82 research outputs found
Large Polarization Degree of Comet 2P/Encke Continuum Based on Spectropolarimetric Signals During Its 2017 Apparition
Spectropolarimetry is a powerful technique for investigating the physical
properties of gas and solid materials in cometary comae without mutual
contamination, but there have been few spectropolarimetric studies to extract
each component. We attempt to derive the continuum polarization degree of comet
2P/Encke, free from influence of molecular emissions. The target is unique in
that it has an orbit dynamically decoupled from Jupiter like main-belt
asteroids, while ejecting gas and dust like ordinary comets. We observed the
comet using the Higashi-Hiroshima Optical and Near-Infrared Camera attached to
the Cassegrain focus of the 150-cm Kanata telescope on UT 2017 February 21 when
the comet was at the solar phase angle of 75.7 deg. We find that the continuum
polarization degree with respect to the scattering plane is 33.8+/-2.7 % at the
effective wavelength of 0.815 um, which is significantly higher than those of
cometary dust in a high-Pmax group at similar phase angles. Assuming that an
ensemble polarimetric response of 2P/Encke's dust as a function of phase angle
is morphologically similar with those of other comets, its maximum polarization
degree is estimated to > 40 % at the phase angle of ~100 deg. In addition, we
obtain the polarization degrees of the C2 swan bands (0.51-0.56 um), the NH2
alpha bands (0.62-0.69 um) and the CN-red system (0.78-0.94 um) in a range of
3-19 %, which depend on the molecular species and rotational quantum numbers of
each branch. The polarization vector aligns nearly perpendicularly to the
scattering plane with the average of 0.4 deg over a wavelength range of
0.50-0.97 um. From the observational evidence, we conjecture that the large
polarization degree of 2P/Encke would be attributable to a dominance of large
dust particles around the nucleus, which have remained after frequent
perihelion passages near the Sun.Comment: 9 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales
Here we report on the results of the WEBT photo-polarimetric campaign
targeting the blazar S5~0716+71, organized in March 2014 to monitor the source
simultaneously in BVRI and near IR filters. The campaign resulted in an
unprecedented dataset spanning \,h of nearly continuous, multi-band
observations, including two sets of densely sampled polarimetric data mainly in
R filter. During the campaign, the source displayed pronounced variability with
peak-to-peak variations of about and "bluer-when-brighter" spectral
evolution, consisting of a day-timescale modulation with superimposed hourlong
microflares characterized by \,mag flux changes. We performed an
in-depth search for quasi-periodicities in the source light curve; hints for
the presence of oscillations on timescales of \,h and \,h do
not represent highly significant departures from a pure red-noise power
spectrum. We observed that, at a certain configuration of the optical
polarization angle relative to the positional angle of the innermost radio jet
in the source, changes in the polarization degree led the total flux
variability by about 2\,h; meanwhile, when the relative configuration of the
polarization and jet angles altered, no such lag could be noted. The
microflaring events, when analyzed as separate pulse emission components, were
found to be characterized by a very high polarization degree () and
polarization angles which differed substantially from the polarization angle of
the underlying background component, or from the radio jet positional angle. We
discuss the results in the general context of blazar emission and energy
dissipation models.Comment: 16 pages, 17 Figures; ApJ accepte
DISCOVERY OF DRAMATIC OPTICAL VARIABILITY IN SDSS J1100+4421: A PECULIAR RADIO-LOUD NARROW-LINE SEYFERT 1 GALAXY?
We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3 x 10^3, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxie
Robo2-Slit1 dependent cell-cell interactions mediate assembly of the trigeminal ganglion
Vertebrate cranial sensory ganglia, responsible for sensation of touch, taste and pain in the face and viscera, are composed of both ectodermal placode and neural crest cells. The cellular and molecular interactions allowing generation of complex ganglia remain unknown. Here, we show that proper formation of the trigeminal ganglion, the largest of the cranial ganglia, relies on reciprocal interactions between placode and neural crest cells in chick, as removal of either population resulted in severe defects. We demonstrate that ingressing placode cells express the Robo2 receptor and early migrating cranial neural crest cells express its cognate ligand Slit1. Perturbation of this receptor-ligand interaction by blocking Robo2 function or depleting either Robo2 or Slit1 using RNA interference disrupted proper ganglion formation. The resultant disorganization mimics the effects of neural crest ablation. Thus, our data reveal a novel and essential role for Robo2-Slit1 signaling in mediating neural crest–placode interactions during trigeminal gangliogenesis
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