Hubble Space Telescope observations of the old pulsar PSR J0108-1431

Abstract

We present results of optical-UV observations of the 200 Myr old rotation-powered radio pulsar J0108−1431 with the Hubble Space Telescope. We found a putative candidate for the far-UV (FUV) pulsar counterpart, with the flux density f ν = 9.0±3.2 nJy at λ = 1528 Å. The pulsar was not detected, however, at longer wavelengths, with 3σ upper limits of 52, 37, and 87 nJy at λ = 4326, 3355, and 2366 Å, respectively. Assuming that the pulsar counterpart was indeed detected in FUV, and the previously reported marginal U and B detections with the Very Large Telescope were real, the optical-UV spectrum of the pulsar can be described by a power-law model with a nearly flat f ν spectrum. Similar to younger pulsars detected in the optical, the slope of the nonthermal spectrum steepens in the X-ray range. The pulsar's luminosity in the 1500-6000 Å wavelength range, L ∼ 1.2 × 10 27 (d/210 pc) 2 erg s −1 , corresponds to a high efficiency of conversion of pulsar rotation energy loss rate Ė to the optical-UV radiation, η = L/ Ė ∼ (1-6) × 10 −4 , depending on somewhat uncertain values of distance and spectral slope. The brightness temperature of the bulk neutron star surface does not exceed 59,000 K (3σ upper bound), as seen by a distant observer. If we assume that the FUV flux is dominated by a thermal component, then the surface temperature can be in the range of 27,000-55,000 K, requiring a heating mechanism to operate in old neutron stars.