X-ray supernova remnants in the starburst region of M 82

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain

Received: 22 December 2020
Accepted: 18 May 2021

Abstract

We searched for X-ray supernova remnants (SNRs) in the starburst region of M 82, using archival data from the Chandra X-ray Observatory with a total effective exposure time of 620 ks with an X-ray spectroscopic selection. Strong line emission from Fe XXV at 6.7 keV is a characteristic spectral feature of hot, shocked gas of young SNRs and is distinctive among the discrete sources in the region populated by X-ray binaries. We selected candidates using narrow-band imaging aimed at the line excess and identified six (and possibly a seventh) X-ray SNRs. Two previously known examples were recovered by our selection. Five of them have radio counterparts, including the radio supernova SN2008iz, which was discovered as a radio transient in 2008. It shows a hard X-ray spectrum with a blueshifted Fe K feature with v ≈ −2700 km s⁻¹, both of which suggest its youth. The 4–8 keV luminosities of the selected SNRs are in the range of (0.3–2.5)×10³⁸ erg s⁻¹. We made a crude estimate of the supernova rate, assuming that more luminous SNRs are younger, and found ν_SN = 0.06 (0.03–0.13) yr⁻¹, in agreement with the supernova rates estimated by radio observations and the generally believed star formation rate of M 82, although the validity of the assumption is questionable. A sum of the Fe XXV luminosity originating from the selected X-ray SNRs consists of half of the total Fe XXV luminosity observed in the central region of M 82. We briefly discuss its implications for starburst winds and the Fe XXV emission in more luminous starburst galaxies.