PKS 1257-326: a scintillating quasar

H. E. Bignall (University of Adelaide/ATNF); D. L. Jauncey, J. E. J. Lovell, A. K. Tzioumis (ATNF); L. Kedziora- Chudczer (AAO/ATNF); J.-P. Macquart (University of Groningen); S. J. Tingay, D. P. Rayner (ATNF); R. W. Clay (University of Adelaide)

Radio variability on time scales shorter than a day has been observed for the past 20 years in a number of compact, extragalactic radio sources. There has been much debate over the origin of this intraday variability (IDV). On the one hand, many radio sources vary at all observed wavelengths, from radio to gamma-ray, and this broad-band variability may thus be explained as being intrinsic to the source. The main problem with very rapid radio variability, if intrinsic, is that it implies a very high source brightness temperature, much higher than allowed by the conventional synchrotron emission mechanism for nonthermal radio emission.

On the other hand, it is known that sources with sufficiently small angular size can be observed to vary as a result of interstellar scintillation, a propagation effect produced in the turbulent, ionized interstellar medium (ISM) of our own Galaxy, which affects only radio wavelengths. An extragalactic radio source small enough to vary on time scales shorter than a day is certainly small enough to scintillate. Interpreting the variability as scintillation generally allows larger source sizes and hence lower source brightness temperatures.

How do we know for sure whether the intraday variability is scintillation, or intrinsic?

Recent Compact Array observations have now confirmed that the principal cause of this radio IDV is interstellar scintillation. Key to this has been the discovery and detailed monitoring of the variability in the recently discovered very rapid variable, PKS 1257-326. This source is one of the three most rapidly variable quasars known. Its flux density has been observed to vary by 40% in 45 minutes.

We have monitored the variability of PKS 1257-326 every six weeks with the Compact Array over the course of the last year, and found an annual cycle in the characteristic time scale of variability, as shown in Figure 1. This annual cycle occurs because the velocity of the interstellar medium, as seen by an observer on Earth, changes due to the Earth's orbital motion. The scintillation pattern is produced by focusing and defocusing the radio emission, as it passes through patches of turbulence in the interstellar medium. The time scale of variability is set by the speed at which this series of patches moves past the observer. For some periods of the year, the relative Earth/ISM velocity can be quite large, so that the scintillation pattern moves rapidly past the observer, and the observed time scale of variability is short. Six months later the ISM and the Earth are moving in much the same direction, so that the scintillation pattern is observed to pass by quite slowly, and the observed time scale of variability is long. The presence of such an annual cycle in PKS 1257-326 shows unequivocally that the IDV in this source is due to interstellar scintillation.

Importantly, this interstellar scintillation can be used as a probe of micro-arcsecond source structure, and of the scale and structure of turbulence in the local ISM. Our data tell us that the angular extent of the scintillating source is at most a few tens of micro-arcseconds. At the distance of PKS 1257-326, 10 micro-arcseconds corresponds to a linear distance of around three light months. We find that the scattering material along the line-of-sight to PKS 1257-326 is likely to be very nearby, less than 100 light years away from the solar system. Modelling the annual cycle also shows strong evidence for a highly anisotropic scintillation pattern.

Thus, with the Compact Array, which has an angular resolution of arcseconds, we can use the ISM and the Earth's orbit to achieve a resolution of tens of micro-arcseconds. That's like extending the Compact Array's railway tracks all the way to the moon!

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Figure 1 Flux density measurements of PKS 1257-326 over the course of a year, showing the clear annual cycle in variability time scale. From June through September, the rapid variability slows down, then speeds up again towards the end of the year. Six-cm data are plotted in black, and 3-cm data in red. Each box shows one day's data, and represents the same range of values, shown in the bottom left-hand corner.
Figure 2 The characteristic time scale of variability at wavelength 6-cm, defined from autocorrelation functions, measured over 2001. Plotted underneath is the speed, observed from Earth, of a medium moving with the local standard of rest, in the direction of PKS 1257-326. The time scale of variations shows a clear signature of the Earth's orbital motion.
Figure 3 The Earth's velocity with respect to the interstellar medium, for each day of the year, projected onto the plane of the sky. This shows that over the course of a year, there is a large change in the direction of the scintillation speed, which allows us to probe the two-dimensional structure of the scintillation pattern.
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