Spectroscopy for the FSRQ Zoo

May 5, 2013

KIPAC scientists have amassed detailed optical light spectra for 165 FSRQ-type blazars seen by the Fermi Gamma-ray Space Telescope. The observations can be used to learn about their violent central regions where a supermassive black hole holds sway.

Examples of optical spectra of Fermi FSRQ-type blazars as measured and presented by Shaw, Romani, and colleagues. In some the emission lines are clearly visible.

Active galactic nuclei (AGN) are nature's extreme particle accelerators, where twin beams of high energy particles and radiation fly away from a supermassive black hole in the center of a distant galaxy. When one of the beams is pointed at us, we see the phenomenon of a blazar, which can be seen prominently in X-rays or gamma rays, the highest energy kinds of light. Observations with the Large Area Telescope of the Fermi Gamma-ray Space Telescope - which was built at SLAC and in which KIPAC is the main science institution - have revolutionized the study of blazars by cataloging their gamma-ray emission.
But gamma rays aren't the only way to learn about blazars. A full picture requires knowledge of their light output across the electromagnetic spectrum, and in particular the unique insight provided by optical spectra - where one can see the intensity differences in small wavelength bins. Obtaining optical spectra requires lengthy, careful observations with some of the world's best telescopes. Up to the challenge, KIPAC scientists led by professor Roger Romani have set about to obtain spectra for Fermi's hundreds of blazars.
In a recent paper authored by KIPAC graduate student Michael Shaw, with Romani, postdoc Stephen Healey, and professor and Fermi principal investigator Peter Michaelson, along with several colleagues from the University of Oxford and Caltech, the team reports on their analysis of optical spectra for 165 of Fermi's observed blazars. These blazars are of the Flat Spectrum Radio Quasar class, known as FSRQs, which are characterized by the lower frequency peak of their radio emission and the presence of broad spectral lines in optical light. The spectra were taken at different telescopes around the world, including the Hobby-Eberley Telescope and the Large Cass Spectrometer at McDonald Observatory, both in West Texas, the Hale Telescope at Mt. Palomar in Southern California, the W. M. Keck Observatory in Hawaii, and the New Technology Telescope at La Silla Observatory and the Very Large Telescope at Paranal Observatory, both in Chile.
Shaw, Romani, and colleagues used the spectra, particularly atomic emission lines associated with Hydrogen, Magnesium, and Carbon, to determine the mass of the black hole and properties of the accretion disk of matter surrounding it. Because of doppler shift effects, wider emission lines indicate higher velocities of matter, and more prominent emission lines indicate higher intrinsic brightness, which can be used in combination to probe total brightness of the accretion disk and the mass of the black hole, even though these can't be seen directly.
The team finds that the black hole masses determined in this way solely from the emission line proerties are significantly lower, and the efficiencies of conversion of accretion disk mass into light out are higher, than those obtained in a more traditional manner. They also find, using these measures, that the Fermi FSRQ blazars are more 'active' on average, that is that they have more mass falling onto the supermassive black hole, than AGN generally. This is intriguing in light of theories that unite FSRQs with the most powerful radio galaxies as part of the same underlying population, and the other class of blazars, BL Lacs, with other AGNs. They also detail a number of other findings that allow comparison of Fermi FSRQs with AGN selected by other means. The team is already obtaining optical spectra for Fermi's BL Lac type blazars, which will enable important comparisons between the two populations toward the goal of further understanding the violent hearts of blazars.
This work is described in a paper to appear in Astrophysical Journal and available from astro-ph at arXiv:1201.0999.
Science Contact:
Michael Shaw
email: msshaw@stanford.edu