Results from an extensive simultaneous broadband campaign on the underluminous active nucleus M81*: Further evidence for mass-scaling accretion in black holes

We present the results of a broadband simultaneous campaign on the nearby low-luminosity active galactic nucleus M81*. From 2005 February through August, we observed M81* five times using the Chandra X-Ray Observatory with the HETGS, complemented by ground-based observations with the Giant Meterwave Radio Telescope, the Very Large Array and Very Large Baseline Array, the Plateau de Bure Interferometer at IRAM, the Submillimeter Array, and Lick Observatory. We discuss how the resulting spectra vary over short and longer timescales compared to previous results, especially in the X-rays where this is the first ever longer term campaign at spatial resolution high enough to nearly isolate the nucleus (17 pc). We compare the spectrum to our Galactic center weakly active nucleus Sgr A*, which has undergone similar campaigns, as well as to weakly accreting X-ray binaries in the context of outflow-dominated models. In agreement with recent results suggesting that the physics of weakly accreting black holes scales predictably with mass, we find that the exact same model that successfully describes hard-state X-ray binaries applies to M81*, with very similar physical parameters.