Universe Today
How well would you know someone from four photographs taken a decade apart? That, roughly, is the position astronomers have been in with blazars. A blazar is an active galaxy in which a jet of ionised matter, launched from the region around a supermassive black hole, happens to point almost straight at us. From that angle the galaxy collapses to a single brilliant point, and it shines across the entire electromagnetic spectrum, from radio waves to gamma rays.
It also varies constantly, and not in step with itself, the X-rays can be doing one thing while the optical light does another, sometimes within a single observation. Studying that properly would need instruments covering the whole spectrum continuously, and no such thing exists. So blazars are watched in campaigns: a few days at a time, every few months or years, in whichever narrow energy band a given telescope happens to cover.
An IXPE study found the best explanation for particle acceleration in these jets is a shock wave travelling through them. The inset shows high energy particles in blue, the electrons, in the standard picture, produce everything we see (Credit : NASA/Pablo Garcia)
Alicja Wierzcholska at the Institute of Nuclear Physics in Cracow, working with Michael Zacharias at Heidelberg, has now managed to take a much longer look. Their target is PKS 2155-304, an unremarkable sounding object a billion and a half light years away in the southern constellation of Piscis Austrinus. Their data covers almost two decades, drawn from NASA's Swift observatory for the optical, ultraviolet and X-ray, and Fermi for the gamma rays.
The standard picture has the radiation coming from one region of the jet, produced by one population of electrons. If that were true, the optical and X-ray brightness ought to rise and fall together, perhaps with a lag. Over twenty years, they simply don't. No long term correlation appears at all.
When a blazar flares in X-rays it seems the harder, higher energy photons usually brighten more sharply than the softer ones. Look at any individual outburst of PKS 2155-304 and you can see that behaviour but the details differ from flare to flare, and across two decades the pattern doesn't hold. Each outburst appears to be driven by something slightly different.
TXS 0506+056 in gamma rays, before its 2017 outburst. When a high energy neutrino arrived at the South Pole that September, Fermi found this blazar was the most active it had been in a decade, the one solid link we have between a cosmic neutrino and its source (Credit: NASA/DOE/Fermi LAT Collaboration)
Then came the odd set of observations that puzzled everyone. In two runs from 2012, an extra dip appears in the spectrum, statistically solid, and at a time when the object wasn't flaring at all. Something switched on between those two measurements. Wierzcholska's team think the likeliest culprit is hadronic, with protons rather than electrons doing the work.
This is the really interesting bit because hadronic processes are how you make neutrinos. High-energy neutrinos have been arriving at Earth for years from sources nobody can identify, and the one solid lead we now have is a blazar, TXS 0506+056, caught mid-outburst.
Source : Two decades of blazar observations, and the mysteries keep piling up
