Obituary for fallen scintillator J1819+3845
21 January 2014
The garden-variety z=0.54 quasar J1819+3845 was once celebrated as the fastest varying radio quasar in the sky. Its flux density sometimes tripled over the course of twenty minutes! This object was the poster-child for the phenomenon of radio intra-day variability. The timescale of the quasar's variations changed according to an annual cycle, attributed to the fact that the Earth's orbital velocity changed direction with respect to the interstellar medium responsible for the scintillations. Its annual cycle demonstrated a means of proving that the variations that had been observed in a number of other intra-day variable quasars were also unambiguously due to scintillation. Thus J1819 was pivotal in proving that radio quasar intra-day variability is caused primarily by interstellar scintillation, and that the brightness temperatures inferred from the variations under the assumption they were intrinsic were fictitious.
But somewhere between 2006 and early 2007 the scintillations stopped, and J1818+3845 flatlined. Why? One possible explanation is that the source suddenly expanded and became too large to exhibit interstellar scintillation. However, this would have required the source to have expanded at an apparent speed of at least 40 times the speed of light, an untenable hypothesis given that VLBI observations showed that the source remained unresolved.
In their 2013 publication, ASTRON’s Ger de Bruyn and CAASTRO member Jean-Pierre Macquart (Curtin University) could prove that the cessation of scintillations was due to the fact that the scattering cloud that previously sat along the line of sight to the quasar had moved away. They also examined the polarisation variations in J1819, which often look identical to the unpolarised variations, but lags them by up to 100 minutes. This suggested that the polarisation structure in the source was offset from the unpolarised emission, and comparison of VLBI polarisation images with the scintillation variations enabled the researchers to determine the distance to the scattering cloud responsible for J1819's once-magnificent variability to be an astounding 1.5 +/- 0.5 parsec. This turbulent cloud is one of the closest objects to Earth outside the solar system!
Armed with this distance, they estimated some of the physical properties of the cloud. Its ~ 100 cm-3 density is remarkably large for an object embedded in the ionised interstellar medium, and it has a physical extent of >100 Astronomical Units, so this cloud should subtend at least 50" on the sky. Furthermore, if the cloud is in pressure balance with its magnetic field, one expects the cloud to increment the rotation measure in the sky near J1819 by at least 2 rad/m2. Given the direction in which the cloud is moving on the sky, de Bruyn and Macquart conducted a preliminary search for variations in other sources near J1819 but found nothing remarkable. They also searched for this RM structure in the cloud, but found no definitive evidence for its signature in RM synthesis cubes.
by J.-P. Macquart