Deep XMM-Newton Observations Reveal the Origin of Recombining Plasma in the Supernova Remnant W44

Abstract

Recent X-ray studies revealed over-ionized recombining plasmas (RPs) in a dozen mixed-morphology (MM) supernova remnants (SNRs). However, the physical process of the over-ionization has not been fully understood yet. Here we report on spatially resolved spectroscopy of X-ray emission from W44, one of the over-ionized MM-SNRs, using XMM-Newton data from deep observations, aiming to clarify the physical origin of the over-ionization. We find that combination of low electron temperature and low recombination timescale is achieved in the region interacting with dense molecular clouds. Moreover, a clear anti-correlation between the electron temperature and the recombining timescale is obtained from each of the regions with and without the molecular clouds. The results are well explained if the plasma was over-ionized by rapid cooling through thermal conduction with the dense clouds hit by the blast wave of W44. Given that a few other over-ionized SNRs show evidence for adiabatic expansion as the major driver of the rapid cooling, our new result indicates that both processes can contribute to over-ionization in SNRs, with the dominant channel depending on the evolutionary stage.

arXiv comment10 pages, 7 figures Accepted for publication in ApJ

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• ALMA CO Observations of the Mixed-Morphology Supernova Remnant W49B: Efficient Production of Recombining Plasma and Hadronic Gamma-rays via Shock-Cloud Interactions
• Radiative Supernova Remnants and Supernova Feedback
• Modeling the mixed-morphology supernova remnant IC 443. Origin of its complex morphology and X-ray emission
• Unveiling pure-metal ejecta X-ray emission in supernova remnants through their radiative recombination continuum
• Spatially Resolved Chandra Spectroscopy of the Large Magellanic Cloud Supernova Remnant N132D
• Non-equilibrium ionisation and radiative transport in an evolving supernova remnant
• Discovery of a Wind-Blown Bubble Associated with the Supernova Remnant G346.6-0.2: A Hint for the Origin of Recombining Plasma
• Plasma heating and particle acceleration in collisionless shocks through astrophysical observations
• Progenitor constraint with circumstellar material for the magnetar-hosting supernova remnant RCW 103
• Estimating Global Ejecta Mass Ratios in Tycho's SNR
• On the formation of over-ionized plasma in evolved supernova remnants
• Investigation of the Physical Origin of Overionized Recombining Plasma in the Supernova Remnant IC 443 with XMM-Newton
• The pulsar wind nebula around B1853+01 in X-rays