Multiwavelength constraints on the unidentified Galactic TeV sources HESS J1427$-$608, HESS J1458$-$608, and new VHE $\gamma$-ray source candidates

A&A 647, A68 (2021) The H.E.S.S. Galactic Plane Survey (HGPS) revealed 78 TeV sources among which 47 are not clearly associated with a known object. We present a multiwavelength approach to constrain the origin of the emission from unidentified HGPS sources. We present a generic pipeline that explores a large database of multiwavelength archival data toward any region in the Galactic plane. Along with a visual inspection of the retrieved multiwavelength observations to search for faint and uncataloged counterparts, we derive a radio spectral index that helps disentangle thermal from nonthermal emission and a mean magnetic field through X-ray and TeV data in case of a leptonic scenario. We also search for a spectral connection between the GeV and the TeV regimes with the Fermi-LAT cataloged sources that may be associated with the unidentified HGPS source. We complete the association procedure with catalogs of known objects and with the source catalogs from instruments whose data are retrieved. The method is applied on two unidentified sources, namely HESS J1427$-$608 and HESS J1458$-$608, for which the multiwavelength constraints favor the pulsar wind nebula (PWN) scenario. We model their broadband nonthermal spectra in a leptonic scenario with a magnetic field $B \lesssim 10$ $\mu$G, which is consistent with that obtained from ancient PWNe. We place both sources within the context of the TeV PWN population to estimate the spin-down power and the characteristic age of the putative pulsar. We also shed light on two possibly significant $\gamma$-ray excesses in the HGPS: the first is located in the south of the unidentified source HESS J1632$-$478 and the second is spatially coincident with the synchrotron-emitting supernova remnant G28.6$-$0.1. The multiwavelength counterparts found toward both $\gamma$-ray excesses make these promising candidates for being new very-high energy $\gamma$-ray sources.