Nonthermal emission properties of the northwestern rim of supernova remnant RX J0852-4622

TL;DR

This study analyzes the nonthermal X-ray emission of the northwestern rim of supernova remnant RX J0852-4622, revealing spectral softening due to synchrotron cooling and estimating magnetic field strengths, with implications for gamma-ray emission mechanisms.

Contribution

It provides the first detailed spatial analysis of nonthermal X-ray spectral softening in RX J0852-4622's rim, linking it to synchrotron cooling and magnetic field estimates.

Findings

Spectral softening from photon index 2.56 to 2.96 across the rim.

Gradual decrease of electron cutoff energy due to synchrotron cooling.

Magnetic field strength estimated to be less than several tens of microgauss.

Abstract

The supernova remnant (SNR) RX J0852-4622 (Vela Jr., G266.6-1.2) is one of the most important SNRs for investigating the acceleration of multi-TeV particles and the origin of Galactic cosmic rays because of its strong synchrotron X-ray and TeV gamma-ray emission, which show a shell-like morphology similar to each other. Using the XMM-Newton archival data consisting of multiple pointing observations of the northwestern rim of the remnant, we investigate the spatial properties of the nonthermal X-ray emission as a function of distance from an outer shock wave. All X-ray spectra are well reproduced by an absorbed power-law model above 2 keV. It is found that the spectra show gradual softening from a photon index 2.56 in the rim region to 2.96 in the interior region. We show that this radial profile can be interpreted as a gradual decrease of the cutoff energy of the electron spectrum due to synchrotron cooling. By using a simple spectral evolution model that includes continuous synchrotron losses, the spectral softening can be reproduced with the magnetic field strength in the post-shock flow to less than several tens of uG. If this is a typical magnetic field in the SNR shell, gamma-ray emission would be accounted for by inverse Compton scattering of high-energy electrons that also produce the synchrotron X-ray emission. Future hard X-ray imaging observations with Nustar and ASTRO-H and TeV gamma-ray observations with the Cherenkov Telescope Array (CTA) will allow to us to explore other possible explanations of the systematic softening of the X-ray spectra.