George's Random Astronomical Object

Object 64: VER J0521+211

Podcast release date: 10 January 2022

Right ascension: 05:21:46.0

Declination: +21:12:51

Epoch: J2000

Constellation: Taurus

Corresponding Earth location: Kotara, Maharashtra, India

VER J0521+211 is another astronomical object where the numbers in the name refer to the coordinates of the object. The VER in the name refer to the VERITAS telescope, which was used to discover this object [1]. This thing was identified by the very high energy gamma rays that it produced. These gamma rays don't penetrate the Earth's atmosphere, but the VERITAS telescope, which is located on a mountain in Arizona that isn't particularly high [2,3], was still able to identify that VER J0521+211 was a source of gamma ray emission. To understand why, I'll need to take some time to explain how VERITAS works and how it can detect gamma rays before I can explain what VER J0521+211 is.

VERITAS is an acronym that stands for Very Energetic Radiation Imaging Telescope Array System. I am fairly certain that the telescope was named this because many astronomers like naming things using clever acronyms (or at least acronyms that they think are clever). The telescope is a set of four reflectors that sort of look like satellite dishes or radio telescopes [2,3]. Each reflector has an array of hexagonal mirrors covering a 12 m circular area that reflect visible light to a box supported by four struts in front of the reflector. The box contains very sensitive photomultiplier tubes that can't necessarily make very good images of anything in the sky but that can detect changes in light on timescales of 2 nanoseconds [2]. To help people understand how short this timescale is, I tried to come up with some sort of comparison between 2 nanoseconds and something else that people might be familiar with, but I couldn't really think of anything that's even remotely comparable. Like I indicated before, VERITAS is unable to actually detect gamma rays, and that is because gamma rays don't make it through the Earth's atmosphere. Instead, VERITAS detects the effects of high energy gamma rays hitting the Earth's atmosphere.

To understand what happens, it's really important to keep in mind the equation E=mc2 by Albert Einstein. Many people have probably heard of this equation before but may not have understood what the heck it's supposed to mean, which was definitely the case for me the few dozen of times I encountered this equation as a kid. People who do understand this equation will know that it's supposed to relate mass to energy and will probably be kind of familiar with how it applies to nuclear physics when atoms either split apart in nuclear fission or join together in nuclear fusion. The resulting atoms from these fission or fusion processes will have less mass than the original atoms, and the excess mass will have been converted to energy in the fission or fusion processes, producing either heat in controlled reactions like in a nuclear reactor or an explosion like in a nuclear bomb or a nuclear meltdown.

That equation E=mc2 also applies to gamma rays, which have an amount of energy that is multiple times that of an electron. This means that, under the right conditions, a gamma ray's energy can be converted into an electron and its antimatter pair, a positron. A gamma ray travelling through empty space won't do this, but when a gamma ray hits the nucleus of an atom in our atmosphere, it will transform into an electron and a positron. However, gamma rays typically have much more energy than what is needed to form the mass of these two particles, so the rest of the gamma ray's energy ends up being transformed into kinetic energy that propels the electron and positron at speeds close to the speed of light in a vacuum. When these particles pass by the nuclei of other atoms, the electromagnetic interactions could produce more gamma rays, and these gamma rays would also produce more pairs of electrons and positrons when they hit the nuclei of other atoms.

Now, if you noticed, I carefully said that these electrons and positrons are moving close to the speed of light in a vacuum; they are actually moving at faster than the speed that light can travel through the Earth's atmosphere. When this happens, the particles actually produce the electromagnetic equivalent of the sonic boom created by a jet or something else moving at faster than the speed of sound. This electromagnetic shockwave produces a form of blue light called Cherenkov radiation. Lots of electrons and positrons doing this at the same time can produce a brief flash of blue light. What VERITAS does is it detects this flash of blue light to infer that a high energy gamma ray hit an atom at the top of the Earth's atmosphere. The flash only last a few nanoseconds, which is why VERITAS needs photomultiplier tunes that can make measurements on such short timescales.

So, VER J0521+211 was detected in a set of observations between October 22, 2009, and January 16, 2010 [1]. The object was not identified in a single night just because high energy gamma rays are extremely rare. Instead, the astronomers using VERITAS (called the VERITAS Collaboration) detected about 100 flashes of light from gamma rays in the constellation of Taurus and concluded that they had to be seeing a source of high energy gamma rays from the location of VER J0521+211. The VERITAS Collaboration then used the Swift spacecraft, which was a space telescope designed to detect gamma ray sources, to confirm that an object existed at this location, and the X-ray telescope on the spacecraft was able to detect X-ray emission from the object [1]. Next, the astronomers determined that this gamma ray source coincided with an object that had been identified in the visible part of the electromagnetic spectrum in computer scans of old photographic plates from the 1950s, although no one had ever paid attention to it before [1]. They also obtained some spectra in the visible part of the electromagnetic spectrum to understand what they were looking at better, and they even made some high resolution radio images of the object [1].

With all of this information, the VERITAS Collaboration reached the conclusion that the source of these gamma rays was a galaxy at some unknown distance with an active galactic nucleus (AGN) [1]. An AGN consists of a supermassive black hole millions or billions the times the mass of the Sun, a disk of intersteller gas falling into that black hole, and jets of gas that appear above the poles of the black hole. The jets originate from gas in the disk that got too hot as it was gravitationally compressed by the gravity of the black hole and that blew away out of the poles of the black hole at close to the speed of light rather than falling into the black hole itself. In VER J0521+211, it looks like the galaxy has an AGN where one of these jets is aimed directly at the Earth. These types of objects are classified as blazars by astronomers.

As I indicated before, gamma rays are very rare. They are even very rare from AGN, despite the fact that galaxies that contain AGN with supermassive black holes are very common. This is because it is rare to see the jets from these AGN aimed directly at Earth; they are usually aimed in some other random direction. It's not known whether the gamma rays come from either near the supermassive black hole or somewhere in the jets further away, but it's fairly clear that the jets need to be beamed towards Earth for astronomers to detect the gamma ray emission effectively [4]. These gamma rays allow astronomers to construct and to test models of how AGN work, and every new discovery of an AGN that produces gamma rays will lead to new advancements in understanding AGN more generally. So, VER J0521+211 is not just exciting because it produces gamma rays with so much energy that we need to use Einstein's really famous equation about matter and energy to understand it. The gamma rays from VER J0521+211 are really important in helping astronomers understand how AGN work.


[1] Archambault, S. et al., Discovery of a New TeV Gamma-Ray Source: VER J0521+211, 2013, Astrophysical Journal, 776, 69

[2] Holder, J. et al., The first VERITAS telescope, 2006, Astroparticle Physics, 25, 391

[3] Holder, J. et al., Status of the VERITAS Observatory, 2008, in American Institute of Physics Conference Series, 1085, 657

[4] Madejski, Grzegorz (Greg) and Sikora, Marek, Gamma-Ray Observations of Active Galactic Nuclei, 2016, Annual Reviews of Astronomy and Astrophysics, 54, 725

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Last update: 25 April 2022