2010ApJ...720.1635M

PSR B1055-52 is a middle-aged (τ = 535 kyr) radio, X-ray, and γ-ray pulsar showing X-ray thermal emission from the neutron star (NS) surface. A candidate optical counterpart to PSR B1055-52 was proposed by Mignani and coworkers based on Hubble Space Telescope (HST) observations performed in 1996, in one spectral band only. We report on HST observations of this field carried out in 2008, in four spectral bands. The astrometric and photometric analyses of these data confirm the identification of the proposed candidate as the pulsar's optical counterpart. Similar to other middle-aged pulsars, its optical-UV spectrum can be described by the sum of a power-law (PL_O) component ({F_ν ∝ν^{-α_O}}), presumably emitted from the pulsar magnetosphere, and a Rayleigh-Jeans (RJ) component emitted from the NS surface. The spectral index of the PL_Ocomponent, α_O = 1.05±0.34, is larger than for other pulsars with optical counterparts. The RJ component, with a brightness temperature T_O= (0.66±0.10) d ² ₃₅₀ R ^–2_O,13MK (where d₃₅₀and R_O,13are the distance to the pulsar in units of 350 pc and the radius of the emitting area in units of 13 km, respectively), shows a factor of 4 excess with respect to the extrapolation of the X-ray thermal component into the UV-optical. This hints that the RJ component is emitted from a larger, colder area, and suggests that the distance to the pulsar is smaller than previously thought. From absolute astrometry of the HST images, we measured the pulsar coordinates with a position accuracy of 0{farcs}15. From comparison with previous observations, we measured the pulsar proper motion, µ = 42±5 mas/yr, which corresponds to a transverse velocity V_t= (70±8) d₃₅₀ km/s.