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

TL;DR

This study conducted deep optical observations of the gamma-ray pulsar PSR J0007+7303 in the CTA 1 supernova remnant, setting new limits on its optical emission and comparing these with X-ray data to understand its emission properties.

Contribution

First deep optical limits for PSR J0007+7303 are established, constraining its optical efficiency and comparing it with X-ray spectra to explore emission mechanisms.

Findings

Optical emission not detected down to r'~27.6 and V~26.9 limits.

Optical efficiency constrained to < 9.4 x 10^{-8}.

Optical spectrum compatible with X-ray power-law extrapolation.

Abstract

The Fermi Large Area Telescope (LAT) 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 gamma-ray pulsar PSR J0007+7303 (P=315.8 ms; dot{P}\sim3.6 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 (\sim 1.4 kpc), interstellar absorption (A_V\sim 1.6), and relatively high energy loss rate (dot{E} \sim4.5 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 (GTC) images down to a limit of r'\sim 27.6 (3 sigma), the deepest ever obtained for this pulsar, while William Herschel Telescope (WHT) images yield a limit of V \sim 26.9. Our r'-band limit corresponds to an optical emission efficiency \eta_{opt}= 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.