J-E-T-S, Jets, Jets, Jets!

Bipolar jet from a young stellar object (YSO). Credit: Gemini Observatory, artwork by Lynette Cook

It seems oddly appropriate to be writing about astrophysical jets on Thanksgiving Day, when the New York football Jets will be featured on television. In the most recent issue of Science, Carlos Carrasco-Gonzalez and collaborators write about how their observations of radio emissions from young stellar objects (YSOs) shed light one of the unsolved problems in astrophysics; what are the mechanisms that form the streams of plasma known as polar jets? Although we are still early in the game, Carrasco-Gonzalez et al have moved us closer to the goal line with their discovery.

Astronomers see polar jets in many places in the Universe. The largest polar jets are those seen in active galaxies such as quasars. They are also found in gamma-ray bursters, cataclysmic variable stars, X-ray binaries and protostars in the process of becoming main sequence stars. All these objects have several features in common: a central gravitational source, such as a black hole or white dwarf, an accretion disk, diffuse matter orbiting around the central mass, and a strong magnetic field.

Relativistic jet from an AGN.
Credit: Pearson Education, Inc.

When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. These are normally the jets produced by supermassive black holes in active galaxies. These jets emit energy in the form of radio waves produced by electrons as they spiral around magnetic fields, a process called synchrotron emission. Extremely distant active galactic nuclei (AGN) have been mapped out in great detail using radio interferometers like the Very Large Array in New Mexico. These emissions can be used to estimate the direction and intensity of AGNs magnetic fields, but other basic information, such as the velocity and amount of mass loss, are not well known.

On the other hand, astronomers know a great deal about the polar jets emitted by young stars through the emission lines in their spectra. The density, temperature and radial velocity of nearby stellar jets can be measured very well. The only thing missing from the recipe is the strength of the magnetic field. Ironically, this is the one thing that we can measure well in distant AGN. It seemed unlikely that stellar jets would produce synchrotron emissions since the temperatures in these jets are usually only a few thousand degrees. The exciting news from Carrasco-Gonzalez et al is that jets from young stars do emit synchrotron radiation, which allowed them to measure the strength and direction of the magnetic field in the massive Herbig-Haro object, HH 80-81, a protostar 10 times as massive and 17,000 times more luminous than our Sun.

Finally obtaining data related to the intensity and orientation of the magnetic field lines in YSO's and their similarity to the characteristics of AGN suggests we may be that much closer to understanding the common origin of all astrophysical jets. Yet another thing to be thankful for on this day.

Twinkle, Twinkle, Quasi-Star

 

"Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns
Twinkle, twinkle, quasi-star
How I wonder what you are."

George Gamow, "Quasar" 1964.

The AAVSO recently announced a special observing campaign on several blazars, including the unusual variable object 3C 66A. So, what the devil is 3C 66A, and what is a blazar?

In the 1960’s advances in radio and x-ray astronomy opened our eyes to new classes of objects we’d never even imagined before. Some of these early discoveries were radio sources we believed were associated with stellar objects. The Third Cambridge Catalogue of Radio Sources (3C) is an astronomical catalogue of celestial radio sources. It was published by the Radio Astronomy Group of the University of Cambridge in 1959. Entries in this catalogue use the prefix 3C followed by a space, then the sequential discovery number, such as 3C 48. In the case of 3C 66A there are two sources very close together in the sky, so they are given an additional letter suffix, resulting in the names 3C 66A and 3C 66B.

3C 66A was one of these radio stars. As hints of their true nature began to unfold, astronomers began calling them Quasi Stellar Radio Sources, which was eventually shortened to ‘quasars’. The spectrum of 3C 273 taken during an occultation by the moon finally revealed that these radio stars were actually galaxies. Even more remarkably, these were the most distant galaxies known, billions of light years away.

Like quasars, blazars appear star-like optically. They emit energy in radio wavelengths as well as all other wavelengths up to gamma-rays. Due to their variability in optical and other wavelengths, these objects have all come to be known as Active Galactic Nuclei, or AGNs. Blazars are the most variable of all the AGNs and can change in brightness by up to a factor of 100 in a few months. BL Lacertae is the prototype of this class, and as you can guess by the name was first thought to be a variable star.

AAVSO 1000 day light curve of 3C 66A

The engines that power these active galaxies are believed to be supermassive black holes residing in the nucleus of the galaxy. These super compact objects can possess the mass and gravitational pull of a million to a few billion Suns. Surrounding the massive central region is an accretion disk. Beyond that is a doughnut-shaped torus of dust and gas extending out another couple of light-years, which glows in the infrared.

Due to the physics of accretion disks, the inner disk rotates more quickly than the outer portions. The inner parts near the black hole are spinning so rapidly and are so hot that very high-energy wavelengths are generated. Gas and dust spiral in towards their eventual doom, like water circling a giant cosmic drain. A massive amount of energy is emitted when matter accretes onto the black hole via the accretion disk, and vast amounts of gravitational energy are released as the matter gets sucked down the drain and disappears from the universe.

In some AGNs, radio jets are produced which protrude perpendicular from the disk, spewing energetic particles at nearly the speed of light. Our point of view relative to these jets is what distinguishes the different types of AGN.

Looking at the jet straight on, right down the barrel of the beast, we see blazars and quasars. However, if the jet is not pointed in our direction, the dusty disk of the galaxy lies in our line of sight, and we see what are called Seyfert galaxies.

So the answer to our initial question, what is 3C 66A?
It is an active galactic nucleus, a quasar, a blazar, and a variable source of radiation in optical and other wavelengths, powered by a supermassive black hole in a galaxy billions of light years away.

Why has the AAVSO asked observers to monitor this crazy cosmic catastrophy?

Observations have been requested by Dr. Markus Boettcher, from Ohio University, in a study he and his colleagues are making of several blazars. These AGN are being intermittently monitored by VERITAS (the Very Energetic Radiation Imaging Telescope Array System), a four-telescope collection designed to detect sources of very high energy (VHE) gamma-rays. If a VHE gamma-ray outburst is detected by VERITAS, target-of-opportunity observations with the Newton X-ray Multi-Mirror (XMM) telescope will be triggered.

The VERITAS telescope array

Since VERITAS is not regularly monitoring the targets, optical monitoring by observers on the ground is crucial to alert the VERITAS collaboration if any one of the blazars on their list enters a high state of activity. AAVSO observers will essentially act as fire spotters, and if we see smoke, professional astronomers will turn the big guns on these blazing beasts.

Details of the campaign and the list of targets can be found in AAVSO Alert Notice 353.

I have observed 3C 66A for years. Partly because it is a variable object, but also because it is remarkable as the most distant thing I can actually see in my 12” telescope. Now I have an even better reason to watch it closely in the coming months. My observations, from my humble back yard observatory, could trigger target of opportunity observations of the XMM Newton satellite. You have to admit, that’s cool.