Modeling SN 1996cr's X-ray lines at high-resolution: Sleuthing the ejecta/CSM geometry

TL;DR

This paper models the high-resolution X-ray emission lines of SN 1996cr to understand its ejecta and circumstellar medium geometry, revealing an axisymmetric structure with polar-enhanced CSM.

Contribution

It introduces a detailed line-shape fitting approach using axisymmetric emission models to infer the geometry of SN 1996cr's ejecta and CSM from high-resolution X-ray spectra.

Findings

Fe XXVI emission originates from high latitudes, indicating polar-enhanced CSM.

Line shapes are well fit by axisymmetric models with a ~55-degree orientation.

Progenitor likely a massive star with a complex wind history.

Abstract

SN 1996cr, located in the Circinus Galaxy (3.7 Mpc, z ~ 0.001) was non-detected in X-rays at ~ 1000 days yet brightened to ~ 4 x 10^{39} erg/s (0.5-8 keV) after 10 years (Bauer et al. 2008). A 1-D hydrodynamic model of the ejecta-CSM interaction produces good agreement with the measured X-ray light curves and spectra at multiple epochs. We conclude that the progenitor of SN 1996cr could have been a massive star, M > 30 M_solar, which went from an RSG to a brief W-R phase before exploding within its ~ 0.04 pc wind-blown shell (Dwarkadas et al. 2010). Further analysis of the deep Chandra HETG observations allows line-shape fitting of a handful of bright Si and Fe lines in the spectrum. The line shapes are well fit by axisymmetric emission models with an axis orientation ~ 55 degrees to our line-of-sight. In the deep 2009 epoch the higher ionization Fe XXVI emission is constrained to high lattitudes: the Occam-est way to get the Fe H-like emission coming from high latitude/polar regions is to have more CSM at/around the poles than at mid and lower lattitudes, along with a symmetric ejecta explosion/distribution. Similar CSM/ejecta characterization may be possible for other SNe and, with higher-throughput X-ray observations, for gamma-ray burst remnants as well.