The Ultra-luminous M81 X-9 source: 20 years variability and spectral states

Abstract

The source X-9 was discovered with the {\it Einstein Observatory} in the field of M81, and is located in the dwarf galaxy Holmberg IX. X-9 has a 0.2-4.0 keV luminosity of $\sim 5.5\times 10^{39}$ ergs~s$^{-1}$, if it is at the same distance as Holmberg IX (3.4 Mpc). This luminosity is above the Eddington luminosity of a 1~$M_{\odot}$ compact accreting object. Past hypotheses on the nature of this Super-Eddington source included a SNR or supershell, an accreting compact object and a background QSO. To shed light on the nature of this source, we have obtained and analyzed archival data, including the {\it Einstein} data, 23 ROSAT observations, Beppo-SAX and ASCA pointings. Our analysis reveals that most of the emission of X-9 arises from a point-like highly-variable source, and that lower luminosity extended emission may be associated with it. The spectrum of this source changes between low and high intensity states, in a way reminiscent of the spectra of galactic Black Hole candidates. Our result strongly suggest that X-9 is not a background QSO, but a bonafide `Super-Eddington' source in Ho IX, a dwarf companion of M81.