Irradiated but not eclipsed, the case of PSR J0610-2100

© E. van der Wateren, C. G. Bassa, C. J. Clark, R. P. Breton, I. Cognard, L. Guillemot, G. H. Janssen, A. G. Lyne, B. W. Stappers, G. Theureau

Black widow pulsars, a class of pulsars with very low-mass companions in a tight orbit, typically display certain observational phenomena not seen in most other binary millisecond pulsars. They often display radio eclipses, where the pulsed radio emission disappears over a fraction of the orbit. The companions commonly show sinusoidal variations in their brightness and colour. In many cases, the systems exhibit variability in orbital parameters. These phenomena are the result of the energetic wind from the radio pulsar and the compactness of the binary orbit, resulting in the companion being irradiated, which drives an evaporative wind from the companion.

PSR J0610−2100 is the third black widow system discovered. While the companion in this system displays optical variations, no evidence of eclipses or orbital variations has been shown to date.

In a new paper, we analysed timing observations of PSR J0610−2100 from Parkes, Lovell, Nançay and Westerbork, extending the timing baseline to 16 years. We find no significant orbital period variations (top panel) and the timing data show no evidence of radio eclipses at observing frequencies ranging from 310 to 2700MHz. The lower right panel displays the optical VRI light curves over the orbital period, showing maximum brightness at superior conjunction of the companion, which confirms irradiation of the companion by the pulsar. We modelled the light curve of the companion and found an unusually low temperature and irradiation of the irradiated hemisphere. Additionally, the companion is likely not filling its Roche lobe, which means there is no mass loss, while having a moderate inclination. We investigated the absence of eclipses in PSR J0610-2100 and other black widow systems in relation to their companion properties (lower left panel) and found that eclipsing systems tend to have higher Roche-lobe filling factors and irradiation luminosities compared to non-eclipsing systems.