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

Abstract

Context. The nearby middle-aged gamma-ray pulsar J1741-2054 and its pulsar wind nebula (PWN) have been studied in X-rays, and its bow-shock nebula (BSN) has been investigated in the Balmer lines, but they have never been observed in far ultraviolet (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's ACS/SBC detector. We also re-analyzed 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 nonthermal components. A joint fit of the FUV-optical and X-ray spectra with combinations of a nonthermal and thermal components showed a hard optical nonthermal spectrum with a photon index $\Gamma_{opt} \approx 1.0-1.2$ and a softer X-ray component, $\Gamma_X \approx 2.6-2.7$. The thermal emission is dominated by the cold component with the temperature $kT_{cold}\approx 40-50$ eV and emitting sphere radius $R_{cold}\approx 8-15$ km, at $d=270$ pc. An additional hot thermal component, with $kT_{hot}\sim 80$ eV and $R_{hot}\sim 1$ 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$\alpha$ BSN morphology, while its FUV flux, $\sim10^{-13}$ erg cm$^{-2}$ s$^{-1}$, is a factor of $\sim 4$ higher than the H$\alpha$ flux. This FUV BSN has a higher surface brightness than the two previously known ones.