The Inner Jet of an Active Galactic Nucleus as Revealed by a Radio-to-Gamma-ray Outburst: The Quasar-like Object BL Lacertae

Animation of BL Lac during first flare

Source: Cosmovision, a group led by Dr. Wolfgang Steffen of the Instituto de Astronomia, UNAM, Ensenada, Mexico

Above: Still frame (high resolution) from animation that illustrates the discovery discussed in our paper "The Inner Jet of an Active Galactic Nucleus as Revealed by a Radio-to-Gamma-ray Outburst" in the journal Nature (Marscher, A.P., et al., 2008, vol. 453, 24 April 2008 issue). See the description below. (It is possible to rotate the image, since there is no up-down or left-right in space. In fact, in BL Lac, the jet points to the south as we see it.) You can skip down to where you can click to download the movie. The movie, which is a conceptual interpretation of the data, was made by Cosmovision, a group led by Dr. Wolfgang Steffen of the Instituto de Astronomia, UNAM, Ensenada, Mexico (see below). The animation features a bright "knot" that moves through the jet away from the black hole. An outburst of visible light, X-rays, and gamma-rays is seen as the knot reaches maximum speed at the edge of the region with coiled magnetic field (twisted light-blue lines). A second outburst occurs when the knot passes through and is compressed by a stationary shock wave (the "X" shaped feature).

Caption: Illustration of a shock wave (bright "blob" in the upper jet) following a
spiral path (in yellow) as it moves away from the black hole and through a section of
the jet where the magnetic field (light blue curved lines) is wound up in a coil. This caused the
first brightening seen in visible, X-ray, and gamma-ray light; later, the
shock passed through the stationary X-shaped compression in the jet and
brightened a second time.
shaped

Note: All images, movies, and figures presented here are copyrighted. However, publication or exhibition in the news media as well as public or private viewing for educational purposes is fair use and does not require permission. Any publication of the images or movies in a scientific paper, book, or textbook requires written permission by the authors, which they will generally grant if the source of the material is indicated in the publication. The PDF file of the Nature paper is copyrighted by Nature Publishing Group and only available through Nature until 24 November 2008, after which it will be posted here. Requests for permission to use the material in the paper should be sent to permissions@nature.com. Send other permission requests to Prof. Alan Marscher, who will refer you to the appropriate copyright holder. The work described here was funded in part by NASA and the National Science Foundation; however, the processing of the data and interpretation are the responsibility of the authors and do not necessarily represent the views of either agency.

Here is a description of our findings. This is an expanded version (and with a different title) of a press release written by David Finley of the National Radio Astronomy Observatory

BURSTS OF RADIATION REVEAL HOW BLACK HOLES MAKE JETS

At the cores of many galaxies, supermassive black holes, millions of times more massive than the Sun, propel powerful jets of charged particles outward at nearly the speed of light. Just how they perform that feat has long been one of the great mysteries of astrophysics. The leading theory says that the particles are accelerated by tightly-twisted magnetic fields close to the black hole, but confirming that idea required an elusive close-up view of the jet's inner throat. Now, using the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope, along with NASA's Rossi X-ray Timing Exporer and a number of telescopes observing at visible wavelengths, astronomers have watched material winding a corkscrew outward path and behaving exactly as predicted by the theory.

"We have gotten the clearest look yet at the innermost portion of the jet, where the stream of particles is actually accelerated, and everything we see supports the idea that twisted, coiled magnetic fields are propelling the material outward," said Alan Marscher of Boston University, leader of an international research team. "This is a major advance in our understanding of a remarkable process that occurs throughout the Universe," he added.

