26 research outputs found
Multi-wavelength aperture polarimetry of debris disc host stars
Debris discs around main sequence stars have been extensively characterised
from infrared to millimetre wavelengths through imaging, spectroscopic, and
total intensity (scattered light and/or thermal emission) measurements.
Polarimetric observations have only been used sparingly to interpret the
composition, structure, and size of dust grains in these discs. Here we present
new multi-wavelength aperture polarisation observations with parts-per-million
sensitivity of a sample of twelve bright debris discs, spanning a broad range
of host star spectral types, and disc properties. These measurements were
mostly taken with the HIgh Precision Polarimetric Instrument on the
Anglo-Australian Telescope. We combine these polarisation observations with the
known disc architectures and geometries of the discs to interpret the
measurements. We detect significant polarisation attributable to circumstellar
dust from HD 377 and HD 39060, and find tentative evidence for HD 188228 and HD
202628.Comment: 25 pages, 8 figures, 9 tables, accepted for publication in MNRA
Angular Broadening of Intraday Variable AGN. II. Interstellar and Intergalactic Scattering
We analyze a sample of 58 multi-wavelength, Very Long Baseline Array
observations of active galactic nuclei (AGN) to determine their scattering
properties. Approximately 75% of the sample consists of AGN that exhibit
centimeter-wavelength intraday variability (interstellar scintillation) while
the other 25% do not show intraday variability. We find that interstellar
scattering is measurable for most of these AGN, and the typical broadening
diameter is 2 mas at 1 GHz. We find that the scintillating AGN are typically at
lower Galactic latitudes than the non-scintillating AGN, consistent with the
scenario that intraday variability is a propagation effect from the Galactic
interstellar medium. The magnitude of the inferred interstellar broadening
measured toward the scintillating AGN, when scaled to higher frequencies, is
comparable to the diameters inferred from analyses of the light curves for the
more well-known intraday variable sources. However, we find no difference in
the amount of scattering measured toward the scintillating versus
non-scintillating AGN. A consistent picture is one in which the scintillation
results from localized regions ("clumps") distributed throughout the Galactic
disk, but which individually make little contribution to the angular
broadening. Of the 58 AGN observed, 37 (64%) have measured redshifts. At best,
a marginal trend is found for scintillating (non-scintillating) AGN to have
smaller (larger) angular diameters at higher redshifts. We also use our
observations to try to constrain the possibility of intergalactic scattering.
While broadly consistent with the scenario of a highly turbulent intergalactic
medium, our observations do not place significant constraints on its
properties.Comment: 13 pages, 4 figures; AASTeX format; ApJ in pres
Rapid interstellar scintillation of PKS B1257-326: two-station pattern time delays and constraints on scattering and microarcsecond source structure
We report measurements of time delays of up to 8 minutes in the centimeter
wavelength variability patterns of the intra-hour scintillating quasar PKS
1257-326 as observed between the VLA and the ATCA on three separate epochs.
These time delays confirm interstellar scintillation as the mechanism
responsible for the rapid variability, at the same time effectively ruling out
the coexistence of intrinsic intra-hour variability in this source. The time
delays are combined with measurements of the annual variation in variability
timescale exhibited by this source to determine the characteristic length scale
and anisotropy of the quasar's intensity scintillation pattern, as well as
attempting to fit for the bulk velocity of the scattering plasma responsible
for the scintillation. We find evidence for anisotropic scattering and highly
elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >
10:1, extended in a northwest direction on the sky. The characteristic scale of
the scintillation pattern along its minor axis is well determined, but the high
anisotropy leads to degenerate solutions for the scintillation velocity. The
decorrelation of the pattern over the baseline gives an estimate of the major
axis length scale of the scintillation pattern. We derive an upper limit on the
distance to the scattering plasma of no more than 10 pc.Comment: 27 pages, 6 figures, accepted for publication in Ap
Interstellar Scintillation of the Polarized Flux Density in Quasar, PKS 0405-385
The remarkable rapid variations in radio flux density and polarization of the
quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis,
from which characteristic time scales and amplitudes are derived. The
variations are interpreted as interstellar scintillations. The cm wavelength
observations are in the weak scintillation regime for which models for the
various auto- and cross-correlations of the Stokes parameters are derived and
fitted to the observations. These are well modelled by interstellar
scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree
rotation of the polarization angle along its long dimension. This success in
explaining the remarkable intra-day variations (IDV)in polarization confirms
that ISS gives rise to the IDV in this quasar. However, the fit requires the
scintillations to be occurring much closer to the Earth than expected according
to the standard model for the ionized interstellar medium (IISM). Scattering at
distances in the range 3-30 parsec are required to explain the observations.
