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Title: Unveiling the nature of the  $$\gamma$$-ray emitting active galactic nucleus PKS 0521-36

PKS 0521-36 is an active galactic nucleus (AGN) with uncertain classification. Here, we investigate the properties of this source from radio to γ-rays. The broad emission lines in the optical and ultraviolet bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the γ-ray properties are similar to those estimated for other SSRQ and BLRG detected in γ-rays, suggesting an intermediate viewing angle with respect to the observer. In this context the flaring activity detected from this source by Fermi-Large Area Telescope between 2010 June and 2012 February is very intriguing. We discuss the γ-ray emission of this source in the framework of the structured jet scenario, comparing the spectral energy distribution (SED) of the flaring state in 2010 June with that of a low state. We present three alternative models corresponding to three different choices of the viewing angles θv = 6°, 15°, and 20°. We obtain a good fit for the first two cases, but the SED obtained with θv = 15° if observed at a small anglemore » does not resemble that of a typical blazar since the synchrotron emission should dominate by a large factor (~100) the inverse Compton component. This suggests that a viewing angle between 6° and 15° is preferred, with the rapid variability observed during γ-ray flares favouring a smaller angle. However, we cannot rule out that PKS 0521-36 is the misaligned counterpart of a synchrotron-dominated blazar.« less
Authors:
 [1] ;  [2] ;  [3] ;  [3] ;  [4] ;  [2] ;  [5] ;  [4] ;  [6] ;  [6] ;  [7] ;  [4] ;  [8] ;  [9]
  1. Univ. degli Studi di Bologna (Italy). Dip. Di Fisica e Astronomia; Istituto Nazionale di Astrofisica (INAF), Bologna (Italy). Istituto di Radioastronomia
  2. Istituto Nazionale di Astrofisica (INAF), Bologna (Italy). Istituto di Radioastronomia
  3. Istituto Nazionale di Astrofisica (INAF), Merate (Italy). Osservatorio Astronomico di Brera
  4. Istituto Nazionale di Astrofisica (INAF), Bologna (Italy). Istituto di Astrofisica Spaziale e Fisica Cosmica
  5. Istituto Nazionale di Astrofisica (INAF), Pino Torinese (Italy). Osservatorio Astrofisico di Torino
  6. Univ. of Michigan, Ann Arbor, MI (United States)
  7. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  8. Scuola Normale Superiore Pisa, Pisa (Italy)
  9. Univ. degli Studi di Perugia (Italy)
Publication Date:
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Volume: 450; Journal Issue: 4; Journal ID: ISSN 0035-8711
Publisher:
Royal Astronomical Society
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; gamma-rays; galaxies; galaxies nuclei; quasars; PKS 0521−36
OSTI Identifier:
1354934

D'Ammando, F., Orienti, M., Tavecchio, F., Ghisellini, G., Torresi, E., Giroletti, M., Raiteri, C. M., Grandi, P., Aller, M., Aller, H., Gurwell, M. A., Malaguti, G., Pian, E., and Tosti, G.. Unveiling the nature of the  $\gamma$-ray emitting active galactic nucleus PKS 0521-36. United States: N. p., Web. doi:10.1093/mnras/stv909.
D'Ammando, F., Orienti, M., Tavecchio, F., Ghisellini, G., Torresi, E., Giroletti, M., Raiteri, C. M., Grandi, P., Aller, M., Aller, H., Gurwell, M. A., Malaguti, G., Pian, E., & Tosti, G.. Unveiling the nature of the  $\gamma$-ray emitting active galactic nucleus PKS 0521-36. United States. doi:10.1093/mnras/stv909.
D'Ammando, F., Orienti, M., Tavecchio, F., Ghisellini, G., Torresi, E., Giroletti, M., Raiteri, C. M., Grandi, P., Aller, M., Aller, H., Gurwell, M. A., Malaguti, G., Pian, E., and Tosti, G.. 2015. "Unveiling the nature of the  $\gamma$-ray emitting active galactic nucleus PKS 0521-36". United States. doi:10.1093/mnras/stv909. https://www.osti.gov/servlets/purl/1354934.
@article{osti_1354934,
title = {Unveiling the nature of the  $\gamma$-ray emitting active galactic nucleus PKS 0521-36},
author = {D'Ammando, F. and Orienti, M. and Tavecchio, F. and Ghisellini, G. and Torresi, E. and Giroletti, M. and Raiteri, C. M. and Grandi, P. and Aller, M. and Aller, H. and Gurwell, M. A. and Malaguti, G. and Pian, E. and Tosti, G.},
abstractNote = {PKS 0521-36 is an active galactic nucleus (AGN) with uncertain classification. Here, we investigate the properties of this source from radio to γ-rays. The broad emission lines in the optical and ultraviolet bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the γ-ray properties are similar to those estimated for other SSRQ and BLRG detected in γ-rays, suggesting an intermediate viewing angle with respect to the observer. In this context the flaring activity detected from this source by Fermi-Large Area Telescope between 2010 June and 2012 February is very intriguing. We discuss the γ-ray emission of this source in the framework of the structured jet scenario, comparing the spectral energy distribution (SED) of the flaring state in 2010 June with that of a low state. We present three alternative models corresponding to three different choices of the viewing angles θv = 6°, 15°, and 20°. We obtain a good fit for the first two cases, but the SED obtained with θv = 15° if observed at a small angle does not resemble that of a typical blazar since the synchrotron emission should dominate by a large factor (~100) the inverse Compton component. This suggests that a viewing angle between 6° and 15° is preferred, with the rapid variability observed during γ-ray flares favouring a smaller angle. However, we cannot rule out that PKS 0521-36 is the misaligned counterpart of a synchrotron-dominated blazar.},
doi = {10.1093/mnras/stv909},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 4,
volume = 450,
place = {United States},
year = {2015},
month = {5}
}