An Ejecta Kinematics Study of Kepler’s Supernova Remnant with High-Resolution Chandra HETG Spectroscopy

Abstract

We report measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Kepler's Supernova Remnant (SNR). We measure the Doppler shift of the He-like Si Kα line center energy in the spectra of these knots from our Chandra High-Energy Transmission Grating Spectrometer (HETGS) observation to estimate their radial velocities. We estimate high radial velocities of up to ∼ 8,000 km s⁻¹ for some of these ejecta knots. We also measure proper motions for our sample based on the archival Chandra Advanced CCD Imaging Spectrometer (ACIS) data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are nearly freely-expanding after ∼ 400 years since the explosion. The fastest moving knots showed proper motions up to ∼ 0.2 arcseconds per year. Based on our radial velocity measurements, we estimate a distance to Kepler's SNR, d ∼ 4.8 to 8.2 kpc. We find that the ejecta knots in our sample have an average space velocity of ∣Vₛ∣∼ 4,600 km s⁻¹ (at a distance of 6 kpc). We note that 8 out of the 15 ejecta knots from our sample show a statistically significant (at the 90% confidence level) redshifted spectrum, compared to only 2 with a blueshifted spectrum, suggesting an asymmetry in the ejecta distribution in Kepler's SNR along the line of sight.