The northwestern ejecta knot in SN 1006

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
e-mail: s.broersen@uva.nl
² Dipartimento di Fisica, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ Université de Savoie, 27 rue Marcoz, BP 1107, 73011 Chambery Cedex, France
⁵ Laboratoire AIM, CEA-IRFU/CNRS/ Univ. Paris Diderot, Service d’Astrophysique/IRFU/DSM/CEA Saclay, Gif-sur-Yvette Cedex, France

Received: 9 October 2012
Accepted: 5 December 2012

Abstract

Aims. We want to probe the physics of fast collision-less shocks in supernova remnants. We are interested in the non-equilibration of temperatures and particle acceleration. Specifically, we aim to measure the oxygen temperature with regards to the electron temperature. In addition, we search for synchrotron emission in the northwestern thermal rim.

Methods. This study is part of a dedicated deep observational project of SN 1006 using XMM-Newton, which provides us with the currently best resolution spectra of the bright northwestern oxygen knot. We aim to use the reflection grating spectrometer to measure the thermal broadening of the O vii line triplet by convolving the emission profile of the remnant with the response matrix.

Results. The line broadening was measured as σ_e = 2.4 ± 0.3 eV, corresponding to an oxygen temperature of 275_-63⁺⁷² keV. From the EPIC spectra we obtain an electron temperature of 1.35 ± 0.10 keV. The difference in temperature between the species provides further evidence of non-equilibration of temperatures in a shock. In addition, we find evidence of a bow shock that emits X-ray synchrotron radiation, which is at odds with the general idea that because of the magnetic field orientation only in the NE and SW region, X-ray synchrotron radiation should be emitted. We find an unusual Hα and X-ray synchrotron geometry, in that the Hα emission peaks downstream of the synchrotron emission. This may be an indication of a peculiar Hα shock in which the density is lower and the neutral fraction is higher than in other supernova remnants, resulting in a peak in Hα emission further downstream of the shock.