The Nonstandard Properties of a "Standard" PWN: Unveiling the Mysteries of PWN G21.5-0.9 Using its IR and X-ray emission

TL;DR

This study reanalyzes IR and X-ray observations of PWN G21.5-0.9, revealing dust/gas contributions, complex X-ray spectra, and modeling that suggests low supernova energy and environmental density variations influence its properties.

Contribution

It provides a new comprehensive analysis combining IR and X-ray data with hydrodynamical modeling to better understand PWN G21.5-0.9's properties and evolution.

Findings

IR emission mainly from dust and gas, not synchrotron radiation

X-ray spectrum best described by multiple power laws with softening at higher energies

Hydrodynamical model reproduces morphology with low supernova energy and ambient density increase

Abstract

The evolution of a pulsar wind nebula (PWN) depends on properties of the progenitor star, supernova, and surrounding environment. As some of these quantities are difficult to measure, reproducing the observed dynamical properties and spectral energy distribution (SED) with an evolutionary model is often the best approach in estimating their values. G21.5-0.9, powered by the pulsar J1833-1034, is a well observed PWN for which previous modeling efforts have struggled to reproduce the observed SED. In this study, we reanalyze archival infrared (IR; Herschel, Spitzer) and X-ray (Chandra, NuSTAR, Hitomi) observations. The similar morphology observed between IR line and continuum images of this source indicates that a significant portion of this emission is generated by surrounding dust and gas, and not synchrotron radiation from the PWN. Furthermore, we find the broadband X-ray spectrum of this source is best described by a series of power laws fit over distinct energy bands. For all X-ray detectors, we find significant softening and decreasing unabsorbed flux at higher energy bands. Our model for the evolution of a PWN is able to reproduce the properties of this source when the supernova ejecta has a low initial kinetic energy $E_{\mathrm{sn}} \approx 1.2 \times 10^{50}\,\mathrm{ergs}$ and the spectrum of particles injected into the PWN at the termination shock is softer at low energies. Lastly, our hydrodynamical modeling of the SNR can reproduce its morphology if there is a significant density increase of the ambient medium ${\sim} 1.8$ pc north of the explosion center.