2021A&A...651A..40M

Context. Central compact objects (CCOs) are a peculiar class of neutron stars, primarily encountered close to the center of young supernova remnants (SNRs) and characterized by thermal X-ray emission. Measurements of their proper motion and the expansion of the parent SNR are powerful tools for constraining explosion kinematics and the age of the system.
Aims. Our goal is to perform a systematic study of the proper motion of all known CCOs with appropriate data available. From this, we hope to obtain constraints on the violent kick acting on the neutron star during the supernova explosion and infer the exact site of the explosion within the SNR. In addition, we aim to measure the expansion of three SNRs within our sample to obtain a direct handle on their kinematics and age.
Methods. We analyze multiple archival Chandra data sets that consist of HRC and ACIS observations separated by temporal baselines of between 8 and 15 years. We achieve accurate source positions by fitting the imaging data with ray-tracing models of the Chandra point spread function. In order to correct for Chandra's systematic astrometric uncertainties, we establish a reference frame using X-ray-detected sources in Gaia DR2 to provide accurate proper motion estimates for our target CCOs. Complementarily, we use our coaligned data sets to trace the expansion of three SNRs by directly measuring the spatial offset of various filaments and ejecta clumps between different epochs.
Results. In total, we present new proper motion measurements for six CCOs. Within our sample, we do not find any indication of a hypervelocity object, and we determine comparatively tight upper limits (<230km/s) on the transverse velocities of the CCOs in G330.2+1.0 and RX J1713.7-3946. We tentatively identify direct signatures of expansion for the SNRs G15.9+0.2 and Kes 79, at an estimated significance of 2.5σ and 2σ, respectively. Moreover, we confirm recently reported results, measuring the rapid expansion of G350.1-0.3 at almost 6000km/s, which places its maximal age at 600-700 years, making this object one of the youngest Galactic core-collapse SNRs. The observed expansion, combined with the proper motion of its CCO, which is much slower than previously predicted, implies the need for a very inhomogeneous circumstellar medium to explain the highly asymmetric appearance of the SNR. Finally, for the SNR RX J1713.7-3946, we combine previously published expansion measurements with our measurement of the CCO's proper motion to obtain a constraining upper limit of 1700 years on the system's age.