A possible source of γ-ray photons observed from the jets of blazars is inverse Compton scattering by relativistic electrons of infrared seed photons from a hot, dusty torus in the nucleus. We use observations from the Spitzer Space Telescope to search for signatures of such dust in the infrared spectra of four γ-ray bright blazars, the quasars 4C 21.35, CTA102, and PKS 1510-089, and the BL Lacertae object ON231. The spectral energy distribution (SED) of 4C 21.35 contains a prominent infrared excess indicative of dust emission. After subtracting a non-thermal component with a power-law spectrum, we fit a dust model to the residual SED. The model consists of a blackbody with temperature ~1200 K, plus a much weaker optically thin component at ~660 K. The total luminosity of the thermal dust emission is 7.9 ± 0.2 × 1045 erg s-1. If the dust lies in an equatorial torus, the density of infrared photons from the torus is sufficient to explain the γ-ray flux from 4C 21.35 as long as the scattering occurs within a few parsecs of the central engine. We also report a tentative detection of dust in the quasar CTA102, in which the luminosity of the infrared excess is 7 ± 2 × 1045 erg s-1. However, in CTA102 the far-infrared spectra are too noisy to detect the 10 μm silicate feature. Upper limits to the luminosity from thermal emission from dust in PKS 1510-089, and ON231, are 2.3 × 1045, and 6.6 × 1043 erg s-1, respectively. These upper limits do not rule out the possibility of inverse Compton upscattering of infrared photons to γ-ray energies in these two sources. The estimated covering factor of the hot dust in 4C 21.35, 22%, is similar to that of non-blazar quasars; however, 4C 21.35 is deficient in cooler dust.

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Published in The Astrophysical Journal, v. 732, no. 2, 116, p. 1-8.

© 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

The copyright holder has granted permission for posting the article here.

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Funding for this project was supplied by JPL through Spitzer grant 1289764, by NASA through Fermi Guest Investigator grant NNX08AV65G, and by National Science Foundation (NSF) grant AST-0907893. This study included observations with the PRISM camera (PI: Kenneth Janes), developed at Boston University and Lowell Observatory, with financial support from NSF, Boston University, and Lowell Observatory. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, NSF, the U.S. Department of Energy, NASA, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/.

apj387127t2_mrt.txt (160 kB)
Table 2. Data for 4C 21.35, PKS 1510−089, CTA102, and ON231: Machine-Readable Version