Interstellar scintillation and PKS 1257-326

Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns.
Twinkle, twinkle quasi-star
How I wonder what you are.

George Gamow, "Quasar" 1964.

George Gamow coined the above in 1964 to commemorate the discovery of the first quasar, 3C273, in 1963 by Hazard and Schmidt. Cyril Hazard supplied the accurate radio position from Parkes lunar occultations (Hazard et al. 1963), and Maarten Schmidt provided the optical spectroscopy and redshift from the 200-inch telescope in California (Schmidt 1963). While the above words were written tongue-in-cheek some forty years ago, the "twinkling" part of the traditional childrens' verse has become a powerful tool for radio astronomers in the new millennium.

Figure 1: The observed "scintillation pattern" of PKS 1257-326 measured approximately every six weeks during a 12-month period, 2001/2002.

That old adage "stars twinkle, planets don't" describes very well the results of one area of research that's presently very active at the ATNF. Stars twinkle in optical light because they have very small angular diameters, milli-arcseconds or less, and hence suffer interference as the light passes through the Earth's turbulent atmosphere. Planets don't, since they subtend a very large angular diameter, many-arcseconds or more, and hence they lose their twinkle.

Much the same applies to radio sources as their radio light passes through the turbulent ionised interstellar medium of our Galaxy, where the "cells" of turbulence allow several ray paths to interfere. If the quasar is small enough, then it will twinkle, if it's too large then there will be no twinkling. Trouble is, radio quasars need to be microarcseconds in angular size to twinkle, and that's pretty small.

Detailed studies of this sort of radio twinkling in the interstellar medium are relatively new. There is another form of radio twinkling that has been known and studied for several decades, and that is the twinkling, or scintillation, of quasars in the interplanetary medium of our solar system. This interplanetary scintillation is strongest at frequencies less than ~1 GHz, and occurs predominantly when the sun, on its annual journey, passes close on the sky to the quasar. The turbulence in the solar wind then causes rapid variations in the apparent source intensity as the radio waves coming from the quasar suffer interference as they pass through the turbulent medium. This phenomenon was used decades ago to probe the centi-arcsecond structure of quasars (e.g. Cohen 1969). Before the advent of VLBI in the late 1960s, inter-planetary scintillation was one of the principal tools for high-resolution radio studies, although it has now been replaced by VLBI.

Now, interstellar scintillation is becoming a particularly useful tool, since it is capable of probing microarcsecond angular sizes that are much smaller than can be reached by any other existing technique, including VLBI and even Space VLBI. For a quasar to scintillate in the turbulent interstellar medium, its angular size must be of the same order as the angular size of the first Fresnel zone. Centimetre wavelengths, typically 3 to 20 cm, are where the variability due to interstellar scintillation is greatest (see Rickett 2002, for an excellent discussion of the effects of ISS). So for a screen at a distance of, say, 100 pc, and a wavelength of 6 cm, a source needs to have an angular size of around 10 microarcseconds.

To get an idea of how small an angle this is, it's about the size of the angle subtended by Neil Armstrong's big toe when he stepped on to the moon, as seen from the Earth! It's about ten thousand times finer resolution than is currently achieved by the Hubble Space Telescope operating at its shortest wavelength. And, for astronomers, it's about 3 light months on a quasar with a redshift of 1, half way across the Universe.

What has revolutionised the study of interstellar scintillation (ISS) has been the discovery of three remarkable sources that scintillate very rapidly. In order of discovery, these are PKS 0405-385, discovered by Lucyna Kedziora-Chudczer as part of her PhD thesis program (Kedziora-Chudczer et al., 1997), J1819+3845 discovered by Jane Dennett-Thorpe at Westerbork in the Netherlands (Dennett-Thorpe & de Bruyn 2000), and PKS 1257-326, discovered by Hayley Bignall, again as part of her PhD program (Bignall et al., 2002). Each of these three quasars has been found to vary by upwards of 40% or more, and to go from a minimum to a maximum in times as short as about half an hour.

The scintillation has a remarkably smooth, quasi-periodic nature at both 4.8 and 8.6 GHz, which we describe by a single time-scale parameter, TISS. This parameter is determined from the autocorrelation function, but can be most easily understood as about half the time it takes to go between a peak and a trough in the light curve. A strong correlation also exists between the variations at the two frequencies, a characteristic that PKS 1257-326 shares with many of the other known scintillators, and with PKS 0405-385 and J1819+3845 in particular.

The ATCA is the perfect telescope for these sorts of measurements, since it tracks the quasar all the time, giving effectively continuous and precise flux density measurements for the 12 hours that it is above the horizon at Narrabri. The first measurements of this sort of rapid intra-day variability (IDV) as it was called, were made with some of the world's largest single dish telescopes, the 300-foot telescope that was in Green Bank, in wild and wonderful West Virginia in the USA, and the Effelsberg 100-m telescope in Germany. However, the situation has changed and the large array telescopes of the world, the ATCA, the VLA and Westerbork are now the radio telescopes of choice.

It has been known for nearly 40 years that many radio sources vary in intensity, so how do we know that these fast variations are not intrinsic to the quasars themselves? How can we be sure that this is scintillation?

