The central engines of radio-quiet quasars

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.Comment: 7 pages, 8 figures. Accepted for publication in MNRA

Evidence for a black hole in a radio-quiet quasar nucleus

We present the first milli-arcsecond resolution radio images of a radio-quiet quasar, detecting a high brightness temperature core with data from the VLBA. On maps made with lower-frequency data from MERLIN and the VLA jets appear to emanate from the core in opposite directions, which correspond to radio-emission on arcsecond scales seen with the VLA at higher frequencies. These provide strong evidence for a black-hole--based jet-producing central engine, rather than a starburst, being responsible for the compact radio emission in this radio-quiet quasar.Comment: 10 pages including 1 postscript figure; uses aaspp4.sty. Accepted for publication in Ap. J. Lett. Also available from http://www-astro.physics.ox.ac.uk/research/preprints

The radio remnant of SN1993J: an instrumental explanation for the evolving complex structure

We present simulated images of Supernova 1993J at 8.4 GHz using Very Long Baseline Interferometry (VLBI) techniques. A spherically symmetric source model is convolved with realistic uv-plane distributions, together with standard imaging procedures, to assess the extent of instrumental effects on the recovered brightness distribution. In order to facilitate direct comparisons between the simulations and published VLBI images of SN1993J, the observed uv-coverage is determined from actual VLBI observations made in the years following its discovery. The underlying source model only exhibits radial variation in its density profile, with no azimuthal dependence and, even though this model is morphologically simple, the simulated VLBI observations qualitatively reproduce many of the azimuthal features of the reported VLBI observations, such as appearance and evolution of complex azimuthal structure and apparent rotation of the shell. We demonstrate that such features are inexorably coupled to the uv-plane sampling. The brightness contrast between the peaks and the surrounding shell material are not as prominent in the simulations (which of course assume no antenna- or baseline-based amplitude or phase errors, meaning no self-calibration procedures will have incorporated any such features in models). It is conclusive that incomplete uv-plane sampling has a drastic effect on the final images for observations of this nature. Difference imaging reveals residual emission up to the 8 sigma level. Extreme care should be taken when using interferometric observations to directly infer the structure of objects such as supernovae.Comment: 14 pages, 10 figures, 2 tables, accepted for publication in MNRA

CARMA: specifications and status

The Combined Array for Research in Millimeter-wave Astronomy (CARMA) is a 23-antenna heterogeneous millimeter array under construction in the White/Inyo Mountains of eastern California. CARMA will merge the existing Owens Valley and Berkeley-Illinois-Maryland Association arrays into a single instrument focusing on pure research, technology development and student training. A new high-altitude site will enable routine 205-265 GHz observing, and may allow observations in the 345 GHz window. Eight additional 3.5-m antennas from the University of Chicago will also be integrated into CARMA when not imaging the Sunyaev-Zel'dovich effect towards clusters of galaxies. At first light, the array will observe at 12, 3 and 1.3 mm using a mix of SIS and MMIC-based receivers. A new, highly flexible correlator incorporating reprogrammable FPGA technology will process configurable subsets of the antennas specified according to the science objectives. Leading-edge water vapor radiometers will be used to correct for atmospheric opacity and signal phase fluctuations. CARMA will be capable of both high resolution and wide-field imaging, covering a range of angular scales unmatched by any current or planned millimeter-wave instrument. The high sensitivity, sub-arcsecond angular resolution and excellent uv-coverage of CARMA will ensure major advances in studies of the universe. The array will provide high-fidelity resolved images of solar-system objects, protostars, protoplanetary disks, and galaxies both nearby and at high redshift - directly addressing many key research areas in astronomy and astrophysics

Acute effects of contract-relax stretching, static stretching and isometric contractions on muscle-tendon mechanics

