Abstract
Aims. Among the γ-ray sources discovered at high and very-high energies, a large fraction still lack a clear identification. In particular, the H.E.S.S. Galactic Plane Survey (HGPS) revealed 78 TeV sources among which 47 are not clearly associated with a known object. Multiwavelength data can help identify the origin of the very-high energy γ-ray emission, although some bright TeV sources have been detected without clear counterparts. We present a multiwavelength approach to constrain the origin of the emission from unidentified HGPS sources. Methods. 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 (supernova remnants, pulsar wind nebulae, H II regions, etc.) and with the source catalogs from instruments whose data are retrieved. Results. 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 ≲ 10 μG, 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 γ-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 γ-ray excesses make these promising candidates for being new very-high energy γ-ray sources.
Highlights
Supernova remnants (SNRs) and pulsar wind nebulae (PWNe) are considered the best candidates to accelerate the bulk of Galactic cosmic rays (CRs) at least up to the knee of the CR spectrum (∼3 × 1015 eV)
With nonthermal radio emission located in the western part of the H.E.S.S. source, which is spatially coincident with a GeV source exhibiting a hard spectrum (4FGL J1456.7−6050c), part of the emission from HESS J1458−608 could originate from a PWN
Given the large number of H.E.S.S. Galactic Plane Survey (HGPS) sources that are not firmly identified, we developed a generic code aiming to constrain the origin of their TeV emission through the exploitation of multiwavelength data
Summary
Supernova remnants (SNRs) and pulsar wind nebulae (PWNe) are considered the best candidates to accelerate the bulk of Galactic cosmic rays (CRs) at least up to the knee of the CR spectrum (∼3 × 1015 eV). While catalogs of known γ-ray emitters in the Galaxy such as SNRs, PWNe, and pulsars can provide hints of the origin of the emission, multiwavelength data exploration is often necessary to pinpoint the nature of the HE/VHE sources (Aliu et al 2014; MAGIC Collaboration 2014, 2020). HESS J1356−645 was classified as an evolved PWN as a result of the exploitation of multiwavelength data that revealed a nonthermal, center-filled radio and X-ray emission surrounding the energetic pulsar PSR J1357−6429 and spatially coincident with the H.E.S.S. source (H.E.S.S. Collaboration 2011). Multiwavelength associations relying on spatial correlations is challenging if the γ-ray emission is produced by an ancient PWN or a molecular cloud (MC) illuminated by CRs that have escaped from a nearby SNR In the former case, the pulsar can be largely offset from the TeV emission produced by old electrons (de Jager et al 2009) and, in the latter case, the MC can be distanced from the SNR from which CRs have escaped (Gabici et al 2009).
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