Over the last decade and a half there has been considerable debate as to just what the answers were to these questions. On one hand, it seemed that there was evidence to support the variations being intrinsic to the quasars, but the major concern was that, if it were intrinsic, then the quasars would be much too hot, up to 1021 degrees in fact. PKS 0405-385 changed the picture, when measurements of its variations at two widely separated telescopes 10,000 km apart, the ATCA in Australia and the VLA in New Mexico USA, showed clearly that the variation pattern appeared at different times at the two telescopes; in fact they appeared about two minutes apart. If the changes were intrinsic to the sources then the changes should have appeared simultaneously, well actually separated by the light travel time, that is milliseconds, not minutes.

Such measurements can only be made on these very rapidly variable quasars, since accurate timing of the pattern of variations requires rapid changes. Here J1819+3845 and PKS 1257-326 really come into their own; unlike PKS 0405-385, their rapid variability is long-lived, since they have both been scintillating now for three to five years. We prefer to use the term "scintillating" since it focuses on the physical phenomenon, rather than the earlier term "IDV" which merely emphasizes the observational properties, not the underlying astrophysics. For both of these sources, measurements have been made of the variability patterns at widely separated radio telescopes, and for both sources, significant time delays of many minutes found (Dennett-Thorpe & de Bruyn 2002, Bignall et al 2002, in preparation). The time delay for J1819+3845 was measured between the VLA and Westerbork, and for PKS 1257-326, between the ATCA and VLA in May this year.

Remember that the scintillations are caused by the focusing and defocusing patches of the turbulence in the interstellar medium as it moves by the Earth. Living on Earth may be expensive, but it includes an annual free trip around the sun. This gives rise to another remarkable aspect of scintillation that leaves an indelible signature in the observations. This is the change in the time-scale of the scintillation pattern throughout the course of the year. It happens because the speed of the Earth in its orbit around the sun is very close to 30 km per second.

The interesting coincidence is that much of the time the velocity of the ISM is also close to 30 km per second. For roughly half the year the Earth is moving parallel to the ISM, then six months later it's moving against the ISM. So when the velocities are parallel, the relative velocity is low, and hence the scintillation pattern speed is correspondingly low, and the scintillation as viewed from our radio telescopes here on Earth is very slow. Then six months later the situation is reversed, the relative speed is high and the variations appear much faster on Earth. This "annual cycle" as its come to be called, is not only conclusive proof that the variations are caused by interstellar scintillation, but it's a beautiful demonstration that the Earth really does go round the sun, and that Copernicus was right after all.

Figure 1 (page 1) shows the beautiful results of our ATCA monitoring program from 2001 and 2002, where we have plotted the observed "scintillation pattern" of PKS 1257-326 measured approximately every six weeks.

This is a remarkable diagram! What is most noticeable is the dramatic change in the time-scale of the variations over the course of the 12 months. From February through May, the flux density varies rapidly with less than an hour between excursions. In June the variations have begun a slow down that lasts through September. November sees them speed up again, while by January 2002 they have returned to much the same rate as February 2001. That the variations show such a clear signature of the Earth's orbital motion, demonstrates unequivocally that the mechanism that is responsible for the variations cannot be intrinsic to the quasar, but resides right here in our Galactic neighborhood. As Shakespeare has Cassius say in Julius Caesar:

The fault, dear Brutus, is not in our stars,
but in ourselves.

This annual cycle behaviour has been found now in close to half a dozen quasars, including J1819+3845, which was the first time it was recognised (Dennett-Thorpe & de Bruyn 2001), and 0917+624, where its presence was recognised independently by Rickett et al., (2001) and Jauncey & Macquart (2001).

To better quantify this annual cycle, Fig. 2 shows the characteristic timescale of the variations (defined as the HWHM of the autocorrelation function) as determined from the data in Fig. 1, plotted against day of year, at both 4.8 and 8.6 GHz (Bignall et al., 2002). Also plotted is the expected scintillation speed, VISS, against day of year, for a scattering medium moving with the local standard of rest. There is a very close, but not exact, alignment between the characteristic time of the annual cycle and the scintillation speed, confirming the scintillation origin of the variability.

Figure 2: The characteristic timescale of the variations as determined from the data in Fig. 1 for 4.8 GHz (a - top panel) and 8.6 GHz (b - top panel). Also plotted is the expected scintillation speed, VISS, against day of year, for a scattering medium moving with the local standard of rest (c - top panel) and the variation in scintillation velocity projected against the plane of the sky (lower panel).

Also plotted is the variation in scintillation velocity projected against the plane of the sky. The velocity is slowest at day 221 ( 9 August), then changes direction and speeds up through the end of the year, and reaches its maximum value around day 100 (10 April), before swinging round through a further 60 degrees and slowing down again.

As the relative velocity of the interstellar medium changes direction over the year, Fig. 2 shows a change of more than 120 degrees for PKS 1257-326, the ISM not only acts like an "interference screen" producing scintillation, but also much like a synthesis radio telescope such as the ATCA. This particular property has been recognised as a means of "imaging" these scintillating quasars with unprecedented angular resolution. Some of the theory underlying this "imaging" has been outlined by Macquart & Jauncey (2002) and has been applied in some detail very successfully to "image" the linearly polarized structure in PKS 0405-385 present during its discovery scintillation episode (Rickett et al., 2002).

Radio astronomy has certainly come a long way when it is possible for a modest telescope, like the ATCA, to be able to achieve such "imaging" with micro-arcsecond resolution. It's like extending the ATCA's railway tracks all the way to the moon!


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Dave Jauncey, Hayley Bignall, Jim Lovell, Tasso Tzioumis, Lucyna Kedziora-Chudczer, J-P Macquart, Steven Tingay, Dave Rayner and Roger Clay