Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userRapid interstellar scintillation of PKS B1257-326: two-station pattern time delays and constraints on scattering and microarcsecond source structure
251
0
0.0
(
0
)
Authors
H. E. Bignall
Ask ChatGPT about the research
No Arabic abstract
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 quasars 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.
rate research
Read More
We have used the broadband backend available at the ATCA to study the fast interstellar scintillation of quasar PKS 1257-326, resolving the core shift as a function of frequency on scales less than 10 microarcseconds. In this short paper we discuss the jet direction implied from the microarcsecond-scale core shift in PKS 1257-326.
Rapid radio intra-day variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405-385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year, and find a clear annual cycle in the characteristic time-scale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material, and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (~10 microarcsecond) offset between the centroids of the 4.8 and 8.6 GHz components, and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of ~10 microarcseconds, resolution orders of magnitude higher than current VLBI techniques allow. General implications of scintillation for the physical properties of sources and the turbulent ISM are discussed.
PKS 1257-326 is a quasar showing extremely unusual, rapid interstellar scintillation (ISS), which has persisted for at least a decade. Simultaneous observations with the VLA and ATCA, combined with ATCA monitoring over several years, have revealed some properties of the turbulent ionized medium responsible for the ISS of PKS 1257-326. The scattering occurs in an unusually nearby (~10 pc), localized screen. The scintillation pattern is highly anisotropic with axial ratio more than 10:1 elongated in a northwest direction on the sky. Recent findings and implications for small-scale ionized structures in the ISM are discussed.
We expand our Bayesian Monte Carlo method for analyzing the light curves of gravitationally lensed quasars to simultaneously estimate time delays and quasar structure including their mutual uncertainties. We apply the method to HE1104-1805 and QJ0158-4325, two doubly-imaged quasars with microlensing and intrinsic variability on comparable time scales. For HE1104-1805 the resulting time delay of (Delta t_AB) = t_A - t_B = 162.2 -5.9/+6.3 days and accretion disk size estimate of log(r_s/cm) = 15.7 -0.5/+0.4 at 0.2 micron in the rest frame are consistent with earlier estimates but suggest that existing methods for estimating time delays in the presence of microlensing underestimate the uncertainties. We are unable to measure a time delay for QJ0158-4325, but the accretion disk size is log(r_s/cm) = 14.9 +/- 0.3 at 0.3 micron in the rest frame.
We have invented a novel technique to measure the radio image of a pulsar scattered by the interstellar plasma with 0.1 mas resolution. We extend the secondary spectrum analysis of parabolic arcs by Stinebring et al. (2001) to very long baseline interferometry and, when the scattering is anisotropic, we are able to map the scattered brightness astrometrically with much higher resolution than the diffractive limit of the interferometer. We employ this technique to measure an extremely anisotropic scattered image of the pulsar B0834+06 at 327 MHz. We find that the scattering occurs in a compact region about 420 pc from the Earth. This image has two components, both essentially linear and nearly parallel. The primary feature, which is about 16 AU long and less than 0.5 AU in width, is highly inhomogeneous on spatial scales as small as 0.05 AU. The second feature is much fainter and is displaced from the axis of the primary feature by about 9 AU. We find that the velocity of the scattering plasma is 16+-10 km/s approximately parallel to the axis of the linear feature. The origin of the observed anisotropy is unclear and we discuss two very different models. It could be, as has been assumed in earlier work, that the turbulence on spatial scales of ~1000 km is homogeneous but anisotropic. However it may be that the turbulence on these scales is homogeneous and isotropic but the anisotropy is produced by highly elongated (filamentary) inhomogeneities of scale 0.05-16 AU.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
