3,406 research outputs found
The broad-band radio spectrum of LSI+61303 in outburst
Aims: Our aim is to explore the broad-band radio continuum spectrum of
LSI+61303 during its outbursts by employing the available set of secondary
focus receivers of the Effelsberg 100 m telescope. Methods: The clear
periodicity of the system LSI+61303 allowed observations to be scheduled
covering the large radio outburst in March-April 2012. We observed LSI+61303 on
14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32 GHz with a
cadence of about 12 hours followed by two additional observations several days
later. Based on these observations we obtained a total of 24 quasi-simultaneous
broad-band radio spectra. Results: During onset, the main flare shows an almost
flat broad-band spectrum, most prominently seen on March 27, 2012, where - for
the first time - a flat spectrum (alpha=0.00+/-0.07, S nu^alpha) is observed up
to 32 GHz (9 mm wavelength). The flare decay phase shows superimposed
'sub-flares' with the spectral index oscillating between -0.4 and -0.1 in a
quasi-regular fashion. Finally, the spectral index steepens during the decay
phase, showing optically thin emission with values alpha -0.5 to -0.7.
Conclusions: The radio characteristics of LSI+61303 compare well with those of
the microquasars XTE J1752-223 and Cygnus X-3. In these systems the flaring
phase is actually also composed of a sequence of outbursts with clearly
different spectral characteristics: a first outburst with a flat/inverted
spectrum followed by a bursting phase of optically thin emission.Comment: 4 pages, 3 figures, accepted for publication in A&
Galaxy evolution across the optical emission-line diagnostic diagrams?
The discovery of the M-sigma relation, the local galaxy bimodality, and the
link between black-hole and host-galaxy properties, have raised the question
whether AGN play a role in galaxy evolution. Several theoretical models
implement AGN feedback to explain the observed galaxy luminosity function, and
possibly the color and morphological transformation of spiral galaxies into
passive ellipticals. To understand the importance of AGN feedback, a study of
the AGN populations in the radio-optical domain is crucial. A mass sequence
linking star-forming galaxies and AGN has been already noted in previous works,
and it is now investigated as possible evolutionary sequence. We observed a
sample of 119 intermediate-redshift (0.04<z<0.4) SDSS-FIRST radio emitters with
the Effelsberg 100-m telescope at 4.85 and 10.45 GHz and obtained spectral
indices. We find indications of spectral index flattening in high-metallicity
star-forming galaxies, composite galaxies, and Seyferts. This "flattening
sequence" along the [NII]-based emission-line diagnostic diagram is consistent
with the hardening of galaxy ionizing field, due to nuclear activity. After
combining our data with FIRST measurements at 1.4 GHz, we find that the
three-point radio spectra of Seyferts and LINERs show substantial differences,
attributable to small radio core components and larger (arcsecond sized)
jet/lobe components, respectively. A visual inspection of FIRST images seems to
confirm this hypothesis. Galaxies along this sequence are hypothesized to be
transitioning from the active star-forming galaxies (blue cloud) to the passive
elliptical galaxies (red sequence). This supports the suggestion that AGN play
a role in shutting down star-formation, and allow the transition from one
galaxy class to the other.Comment: 20 pages, 19 figures, accepted for publication in A&
Learning Design: reflections on a snapshot of the current landscape
The mounting wealth of open and readily available information and the swift evolution of social, mobile and creative technologies warrant a re-conceptualisation of the role of educators: from providers of knowledge to designers of learning. This need is being addressed by a growing trend of research in Learning Design. Responding to this trend, the Art and Science of Learning Design workshop brought together leading voices in the field and provided a forum for discussing its key issues. It focused on three thematic axes: practices and methods, tools and resources, and theoretical frameworks. This paper reviews some definitions of Learning Design and then summarises the main contributions to the workshop. Drawing upon these, we identify three key challenges for Learning Design that suggest directions for future research
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
Dynamic modulation of photonic crystal nanocavities using gigahertz acoustic phonons
Photonic crystal membranes (PCM) provide a versatile planar platform for
on-chip implementations of photonic quantum circuits. One prominent quantum
element is a coupled system consisting of a nanocavity and a single quantum dot
(QD) which forms a fundamental building block for elaborate quantum information
networks and a cavity quantum electrodynamic (cQED) system controlled by single
photons. So far no fast tuning mechanism is available to achieve control within
the system coherence time. Here we demonstrate dynamic tuning by monochromatic
coherent acoustic phonons formed by a surface acoustic wave (SAW) with
frequencies exceeding 1.7 gigahertz, one order of magnitude faster than
alternative approaches. We resolve a periodic modulation of the optical mode
exceeding eight times its linewidth, preserving both the spatial mode profile
and a high quality factor. Since PCMs confine photonic and phononic
excitations, coupling optical to acoustic frequencies, our technique opens ways
towards coherent acoustic control of optomechanical crystals.Comment: 11 pages 4 figure
Planet-Induced Emission Enhancements in HD 179949: Results from McDonald Observations
We monitored the Ca II H and K lines of HD 179949, a notable star in the
southern hemisphere, to observe and confirm previously identified planet
induced emission (PIE) as an effect of star-planet interaction. We obtained
high resolution spectra (R ~ 53,000) with a signal-to-noise ratio S/N >~ 50 in
the Ca II H and K cores during 10 nights of observation at the McDonald
Observatory. Wide band echelle spectra were taken using the 2.7 m telescope.
Detailed statistical analysis of Ca II K revealed fluctuations in the Ca II K
core attributable to planet induced chromospheric emission. This result is
consistent with previous studies by Shkolnik et al. (2003). Additionally, we
were able to confirm the reality and temporal evolution of the phase shift of
the maximum of star-planet interaction previously found. However, no
identifiable fluctuations were detected in the Ca II H core. The Al I lambda
3944 A line was also monitored to gauge if the expected activity enhancements
are confined to the chromospheric layer. Our observations revealed some
variability, which is apparently unassociated with planet induced activity.Comment: 11 pages, 11 figures, 5 tables; Publications of the Astronomical
Society of Australia (in press
- …