X-ray Spectroscopy of the Mixed Morphology Supernova Remnant W28 with XMM-Newton

Abstract

We report on spatially resolved X-ray spectroscopy of the north-eastern part of the mixed morphology supernova remnant (SNR) W28 with {\it XMM-Newton}. The observed field of view includes a prominent and twisted shell emission forming the edge of this SNR as well as part of the center-filled X-ray emission brightening toward the south-west edge of the field of view. The shell region spectra are in general represented by an optically thin thermal plasma emission in collisional ionization equilibrium with a temperature of $\sim$0.3~keV and a density of $\sim$10~cm${}^{-3}$, which is much higher than the density obtained for inner parts. In contrast, we detected no significant X-ray flux from one of the TeV ${\hat{I}}^3$-ray peaks with an upper-limit flux of 2.1$\times$10${}^{-14}$ erg cm${}^{-2}$ s${}^{-1}$ in the 2--10~keV band. The large flux ratio of TeV to X-ray, larger than 16, and the spatial coincidence of the molecular cloud and the TeV ${\hat{I}}^3$-ray emission site indicate that the TeV ${\hat{I}}^3$-ray of W28 is $\ddot{I}{€}^0$-decay emission originating from collisions between accelerated protons and molecular cloud protons. Comparing the spectrum in the TeV band and the X-ray upper limit, we obtained a weak upper limit on the magnetic field strength $B\lesssim$ 1500~$\hat{I}¼$G.

16 pages, 12 figures, Publications of the Astronomical Society of Japan, in press