Brightening and Fading in the Youngest Galactic Supernova Remnant G1.9+0.3: 13 years of monitoring with the Chandra X-ray Observatory

TL;DR

This 13-year Chandra study of the youngest Galactic supernova remnant G1.9+0.3 reveals spatially varied X-ray brightness changes, spectral stability, and insights into particle acceleration and magnetic fields, highlighting complex local dynamics.

Contribution

First long-term X-ray monitoring revealing spatially varied brightness evolution and spectral changes in G1.9+0.3, informing models of supernova remnant evolution and particle acceleration.

Findings

Global flux increases by 1.2% per year

Spatial variations include decreases up to -3% and increases up to 7% per year

Remnant age estimated around 1900 CE with shock parameters consistent with observations

Abstract

We report results from 13 years of Chandra monitoring of nonthermal X-ray emission from the youngest Galactic supernova remnant G1.9+0.3, the only remnant known to be increasing in brightness. We confirm the spatially-integrated flux increase rate of $(1.2 \pm 0.2)$% yr$^{-1}$ between 1 and 7 keV, but find large spatial variations, from decreases of $-3$% yr$^{-1}$ to increases of 7% yr$^{-1}$, over length scales as small as $10''$ or smaller. We observe relatively little change in spectral slope, though one region shows significant hardening (photon index $\Delta \Gamma \sim 0.4$) as it brightens by 1% yr$^{-1}$. Such rates of change can be accommodated by any of several explanations, including steady evolution of the blast wave, expansion or compression of discrete plasma blobs, strong magnetic turbulence, or variations in magnetic-field aspect angle. Our results do not constrain the mean magnetic-field strength, but a self-consistent picture of the spatially averaged rate of increase can be produced in which the maximum energies of accelerated particles are limited by the remnant age (applying both to electrons and to ions) to about 20 TeV, and the remnant-averaged magnetic field strength is about 30 $\mu$G. The deceleration parameter $m$ (average shock radius varying as $t^m$) is about 0.7, consistent with estimates from overall expansion dynamics, and confirming an explosion date of about 1900 CE. Shock-efficiency factors $\epsilon_e$ and $\epsilon_B$ (fractions of shock energy in relativistic electrons and magnetic field) are 0.003 and 0.0002 in this picture. However, the large range of rates of brightness change indicates that such a global model is oversimplified. Temporal variations of photon index, expected to be small but measurable with longer time baselines, can discriminate among possible models.