Evidence for "Propeller" Effects In X-ray Pulsars GX 1+4 And GROJ1744-28

We present observational evidence for "propeller" effects in two X-ray pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when total X-ray flux had dropped below a certain threshold. Such a phenomenon is a clear indication of the propeller effects which take place when pulsar magnetosphere grows beyond the co-rotation radius as a result of the decrease in mass accretion rate and centrifugal force prevents accreting matter from reaching the magnetic poles. The entire process should simply reverse as the accretion rate increases. Indeed, steady X-ray pulsation was reestablished as the sources emerged from the non-pulsating faint state. These data allow us to directly derive the surface polar magnetic field strength for both pulsars: 3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to be accurate to within a factor of 2, with the total uncertainty dominated by the uncertainty in estimating the distances to the sources. Possible mechanisms for the persistent emission observed in the faint state are discussed in light of the extreme magnetic properties of the sources.Comment: 12 pages including 3 ps figures. To appear in ApJ Letters Vol. 48

The alignment of disk and black hole spins in active galactic nuclei

The inner parts of an accretion disk around a spinning black hole are forced to align with the spin of the hole by the Bardeen-Petterson effect. Assuming that any jet produced by such a system is aligned with the angular momentum of either the hole or the inner disk, this can, in principle provide a mechanism for producing steady jets in AGN whose direction is independent of the angular momentum of the accreted material. However, the torque which aligns the inner disk with the hole, also, by Newton's third law, tends to align the spin of the hole with the outer accretion disk. In this letter, we calculate this alignment timescale for a black hole powering an AGN, and show that it is relatively short. This timescale is typically much less than the derived ages for jets in radio loud AGN, and implies that the jet directions are not in general controlled by the spin of the black hole. We speculate that the jet directions are most likely controlled either by the angular momentum of the accreted material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter

Long term time-lapse microgravity and geotechnical monitoring of relict salt-mines, Marston, Cheshire, UK.

The area around the town of Northwich in Cheshire, U. K., has a long history of catastrophic ground subsidence caused by a combination of natural dissolution and collapsing abandoned mine workings within the underlying Triassic halite bedrock geology. In the village of Marston, the Trent and Mersey Canal crosses several abandoned salt mine workings and previously subsiding areas, the canal being breached by a catastrophic subsidence event in 1953. This canal section is the focus of a long-term monitoring study by conventional geotechnical topographic and microgravity surveys. Results of 20 years of topographic time-lapse surveys indicate specific areas of local subsidence that could not be predicted by available site and mine abandonment plan and shaft data. Subsidence has subsequently necessitated four phases of temporary canal bank remediation. Ten years of microgravity time-lapse data have recorded major deepening negative anomalies in specific sections that correlate with topographic data. Gravity 2D modeling using available site data found upwardly propagating voids, and associated collapse material produced a good match with observed microgravity data. Intrusive investigations have confirmed a void at the major anomaly. The advantages of undertaking such long-term studies for near-surface geophysicists, geotechnical engineers, and researchers working in other application areas are discussed

Competitive accretion in embedded stellar cluster

We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi-Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity that results means that the tidal radius is smaller than the Bondi-Hoyle radius in these systems. In contrast, when the stars dominate the potential and are virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus Bondi-Hoyle accretion is a better fit to the accretion rates.Comment: 11 pages, 11 figures, MNRAS in pres

Simulating spatial variability of cereal yields from historical yield maps and satellite imagery

[Abstract]: The management of spatial variability of crop yields relies on the availability of affordable and accurate spatial data. Yield maps are a direct measure of the crop yields, however, costs and difficulties in collection and processing to generate yield maps results in poor availability of such data in Australia. In this study, we used historical mid-season normalised difference vegetation index (NDVI), generated from Landsat imagery over 4 years. Using linear regression model, the NDVI was compared to the actual yield map from a 257 ha paddock. The difference between actual and predicted yield showed that 77% and 93% of the paddock area had an error of <20% and <30%, respectively. The linear model obtained in the paddock was used to simulate crop yield for an adjoining paddock of 162 ha. On an average of 4 years, the difference between actual and simulated yield showed that 87% of the paddock had an error of <20%. However, this error varied from season to season. Paddock area with <20% error increased exponentially with decreasing in-crop rainfall between anthesis and crop maturity. Furthermore, the error in simulating crop yield also varied with the soil constraints. Paddock zones with high concentrations of subsoil chloride and surface soil exchangeable sodium percentage generally had higher percent of error in simulating crop yields. Satellite imagery consistently over-predicted cereal yields in areas with subsoil constraints, possibly due to chloride-induced water stress during grain filling. The simulated yield mapping methodology offers an opportunity to identify within-field spatial variability using satellite imagery as a surrogate measure of biomass. However, the ability to successfully simulate crop yields at farm scale or regional scale requires wider evaluation across different soil types and climatic conditions

Are there brown dwarfs in globular clusters?

We present an analytical method for constraining the substellar initial mass function in globular clusters, based on the observed frequency of transit events. Globular clusters typically have very high stellar densities where close encounters are relatively common, and thus tidal capture can occur to form close binary systems. Encounters between main sequence stars and lower-mass objects can result in tidal capture if the mass ratio is > 0.01. If brown dwarfs exist in significant numbers, they too will be found in close binaries, and some fraction of their number should be revealed as they transit their stellar companions. We calculate the rate of tidal capture of brown dwarfs in both segregated and unsegregated clusters, and find that the tidal capture is more likely to occur over an initial relaxation time before equipartition occurs. The lack of any such transits in recent HST monitoring of 47 Tuc implies an upper limit on the frequency of brown dwarfs (< 15 % relative to stars) which is significantly below that measured in the galactic field and young clusters.Comment: MNRAS in pres

Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231

The VLBA and the VLA have been used to image the continuum radio emission from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local universe. The smallest scales reveal a double source less than 2 pc in extent. The components of this central source have minimum brightness temperatures of 10^9 to 10^{10} K, spectral turnovers between 2 and 10 GHz, and appear to define the galaxy nucleus plus the inner regions of a jet. The components may be free-free absorbed or synchtrotron self-absorbed. On larger scales, the images confirm a previously known north-south triple source extending 40 pc and elongated perpendicular to a 350-pc starburst disk. Both lobes show evidence for free-free absorption near 2 GHz, probably due to ionized gas with a density of 1-2 X 10^3 cm^{-3} in the innermost parts of the starburst disk. The absorbing gas may be ionized by the active nucleus or by local regions of enhanced star formation. The elongation of the 40-pc triple differs by 65 deg from that of the 2-pc source. The different symmetry axes on different scales imply strong curvature in the inner part of the radio jet. The radio continuum from the 350-pc disk has a spectral index near -0.4 above 1.4 GHz and is plausibly energized by a massive burst of star formation. On VLA scales, asymmetric and diffuse emission extends for more than 25 kpc. This emission has a steep spectrum, linear polarization exceeding 50% at some locations, and shares the symmetry axis of the 40-pc triple. The diffuse radio source is probably generated by energy deposition from a slow-moving nuclear jet, which conceivably could help energize the off-nuclear starburst as well.Comment: 34 pages, 7 Postscript figures, LaTeX file in AASTeX format, accepted in ApJ, Vol. 516, May 1, 199