On the nature of the intermittent pulsar PSR B1931+24

PSR B1931+24 is the first intermittent radio pulsar discovered to date, characterized by a 0.8-s pulsation, which turns on and off quasi-periodically every similar to 35 d, with a duty cycle of similar to 10 per cent. Here, we present X-ray and optical observations of PSR B1931+24 performed with the Chandra X-ray Observatory and the Isaac Newton Telescope, respectively. Simultaneous monitoring from the Jodrell Bank Observatory showed that this intermittent pulsar was in the radio-on phase during our observations. We do not find any X-ray or optical counterpart to PSR B1931+24, translating into an upper limit of similar to 2 x 10(31) erg s(-1) on the X-ray luminosity, and of g' > 22.6 on the optical magnitude. If the pulsar is isolated, these limits cannot constrain the dim X-ray and optical emission expected for a pulsar of this age (similar to 1.6 Myr). We discuss the possibility that the quasi-periodic intermittent behaviour of PSR B1931+24 is a result of the presence of a low-mass companion star or gaseous planet, tight with the pulsar in an eccentric orbit. In order to constrain the parameters of this putative binary system, we re-analysed the pulsar radio timing residuals. We found that (if indeed hosted in a binary system) PSR B1931+24 should have a very low-mass companion and an orbit of low inclination.