The Ultraluminous M81 X-9 Source: 20 Years' Variability and Spectral States

The source X-9 was discovered with the 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 in excess of the Eddington limit for a 1 M_solar compact accreting object, if it is at the same distance as Holmberg IX (3.4 Mpc). Past hypotheses on the nature of this super-Eddington source included a supernova remnant or supershell, an accreting compact object, and a background QSO. To shed light on the nature of this source, we have analyzed archival data, including the Einstein data, 23 ROSAT observations, and BeppoSAX and ASCA pointings. Our analysis reveals that most of the emission of X-9 arises from a pointlike highly variable source (0.5-2.4 keV L_X~2-8×10³⁹ ergs s^-1) 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 bona fide ``super-Eddington'' source in Ho IX, a dwarf companion of M81.