Showing papers by "Jean-Pierre Macquart published in 2005"
TL;DR: In this article, the authors determine the characteristics of the 7mm to 20cm wavelength radio variability in Sgr A* on time scales from days to three decades, showing that the amplitude of the intensity modulation is between 30 and 39% at all wavelengths.
Abstract: We determine the characteristics of the 7mm to 20cm wavelength radio variability in Sgr A* on time scales from days to three decades. The amplitude of the intensity modulation is between 30 and 39% at all wavelengths. Analysis of uniformly sampled data with proper accounting of the sampling errors associated with the lightcurves shows that Sgr A* exhibits no 57- or 106-day quasi-periodic oscillations, contrary to previous claims. The cause of the variability is investigated by examining a number of plausible scintillation models, enabling those variations which could be attributed to interstellar scintillation to be isolated from those that must be intrinsic to the source. Thin-screen scattering models do not account for the variability amplitude on most time scales. However, models in which the scattering region is extended out to a radius of 50-500pc from the Galactic Center account well for the broad characteristics of the variability on >4-day time scales. The ~ 10% variability on <4-day time scales at 0.7-3cm appears to be intrinsic to the source. The degree of scintillation variability expected at millimeter wavelengths depends sensitively on the intrinsic source size; the variations, if due to scintillation, would require an intrinsic source size smaller than that expected.
36 citations
TL;DR: In this article, the authors reported the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at a wavelength of 21cm which resemble diffractive interstellar scintillation observed in pulsars.
Abstract: We report the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at a wavelength of 21cm which resemble diffractive interstellar scintillations observed in pulsars. The timescale (down to 20 min) and the bandwidth (frequency decorrelation bandwidth of 160 MHz) of the observed variations jointly imply that the component of the source exhibiting this scintillation must possess a brightness temperature well in excess of the inverse Compton limit. A specific model in which both the source and scintillation pattern are isotropic implies a brightness temperature 0.5 x 10^13 z_{pc} K, where previous estimates place the distance to the scattering medium in the range z_{pc}=4-12pc, yielding a minimum brightness temperature >20 times the inverse Compton limit. An independent estimate of the screen distance using the 21cm scintillation properties alone indicates a minimum screen distance of z approx 40pc and a brightness temperature above 2 x 10^14 K. There is no evidence for anisotropy in the scattering medium or source from the scintillation characteristics, but these estimates may be reduced by a factor comparable to the axial ratio if the source is indeed elongated.
23 citations
TL;DR: In this paper, the authors point out that spectral variability can be explained by refractive scintillation caused by the interstellar medium of our Galaxy, which can cause independent variations between closely-spaced spectral lines and can even alter the line profile.
Abstract: Spectral features absorbed against some radio quasars exhibit similar to 50 mJy variations, with the lines varying both relative to the continuum and, when several lines are present, even relative to one another. We point out that such variability can be expected as a consequence of refractive scintillation caused by the interstellar medium of our Galaxy. Scintillation can cause independent variations between closely-spaced spectral lines, and can even alter the line profile. The background source need not be compact to exhibit spectral variability. The variability can be used to infer the parsec to sub-parsec scale structure of the intervening absorbing material. We discuss the importance of scintillation relative to other possible origins of spectral variability. The present theory is applied to account for the variations observed in the HI-absorbed quasar PKS1127-145.
5 citations
TL;DR: In this paper, the authors measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source.
Abstract: PKS 1257-326 is one of three quasars known to show unusually large and rapid, intra-hour intensity variations, as a result of scintillation in the turbulent Galactic interstellar medium. We have measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source. Results of the two-station time delay observations are presented here. Implications for the, scintillation of this source axe discussed in the light of these results, together with results from two years of monitoring with the ATCA.
1 citations
Journal Article•
TL;DR: In this paper, the first results of the MASIV 5 GHz VLA survey were reported, showing that the fraction of sources that scintillate and their fractional scintillation amplitudes are larger among the weaker, similar to 100 mJy, sources.
Abstract: We report on the first results of the recent large-scale Micro-Arcsecond Scintillation Induced Variability (MASIV) 5 GHz VLA Survey of the northern sky. Our objective is to assemble a sample of 100-150 scintillating flat-spectrum extragalactic sources, which will be used to examine both the micro-arcsecond structure and parent populations of these sources, and to probe the turbulent interstellar medium responsible for the scintillation. One of the first findings of the MASIV Survey is that both the fraction of sources that scintillate, and their fractional scintillation amplitudes, are larger among the weaker, similar to 100 mJy, sources.