Marscher's team includes Boston University senior research associate Svetlana Jorstad, graduate student Francesca D'Arcangelo, and undergraduate students Haruki Oh and Alice Olmstead, as well as researchers from the University of Arizona's Steward and Multiple Mirror observatories, the University of Michigan, Georgia State University, St. Petersburg State University in Russia (with a telescope at the Crimean Astrophysical Observatory), the University of Southampton and Cardiff University in the UK, Metsahovi Radio Observatory of the Helsinki University of Technology (TKK), and University of Turku in Finland, Perugia University Observatory in Italy, and Abastumani Astrophysical Observatory in the Republic of Georgia.

The astronomers studied a galaxy called BL Lacertae (BL Lac), located about 950 million light-years from Earth. BL Lac is a blazar, the most energetic type of black-hole-powered galactic core. A black hole is a concentration of mass so dense that not even light can escape its gravitational pull. Supermassive black holes - about 200 million times the mass of our Sun in the case of BL Lac - in these galaxies' cores power jets of electrically charged particles and intense radiation.

Material pulled inward toward the black hole forms a flattened, rotating disk, called an accretion disk. As the material moves from the outer edge of the disk inward, magnetic field lines perpendicular to the disk are twisted, forming a tightly-coiled bundle that, astronomers believe, propels and confines the ejected particles. Closer to the black hole, space itself, including the magnetic fields, is twisted by the strong gravitational pull and rotation of the black hole.

Theorists predicted that material moving outward in this close-in acceleration region would follow a corkscrew-shaped path inside the bundle of twisted magnetic fields. They also predicted that light and other radiation emitted by the moving material would brighten when its rotating path was aimed most directly toward Earth. Marscher and his colleagues anticipated that there might also be a flare later when the material hits a stationary shock wave called the "core" some time after it has emerged from the acceleration region. "That behavior is exactly what we saw," Marscher said, when his team followed an outburst of radiation from BL Lac. In late 2005 and early 2006, the astronomers watched BL Lac with an international collection of ground- and space-based telescopes as a bright knot of condensed material was ejected outward through the jet. As the material sped out from the neighborhood of the black hole, the VLBA could pinpoint its location, while other telescopes measured the properties of the radiation emitted from the knot.

Bright bursts of light, X rays, and gamma rays came when the knot was precisely at locations where the theories said such bursts would be seen. In addition, the property of the radio and light waves called polarization rotated as the knot wound its corkscrew path inside the tight throat of twisted magnetic fields.

"We got an unprecedented view of the inner portion of one of these jets and gained information that's very important to understanding how these tremendous particle accelerators work," Marscher said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement
by Associated Universities, Inc. The US-based portion of the research was funded by NASA and the National Science Foundation.

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MOVIE!! that you can download in any of 3 formats. The movie illustrates what we think is going on near the center of BL Lac. This is an animation (not from real data) created by Cosmovision*. [Note: an improved version of the movie, as well as versions that require less disk space, are expected to be available later in April.]

Warning: the size of these files is about 90 MB, so download it to your computer (right click - choose 'save target as') and then play it on your computer.

360 sq. pixels best for computer screens)

ntsc version (for US television, video codec: DVC-PRO)  

pal version (for European television)

Description of the movie: The supermassive black hole (with a mass of about 200 millions times that of the Sun, which corresponds to an event horizon that is about twice as large as the size of the Earth's orbit around the Sun) is just a very tiny black dot at the center. Surrounding it is an accretion disk of gas and dust from interstellar space that is slowly falling into the black hole while rotating around it in nearly circular orbits. (Despite the black hole's strong gravity, the gas has too much inertia because of the rotation of the disk to fall directly into the black hole. Instead, it swirls around until a process similar to friction slows it enough to fall past the event horizon.) The accretion disk contains a magnetic field that is twisted by the rotation (which is faster closer to the black hole). A coiled magnetic field creates a pinching force that focuses the plasma (charged particles that move together) into a narrow jet as it flows away from the black hole. (The magnetic field itself is probably kept from expanding excessively by pressure from a wind that we think surrounds the jet.)

