A Flare in the Jet of Pictor A

May 5, 2015

Long (up to Megaparsec scale), highly collimated jets of magnetized plasma emanating from the active nuclei of galaxies pose many astrophysical puzzles - including the mechanism by which those outflows are accelerated to relativistic velocities, and the structure of the jet magnetic field. Recent high resolution X-ray imaging of the jet in famous radio galaxy Pictor A shows some surprising and unexpected variability. This suggests that the local magnetic field within the jet of Pictor A may have been much stronger than previously thought, affecting dramatically the high energy emission of extremely energetic, ultrarelativistic jet particles.

Figure: X-ray images of the Pictor A jet at several epochs: 2000 (top), 2002 (second from top), 2009 (second from bottom), and the total (bottom). Possible knot flares are indicated with magenta arrows. From Marshall et al (2010), ApJL, 714, 213, Figure 2



An international team of high energy astrophysicists, including KIPAC's Lukasz Stawarz, analysed high resolution images from the Chandra X-ray telescope taken at three observing epochs: 2000, 2002 and 2009. The jet, which extends nearly 2 arcmin (or about 80 kpc) from the centre of the galaxy, is barely resolved in its transverse dimension (width ~ 1 arcsec, translating to about 1 kpc). Within such an extended outflow, no variability was expected to be detected at the timescale of tens, or even hundreds of years. Surprisingly, however, a careful analysis of the X-ray data revealed an X-ray flare in the jet, visible as a pointlike object in the 2000 image, which had faded by 2002, and was found by the team not to be present in the most recent image, confirming its highly transient nature.

For this part of the jet to vary by more than a factor of two in brightness in only 2 years, the emission site must be very small - much smaller than the width of the jet itself. Translating - by light travel time arguments - the variability timescale into a characteristic size, the team find that the emission region could only have been about 1 milliarcsecond in apparent size, and as such could occupy only a tiny fraction of the total cross sectional area of the outflow. This tiny emission site would need on the other hand a very large magnetic field - 100 times stronger than the average field in the jet - to accelerate the jet particles to sufficiently high energies to produce the observed amount of the synchrotron X-ray radiation. Could this have been a burst of intense radiation produced as a very tightly wound magnetic field suddenly "reconnected"?