Maximum joint range of motion (ROM) and resistance to joint rotation within that range (i.e. resistance to stretch) are important physical characteristics that influence the capacity to perform activities of daily living and athletic tasks [1], and are affected considerably by aging and disease [2]. The loading characteristics of different muscle stretching techniques likely influence the specific mechanisms responsible for acute increases in range of motion (ROM). Identifying mechanisms allows the determination a priori as to whether these interventions might be useful in different clinical populations, why such stretch interventions elicit different responses in different individuals, and to optimize/improve the methodology of, and response to, the stretch technique. Therefore, the effects of contract-relax proprioceptive neuromuscular facilitation (CR) stretching, static stretching (SS) and maximal isometric contraction (Iso) interventions were studied in 17 healthy human volunteers. Methods: Passive ankle moment was recorded on an isokinetic dynamometer with electromyographic (EMG) recording from the triceps surae, simultaneous real-time motion analysis, and ultrasound imaging recorded gastrocnemius medialis muscle and Achilles tendon elongation. The subjects then performed each intervention randomly on separate days before reassessment. The SS condition included 4 x 15 s plantar flexor stretches with 15 s rest between stretches, the CR condition included 10 s stretches followed immediately by a 5-s ramped maximal isometric plantar flexor contraction in a fully stretched position, repeated four times giving a total duration of 60 s; identical to the SS condition. The Iso condition included 4 x 5 s ramped maximal isometric plantar flexor contractions performed in the anatomical position, with 15 s rest between contractions. Results: Significant increases in dorsiflexion ROM (2.5-5.3°; P 0.05), while significant reductions in muscle stiffness (see Figure 1, B) occurred after CR and SS (16.0-20.5%; P 0.05). Increases in peak passive moment (stretch tolerance) occurred after Iso (6.8%; P 0.05). Significant correlations (rs = 0.69-0.82; P < 0.01) were observed between the changes in peak passive moment and maximum ROM in all conditions. Conclusion: While similar ROM increases occurred after isometric contractions and static stretching, changes in muscle and tendon stiffness were distinct. The concomitant reductions in muscle and tendon stiffness after contract-relax stretching suggest a broader adaptive mechanical response that likely explains its superior efficacy to acutely increase ROM. These are the first data to confirm that acute changes in tendon stiffness may contribute to PNF stretching’s superior efficacy to increase ROM, above other stretching modes. However, while mechanical changes appear tissue-specific between interventions, significant correlations between the change in peak passive moment and the change in ROM were observed in all conditions, indicative of a neurological adaptation (i.e. increased stretch tolerance) also being important for the acute increases in ROM. The reduction in tendon stiffness and increase in ROM detected following the Iso condition have important methodological implications as these data indicate that the performance of contractions ‘on stretch’ during PNF stretching may not be needed. Modifying PNF stretching to perform the contraction phase in the anatomical, rather than highly stretched position, removes the need for partner assistance, decreases the likelihood of inducing pain, tissue damage and muscle strain injury during the technique, which may enhance PNF’s suitability and practicality for athletic and clinical populations

Effects of contract–relax, static stretching, and isometric contractions on muscle–tendon mechanics

Introduction: Loading characteristics of stretching techniques likely influence the specific mechanisms responsible for acute increases in range of motion (ROM). Therefore, the effects of a version of contract–relax (CR) proprioceptive neuromuscular facilitation stretching, static stretching (SS), and maximal isometric contraction (Iso) interventions were studied in 17 healthy human volunteers. Methods: Passive ankle moment was recorded on an isokinetic dynamometer, with EMG recording from the triceps surae, simultaneous real-time motion analysis, and ultrasound-imaging-recorded gastrocnemius medialis muscle and Achilles tendon elongation. Subjects then performed each intervention randomly on separate days before reassessment. Results: Significant increases in dorsiflexion ROM (2.5°–5.3°; P 0.05), whereas significant reductions in muscle stiffness occurred after CR stretching and SS (16.0%–20.5%; P 0.05). Increases in peak passive moment (stretch tolerance) occurred after Iso (6.8%; P 0.05). Significant correlations (rs = 0.69–0.82; P < 0.01) were observed between changes in peak passive moment and maximal ROM under all conditions. Conclusions: Although similar ROM increases occur after Iso and SS, changes in muscle and tendon stiffness are distinct. Concomitant reductions in muscle and tendon stiffness after CR stretching suggest a broader adaptive response that likely explains its superior efficacy in acutely increasing ROM. Although mechanical changes appear tissue-specific between interventions, similar increases in stretch tolerance after all interventions are strongly correlated with changes in ROM

CARMA: specifications and status

The Combined Array for Research in Millimeter-wave Astronomy (CARMA) is a 23-antenna heterogeneous millimeter array under construction in the White/Inyo Mountains of eastern California. CARMA will merge the existing Owens Valley and Berkeley-Illinois-Maryland Association arrays into a single instrument focusing on pure research, technology development and student training. A new high-altitude site will enable routine 205-265 GHz observing, and may allow observations in the 345 GHz window. Eight additional 3.5-m antennas from the University of Chicago will also be integrated into CARMA when not imaging the Sunyaev-Zel'dovich effect towards clusters of galaxies. At first light, the array will observe at 12, 3 and 1.3 mm using a mix of SIS and MMIC-based receivers. A new, highly flexible correlator incorporating reprogrammable FPGA technology will process configurable subsets of the antennas specified according to the science objectives. Leading-edge water vapor radiometers will be used to correct for atmospheric opacity and signal phase fluctuations. CARMA will be capable of both high resolution and wide-field imaging, covering a range of angular scales unmatched by any current or planned millimeter-wave instrument. The high sensitivity, sub-arcsecond angular resolution and excellent uv-coverage of CARMA will ensure major advances in studies of the universe. The array will provide high-fidelity resolved images of solar-system objects, protostars, protoplanetary disks, and galaxies both nearby and at high redshift - directly addressing many key research areas in astronomy and astrophysics