Relativistic Outflows

Relativistic Outflows
The Fermi-LAT detects gamma-rays with energies several hundred times the rest mass of an electron. The generation of gamma-rays with these energies requires extremely energetic particles.  In some galactic sources, the radiating particles are energized by acceleration from shocks formed in the wake of stellar explosions.  Particle acceleration can also occur in relativistic outflows. These collimated streams of relativistic plasma are commonly seen in active galactic nuclei, gamma-ray bursts, and even in galactic binary sources.

Beyond our own galaxy, the greatest sources of gamma-rays detected by the Fermi-LAT are relativistic jets associated with Active Galactic Nuclei (AGN).  Such nuclei are found in the cores of galaxies and are powered by actively accreting supermassive black holes with masses at least a million times greater than the Sun.  The accretion of material onto the black hole often leads to the formation of a collimated jet of plasma that travels outward along the axis of the accretion disk.  The formation, internal structure, and evolution of collimated jets are still not fully understood.

The majority of the gamma-ray emission from an AGN is tightly focused along the jet axis and therefore the Fermi-LAT is most sensitive to those jets that are directed toward our line of sight. These objects are known as blazars. The observed broad-band spectrum of blazars is dominated by gamma-ray emission. Thus, gamma-ray studies are crucial for understanding the jet energetics as well as the relation of the jet to other constituents of the nucleus such as the black hole and the accretion disk.  In addition to their gamma ray emissions, blazars are strongly variable in all observable bands of the electromagnetic spectrum. Therefore, simultaneous observations with instruments sensitive to different wavelengths of radiation are critical for studying them.

Recently the Fermi-LAT detected one of the brightest gamma-ray outbursts ever observed from any blazar; the culprit was called 3C 454.3 located at a redshift of 0.895. For four days in November 2010, this blazar was observed to flare up to three times its greatest brightness, with additional variability during, before, and after the flare. During this period it became the brightest object in the gamma-ray sky. Given its large distance from Earth, this means that 3C 454.3 is not only the most luminous blazar. Aside from the momentary explosions known as gamma-ray bursts, 3C 454.3 is the most luminous object ever observed in any spectral band in the universe. Studies of this flaring -- from overall flux to changes in the spectral index of the gamma-ray emission -- will be important for understanding the extreme physics in AGNs, black holes, and the high energy universe.

Blazars compose a significant fraction of the diffuse extragalactic gamma-ray background (EGB), an isotropic component of the gamma-ray sky extensively measured by the Fermi-LAT. The diffuse X-ray background is known to be entirely made up of the unresolved astrophysical sources, and in particular AGN.  However, by extrapolating the number density of the Fermi-resolved AGN population to undetectably faint fluxes, the Fermi-LAT team at KIPAC has shown that their contribution to the gamma-ray background is limited to just 30 percent. This means that other gamma-ray sources must contribute to the extragalactic diffuse emission observed by Fermi. But its brightness suggests that these mysterious sources, too faint to be detected individually by the LAT, must be as numerous as normal galaxies, which are thought to contribute an additional 15-20 percent of the total diffuse flux.

These are tantalizing hints that the EGB might arise from processes beyond standard model physics such as dark matter annihilation in the Milky Way halo and external galaxies.  More studies  incorporating longer observation times and improved foreground and background subtraction are now needed to firmly identify the gamma-ray signal, and to ascertain if dark matter may be the source. However, we are already at the stage where we can derive scientifically compelling upper limits on the annihilation cross-section of cosmological dark matter contributing to the EGB.


Multiwavelength light curve of the distant gamma-ray blazar 3C279 (Abdo et al. 2010, Nature 463, 919).  The top two frames (a and b) show the flux and spectral index measured by the Fermi-LAT.
2. (optical for M87;  Chandra image of Pic A radio galaxy;  possibly the light curve in many bands for 3C279;  image of numerical simulations from J McKinney;  ) 251658240


Representation of the components of the Extragalactic Gamma-ray Background (EGB) as inferred from population studies of AGN and star-forming galaxies.  As much as 50 percent of the EGB is believed to arise from an as yet unidentified source class.  A potential candidate for this unknown component would be the radiation emitted by annihilating DM particles.