Identifying Gamma Ray Blazars

A blazar is a galaxy with an intensely bright central nucleus containing a supermassive black hole, much like a quasar. The difference is that a blazar can emit extremely high energy gamma rays -- light that is over a hundred million times more energetic than the highest energy X-rays as seen, for example, by the Chandra X-ray Observatory. Blazars are also generally characterized by having rapid and strong variability and a host of effects that result from its producing electrons moving close to the speed of light. Astronomers suspect that the bizarre behavior of blazars results when matter falling onto the vicinity of the massive black hole erupts into a powerful, narrow beam of high velocity charged particles. The intense X-ray, gamma ray, and infrared emission, and their variability as well, are thought to be the result of our fortuitously staring right down the throats of the jets. The orientation makes these objects unique probes of exotic physical activity. In most other galaxies, the infrared radiation comes from dust heated either by star formation or ultraviolet radiation from the vicinity of the massive black hole, rather than a blazar jet.

One of the major problems in the study of gamma-ray blazars is identifying the source in the sky for followup study. Transient gamma-rays are spotted by satellites like Fermi that have a typical pointing uncertainly roughly about the size of the full moon, leaving detailed followup studies (using optical or radio telescopes) with the problem of identifying which of the hundreds of candidates in the field is the actual one of interest. The gamma-rays for example, might not even be associated with a blazar jet but some other mechanism. CfA astronomers recently discovered that the infrared colors of blazars, as measured by the recent NASA WISE survey satellite with its much higher spatial precision, are so unusual that any objects with these colors are probable blazars. Indeed, ninety-seven percent of known blazars were easily picked out from thousands of other WISE sources by their infrared colors.

Harvard graduate student Philip Cowperthwaite and his CfA collaborators Raffaele D'Abrusco, Alessandro Paggi, and Howard Smith, and two colleagues, have now published a paper in which they use the WISE infrared colors to identify possible blazars in the fields of previously mysterious, unidentified gamma-ray sources. From 102 unclassified gamma-ray objects in the published literature they found thirteen clear, likely candidate galaxies (in the other cases the WISE data were not sensitive enough and/or ambiguous). They were then able to search the data archives at many wavelengths for additional information about these candidates. All of them emit strong radio radiation, as expected from blazars, and eleven have a known X-ray counterpart as well. Seven of them (so far) also have optical spectra indicating that they are blazars; the scientists are in the process of obtaining optical spectra of some others. The new paper confirms the powerful utility of the infrared colors in identifying gamma-ray blazars, and marks the beginning of solving the puzzle of unidentified gamma-ray blazars.

"Identification of New Gamma-Ray Blazar Candidates with Multifrequency Archival Observations," Philip S. Cowperthwaite, F. Massaro, R. D'Abrusco, A. Paggi, G. Tosti, and Howard A. Smith, AJ 146, 110, 2013.