Dynamics of X-Ray-emitting Ejecta in the Oxygen-rich Supernova Remnant Puppis A Revealed by the XMM-Newton Reflection Grating Spectrometer

Using the unprecedented spectral resolution of the reflection grating spectrometer (RGS) on board XMM-Newton, we reveal dynamics of X-ray-emitting ejecta in the oxygen-rich supernova remnant Puppis A. The RGS spectrum shows prominent K-shell lines, including O VII Heα forbidden and resonance, O VIII Lyα, O VIII Lyβ, and Ne IX Heα resonance, from an ejecta knot positionally coincident with an optical oxygen-rich filament (the so-called Ω filament) in the northeast of the remnant. We find that the line centroids are blueshifted by 1480 ± 140 ± 60 km s^-1 (the first and second term errors are measurement and calibration uncertainties, respectively), which is fully consistent with that of the optical Ω filament. Line broadening at 654 eV (corresponding to O VIII Lyα) is obtained to be σ <~ 0.9 eV, indicating an oxygen temperature of <~ 30 keV. Analysis of XMM-Newton MOS spectra shows an electron temperature of ~0.8 keV and an ionization timescale of ~2 × 10¹⁰ cm^-3 s. We show that the oxygen and electron temperatures as well as the ionization timescale can be reconciled if the ejecta knot was heated by a collisionless shock whose velocity is ~600-1200 km s^-1 and was subsequently equilibrated due to Coulomb interactions. The RGS spectrum also shows relatively weak K-shell lines of another ejecta feature located near the northeastern edge of the remnant, from which we measure redward Doppler velocities of 650 ± 70 ± 60 km s^-1.