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

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.

Search for continuous gravitational waves from ten H.E.S.S. sources using a hidden Markov model

“…emission region and has a hard, pulsar-like spectrum which connects smoothly to the TeV γ−ray spectrum measured by H.E.S.S. Although the exact mechanism responsible for the generation of γ−ray emission is not yet clear, Devin et al [28] conclude that the presence of coincident, extended X-ray emission region and the hard spectral shape of the Fermi-LAT source suggests a leptonic scenario. In this scenario, the TeV γrays are generated by inverse Compton scattering of lowenergy photons by relativistic non-thermal electrons or positrons.…”

“…Multiple groups have attempted to estimate the age of HESS J1427-608. Assuming that the TeV source is an evolved PWN, Devin et al [28] estimate a characteristic age between 4.9−13.6 kyr. Fujinaga et al used the correlation between the ratio of γ−ray to X-ray flux (F γ /F X ) and τ c to obtain τ c ∼ 6.4 kyr [84].…”

“…Recent analysis of the X-ray Multi-Mirror Mission (XMM-Newton) source catalogue found a point-like object, 4XMM J142756.7 − 605214, located near the center of the H.E.S.S. emission region [28,29]. An extended non-thermal X-ray emission (Suzaku J1427−6051) is also associated with this TeV source [30].…”

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843–033 Region

“…Similarly, the extension of HESS J1843-033 in the TeV range can be explained by ICS off the CMB photons by ;10 TeV electrons diffusing for ∼8 kyr. Devin et al (2021) found no radio or X-ray emission that indicates the existence of a PWN. Given the characteristic age of PSR J1844-0346, synchrotron emission from the PWN would not be bright enough to be observed owing to the decay of the magnetic field (Tanaka & Takahara 2010).…”

“…This pulsar, also named 4FGL J1844.4-0345 (Abdollahi et al 2020), was discovered by Fermi-LAT (Clerk et al 2017), and its spin-down luminosity, characteristic age, and spin period are 4.2 × 10 36 erg s −1 , 12 kyr, and 113 ms, respectively. Assuming that the distance to the pulsar is 4.3 kpc (Devin et al 2021), the gamma-ray energy flux of 1.1 × 10 −11 erg cm −2 s −1 in 1 TeV < E < 10 TeV (H.E.S.S. Collaboration 2018a) is translated into the luminosity of 2.4 × 10 34 erg s −1 .…”