BY DR EMILY BALDWIN
Posted: 20 March, 2009
By taking advantage of telescopes in space and on the ground, astronomers have uncovered surprising changes in radiation emitted by an active galaxy.
The observations made use of the High Energy Stereoscopic System (HESS) - an array of telescopes located in Namibia - and the Large Area Telescope (LAT) aboard NASA’s orbiting Fermi Gamma-ray Space Telescope, with follow-up observations from Swift and Rossi. The observation campaign is the first time that simultaneous measurements of an active galaxy have been made with optical, X-ray and gamma-ray telescopes.
“The launch of Fermi gives us the opportunity to measure this powerful galaxy across as many wavelengths as possible for the first time,” says Werner Hofmann, spokesperson for the HESS team at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.
In the heart of an active galaxy, matter falling into a supermassive black hole somehow creates jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams right toward Earth. Image: NASA/Goddard Space Flight Center Conceptual Image Labs.
The international team of astronomers studied active galaxy PKS 2155-304, which is located 1.5 billion light years away in the southern constellation of Piscis Austrinus. PKS 2155-304 is a blazar, which emits oppositely directed jets of particles traveling near the speed of light as a result of matter falling into a central supermassive black hole. In the case of blazars, the galaxy is oriented such that we’re looking right down the jet.
PKS 2155-304 is usually detectable as a faint gamma-ray source, but in 2006 it underwent a major outburst, lighting up in the highest gamma ray energies at 50 trillion times the energy of visible light. When this energy strikes the top of the Earth’s atmosphere, absorption of the gamma rays creates a short lived shower of subatomic particles. As these fast moving particles career through the atmosphere, they produce a faint flash of blue light, which in this case, were picked up by HESS, while the lower energy gamma rays were monitored by Fermi.
With the gamma-ray regime fully covered, the team turned to NASA’s Swift and Rossi X-ray Timing Explorer (RXTE) satellites to provide data on the galaxy’s X-ray emissions. In addition, the HESS Automatic Telescope for Optical Monitoring recorded the galaxy’s activity in visible light. The multi-spectral analysis paints a complex picture that challenges current theories of how the blazar’s radiation is generated.
The four identical telescopes of the High Energy Stereoscopic System in Namibia detect faint atmospheric flashes caused by the absorption of ultrahigh-energy gamma rays. Image: H.E.S.S.
Between August 25 and September 6, 2008, the telescopes monitored PKS 2155-304 in its quiet, non-flaring state, but this threw up surprising results. Usually, during flaring episodes, the X-ray and gamma-ray emission rise and fall together, but not for PKS 2155-304 in its quiet state, where the galaxy’s visible light rises and falls with its gamma-ray emission.
“It’s like watching a blowtorch where the highest temperatures and the lowest temperatures change in step, but the middle temperatures do not,” says Berrie Giebels, an astrophysicist at France’s École Polytechnique who works with both the HESS and Fermi LAT teams.
Astronomers may not have all the answers to this mysterious behaviour yet, but what it does show is that the various constituents of blazar jets interact in fairly complicated ways to produce the radiation that is observed. “These observations may contain the first clues to help us untangle what’s really going on deep in the heart of a blazar,” concludes Fermi team member Jim Chiang at Stanford University.
The findings have been submitted to The Astrophysical Journal.