April 19, 2013
Puzzling Blazar Found To Be The Most Distant Known Source Of Very High-Energy Gamma Rays
April Flowers for redOrbit.com - Your Universe Online
Of all the active galactic nuclei, blazars are the brightest and emit very high-energy gamma rays. A team led by physicists from the University of California, Santa Cruz (UCSC), has made new observations of the blazar known as PKS 1424+240 that reveal it is the most distant known source of very high-energy gamma rays. The emission spectrum of PKS 1424+240 now appears highly unusual in the light of new data.
Data from the Hubble Space Telescope was used to set a lower limit for the blazar's redshift (z 0.6035) that corresponds to a distance of at least 7.4 billion light-years. A substantial portion of the gamma rays should be absorbed by extragalactic background light at such a great distance. The calculations that account for the expected absorption, however, yield an unexpected emission spectrum for the blazar.
"We're seeing an extraordinarily bright source which does not display the characteristic emission expected from a very high-energy blazar," said Amy Furniss, a graduate student at the Santa Cruz Institute for Particle Physics (SCIPP) at UCSC.
The findings, published in Astrophysical Journal Letters, may indicate something novel about the emission mechanism of blazars, the extragalactic background light, or the propagation of gamma-ray photons over long distances, according to David Williams, adjunct professor of physics at UC Santa Cruz.
"We're finding very high-energy gamma-ray sources at greater distances than we thought we might, and in doing so we're finding some things we don't entirely understand," Williams said. "Having a source at this distance will allow us to better understand how much background absorption there is and test the cosmological models that predict the extragalactic background light."
The diffuse radiation from all stars and galaxies is called the extragalactic background light (EBL). The EBL is a dim but pervasive glow that fills the Universe. When a lower-energy EBL photon collides with a high-energy gamma ray photon, they are both annihilated and an electron-positron pair is created. This mechanism is more likely to absorb gamma rays the farther they have to travel, limiting the distance to which sources of very high-energy gamma rays can be detected.
Because there are so many bright sources of light in our immediate neighborhood, measuring EBL is challenging. Astronomers have used galaxy counts, in addition to estimates based on cosmological models, to set a lower limit for the EBL. Furniss derived an intrinsic gamma-ray emission spectrum for the blazar using a model close to this lower limit. This allowed him to calculate the expected absorption of very high-energy gamma rays from PKS 1424+240.
A relativistic jet of particles, powered by matter falling onto a supermassive black hole at the center of the host galaxy, is thought to be the cause of blazar emission. "There may be something going on in the emission mechanisms of the blazar that we don't understand," Williams said. "There are more exotic explanations as well, but it may be premature to speculate at this point."
The Fermi Gamma-ray Space Telescope first detected gamma rays from PKS 1424+240. These observations were verified by the ground-based instrument VERITAS (Very Energetic Radiation Imaging Telescope Array System). VERITAS is sensitive to gamma rays in the very high-energy (VHE) band from about 100 GeV to more than 10 TeV. Redshift is the measure of how much the light from an object has been stretched to longer wavelengths by the expansion of the Universe. To obtain the redshift for PKS 1424+240, the researchers used data obtained by the Hubble Space Telescope's Cosmic Origins Spectrograph for another research program.