Letter to the Editor

Spatial identification of the overionized plasma in W49B

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Sezione di Astronomia, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: miceli@astropa.unipa.it
² INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ DSM/DAPNIA/Service d'Astrophysique, AIM-UMR 7158, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 20 November 2009
Accepted: 9 April 2010

Abstract

Context. Recent Suzaku X-ray observations of the ejecta-dominated supernova remnant W49B have shown that there is a clear indication in the global spectrum for overionized plasma, whose physical origin is still under debate.

Aims. In order to ascertain the physical origin of this rapidly cooling plasma, we focus on its spatial localization within the X-ray emitting ejecta.

Methods. We confirm a saw-edged excess (interpreted as a strong radiative recombination continuum) in the global spectrum above 8 keV, which emerges above the ionization-equilibrium model. We produce a hardness-ratio map to determine where the plasma is overionized and perform a spectral analysis of the regions with and without strong overionization.

Results. We find that the overionized plasma is localized in the center of the remnant and in its western jet, while it is not detected in the bright eastern jet, where the expansion of the ejecta is hampered by their interaction with a dense interstellar cloud.

Conclusions. The location of overionized plasma suggests that the inner ejecta are rapidly cooling by expansion, unlike the outer ejecta, for which expansion is hampered by interstellar clouds seen in H₂.