Research Note

Near-infrared and gamma-ray monitoring of TANAMI gamma-ray bright sources^⋆

¹ INAF – IAPS, via Fosso del Cavaliere 100, 00133 Roma Italy
e-mail: roberto.nesci@uniroma1.it
² Universita’ La Sapienza, 00133 Roma, Italy
³ Universita’ di Perugia, 06023 Perugia, Italy
⁴ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
⁵ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁶ Institute for Astrophysics & Computational Sciences, Catholic University of America, Washington, DC 20064, USA
⁷ Institut für Theoretische Physik und Astrophysik, Lehrstuhl für Astronomie, Univ. Würzburg, Emil-Fischer Str. 31, 97074 Würzburg, Germany
⁸ Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Sternwartstraße 7, 96049 Bamberg, Germany

Received: 13 January 2013
Accepted: 7 May 2013

Abstract

Context. Spectral energy distribution and its variability are basic tools for understanding the physical processes operating in active galactic nuclei (AGN).

Aims. In this paper we report the results of a one-year near-infrared (NIR) and optical monitoring of a sample of 22 AGN known to be gamma-ray emitters, aimed at discovering correlations between optical and gamma-ray emission.

Methods. We observed our objects with the Rapid Eye Mount (REM) telescope in J,H,K, and R bands nearly twice every month during their visibility window and derived light curves and spectral indexes. We also analyzed the gamma-ray data from the Fermi gamma-ray Space Telescope, making weekly averages.

Results. Six sources were never detected during our monitoring, proving to be fainter than their historical Two micron all sky survey (2MASS) level. All of the sixteen detected sources showed marked flux density variability, while the spectral indexes remained unchanged within our sensitivity limits. Steeper sources showed, on average, a larger variability. From the NIR light curves we also computed a variability speed index for each detected source. Only one source (PKS 0208-512) underwent an NIR flare during our monitoring. Half of the sources showed a regular flux density trend on a one-year time scale, but do not show any other peculiar characteristic. The broadband spectral index α_ro appears to be a good proxy of the NIR spectral index only for BL Lac objects. No clear correlation between NIR and gamma-ray data is evident in our data, save for PKS 0537−441, PKS 0521−360, PKS 2155−304, and PKS 1424−418. The gamma-ray/NIR flux ratio showed a large spread, QSO being generally gamma-louder than BL Lac, with a marked correlation with the estimated peak frequency (ν_peak) of the synchrotron emission.