Constraining leptonic emission scenarios for the PeVatron candidate HESS J1702−420 with deep XMM-Newton observations^⋆

¹ Astroparticule et Cosmologie, Université de Paris-Cité, CNRS/IN2P3, 75013 Paris, France
e-mail: giunti@apc.in2p3.fr
² Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM de Paris-Saclay, 91191 Gif-sur-Yvette, France

Received: 5 August 2022
Accepted: 15 September 2022

Abstract

Aims. We aim to search for a hidden leptonic accelerator, such as a high-Ė pulsar, associated with the unidentified TeV object HESS J1702−420A.

Methods. We carried out a 72 ks X-ray observation with the XMM-Newton satellite and analyzed the resulting data jointly with the publicly available HESS spectral energy distribution (SED) results to derive constraints on the leptonic contribution to the TeV emission of HESS J1702−420A. A set of scripts dedicated to the multi-wavelength modeling of X-ray and γ-ray data, based on Gammapy, Naima, and Xspec, has been developed in the context of this work and made publicly available along with this paper.

Results. No object clearly associated with HESS J1702−420A was found in the XMM-Newton data. After excluding the unidentified object Suzaku src B as a possible X-ray counterpart and classifying it as a new cataclysmic variable source candidate, we derived strict upper limits on the level of diffuse X-ray emission in the HESS J1702−420A region: F(2 − 10 keV)≲5.4  × 10⁻⁵ keV  cm⁻² s⁻¹ at 2σ (≈95.5%) confidence level. A tight constraint on the magnetic field was derived, under a one-zone leptonic scenario, by jointly fitting the XMM-Newton spectra and the HESS SED: B ≲ 1.45 μG at 2σ level. We additionally report the serendipitous discovery of a new extended X-ray source with a hard spectral index of 1.99 ± 0.45, named XMMU J170147.3−421407 which is likely Galactic. Its classification as a high-speed runaway pulsar wind nebula (PWN), possibly associated with HESS J1702−420A, is not obvious but cannot be ruled out either.

Conclusions. The hard γ-ray object HESS J1702−420A remains unidentified, but the absence of a clear X-ray counterpart strongly challenges simple leptonic scenarios. The only remaining possible leptonic counterpart for HESS J1702−420A appears to be a newly discovered X-ray source with extended morphology and hard spectral index, which may be a PWN powered by a high-speed runaway pulsar.