Freely Expanding Knots of X-Ray-emitting Ejecta in Kepler’s Supernova Remnant

Abstract

We report measurements of proper motion, radial velocity, and elemental composition for 14 compact X-ray bright knots in Kepler's supernova remnant (SNR) using archival Chandra data. The highest speed knots show both large proper motions (µ ∼ 0.11-0.14 yr −1 ) and high radial velocities (v ∼ 8,700-10,020 km s −1 ). For these knots the estimated space velocities (9,100 km s −1 v 3D 10,400 km s −1 ) are similar to the typical Si velocity seen in SN Ia near maximum light. High speed ejecta knots appear only in specific locations and are morphologically and kinematically distinct from the rest of the ejecta. The proper motions of five knots extrapolate back over the age of Kepler's SNR to a consistent central position. This new kinematic center agrees well with previous determinations, but is less subject to systematic errors and denotes a location about which several prominent structures in the remnant display a high degree of symmetry. These five knots are expanding at close to the free expansion rate (expansion indices of 0.75 m 1.0), which we argue indicates either that they were formed in the explosion with a high density contrast (more than 100 times the ambient density) or that they have propagated through relatively low density (n H < 0.1 cm −3 ) regions in the ambient medium. X-ray spectral analysis shows that the undecelerated knots have high Si and S abundances, a lower Fe abundance and very low O abundance, pointing to an origin in the partial Si-burning zone, which occurs in the outer layer of the exploding white dwarf for SN Ia models. Other knots show slower speeds and expansion indices consistent with decelerated ejecta knots or features in the ambient medium overrun by the forward shock. Our new accurate location for the explosion site has well-defined positional uncertainties allowing for a great reduction in the area to be searched for faint surviving donor stars under non-traditional single-degenerate SN Ia scenarios; because of the lack of bright stars in the search area the traditional scenario remains ruled out.

A Nucleosynthetic Origin for the Southwestern Fe-rich Structure in Kepler’s Supernova Remnant

“…A model consisting of a power law + several Gaussian models + background model fits the spectrum well (χ 2 /d.o.f = 42.16/55), and we found the spectrum has strong Kshell emission from the Fe-peak elements (Cr, Mn, Fe, Ni) where the significance for each element is above 2σ ( Table 1). The centroid energies of all the lines are slightly higher (∼30-70 eV) than those in the remnant as a whole (Park et al 2013), consistent with a spectrum blueshifted by several thousand km s −1 as reported in Sato & Hughes (2017b). As we show below, spectral fits using a physical NEI model also require a comparable blueshift for the Fe-rich knot.…”

“…Proper motions at the northern rim of Kepler's SNR show values of ∼0.8-0.11 yr −1 (Katsuda et al 2008;Vink 2008) corresponding to speeds of 2,300-3,100 km s −1 at the distance of 6 kpc (e.g., Millard et al 2019). The 3D velocities of associated knots SW1 and SW2 from Sato & Hughes (2017b) are ∼5,000-8,000 km s −1 , significantly higher than the expansion velocity of the northern rim. We can estimate the 3D position of the Fe-rich structure from the explosion center, R 3D as…”

“…The transverse velocity change with the distance to the remnant of 4-7 kpc. Here we assumed the proper motion of µ = 0.1 yr −1 , the radial velocity of vr = 4,000-8.000 km s −1(Sato & Hughes 2017b) and the projected radius of rpro = 57 . The red line shows the typical SNR radius, R = 1.8 Vancura et al 1995;Sato & Hughes 2017a;Yamaguchi et al 2017…”

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

“…For spectral fitting purposes (Section 3.3), we reprocessed the six ObsIDs from the 2006 archival ACIS-S3 data by following standard data reduction procedures with CIAO versions 4.8 to 4.8.2 and CALDB version 4.7.2, which resulted in a total effective exposure of ∼ 733 ks. To make our proper motion measurements, we used the 2000, 2006, and 2014 archival Chandra ACIS data, as previously processed and prepared in Sato & Hughes (2017b). Chandra HETG 3-color image of Kepler.…”

“…A straightforward way to reveal the 3-D structure of ejecta, and ultimately the explosion nature and dynamical evolution of Kepler, is to measure the Doppler shifts in the emission lines from the X-ray-emitting ejecta knots projected over the face of the SNR, and thus their bulk motion radial velocities (v r ) along the line of sight. Recently, Sato & Hughes (2017b) reported measurements of radial velocity for several compact X-ray-bright knots in Kepler's SNR using archival Chandra ACIS data. They measured high radial velocities of up to ∼ 10 4 km s -1 and nearly free expansion rates for some knots.…”