High-energy sources at low radio frequency: the Murchison Widefield Array view of Fermi blazars^⋆

¹ INAF Osservatorio di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
² Physics Department, University of Turin, via Pietro Giuria 1, 10125 Turin, Italy
³ Department of Physical Sciences, University of Naples Federico II, via Cinthia 9, 80126 Naples, Italy
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, NSW 2006, Australia
⁶ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)  
⁷ International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia
⁸ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand
⁹ University of Crete and Foundation for Research and Technology – Hellas, 71003 Heraklion, Greece
¹⁰ CSIRO Astronomy and Space Science (CASS), PO Box 76, Epping, NSW 1710, Australia
¹¹ Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
¹² School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
¹³ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁴ MIT Haystack Observatory, Westford, MA 01886, USA
¹⁵ Raman Research Institute, 560080 Bangalore, India
¹⁶ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁷ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, ON, M5S 3H4, Canada
¹⁸ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹⁹ Department of Physics, University of Washington, Seattle, WA 98195, USA
²⁰ Department of Physics, University of Wisconsin–Milwaukee, Milwaukee, WI 53201, USA
²¹ Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA
²² Institute of Cybernetics at Tallinn University of Technology, Akadeemia tee 21, 12618 Tallinn, Estonia
²³ School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia
²⁴ National Centre for Radio Astrophysics, Tata Institute for Fundamental Research, 411007 Pune, India
²⁵ Netherlands Institute for Radio Astronomy (ASTRON), PO Box 2, 7990 AA Dwingeloo, The Netherlands
²⁶ National Radio Astronomy Observatory, Charlottesville and Greenbank, A 22903, USA
²⁷ SKA Organisation, Macclesfield SK11 9DL, UK

Received: 24 November 2015
Accepted: 9 February 2016

Abstract

Context. Low-frequency radio arrays are opening a new window for the study of the sky, both to study new phenomena and to better characterize known source classes. Being flat-spectrum sources, blazars are so far poorly studied at low radio frequencies.

Aims. We characterize the spectral properties of the blazar population at low radio frequency, compare the radio and high-energy properties of the gamma-ray blazar population, and search for radio counterparts of unidentified gamma-ray sources.

Methods. We cross-correlated the 6100 deg² Murchison Widefield Array Commissioning Survey catalogue with the Roma blazar catalogue, the third catalogue of active galactic nuclei detected by Fermi-LAT, and the unidentified members of the entire third catalogue of gamma-ray sources detected by Fermi-LAT. When available, we also added high-frequency radio data from the Australia Telescope 20 GHz catalogue.

Results. We find low-frequency counterparts for 186 out of 517 (36%) blazars, 79 out of 174 (45%) gamma-ray blazars, and 8 out of 73 (11%) gamma-ray blazar candidates. The mean low-frequency (120–180 MHz) blazar spectral index is ⟨α_low⟩ = 0.57 ± 0.02: blazar spectra are flatter than the rest of the population of low-frequency sources, but are steeper than at ~GHz frequencies. Low-frequency radio flux density and gamma-ray energy flux display a mildly significant and broadly scattered correlation. Ten unidentified gamma-ray sources have a (probably fortuitous) positional match with low radio frequency sources.

Conclusions. Low-frequency radio astronomy provides important information about sources with a flat radio spectrum and high energy. However, the relatively low sensitivity of the present surveys still misses a significant fraction of these objects. Upcoming deeper surveys, such as the GaLactic and Extragalactic All-Sky MWA (GLEAM) survey, will provide further insight into this population.