Deep optical observations of the γ-ray pulsar PSR J0007+7303 in the CTA 1 supernova remnant

The Fermi Large Area Telescope discovered the time signature of a radio-silent pulsar coincident with RX J0007.0+7302, a plerion-like X-ray source at the centre of the CTA 1 supernova remnant. The inferred timing parameters of the γ-ray pulsar PSR J0007+7303 (P = 315.8 ms; dot{P} ∼ 3.6× 10^{-13} s s^-1) point to a Vela-like neutron star, with an age comparable to that of CTA 1. The PSR J0007+7303 low distance (∼1.4 kpc), interstellar absorption (A_V ∼ 1.6), and relatively high energy loss rate (dot{E} ∼ 4.5× 10^{35} erg s^-1), make it a suitable candidate for an optical follow-up. Here, we present deep optical observations of PSR J0007+7303. The pulsar is not detected in the Gran Telescopio Canarias images down to a limit of r^' ∼ 27.6 (3σ), the deepest ever obtained for this pulsar, while William Herschel Telescope images yield a limit of V ∼ 26.9. Our r^'-band limit corresponds to an optical emission efficiency η _opt equiv L_opt/dot{E} ≲ 9.4 × 10^{-8}. This limit is more constraining than those derived for other Vela-like pulsars, but is still above the measured optical efficiency of the Vela pulsar. We compared the optical upper limits with the extrapolation of the XMM-Newton X-ray spectrum and found that the optical emission is compatible with the extrapolation of the X-ray power-law component, at variance with what is observed, e.g. in the Vela pulsar.