AstroJargon of the Week: Blazar

This week’s… last week’s… aw, did I miss a week? Anyway, THIS week’s “astrojargon” has a super-fun name, and it’s a pretty fun object as well. I’m talking about blazars. This is a subclass of AGN, the jargon with which I started my series. A blazar is highly variable, very luminous, and quite polarized. (Polarization means that the light has a preferred orientation.) Whereas 10% of all AGN rare bright in radio light, all blazars emit radio emission. In fact, they emit lots of light all across the electromagnetic spectrum, from radio through to gamma ray. When the radio jet is viewed up-close, the jets often exhibit super-luminal motion. It seems as though they are moving faster than the speed of light! However, it’s all due to a trick of geometry.

An image of the most central part of the radio jet of 3c273, made with the Very Long Baseline Array by Zavala & Taylor (NRAO/AUI.) Also, the subject of my very first research project, where I could see superluminal motion and measured polarization in the jet!

The term blazar encompasses two subclasses of astronomical objects: optically violent variables (OVVs) and BL Lac objects, named after the prototype of the class which distinguished itself with variability and polarization. Many of these objects were first identified as irregularly variable stars, and others as strange “radio stars.” But as it was slowly uncovered that quasars were really the active nuclei of distance galaxies, the true power of blazars was finally understood.

Famous blazar 3C273, taken in the optical with the Kitt Peak 4m. This is one of the rare cases where the jet is visible in the optical! The previous image would fit well within a pixel here.

A picture of active galactic nuclei has emerged that unifies many of the different types. The supermassive black hole is accreting material in the center, sometimes ejecting powerful radio jets, and always ionizing some of the surrounding material. However, there is also a dusty torus surrounding it, and this torus blocks some of the AGN emission, depending on your viewing angle. The variability of the blazar means that you are seeing emission from very near the (relatively) small black hole itself. You are, in fact, looking right down the throat of the beast, along the line of sight of the jets. This allows for the appearance of superluminal motion as well!

Cartoon of the unified theory of AGN, by Urry, C. M., & Padovani, P. Blazars are the “OVV” and “BL Lac” at the top.

Blazars encompass 80% of gamma ray point sources, which are being monitored now by a space telescope called Fermi. At the same time, very high resolution maps of the radio jets can be made withe the Very Long Baseline Array. The Atacama Large Millimeter Array is also expected to discover many fainter blazars during the normal course of calibrator searches. Although these weird and cool objects have been studied for sometime, they still hide some of their secrets. For example, is the variability that some show on a timescale of mere hours due to intrinsic and very powerful changes in flux, or is it an effect imposed by the interstellar material within our own Galaxy along the line of sight? Astronomers aren’t exactly sure how a supermassive black hole blasting away material can make such narrow jets, but maybe one day blazars will tell us how.