The middle-aged pulsar PSR J1741–2054 and its bow-shock nebula in the far-ultraviolet

Context. Far-ultraviolet (FUV) observations of pulsars allow us to measure surface temperatures of neutron stars and study their thermal evolution. Some pulsars can exhibit FUV bow-shock nebulae (BSNe), providing an additional tool for probing the interstellar medium and studying the pulsar’s properties. The nearby middle-aged gamma-ray pulsar J1741–2054 and its pulsar wind nebula (PWN) have been studied in X-rays, and its BSN has been investigated in the Balmer lines, but they have never been observed in the FUV. Aims. To further study the thermal and magnetospheric emission from PSR J1741–2054 and the BSN properties, we observed them in the FUV range with the Hubble Space Telescope (HST). Methods. We imaged the target in two FUV filters of the HST ACS/SBC detector. We also reanalyzed previous optical observations of the pulsar and its BSN. We fit the pulsar’s FUV-optical spectrum separately and together with its X-ray spectrum. Results. We found that the pulsar’s FUV-optical spectrum consists of a thermal and a nonthermal component. A joint fit of the FUV-optical and X-ray spectra with combinations of the nonthermal and thermal components showed a hard optical nonthermal spectrum with a photon index Γ opt ≈ 1.0–1.2 and a softer X-ray component, Γ X ≈ 2.6–2.7. The thermal emission is dominated by the cold component with the temperature kT cold ≈ 40–50 eV and emitting sphere radius R cold ≈ 8–15 km, at d = 270 pc. An additional hot thermal component, with kT hot ∼ 80 eV and R hot ∼ 1 km, is also possible. Such a spectrum resembles the spectra of other middle-aged pulsars, but it shows a harder (softer) optical (X-ray) nonthermal spectrum. We detected the FUV BSN, the first one associated with a middle-aged pulsar. Its closed-shell morphology is similar to the H α BSN morphology, while its FUV flux, ∼10 −13 erg cm −2 s −1 , is a factor of ∼4 higher than the H α flux. This FUV BSN has a higher surface brightness than the two previously known BSNe.