H.E.S.S. detection of very high-energy γ-ray emission from the quasar PKS 0736+017
Authors:   H.E.S.S. Collaboration, Abdalla H. , Adam R. , Aharonian F. , Ait Benkhali F. , Angüner E. O. , Arakawa M. , Arcaro C. , Armand C. , Ashkar H. , Backes M. , Barbosa Martins V. , Barnard M. , Becherini Y. , Berge D. , Bernlöhr K. , Blackwell R. , Böttcher M. , Boisson C. , Bolmont J. , Bonnefoy S. , Bregeon J. , Breuhaus M. , Brun F. , Brun P. , Bryan M. , Büchele M. , Bulik T. , Bylund T. , Capasso M. , Caroff S. , Carosi A. , Casanova S. , Cerruti M. , Chand T. , Chandra S. , Chen A. , Colafrancesco S. , Curyło M. , Davids I. D. , Deil C. , Devin J. , deWilt P. , Dirson L. , Djannati-Ataï A. , Dmytriiev A. , Donath A. , Doroshenko V. , Drury L. O’C. , Dyks J. , Egberts K. , Emery G. , Ernenwein J.-P. , Eschbach S. , Feijen K. , Fegan S. , Fiasson A. , Fontaine G. , Funk S. , Füßling M. , Gabici S. , Gallant Y. A. , Gaté F. , Giavitto G. , Glawion D. , Glicenstein J. F. , Gottschall D. , Grondin M.-H. , Hahn J. , Haupt M. , Heinzelmann G. , Henri G. , Hermann G. , Hinton J. A. , Hofmann W. , Hoischen C. , Holch T. L. , Holler M. , Horns D. , Huber D. , Iwasaki H. , Jamrozy M. , Jankowsky D. , Jankowsky F. , Jardin-Blicq A. , Jung-Richardt I. , Kastendieck M. A. , Katarzyński K. , Katsuragawa M. , Katz U. , Khangulyan D. , Khélifi B. , King J. , Klepser S. , Kluźniak W. , Komin Nu. , Kosack K. , Kostunin D. , Kraus M. , Lamanna G. , Lau J. , Lemière A. , Lemoine-Goumard M. , Lenain J.-P. , Leser E. , Levy C. , Lohse T. , Lypova I. , Mackey J. , Majumdar J. , Malyshev D. , Marandon V. , Marcowith A. , Mares A. , Mariaud C. , Martí-Devesa G. , Marx R. , Maurin G. , Meintjes P. J. , Mitchell A. M. W. , Moderski R. , Mohamed M. , Mohrmann L. , Muller J. , Moore C. , Moulin E. , Murach T. , Nakashima S. , de Naurois M. , Ndiyavala H. , Niederwanger F. , Niemiec J. , Oakes L. , O’Brien P. , Odaka H. , Ohm S. , de Oña Wilhelmi E. , Ostrowski M. , Oya I. , Panter M. , Parsons R. D. , Perennes C. , Petrucci P.-O. , Peyaud B. , Piel Q. , Pita S. , Poireau V. , Priyana Noel A. , Prokhorov D. A. , Prokoph H. , Pühlhofer G. , Punch M. , Quirrenbach A. , Raab S. , Rauth R. , Reimer A. , Reimer O. , Remy Q. , Renaud M. , Rieger F. , Rinchiuso L. , Romoli C. , Rowell G. , Rudak B. , Ruiz-Velasco E. , Sahakian V. , Saito S. , Sanchez D. A. , Santangelo A. , Sasaki M. , Schlickeiser R. , Schüssler F. , Schulz A. , Schutte H. , Schwanke U. , Schwemmer S. , Seglar-Arroyo M. , Senniappan M. , Seyffert A. S. , Shafi N. , Shiningayamwe K. , Simoni R. , Sinha A. , Sol H. , Specovius A. , Spir-Jacob M. , Stawarz Ł. , Steenkamp R. , Stegmann C. , Steppa C. , Takahashi T. , Tavernier T. , Taylor A. M. , Terrier R. , Tiziani D. , Tluczykont M. , Trichard C. , Tsirou M. , Tsuji N. , Tuffs R. , Uchiyama Y. , van der Walt D. J. , van Eldik C. , van Rensburg C. , van Soelen B. , Vasileiadis G. , Veh J. , Venter C. , Vincent P. , Vink J. , Voisin F. , Völk H. J. , Vuillaume T. , Wadiasingh Z. , Wagner S. J. , White R. , Wierzcholska A. , Yang R. , Yoneda H. , Zacharias M. , Zanin R. , Zdziarski A. A. , Zech A. , Ziegler A. , Zorn J. , Żywucka N. , Smith P. S. 
Published: November 2019

Context. Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies (E ≥ 100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects.

Aims. Following the detection of MeV–GeV γ-ray flaring activity from the FSRQ PKS 0736+017 (z = 0.189) with Fermi-LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying the γ-ray emission in the VHE band.

Methods. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi-LAT, the multi-wavelength coverage of the flare includes Swift observations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, and the ATOM, the KAIT, and the ASAS-SN telescopes.

Results. VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015. Fermi-LAT data indicate the presence of a γ-ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. The γ-ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The γ-ray observations with H.E.S.S. and Fermi-LAT are used to put constraints on the location of the γ-ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line region rBLR with a bulk Lorentz factor Γ ≃ 20, or at the level of the radius of the dusty torus rtorus with Γ ≃ 60.

Conclusions. PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and at z = 0.189 is the nearest so far. The location of the γ-ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.