An X-ray Search for Compact Central Sources in Supernova Remnants

Most astronomers now accept that stars more massive than about 9M⊙ explode as supernovae and leave stellar remnants, either neutron stars or black holes, with neutron stars being more prevalent. Recent modeling of the explosions suggests a significant diversity in the key natal properties — rotation rate, velocity, and magnetic field strength — of the resulting neutron stars that account for the association of active radio pulsars, pulsar wind nebulae, and magnetars with supernova remnants (SNRs). The discovery of a central X-ray source in Cas A, the youngest known Galactic SNR, dramatized the expected diversity. However, less than half of the SNRs within 5 kpc have identified central sources, and only three are identified as the remnants of Type Ia SNe. Here, we report a systematic effort to search for compact central sources in the remaining 23 SNRs of this distance limited sample. Our search was inspired, on empirical considerations, by the enigmatic faint X-ray source in Cas A; motivated, on theoretical grounds, by the expectation that young neutron stars emit cooling X-ray emission; and made possible by the superb angular resolution offered by the Chandra X-ray mission and the sensitivity of the XMM-Newton mission. In this first paper we report Chandra observations of four SNRs (G093.3+6.9, G315.4−2.3, G084.2+0.8, and G127.1+0.5). We have undertaken a systematic optical/IR identification program of the X-ray sources detected in the field of each SNR. Foreground (flare stars, active stars) and background (active galactic nuclei) sources have identifiable IR/optical counterparts. In contrast, the counterparts of neutron stars (or black holes) are expected to be very faint. We are able to account for all the well-detected X-ray sources and thus able to state with some confidence that there are no associated central sources down to a level of one tenth of that of the Cas A central source, LX . 10 31 ergs s. We compare our limits with cooling curves for neutron stars and find that any putative neutron stars in these SNRs must be cooling faster than expected for traditional 1.35M⊙ neutron stars and that any putative pulsar must have low spin-down luminosities Ė . 10 ergs s. However, our limits are unable to constrain the presence or absence of more unusual options, such as relatively more massive neutron stars with M & 1.45M⊙, neutron stars with exotic interiors, or quiescent black holes. In subsequent papers, we will report on the X-ray and optical/IR observations of the remaining members of the 5-kpc sample. Subject headings: pulsars: general — stars: neutron — supernova remnants — X-rays: stars