HESS J1427-608: an unusual hard unbroken $\gamma-$ray spectrum in a very wide energy range

TL;DR

This paper reports the detection of a gamma-ray source associated with HESS J1427-608, exhibiting an unbroken spectrum over a wide energy range, suggesting it may be a PeV particle accelerator with possible leptonic or hadronic origins.

Contribution

First detailed GeV-TeV broadband spectral analysis of HESS J1427-608, revealing an unbroken power-law spectrum and discussing its potential nature as a PeV accelerator.

Findings

Spectrum best described by a power-law with index 1.85 in 3-500 GeV

Broadband emission fitted by a single power-law with index 2.0

Possible leptonic or hadronic origin, with magnetic field of a few microGauss

Abstract

We report the detection of a GeV $\gamma$-ray source which is spatially overlapping and thus very likely associated with the unidentified very-high-energy (VHE) $\gamma$-ray source HESS J1427-608 with the Pass 8 data recorded by the Fermi Large Area Telescope. The photon spectrum of this source is best described by a power-law with an index of $1.85\pm0.17$ in the energy range of $3-500$ GeV, and the measured flux connects smoothly with that of HESS J1427-608 at a few hundred GeV. This source shows no significant extension and time variation. The broadband GeV-TeV emission over four decades of energies can be well fitted by a single power-law function with an index of 2.0, without obvious indication of spectral cutoff toward high energies. Such a result implies that HESS J1427-608 may be a PeV particle accelerator. We discuss possible nature of HESS J1427-608 according to the multi-wavelength spectral fittings. Given the relatively large errors, either a leptonic or a hadronic model can explain the multi-wavelength data from radio to VHE $\gamma$-rays. The inferred magnetic field strength is a few $\mu$G, which is smaller than typical values of supernova remnants (SNRs), and is consistent with some pulsar wind nebulae (PWNe). On the other hand, the flat $\gamma$-ray spectrum is slightly different from typical PWNe but similar to that of some known SNRs.