84 research outputs found
Power-law models of totally anisotropic scattering
The interstellar scattering responsible for pulsar parabolic arcs, and for intraday variability of compact radio quasars, is highly anisotropic in some cases. We numerically simulate these observed phenomena using totally anisotropic, power-law models for the electron density fluctuations which cause the scattering. By comparing our results to the scattered image of PSR B0834+06 and, independently, to dual-frequency light curves of the quasar PKS 1257−326, we constrain the nature of the scattering media on these lines of sight. We find that models with spectral indices slightly below β = 3, including the one-dimensional Kolmogorov model, are broadly consistent with both data sets. We confirm that a single physical model suffices for both sources, with the scattering medium simply being more distant in the case of B0834+06. This reinforces the idea that intraday variability and parabolic arcs have a common cause in a type of interstellar structure which, though obscure, is commonplace. However, the implied gas pressure fluctuations are large compared to typical interstellar pressures, and the magnetic stresses are much larger still. Thus while these scattering media may be commonplace, their underlying dynamics appear quite extraordinary
Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability
We present the first results of an ongoing intra-day variability (IDV) flux
density monitoring program of 107 blazars, which were selected from a sample of
RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV
observations were performed with the Effelsberg 100-m radio telescope at
4.8\,GHz, focusing on the statistical properties of IDV in a relatively large
sample of compact active galactic nuclei (AGN). We investigated the dependence
of rapid (3 day) variability on various source properties through a
likelihood approach. We found that the IDV amplitude depends on flux density
and that fainter sources vary by about a factor of 3 more than their brighter
counterparts. We also found a significant difference in the variability
amplitude between inverted- and flat-spectrum radio sources, with the former
exhibiting stronger variations. -ray loud sources were found to vary by
up to a factor 4 more than -ray quiet ones, with 4
significance. However a galactic latitude dependence was barely observed, which
suggests that it is predominantly the intrinsic properties (e.g., angular size,
core-dominance) of the blazars that determine how they scintillate, rather than
the directional dependence in the interstellar medium (ISM). We showed that the
uncertainty in the VLBI brightness temperatures obtained from the space VLBI
data of the RadioAstron satellite can be as high as 70\% due to the
presence of the rapid flux density variations. Our statistical results support
the view that IDV at centimeter wavelengths is predominantly caused by
interstellar scintillation (ISS) of the emission from the most compact,
core-dominant region in an AGN.Comment: 23 pages, 9 figures, published online by MDPI Galaxie
Optical study of PKS B1322-110, the intra-hour variable radio source
Observations with the Australia Telescope Compact Array revealed intra-hour variations in the radio source PKS B1322-110 (Bignall et al. 2019). As part of an optical follow-up, we obtained Gemini Hα and Hα continuum (HαC) images of the PKS B1322-110 field. A robust 19 σ detection of PKS B1322- 110 in the Hα−HαC image prompted us to obtain the first optical spectrum of PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a flat-spectrum radio quasar at a redshift of z = 3.007 ± 0.002. The apparent flux detected in the Hα filter is likely to originate from He ii emission redshifted precisely on the Galactic Hα narrow-band filter. We set upper limits on the emission measure of the Galactic plasma, for various possible cloud geometries
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
Optical study of PKS B1322-110, the intra-hour variable radio source
Observations with the Australia Telescope Compact Array revealed intra-hour
variations in the radio source PKS B1322-110 (Bignall et al. 2019). As part of
an optical follow-up, we obtained Gemini Halpha and Halpha continuum (HalphaC)
images of the PKS B1322-110 field. A robust 19-sigma detection of PKS B1322-110
in the Halpha-HalphaC image prompted us to obtain the first optical spectrum of
PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a
flat-spectrum radio quasar at a redshift of z=3.007 +/- 0.002. The apparent
flux detected in the Halpha filter is likely to originate from HeII emission
redshifted precisely on the Galactic Halpha narrow-band filter. We set upper
limits on the emission measure of the Galactic plasma, for various possible
cloud geometries
The Asymmetry Coefficient for Interstellar Scintillation of Extragalactic Radio Sources
Comparing the asymmetry coefficients and scintillation indices for observed
time variations of the intensity of the radiation of extragalactic sources and
the predictions of theoretical models is a good test of the nature of the
observed variations. Such comparisons can be used to determine whether
flux-density variations are due to scintillation in the interstellar medium or
are intrinsic to the source. In the former case, they can be used to estimate
the fraction of the total flux contributed by the compact component (core)
whose flux-density variations are brought about by inhomogeneities in the
interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS
1257-336, and J1819+3845 demonstrate that the scintillating component in these
objects makes up from 50% to 100% of the total flux, and that the intrinsic
angular sizes of the sources at 5 GHz is 10-40 microarcseconds. The
characteristics of the medium giving rise to the scintillations are presented
Maser Emission in G 339.884-1.259
We present multi-epoch VLBI observations of the methanol and water masers in the high-mass star formation region G 339.884-1.259, made using the Australian Long Baseline Array (LBA). Our sub-milliarcsecond precision measurements trace the proper motions of individual maser features in the plane of the sky. When combined with the direct line-of-sight radial velocity (vlsr), these measure the 3 D gas kinematics of the associated high-mass star formation region, allowing us to probe the dynamical processes to within 1000 AU of the core
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