Evolution of X-ray Gas in SN 1987A from 2007 to 2021: Ring Fading and Ejecta Brightening Unveiled through Differential Emission Measure Analysis

TL;DR

This study uses differential emission measure analysis on X-ray data from SN 1987A (2007-2021) to reveal the fading of the equatorial ring and brightening of the ejecta, aligning with simulation predictions.

Contribution

It introduces a detailed DEM analysis approach to interpret multi-year X-ray observations, linking temperature structures to physical evolution in SN 1987A.

Findings

The major emission measure peak declined after 2014.

The high-temperature tail's emission measure increased, indicating ejecta shock heating.

The Fe K line centroid energy decreased, supporting ejecta shock evidence.

Abstract

As the nearest supernova (SN) observed since Kepler's SN of 1604, SN 1987A provides an unprecedented opportunity to study in detail the early evolution of supernova remnants (SNRs). Despite extensive studies through both observations and simulations, there is still an urgent need for a more effective approach to integrate the results from two sides. In this study, we conducted a detailed differential emission measure (DEM) analysis on the XMM-Newton observations taken in 2007 to 2021 to characterize the continuous temperature structure of SN 1987A, which can be better compared with simulations. The X-ray plasma exhibit a temperature distribution with a major peak at $\sim0.5$-$1$ keV and a high-temperature tail extending to $\gtrsim5$ keV. The emission measure (EM) of the major peak started to decline around 2014, while the EM of the tail continued increasing and appears to have formed a secondary peak at $\sim3$-$5$ keV in recent years. Our DEM results consistent well with simulations, which help to further identify the major peak as originating from the equatorial ring and the secondary peak as arising from the newly shocked ejecta. Together with the simulations, our DEM analysis reveals recent fading of the ring and brightening of the ejecta in X-rays from SN 1987A. Additionally, we observed a recent decrease in the centroid energy of Fe K line, providing further evidence of newly shocked ejecta.