Echo mapping of active galactic nuclei

Echo mapping makes use of the intrinsic variaibility of the continuum source in active galactic nuclei to map out the distribution and kinematics of line‐emitting gas from its light travel time‐delayed response to continuum changes. Echo mapping experiments have yielded sizes for the broad line‐emitting region in about three dozen AGNs. The dynamics of the line‐emitting gas seem to be dominated by the gravity of the central black hole, enabling measurement of the black‐hole masses in AGNs. We discuss requirements for future echo‐mapping experiments that will yield the high‐quality velocity–delay maps of the broad‐line region that are needed to determine its physical nature. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

LRS Bianchi type-I models with constant deceleration parameter in?creation field cosmology

We have studied Locally Rotationally Symmetric (LRS) Bianchi type-I space-time having constant deceleration parameter and filled with perfect fluid in the Hoyle-Narlikar C-field cosmology. Here, the creation field C is a function of time t only. The geometrical and physical aspects for the models are also studied.     

Mass density of the upper atmosphere derived from Starlette’s Precise Orbit Determination with Satellite Laser Ranging

The atmospheric mass density of the upper atmosphere from the spherical Starlette satellite’s Precise Orbit Determination is first derived with Satellite Laser Ranging measurements at 815 to 1115 km during strong solar and geomagnetic activities. Starlette’s orbit is determined using the improved orbit determination techniques combining optimum parameters with a precise empirical drag application to a gravity field. MSIS-86 and NRLMSISE-00 atmospheric density models are compared with the Starlette drag-derived atmospheric density of the upper atmosphere. It is found that the variation in the Starlette’s drag coefficient above 800 km corresponds well with the level of geomagnetic activity. This represents that the satellite orbit is mainly perturbed by the Joule heating from geomagnetic activity at the upper atmosphere. This result concludes that MSIS empirical models strongly underestimate the mass density of the upper atmosphere as compared to the Starlette drag-derived atmospheric density during the geomagnetic storms. We suggest that the atmospheric density models should be analyzed with higher altitude acceleration data for a better understanding of long-term solar and geomagnetic effects.     

A predator–prey model for moon-triggered clumping in Saturn’s rings

UVIS occultation data show clumping in Saturn’s F ring and at the B ring outer edge, indicating aggregation and disaggregation at these locations that are perturbed by Prometheus and by Mimas. The inferred timescales range from hours to months. Occultation profiles of the edge show wide variability, indicating perturbations by local mass aggregations. Structure near the B ring edge is seen in power spectral analysis at scales 200–2000 m. Similar structure is also seen at the strongest density waves, with significance increasing with resonance strength. For the B ring outer edge, the strongest structure is seen at longitudes 90° and 270° relative to Mimas. This indicates a direct relation between the moon and the ring clumping. We propose that the collective behavior of the ring particles resembles a predator–prey system: the mean aggregate size is the prey, which feeds the velocity dispersion; conversely, increasing dispersion breaks up the aggregates. Moons may trigger clumping by streamline crowding, which reduces the relative velocity, leading to more aggregation and more clumping. Disaggregation may follow from disruptive collisions or tidal shedding as the clumps stir the relative velocity. For realistic values of the parameters this yields a limit cycle behavior, as for the ecology of foxes and hares or the “boom-bust” economic cycle. Solving for the long-term behavior of this forced system gives a periodic response at the perturbing frequency, with a phase lag roughly consistent with the UVIS occultation measurements. We conclude that the agitation by the moons in the F ring and at the B ring outer edge drives aggregation and disaggregation in the forcing frame. This agitation of the ring material may also allow fortuitous formation of solid objects from the temporary clumps, via stochastic processes like compaction, adhesion, sintering or reorganization that drives the denser parts of the aggregate to the center or ejects the lighter elements. Any of these more persistent objects would then orbit at the Kepler rate. We would also expect the formation of clumps and some more permanent objects at the other perturbed regions in the rings… including satellite resonances, shepherded ring edges, and near embedded objects like Pan and Daphnis (where the aggregation/disaggregation cycles are forced similar to Prometheus forcing of the F ring).     

Analysis of energetic electron drop-outs in the upper atmosphere of Titan during flybys in the dayside magnetosphere of Saturn

We discuss the high energy electron absorption signatures at Titan during the Cassini dayside magnetospheric encounters. We use the electron measurements of the Low Energy Measurement System of the Magnetospheric Imaging Instrument. We also examine the mass loading boundary based on the ion data of the Ion Mass Spectrometer sensor of the Cassini Plasma Spectrometer. The dynamic motion of the Kronian magnetopause and the periodic charged particle flux and magnetic field variations – associated with the magnetodisk of Saturn – of the subcorotating magnetospheric plasma creates a unique and complex environment at Titan. Most of the analysed flybys (like T25–T33 and T35–T51) cluster at similar Saturn Local Time positions. However the instantaneous direction of the incoming magnetospheric particles may change significantly from flyby to flyby due to the very different magnetospheric field conditions which are found upstream of Titan within the sets of encounters.The energetic magnetospheric electrons gyrate along the magnetic field lines of Saturn, and at the same time bounce between the mirror points of the magnetosphere. This motion is combined with the drift of the magnetic field lines. When these flux tubes interact with the upper atmosphere of Titan, their content is depleted over approximately an electron bounce period. These depletion signatures are observed as sudden drop-outs of the electron fluxes. We examined the altitude distribution of these drop-outs and concluded that these mostly detected in the exo-ionosphere of Titan and sometimes within the ionosphere.However there is a relatively significant scatter in the orbit to orbit data, which can be attributed to the which can be attributed to the variability of the plasma environment and as a consequence, the induced magnetosphere of Titan. A weak trend between the incoming electron fluxes and the measured drop-out altitudes has also been observed.     

The Torun catalogue of 6.7 GHz methanol masers

We report the observations of 289 methanol maser sources at 6.7 GHz obtained over a two month period with the Torun 32 m telescope. The data form a catalogue of all objects north of δ = –22° brighter than 7.5 Jy in the peak emission. The positions of sub‐arcsecond accuracy are updated for 76 % of the objects. We find that about one third of the sources show changes in the peak fluxes by a factor of two or more on time scales of 8.5–9.5 years (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)