Blazars Blare Bright But Below Background

May 5, 2015

The origin of the extragalactic gamma-ray background remains a cosmic and high energy physics enigma as KIPAC scientists have estimated the contribution to it from blazars in two different ways.

Estimate of the total cumulative flux from blazars above a given flux value. The level of the gamma-ray background is shown by the dashed line.

In distant active galactic nuclei, matter falling onto a giant black hole results in powerful jets of particles and radiation, at energies and scales more extreme than can be achieved on Earth. When a jet is pointed right at us, we see a blazar - a distant point of intense gamma-ray, x-ray, and other light. The Fermi Gamma Ray Space Telescope has revolutionized the study of blazars, seeing hundreds of them all over the sky.
Fermi has also honed in on the extragalactic gamma-ray background, the diffuse gamma-ray light that pervades the Universe. The two are intimately connected, as the integrated emission from dimmer blazars is the most obvious candidate for the origin of some of the gamma-ray background. This contribution from dim blazars can be estimated, even though we can't see them directly. The idea is simple and common in astrophysics: if we know how the number of a object scales with their flux from the ones that are bright enough too see, then we can calculate the total from the ones we can't see by applying that scaling.
However, there is a wrinkle with Blazars because their spectra - how the intensity of gamma-rays scales with gamma-ray energy - varies from object to object, and Fermi is more sensitive to those with 'hard' spectra, where the intensity falls off less sharply with increasing energy. Therefore, determining the flux and photon spectral index distributions of blazars seen with Fermi is a coupled problem with incomplete data. Not only is Fermi - or any other instrument - not seeing blazars if they are too dim, it is also not seeing them if they have the wrong spectrum.
In recent analyses of the flux and spectral index distributions of Fermi blazars, KIPAC scientists have dealt with these complications in two different ways. The first, by the Fermi-LAT team and led by KIPAC postdoc Marco Ajello, used Monte Carlo statistical techniques to recover the most likely intrinsic distributions. The second, by KIPAC postdoc Jack Singal, professor Vahe Petrosian, and Ajello, used a technique pioneered by Pertrosian and Stanford statistician Bradley Efron to recover intrinsic distributions from the biased and truncated observed ones.
In both determinations the blazars were seen to have an intrinsic flux distribution whereby at fluxes below a certain 'break' value the number of blazars at a given flux increases with decreasing flux only slowly. Therefore, it appears that there are simply not enough blazars in the Universe to account for all of the gamma-ray background. Absent some unanticipated special population of dim blazars, which Singal, Petrosian, and Ajello plan to investigate next, the gamma-ray background would have to come in large part from some other sources. One of those sources could be annihilation of dark matter particles throughout the Universe, although these results only allow for the possibility. Determining what the remaining components of the gamma-ray background are will be one of the most fascinating challenges for Fermi observations, and will reveal important information about the history of high energy processes in the Universe.
This work is based on a paper published in the Astrophysical Journal (ApJ, 2010, 720, 435) and another submitted to the Astrophysical Journal and available from astro-ph at arXiv:1106.3111.
Science Contact:
Jack Singal