The acquisition and analysis of optical photometric data on INS has been shown to provide an independent dataset from which constraints to the optimum thermal spectral energy distribution may be applied. We have attempted for the first time to apply the phase-resolved optical flux consistent with that expected specifically from the thermal component to the previous integrated analysis of the pulsars Geminga and PSR B0656+14. Despite being upper limits to the unpulsed optical flux in the B band, we find that in both cases, the resulting blackbody spectral distribution is constrained to the extent that we can set upper limits to the parameter for both neutron stars. For the case of Geminga, with a known parallax derived distance of 160 pc and using the lower limit, we suggest that 9.5 km for a blackbody source, and 10.0 km with the presence of a magnetized H atmosphere. Previous work using these unpulsed upper limits has suggested a 5.0 K for the pulsar B0656+14 (Golden, 1998), and under the assumption of 13km, places the pulsar at 210 pc - in contrast to the DM derived distance of 760 pc. Assuming the neutron star has 9.5 km, then this limits the distance to 160 pc. This suggests the possibility that the pulsar may be a viable candidate for a parallax measurement attempt using the HST.
Despite using upper limits to the unpulsed fluxes of these two INS, we have been successful in indicating how important such definitive measurements are in the rigorous derivation of a given neutron star's thermal parameters. In this way, we have independently provided limits to the radius of one, Geminga, and the distance of another, PSR B0656+14 and shown the promise to future studies this observational technique offers. This short report documenting these results is most timely, following on from a recent forum on the study of INS in this energy regime, at which the future importance of such high speed photometric studies was stressed (Romani 1998), and where a recent theoretical analysis based upon the discovery of RXJ185635-3754 has shown that neutron star radii should be ideally 10 km if one can hope to effectively constrain the EOS (An et al. 1998). That our results point to such a radius limit only emphasises the need for further high speed photometry of both pulsars, particularly Geminga, so as to provide definitive, rather than upper limit, fluxes of their unpulsed emission.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998