The associated source model has a peak brightness temperature near 2.0
10^{13}K, which is about twenty-five times smaller than previously derived for
this source. This reduces the implied Doppler factor in the relativistic jet,
presumed responsible to 10-20, high but just compatible with cm wavelength VLBI
estimates for the Doppler factors in Active Galactic Nuclei (AGNs).Comment: 43 pages 15 figures, accepted for ApJ Dec 200
The MASIV Survey - IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs
We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected active galactic nuclei (AGNs) drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, m5 ≥ 0.02) all exhibit strong 15 GHz intrinsic variability (m15 ≥ 0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ~100 μas scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects
Simultaneous infrared and optical observations of the transiting debris cloud around WD 1145+017
We present multiwavelength photometric monitoring of WD 1145+017, a white dwarf exhibiting periodic dimming events interpreted to be the transits of orbiting, disintegrating planetesimals. Our observations include the first set of near-infrared light curves for the object, obtained on multiple nights over the span of 1 month, and recorded multiple transit events with depths varying between ∼20 and 50 per cent. Simultaneous near-infrared and optical observations of the deepest and longest duration transit event were obtained on two epochs with the Anglo-Australian Telescope and three optical facilities, over the wavelength range of 0.5–1.2μm. These observations revealed no measurable difference in transit depths for multiple photometric pass bands, allowing us to place a 2σ lower limit of 0.8μm on the grain size in the putative transiting debris cloud. This conclusion is consistent with the spectral energy distribution of the system, which can be fit with an optically thin debris disc with minimum particle sizes of 10+5−3μm
The Radio Variability of the Gravitational Lens PMN J1838-3427
We present the results of a radio variability study of the gravitational lens
PMN J1838-3427. Our motivation was to determine the Hubble constant by
measuring the time delay between variations of the two quasar images. We
monitored the system for 4 months (approximately 5 times longer than the
expected delay) using the Australia Telescope Compact Array at 9 GHz. Although
both images were variable on a time scale of a few days, no correlated
intrinsic variability could be identified, and therefore no time delay could be
measured. Notably, the fractional variation of the fainter image (8%) was
greater than that of the brighter image (4%), whereas lensed images of a point
source would have the same fractional variation. This effect can be explained,
at least in part, as the refractive scintillation of both images due to the
turbulent interstellar medium of the Galaxy.Comment: To appear in AJ (8 pages, including 4 figures
A Bright Short Period M-M Eclipsing Binary from the KELT Survey: Magnetic Activity and the Mass–Radius Relationship for M Dwarfs
We report the discovery of KELT J041621-620046, a moderately bright (J ~ 10.2) M-dwarf eclipsing binary system at a distance of 39 ± 3 pc. KELT J041621-620046 was first identified as an eclipsing binary using observations from the Kilodegree Extremely Little Telescope (KELT) survey. The system has a short orbital period of ~1.11 days and consists of components with and in nearly circular orbits. The radii of the two stars are and . Full system and orbital properties were determined (to ∼10% error) by conducting an EBOP (Eclipsing Binary Orbit Program) global modeling of the high precision photometric and spectroscopic observations obtained by the KELT Follow-up Network. Each star is larger by 17%–28% and cooler by 4%–10% than predicted by standard (non-magnetic) stellar models. Strong H α emission indicates chromospheric activity in both stars. The observed radii and temperature discrepancies for both components are more consistent with those predicted by empirical relations that account for convective suppression due to magnetic activity
The Micro-Arcsecond Scintillation-Induced Variability (Masiv) Survey. III. Optical Identifications and New Redshifts
Intraday variability (IDV) of the radio emission from active galactic nuclei is now known to be predominantly due to interstellar scintillation (ISS). The MASIV (The Micro-Arcsecond Scintillation-Induced Variability) survey of 443 flat spectrum sources revealed that the IDV is related to the radio flux density and redshift. A study of the physical properties of these sources has been severely handicapped by the absence of reliable redshift measurements for many of these objects. This paper presents 79 new redshifts and a critical evaluation of 233 redshifts obtained from the literature. We classify spectroscopic identifications based on emission line properties, finding that 78% of the sources have broad emission lines and are mainly FSRQs. About 16% are weak lined objects, chiefly BL Lacs, and the remaining 6% are narrow line objects. The gross properties (redshift, spectroscopic class) of the MASIV sample are similar to those of other blazar surveys. However, the extreme compactness implied by ISS favors FSRQs and BL Lacs in the MASIV sample as these are the most compact object classes. We confirm that the level of IDV depends on the 5 GHz flux density for all optical spectral types. We find that BL Lac objects tend to be more variable than broad line quasars. The level of ISS decreases substantially above a redshift of about two. The decrease is found to be generally consistent with ISS expected for beamed emission from a jet that is limited to a fixed maximum brightness temperature in the source rest frame