X‐Ray, Radio, and Optical Observations of the Putative Pulsar in the Supernova Remnant CTA 1

Abstract

A Chandra image of the central X-ray source RX J0007.0+7303 in the supernova remnant CTA 1 reveals a point source, a compact nebula, and a bent jet, all of which are characteristic of energetic, rotation-powered pulsars. Using the MDM 2.4 m telescope we obtain upper limits in the optical at the position of the point source, (J2000.0) 00h07m0156, +73°03'081, determined to an accuracy of 01, of B > 25.4, V > 24.8, R > 25.1, and I > 23.7; these correspond to an X-ray-to-optical flux ratio ≳100. Neither a Very Large Array image at 1425 MHz nor a deep pulsar search at 820 MHz using the NRAO Green Bank Telescope reveal a radio pulsar counterpart to an equivalent luminosity limit at 1400 MHz of 0.02 mJy kpc2, which is equal to the lowest luminosity known for any radio pulsar. The Chandra point source accounts for ≈30% of the flux of RX J0007.0+7303, while its compact nebula plus jet comprise ≈70%. The X-ray spectrum of the point source is fitted with a power law-plus-blackbody model with Γ = 1.6 ± 0.6, kT∞ = 0.13 ± 0.05 keV, and R∞ = 0.37 km, values typical of a young pulsar. An upper limit of T < 6.6 × 105 K to the effective temperature of the entire neutron star surface is derived, which is one of the most constraining data points on cooling models. The 0.5-10 keV luminosity of RX J0007.0+7303 is ≈4 × 1031 (d/1.4 kpc)2 ergs s-1, but the larger (~18' diameter) synchrotron nebula in which it is embedded is 2 orders of magnitude more luminous. These properties allow us to estimate, albeit crudely, that the spin-down luminosity of the underlying pulsar is in the range 1036-1037 ergs s-1, and support the identification of the high-energy γ-ray source 3EG J0010+7309 as a pulsar even though its spin parameters have not yet been determined.