respectively, depending on the assumed geometrical configuration of the cavities. Thus, RBS 797 is apparently among the the most powerful AGN outbursts known in a cluster. The average mass accretion rate needed to power the AGN by accretion alone is $\sim 1 M_{\odot}$ yr$^{-1}$. We show that accretion of cold gas onto the AGN at this level is plausible, but that Bondi accretion of the hot atmosphere is probably not. The BCG harbors an unresolved, non-thermal nuclear X-ray source with a bolometric luminosity of $\approx 2 \times 10^{44}$ erg s$^{-1}$. The nuclear emission is probably associated with a rapidly-accreting, radiatively inefficient accretion flow. We present tentative evidence that star formation in the BCG is being triggered by the radio jets and suggest that the cavities may be driving weak shocks ($M \sim 1.5$) into the ICM, similar to the process in the galaxy cluster MS 0735.6+7421.Comment: Accepted to ApJ; 20 pages, 11 low-resolution figure

A Deep Chandra Observation of the AGN Outburst and Merger in Hickson Compact Group 62

We report on an analysis of new Chandra data of the galaxy group HCG 62, well known for possessing cavities in its intragroup medium (IGM) that were inflated by the radio lobes of its central active galactic nucleus (AGN). With the new data, a factor of three deeper than previous Chandra data, we re-examine the energetics of the cavities and determine new constraints on their contents. We confirm that the ratio of radiative to mechanical power of the AGN outburst that created the cavities is less than 10^-4, among the lowest of any known cavity system, implying that the relativistic electrons in the lobes can supply only a tiny fraction of the pressure required to support the cavities. This finding implies additional pressure support in the lobes from heavy particles (e.g., protons) or thermal gas. Using spectral fits to emission in the cavities, we constrain any such volume-filling thermal gas to have a temperature kT > 4.3 keV. For the first time, we detect X-ray emission from the central AGN, with a luminosity of L(2-10 keV) = (1.1 +/- 0.4) x 10^39 erg s^-1 and properties typical of a low-luminosity AGN. Lastly, we report evidence for a recent merger from the surface brightness, temperature, and metallicity structure of the IGM.Comment: Accepted to MNRAS, 14 pages, 9 figure

A technique to record the sedentary to walk movement during free living mobility : a comparison of healthy and stroke populations

Background Hesitation between moving from a sedentary posture (lying/sitting) to walking is a characteristic of mobility impaired individuals, as identified from laboratory studies. Knowing the extent to which this hesitation occurs during everyday life would benefit rehabilitation research. This study aimed to quantify this transition hesitation through a novel approach to analysing data from a physical activity monitor based on a tri-axial accelerometer and compare results from two populations; stroke patients and age-matched unimpaired controls. Methods Stroke patients living at home with early supported discharge (n=34, 68.9YO ± 11.8) and age matched controls (n=30, 66.8YO ± 10.5) wore a physical activity monitor for 48hrs. The outputs from the monitor were then used to determine the transitions from sedentary to walking. The time delay between a sedentary posture ending and the start of walking classified four transition types: 1) fluent (<=2s), 2) hesitant (>2s<=10s), 3) separated (>10s) and 4) a change from sedentary with no registered walking to a return to sedentary. Results Control participants initiated walking after a sedentary posture on 92% of occasions. Most commonly (43%) this was a fluent transition. In contrast stroke patients walked after changing from a sedentary posture on 68% of occasions with only 9% of transitions classed as fluent, (p<0.05). Discussion/Conclusion A new data analysis technique reports the frequency of walking following a change in sedentary position in stroke patients and healthy controls and characterises this transition according to the time delay before walking. This technique creates opportunities to explore everyday mobility in greater depth

An Energetic AGN Outburst Powered by a Rapidly Spinning Supermassive Black Hole or an Accreting Ultramassive Black Hole

Powering the 10^62 erg nuclear outburst in the MS0735.6+7421 cluster central galaxy by accretion implies that its supermassive black hole (SMBH) grew by ~6x10^8 solar masses over the past 100 Myr. We place upper limits on the amount of cold gas and star formation near the nucleus of <10^9 solar masses and <2 solar masses per year, respectively. These limits imply that an implausibly large fraction of the preexisting cold gas in the bulge must have been consumed by its SMBH at the rate of ~3-5 solar masses per year while leaving no trace of star formation. Such a high accretion rate would be difficult to maintain by stellar accretion or the Bondi mechanism, unless the black hole mass approaches 10^11 solar masses. Its feeble nuclear luminosities in the UV, I, and X-ray bands compared to its enormous mechanical power are inconsistent with rapid accretion onto a ~5x10^9 solar mass black hole. We suggest instead that the AGN outburst is powered by a rapidly-spinning black hole. A maximally-spinning, 10^9 solar mass black hole contains enough rotational energy, ~10^62 erg, to quench a cooling flow over its lifetime and to contribute significantly to the excess entropy found in the hot atmospheres of groups and clusters. Two modes of AGN feedback may be quenching star formation in elliptical galaxies centered in cooling halos at late times. An accretion mode that operates in gas-rich systems, and a spin mode operating at modest accretion rates. The spin conjecture may be avoided in MS0735 by appealing to Bondi accretion onto a central black hole whose mass greatly exceeds 10^10 solar mass. The host galaxy's unusually large, 3.8 kpc stellar core radius (light deficit) may witness the presence of an ultramassive black hole.Comment: Accepted for publication in ApJ. Modifications: adopted slightly higher black hole mass using Lauer's M_SMBH vs L_bulge relation and adjusted related quantities; considered more seriously the consequences of a ultramassive black hole, motivated by new Kormendy & Bender paper published after our submission; other modifications per referee comments by Ruszkowsk