Fermi 10 Blog for "Fermi Fridays" Series: Behind the Scenes: Doing Science Across the Spectrum (author's version)

-       Posted on 3/16/2018 at https://fermi.gsfc.nasa.gov/ssc/

Professor Alan Marscher, Institute for Astrophysical Research, Boston University

The gamma-ray sky seen by Fermi is lit up by blazars, the most extreme examples of super-massive black holes swallowing gas at the centers of large galaxies. A tiny fraction of the charged particles near the black hole are funneled by magnetic forces into narrow jets that are propelled outward at 0.98-0.999 times the speed of light. Despite the low density of particles in the jets, the electrons gain so much energy that they produce gamma rays with a luminosity higher than that of any other object in the universe that shines for more than a few minutes. (Only the short-lived gamma-ray bursts produce more energy.)

Artist's concept of an Active Galactic Nucleus (AGN) with a jet.  Blazars are those AGN for which the jet is pointed toward Earth. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab.

Astrophysicists learn about how these exotic jets form, energize particles, and radiate such an extreme amount of light by studying not only the gamma rays that they produce, but also their light at other wavelengths. While Fermi can measure the changing brightness of gamma rays Ð the most energetic form of light Ð other telescopes are needed to observe their radio, infrared, visible, UV, and X-ray emissions. For example, the Swift satellite measures the brightness of visible, UV, and X-ray light of many blazars at different times. Seeing whether the brightness at these wavelengths changes along with the gamma-ray brightness is an important diagnostic for figuring out whatÕs going on in the jets.

Astronomers can also measure the polarization of the light at visible and radio wavelengths. For light produced by high-energy electrons moving along magnetic fields, the degree of polarization tells us whether the field is well-ordered (high percentage of polarization) or in disarray, as occurs if there is a lot of turbulence in the jet. The direction of the polarization gives us the average direction of the magnetic field. In some blazars, we see the polarization direction rotate with time, which suggests that the magnetic field has a spiral shape, just as predicted by theories for producing jets when magnetic fields are twisted by rotation of the black hole.

 

 

Changes of brightness with time at
gamma-ray, X-ray, and visible wave-
lengths in the blazar 3C 454.3. The
second panel from the bottom displays
the degree of polarization of the
visible light, while the bottom panel
shows the direction of the polarization,
which is perpendicular to the average
direction of the magnetic field in the
blazar's jet. Note that the direction of
the polarization often changes rapidly,
as if it is rotating.

 

 

A particularly powerful technique uses data from the Very Long Baseline Array (VLBA) to make images of the radio emission from the jets. While Fermi and other telescopes can measure how the brightness changes with time, only the VLBA has enough resolution to see the jet in detail. And what these images reveal is spectacular! Bright spots (ÒblobsÓ) appear to move away from the black hole at speeds greater than the speed of light Ð an illusion that comes from our measurement of time being different from that in the blob when the blob moves almost as fast as light. At such high velocities, a jet pointing almost right at us beams light in our direction to make the jet extremely bright; the jet on the other side beams its light away from us, and so is too faint to detect. The movie below, made from actual data, shows a jet in action, with new super-luminal blobs created near times when we see outbursts at gamma-ray and other wavelengths. In the movie, you can see bright blobs move northward (upward on your screen). The figure next to the movie shows how the brightness and polarization change with time. Outbursts of gamma rays generally occur as a new blob emerges. But not all new blobs cause gamma-ray outbursts. Astronomers donÕt know why this is, and hope that by observing more events of this sort, they can figure out exactly what needs to happen in a blazar to make gamma rays.

The movie on the left, created by Dr. S. Jorstad, shows hows the jet in the blazar 4C 21.35 has changed with time. Blobs that seem to move faster than light appear. The figure to the right displays changes of brightness and polarization with time; times when a new superluminal blob appeared are indicated by the this vertical red lines.

Interpretation of the observations requires the development of theoretical models based on gravity, orbits, and our knowledge of how particles and magnetic fields behave. The polarization levels, which are rarely more than 30%, indicate that turbulence scrambles the magnetic field into a spaghetti-like pattern. But there is partial order to the field as well, which can be explained if it is both twisted by rotation and compressed by shock waves. Both turbulence and shock waves are known to energize charged particles like the electrons that make the light at the various wavelengths in blazars. ItÕs probably the case that everything that can happen in such a high-energy situation does happen. The cartoon below sketches the various phenomena that we think occur in a blazar.

Sketch of the complex processes in a blazar. The gamma rays may come from the zone where the jet accelerates to near-light speed, turbulent regions, a standing shock, or a moving shock, or all of these. The photons that electrons in the jet knock up to X-ray and gamma-ray energies can come from the accretion disk, hot dust in the outer disk, hot gas clouds, or the jet itself. The scale in light years, shown on the bottom, increases by ten times with each tick mark so that everything will fit in the sketch. [© 2018 by A. Marscher]