[Paper Review] H.E.S.S. discovery of very-high-energy emission from the blazar PKS 0736+017: on the location of the $γ$-ray emitting region in FSRQs

M. Cerruti, J.-P. Lenain+1 more|arXiv (Cornell University)|Aug 2, 2017

Astrophysics and Cosmic Phenomena4 citations

TL;DR

H.E.S.S. reports the first detection of very-high-energy (VHE; E > 100 GeV) γ-ray emission from the flat-spectrum radio quasar PKS 0736+017 (z = 0.189), confirming it as the sixth known VHE FSRQ. Using opacity, variability, and cooling constraints from contemporaneous Fermi-LAT and H.E.S.S. data, the study locates the γ-ray emitting region at r > 8 × 10¹⁶ cm, excluding emission near the broad-line region, with a minimum bulk Lorentz factor Γ ≳ 10.

ABSTRACT

With the installation of a new 28-m diameter imaging atmospheric Cherenkov telescope in the middle of the array, the H.E.S.S. instrument has entered since 2012 into its Phase II. The fifth large-size telescope is particularly important to lower the threshold energy of the array, and is thus a unique instrument to observe low-frequency-peaked blazars, such as flat-spectrum radio-quasars (FSRQs), which remain rare in the very-high- energy (VHE; E $>$ 100 GeV) $γ$-ray domain. In this contribution, we report on the discovery with the H.E.S.S. telescopes of VHE $γ$-ray emission from the FSRQ PKS 0736+017 (z=0.189). H.E.S.S. observations were triggered as a target-of-opportunity in February 2015 following the detection of $γ$-ray flaring activity in the MeV-GeV energy-band with Fermi-LAT. Significant VHE emission is detected with H.E.S.S. only during one of the nights of the observing campaign, showing at least night-by-night variability in the VHE regime. We discuss the location of the $γ$-ray emitting region within the relativistic jet, using opacity and variability constraints.

Motivation & Objective

• To identify the location of the γ-ray emitting region within the relativistic jet of the FSRQ PKS 0736+017 using multiwavelength γ-ray data.
• To resolve the long-standing ambiguity in the jet emission site for low-frequency-peaked FSRQs, which are rare in the VHE domain.
• To test competing models of γ-ray production—either in a single zone or in multiple regions—using spectral and variability constraints.
• To determine the minimum Lorentz factor and emission radius based on opacity, variability timescale, and cooling timescale constraints.

Proposed method

• Utilized H.E.S.S. Phase II data from a target-of-opportunity campaign triggered by Fermi-LAT flaring activity in February 2015.
• Performed analysis using both monoscopic (28-m telescope only) and stereoscopic (≥2 telescopes) configurations to optimize sensitivity and energy threshold.
• Applied the ImPACT and model analysis chains to reconstruct Cherenkov shower images and extract VHE photon fluxes.
• Used the γγ pair production optical depth to constrain the emission site relative to the broad-line region (BLR), assuming absorption limits the VHE spectrum.
• Applied the variability timescale τ ≈ 12 hours from Fermi-LAT to estimate the emitting region size via R ≈ cτδ/(1+z), linking it to jet bulk Lorentz factor Γ and opening angle θ.
• Imposed cooling timescale τ_c ≤ τ to constrain the maximum energy loss rate, translating into an exclusion region in the Γ–r plane.

Experimental results

Research questions

• RQ1 Where is the γ-ray emitting region located within the relativistic jet of PKS 0736+017, given its VHE emission and MeV–GeV flaring activity?
• RQ2 Can the observed spectral break between Fermi-LAT and H.E.S.S. energies be attributed to γγ absorption in the broad-line region, and what does this imply for the emission site?
• RQ3 What is the minimum bulk Lorentz factor Γ consistent with the observed variability timescale and collimation constraints?
• RQ4 Do the cooling timescale and variability timescale constraints jointly exclude emission regions close to the black hole or within the broad-line region?

Key findings

• H.E.S.S. detected significant VHE γ-ray emission from PKS 0736+017 only on February 19, 2015, indicating strong night-by-night variability in the VHE band.
• The spectral break observed between Fermi-LAT (MeV–GeV) and H.E.S.S. (VHE) energies is consistent with γγ pair production opacity, implying a minimum emission radius r > 8 × 10¹⁶ cm.
• The opacity constraint yields an upper limit on the optical depth of ≃2, which excludes emission sites within the broad-line region.
• The variability timescale of ≈12 hours implies a minimum bulk Lorentz factor of approximately 10, assuming relativistic beaming with Doppler factor δ.
• The cooling timescale constraint, combined with variability, excludes emission regions with high Lorentz factors and small radii, further supporting a location beyond the BLR.
• The results place the VHE emission site outside the broad-line region and the dusty torus, favoring a single-zone emission model with a compact, relativistically moving region at r > 8 × 10¹⁶ cm.