We think that outbursts of radiation from blazars are triggered near the black hole, where some explosive event (such as "reconnection" of magnetic field in places where oppositely directed magnetic fields come in contact) shoots extra energy down the jet. This probably forms a shock wave that moves down the jet along a spiral path. The jet flow velocity increases with distance from the black hole, driven by magnetic forces. As the speed approaches the speed of light, the radiation is beamed more and more in the forward direction, similar to focusing a halogen flashlight. Since the jet points almost in our direction, the radiation from the shock wave gets most intense when the velocity reaches its maximum value, 0.98 times the speed of light. This actually creates an illusion that the bright knot of material made by the shock is moving 5 times faster than light travels. A few weeks later, after the emission has faded as the material in the shock expands and cools, we see a second brightening when the material is compressed by a stationary shock wave created by a pressure difference between the jet and the gas of the surrounding galaxy.

The crucial aspect is what happened during the first flare. The polarization - which indicates the direction of the magnetic field - made about 1.5 rotations. So, that means that the blob passed through a coiled magnetic field. (In order to break the symmetry so that the polarization from different parts of the coil don't cancel, the blob needs to cover less than 100% of the width of the jet and it needs to follow a loose spiral path.) We surmise that the reason for the rise in brightness is the acceleration of the blob as it passes through the region of coiled magnetic field. All of this is exactly what was predicted by theorists, especially Nektarios Vlahakis (U. Athens, Greece) and Arieh Konigl (U. Chicago). But previous observations did not have frequent enough VLBA imaging combined with polarization observations and closely spaced measurements of the radio, visible, and X-ray brightness to fill in enough pieces of the puzzle to determine what is happening physically in the jets.

We hope that we can get an even better look at the jet - maybe see even closer to the black hole - when NASA's GLAST satellite observatory is launched in May, providing us with data showing how the brightness of gamma-rays changes with time. And in 2012, a Japanese radio antenna, VSOP-2, is scheduled for launch into a large Earth orbit. This will allow better resolution of the radio images so that we can see finer detail than is currently possible.

Here are 3 freeze-frames from the movie showing the shock in different parts of the jet as it moves away from the black hole. The first represents the shock wave while it is still in the acceleration zone where the magnetic field is coiled. The 2nd shows the shock as it exits this region, just after it reaches its brightest point. The last frame shows the moving shock passing through the stationary "X" shock that we call the "core."

   

*COSMOVISION is a project to produce scientific and didactic visualization of astrophysical objects and processes by way of 3D-animations. Using methods similar to those applied in the production of documentaries and feature films, they put emphasis in scientific and didactic rigor. They can combine physical computer-simulations with conceptual animations to present scientific results in a wholy new fashion. The research group is based in Ensenada, Mexico.

Cosmovision's motto is: "If we can't go to the stars, we bring them to your home."

COSMOVISION web site: http://www.cosmovision.com.mx

 

SONG!! Alan Marscher (stage name: Cosmos II) has composed and recorded a song about blazars called "Superluminal Lover." You can download the MP3 file and view the lyrics on the Superluminal Lover web page. More science songs by Cosmos II can be found on Marscher's songs web page.

Newer Data

We included BL Lac in our monitoring program starting in 2005. (We also monitored it less intensively in 1999-2000.) The light curve from the more recent period is shown below. (We need to collect optical data from our collaborators for the period after early 2007). We note that there are signs that flares appear in pairs, as in late 2005. Notice the very high X-ray point in late 2006, which coincided with the peak of an optical flare. The X-ray spectrum steepened significantly, which we interpret as a sign that synchrotron radiation becomes dominant over inverse Compton scattering as the cause of the X-ray emission. When this happens, the variations in brightness are often too rapid for us to follow well with our 3-times-per-week monitoring with RXTE. We will fill in the optical light curve and marking the "ejections" of superluminal radio knots seen with the VLBA (the upward arrows in the top panel) when the data become available.

Long-term light curve of BL Lac

 

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