H.E.S.S. detection and multi-wavelength study of the $z \sim$ 1 blazar PKS 0346$-$27

TL;DR

PKS 0346-27, a Low-Synchrotron-Peaked blazar at $z=0.991$, was observed in a High-Energy Stereoscopic System ToO campaign triggered by a Fermi-LAT HE flare. The VHE detection on 3 November 2021 reached $6.3\sigma$, with simultaneous Fermi-LAT, Swift, and ATOM data enabling a broadband SED analysis. A single-zone hadronic model with proton-synchrotron-dominated emission and cascade components successfully reproduces the flare-state SED, at the cost of temporarily super-Eddington jet power; a one-zone leptonic alternative can fit the data but requires extreme Doppler factors ($\Gamma,\delta \gtrsim 80$) and strong departures from equipartition, making it less favorable. The observed ~2-day delay between the HE and VHE flares motivates scenarios such as hadronic synchrotron mirrors or delayed proton acceleration, which are discussed as plausible explanations with testable predictions for future multi-zone or time-dependent modelling. Overall, the work extends VHE blazar detections to the high-redshift regime and provides insights into jet energetics and the role of EBL in shaping distant gamma-ray spectra.

Abstract

PKS 0346-27 is a Low Synchrotron Peaked (LSP) blazar at redshift 0.991. The very-high-energy (VHE, E > 100 GeV) spectra of blazars are always affected by $γγ$ absorption by the Extragalactic Background Light (EBL) and subsequently, no blazars have been detected in VHE $γ$-rays at redshifts exceeding 1. Extending the redshift range of VHE-detected blazars to $z \gtrsim 1$ will yield insights into the cosmological evolution of both the VHE blazar population and the EBL. This is the goal of a target-of-opportunity (ToO) programme by H.E.S.S. to observe flaring high-redshift ($z \gtrsim 1$) blazars. We report on H.E.S.S. ToO and multi-wavelength observations of the blazar PKS\,0346$-$27. Along with H.E.S.S., simultaneous data from {\it Fermi}-LAT, {\it Swift} (XRT and UVOT), and ATOM have been analysed and modelled using single-zone leptonic and hadronic models. PKS~0346-27 has been detected by H.E.S.S at a significance of 6.3$σ$ during one night, on 3 November 2021, while for other nights before and after this day, upper limits on the VHE flux are determined. No evidence for intra-night $γ$-ray variability has been found. A flare in high-energy (HE, $E > 100$~MeV) $γ$-rays detected by {\it Fermi}-LAT preceded the H.E.S.S. detection by 2 days. A fit with a single-zone emission model to the contemporaneous spectral energy distribution during the detection night was possible with a proton-synchrotron-dominated hadronic model, requiring a proton-kinetic-energy-dominated jet power temporarily exceeding the source's Eddington limit, although alternative (e.g. multi-zone) models can not be ruled out. A one-zone leptonic model is, in principle, also able to fit the flare-state SED, however, requiring implausible parameter choices, in particular, extreme Doppler and bulk Lorentz factors of $\gtrsim 80$.

Figures (6)

• Figure 1: November 3, 2021 (MJD 59521.93 - 59521.99) observation night analysis result for $\theta^2$ and significance plots with the 4 selected runs. Top and bottom left are results from CT1-4 data set while top and bottom right are results from CT5 data set. The inset shows the Point spread function (PSF) which is derived by fitting the $\theta^2$ distribution to the KING's function, described in 2013ApJ...765...54A
• Figure 2: Observed spectra and flux points together with intrinsic flux points for CT1-4 (Figure a) and CT5 (Figure b). The observed spectrum and flux points were de-absorbed with finke2010modeling EBL model. The orange bands are the statistical errors only while the purple bands show the systematic together with statistical errors. The error bars of the flux points are statistical only.
• Figure 3: Multi-wavelength light curves from MJD 59515.99 - 59526.10. Panels from top to bottom: H.E.S.S flux for CT1-4 and CT5 in $10$$^{-11}$ cm$^{-2}$ s$^{-1}$, Fermi-LAT flux for both 12-h and 24-h binning in $10^{-6}$ cm$^{-2}$ s$^{-1}$, Fermi-LAT spectral index for 24-h binning, Swift-XRT flux in $10^{-12}$ erg cm$^{-2}$ s$^{-1}$, Swift-UVOT flux in $10^{-12}$ erg cm$^{-2}$ s$^{-1}$ and ATOM flux for V, B and R filters in $10^{-12}$ erg cm$^{-2}$ s$^{-1}$. The double vertical red dotted lines denote the H.E.S.S. detection period, MJD 59521.93 - 59521.99. Only statistical errors are shown.
• Figure 4: SED of PKS 0346-27 during the H.E.S.S. detection night of 3 November 2021, fit with a hadronic single-zone model. The individual radiation components are shown without EBL absorption, adding up to the total, intrinsic model SED shown by the solid blue curve. The black solid line shows the total model curve accounting for EBL absorption following the finke2010modeling model. The Fermi-LAT emission is explained by the proton-synchrotron component while the casacade component explains the H.E.S.S. emission. The archival flux points are from https://tools.ssdc.asi.it/SED/. See Table \ref{['tab:parameters']} for model parameters.
• Figure 5: Representative attempt of single-zone leptonic model fit to the SED of PKS 0346-27 during the H.E.S.S. detection night of 3 November 2021. The individual radiation components are shown without EBL absorption, adding up to the total, intrinsic model SED shown by the solid blue curve. The black solid line shows the total model curve accounting for EBL absorption following the finke2010modeling model. The archival flux points are from https://tools.ssdc.asi.it/SED/. See Table \ref{['tab:leptonic_parameters']} for model parameters.