10,373 research outputs found
Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations
We present 1-second cadence observations of M32 (NGC221) with the CHIMERA
instrument at the Hale 200-inch telescope of the Palomar Observatory. Using
field stars as a baseline for relative photometry, we are able to construct a
light curve of the nucleus in the g-prime and r-prime band with 1sigma=36
milli-mag photometric stability. We derive a temporal power spectrum for the
nucleus and find no evidence for a time-variable signal above the noise as
would be expected if the nuclear black hole were accreting gas. Thus, we are
unable to constrain the spin of the black hole although future work will use
this powerful instrument to target more actively accreting black holes. Given
the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics,
the absence of a contribution from a nuclear time-variable signal places an
upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a
factor of two more stringent than past upper limits from HST. The low mass of
the black hole despite the high stellar density suggests that the gas liberated
by stellar interactions was primarily at early cosmic times when the low-mass
black hole had a small Eddington luminosity. This is at least partly driven by
a top-heavy stellar initial mass function at early cosmic times which is an
efficient producer of stellar mass black holes. The implication is that
supermassive black holes likely arise from seeds formed through the coalescence
of 3-100 Msun mass black holes that then accrete gas produced through stellar
interaction processes.Comment: 8 pages, 3 figures, submitted to the Astrophysical Journal, comments
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High Energy Neutrinos and Photons from Curvature Pions in Magnetars
We discuss the relevance of the curvature radiation of pions in strongly
magnetized pulsars or magnetars, and their implications for the production of
TeV energy neutrinos detectable by cubic kilometer scale detectors, as well as
high energy photons.Comment: 19 pages, 4 figures, to appear in JCA
Use of strategies to improve retention in primary care randomised trials: a qualitative study with in-depth interviews
Objective To explore the strategies used to improve retention in primary care randomised trials.<p></p>
Design Qualitative in-depth interviews and thematic analysis.<p></p>
Participants 29 UK primary care chief and principal investigators, trial managers and research nurses.<p></p>
Methods In-depth face-to-face interviews.<p></p>
Results Primary care researchers use incentive and communication strategies to improve retention in trials, but were unsure of their effect. Small monetary incentives were used to increase response to postal questionnaires. Non-monetary incentives were used although there was scepticism about the impact of these on retention. Nurses routinely used telephone communication to encourage participants to return for trial follow-up. Trial managers used first class post, shorter questionnaires and improved questionnaire designs with the aim of improving questionnaire response. Interviewees thought an open trial design could lead to biased results and were negative about using behavioural strategies to improve retention. There was consensus among the interviewees that effective communication and rapport with participants, participant altruism, respect for participant's time, flexibility of trial personnel and appointment schedules and trial information improve retention. Interviewees noted particular challenges with retention in mental health trials and those involving teenagers.<p></p>
Conclusions The findings of this qualitative study have allowed us to reflect on research practice around retention and highlight a gap between such practice and current evidence. Interviewees describe acting from experience without evidence from the literature, which supports the use of small monetary incentives to improve the questionnaire response. No such evidence exists for non-monetary incentives or first class post, use of which may need reconsideration. An exploration of barriers and facilitators to retention in other research contexts may be justified.<p></p>
Effects of Rotation and Relativistic Charge Flow on Pulsar Magnetospheric Structure
We propose an analytical 3-D model of the open field-line region of a neutron
star (NS) magnetosphere. We construct an explicit analytic solution for
arbitrary obliquity (angle between the rotation and magnetic axes)
incorporating the effects of magnetospheric rotation, relativistic flow of
charges (e.g. primary electron beam) along the open field lines, and E X B
drift of these charges. Our solution employs the space-charge-limited
longitudinal current calculated in the electrodynamic model of Muslimov &
Tsygan (1992) and is valid up to very high altitudes nearly approaching the
light cylinder. We assume that in the innermost magnetosphere, the NS magnetic
field can be well represented by a static magnetic dipole configuration. At
high altitudes the open magnetic field lines significantly deviate from those
of a static dipole and tend to focus into a cylindrical bundle, swept back in
the direction opposite to the rotation, and also bent towards the rotational
equator. We briefly discuss some implications of our study to spin-powered
pulsars.Comment: 24 pages, 3 figures, accepted for publication in Ap
Compton Scattering in Ultra-Strong Magnetic Fields: Numerical and Analytical Behavior in the Relativistic Regime
This paper explores the effects of strong magnetic fields on the Compton
