Evolution and Hydrodynamics of the Very-Broad X-ray Line Emission in SN 1987A

TL;DR

This paper analyzes the evolution of very-broad X-ray emission lines in SN 1987A, modeling the interaction of the blast wave with surrounding material to understand the changing spectral features and flux over time.

Contribution

It introduces a 2x1D hydrodynamic model that successfully reproduces observed spectra, radii, and light curves of SN 1987A with minimal free parameters.

Findings

Broad emission lines have a width of ~9300 km/s, contributing ~20% of the flux.

The interaction with the HII region produces the broad lines and most of the 3-10 keV flux.

The model predicts a 17% annual decline in 0.5-2 keV flux after dense ER material is no longer shocked.

Abstract

Observations of SN 1987A by the Chandra High Energy Transmission Grating (HETG) in 1999 and the XMM-Newton Reflection Grating Spectrometer (RGS) in 2003 show very broad (v-b) lines with a full-width at half-maximum (FWHM) of order 10^4 kms; at these times the blast wave was primarily interacting with the HII region around the progenitor. Since then, the X-ray emission has been increasingly dominated by narrower components as the blast wave encounters dense equatorial ring (ER) material. Even so, continuing v-b emission is seen in the grating spectra suggesting that interaction with HII region material is on-going. Based on the deep HETG 2007 and 2011 data sets, and confirmed by RGS and other HETG observations, the v-b component has a width of 9300 +/-2000 kms FWHM and contributes of order 20% of the current 0.5--2 keV flux. Guided by this result, SN 1987A's X-ray spectra are modeled as the weighted sum of the non-equilibrium-ionization (NEI) emission from two simple 1D hydrodynamic simulations, this "2x1D" model reproduces the observed radii, light curves, and spectra with a minimum of free parameters. The interaction with the HII region (rho_init \sim 130 amu/cc, +/- 15 degrees opening angle) produces the very-broad emission lines and most of the 3-10 keV flux. Our ER hydrodynamics, admittedly a crude approximation to the multi-D reality, gives ER densities of order 10^4 amu/cc, requires dense clumps (x5.5 density enhancement in \sim 30% of the volume), and it predicts that the 0.5-2 keV flux will drop at a rate of \sim 17% per year once no new dense ER material is being shocked.