Abstract
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.
Highlights
The very high-energy (VHE; E ≥ 100 GeV) window on the Universe was opened with the discovery of VHE emission from the Crab Nebula (Weekes et al 1989) using the Whipple 10-m Imaging Atmospheric Cherenkov Telescope (IACT)
All blazars are characterized by a similar spectral energy distribution (SED), which consists of two distinct components peaking in the infrared to X-ray band and in the MeV–TeV band, respectively
With the notable exception of PKS 1510−089 (MAGIC Collaboration 2018), which at z = 0.361 is the nearest, all the other flat-spectrum radio-quasars (FSRQs) have been detected at VHE only during bright flaring activity, and are all characterized by very soft VHE spectra
Summary
The very high-energy (VHE; E ≥ 100 GeV) window on the Universe was opened with the discovery of VHE emission from the Crab Nebula (Weekes et al 1989) using the Whipple 10-m Imaging Atmospheric Cherenkov Telescope (IACT). VHE astronomy has the important property of being limited in redshift, due to the absorption of VHE photons via pair-production over the extragalactic background light (EBL; see Salamon & Stecker 1998) Both intrinsic source properties and propagation effects make FSRQs difficult to be observed with IACTs. So far only six FSRQs have been detected by IACTs: 3C 279 The flaring state in HE γ-rays triggered VHE observations with the High Energy Stereoscopic System (H.E.S.S.), resulting in the first detection of VHE photons from PKS 0736+017, which is reported in this paper. 3 we discuss the implications of the VHE detection, in particular in terms of the location of the γ-ray emitting region, and we present the SED of the source during the flare; we draw our conclusions in Sect. Feb. 18, 2015, 21:21 Feb. 19, 2015, 18:53 Feb. 21, 2015, 20:31 Feb. 24, 2015, 21:43
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