scattering of relativistic electrons. Recent studies of upscattering and energy
loss by relativistic electrons that have used the non-relativistic, magnetic
Thomson cross section for resonant scattering or the Klein-Nishina cross
section for non-resonant scattering do not account for the relativistic quantum
effects of strong fields ( G). We have derived a
simplified expression for the exact QED scattering cross section for the
broadly-applicable case where relativistic electrons move along the magnetic
field. To facilitate applications to astrophysical models, we have also
developed compact approximate expressions for both the differential and total
polarization-dependent cross sections, with the latter representing well the
exact total QED cross section even at the high fields believed to be present in
environments near the stellar surfaces of Soft Gamma-Ray Repeaters and
Anomalous X-Ray Pulsars. We find that strong magnetic fields significantly
lower the Compton scattering cross section below and at the resonance, when the
incident photon energy exceeds in the electron rest frame. The cross
section is strongly dependent on the polarization of the final scattered
photon. Below the cyclotron fundamental, mostly photons of perpendicular
polarization are produced in scatterings, a situation that also arises above
this resonance for sub-critical fields. However, an interesting discovery is
that for super-critical fields, a preponderance of photons of parallel
polarization results from scatterings above the cyclotron fundamental. This
characteristic is both a relativistic and magnetic effect not present in the
Thomson or Klein-Nishina limits.Comment: AASTeX format, 31 pages included 7 embedded figures, accepted for
publication in The Astrophysical Journa
Impacts of climate change on the body size of horse mackerel in the North Sea
Clear biogeographic trends persist in the body sizes of individuals in marine communities, with smaller species and individuals within species found in warmer waters. Based on this trend (Bergmann’s Rule), communities undergoing warming at a given location are likely to become more representative of body sizes seen in warmer waters. Using fisheries independent North Sea survey data with established catchability corrections, we investigated the effect of 30 years of warming on average size and species size distributions. We found, as an example, that horse mackerel (Trachurus trachurus) is consistent with Bergmann’s Rule, with larger average sizes seen at higher latitudes. Over 30 years a significant decline in average size has occurred at several latitudes. Using a Eulerian (grid based) approach and dividing the North Sea into 82 1° x 1° cells, 56% of cells displayed size distributions shifting towards populations with higher proportions of smaller individuals. Changing size distribution correlated significantly with warming over the study period. Horse mackerel’s northern range is in the northern North Sea, indicating populations at species range boundaries may be most affected by further warming. This finding warrants further work on a wider number of species. Changing body sizes and shifts in size distributions has implications for ecosystem functioning, trophic level dynamics and the value of fisheries
Remote-refocusing light-sheet fluorescence microscopy enables 3D imaging of electromechanical coupling of hiPSC-derived and adult cardiomyocytes in co-culture
Improving cardiac function through stem-cell regenerative therapy requires functional and structural integration of the transplanted cells with the host tissue. Visualizing the electromechanical interaction between native and graft cells necessitates 3D imaging with high spatio-temporal resolution and low photo-toxicity. A custom light-sheet fluorescence microscope was used for volumetric imaging of calcium dynamics in co-cultures of adult rat left ventricle cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes. Aberration-free remote refocus of the detection plane synchronously to the scanning of the light sheet along the detection axis enabled fast dual-channel 3D imaging at subcellular resolution without mechanical sample disturbance at up to 8 Hz over a ∼300 µm × 40 µm × 50 µm volume. The two cell types were found to undergo electrically stimulated and spontaneous synchronized calcium transients and contraction. Electromechanical coupling improved with co-culture duration, with 50% of adult-CM coupled after 24 h of co-culture, compared to 19% after 4 h (p = 0.0305). Immobilization with para-nitroblebbistatin did not prevent calcium transient synchronization, with 35% and 36% adult-CM coupled in control and treated samples respectively (p = 0.91), indicating that electrical coupling can be maintained independently of mechanotransduction
Antimatter spectra from a time-dependent modeling of supernova remnants
We calculate the energy spectra of cosmic rays (CR) and their secondaries
produced in a supernova remnant (SNR), taking into account the time-dependence
of the SNR shock. We model the trajectories of charged particles as a random
walk with a prescribed diffusioncoefficient, accelerating the particles at each
shock crossing. Secondary production by CRs colliding with gas is included as a
Monte Carlo process. We find that SNRs produce less antimatter than suggested
previously: The positron/electron ratio and the antiproton/proton ratio are a
few percent and few , respectively. Both ratios do not rise
with energy.Comment: 4 pages, 4 eps figures; v2: results for time-dependent magnetic field
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