X‐Ray Imaging and Spectroscopy of the Supernova Remnant CTB 109 and Its Associated Pulsar 1E 2259+586

Abstract

We present the results of our analysis of the X-ray spectral structure of the evolved supernova remnant CTB 109 (G109.1-1.0), using data from the ROSAT Position Sensitive Proportional Counter (PSPC) and Broad Band X-Ray Telescope (BBXRT). The deep broadband PSPC image shows the same overall remnant morphology as previous X-ray images but reveals many new details. The remnant appears as a hemispherical shell centered on the X-ray-bright central pulsar 1E 2259+586. Despite the substantial improvement in sensitivity of the PSPC image over previous images, no X-ray emission is detected from the western half of the remnant, consistent with the interpretation that the western shock front has been significantly decelerated by a dense molecular cloud. Enhanced emission along the cloud remnant boundary supports this interpretation. Among important new small-scale structures revealed are clumpy substructure within the jetlike lobe running northeastward from the pulsar to the shell, and extended emission, with 2-3 arc minute radius, around 1E 2259+586. Spatially resolved spectroscopy using the PSPC reveals an overall column density variation across the remnant as well as intrinsic spectral variations. In particular, the spectrum over most of the shell is well fitted by a single-component thermal model, while for the lobe, the northern and southern shell, two thermal components are required, with one having parameter values similar to that found in those regions fitted by a single-component model. We conclude that either the thermal conditions vary within the remnant or there exists a second, distinct gas component in some parts of the remnant. A simultaneous fit to BBXRT and PSPC spectra for part of the interior and shell to the south of the pulsar shows that the plasma there is not in ionization equilibrium. The results of fitting these spectra using nonequilibrium ionization models are ambiguous, however: equally acceptable fits were obtained using models with and without electron-ion equipartition, but with very different parameter values. The best nonequipartition model yields shock temperature T_s = 2 × 10⁷ K and ionization parameter n₀t = 430 cm^-3 yr, suggesting extreme departure from ionization equilibrium, while the best-fit equipartition model indicates conditions near ionization equilibrium, with T_s = 1.8 × 10⁶ K and n₀t = 17,000 cm^-3 yr. The thermal nature of the emission from the lobe is counter to the prediction of the hypothesis that it arises from a precessing jet and contrasts with the nonthermal lobes in SS433/W50. Thus, the lobe in CTB 109 is probably not related to or powered by the central pulsar. The similarity of the lobe spectrum to those of the northern and southern shell segments suggests they are physically related. Together they form a plumelike structure whose morphology and location relative to the molecular cloud suggest that they represent gas from the remnant interior, swept up and reheated by a shock reflected off the molecular cloud, as reproduced in hydrodynamical simulations (see Tenorio-Tagle et al.). A timing analysis of the PSPC data for 1E 2259+586 yields a period of 6.978814 ± 9.4 × 10^-6 s, consistent with a constant . Simultaneous fitting of the pulsar spectra from the PSPC, BBXRT, and the ASCA are best modeled by a blackbody (kT = 0.43 keV). The extended emission around 1E 2259+586 has an extent of 2'-3' in both the PSPC and ROSAT HRI images. Its nonthermal spectrum suggests that it arises from a synchrotron nebula, although we cannot rule out the possibility of a dust-scattering halo.