2013MNRAS.436.1278W

We fit the most up-to-date broad-band spectral energy distribution from radio to X-rays for NGC 4258 with a coupled accretion-jet model that surrounds a Kerr black hole (BH). Here, both the jet and the warped H2O maser disc are assumed to be triggered by a spinning BH through the Blandford-Znajek mechanism and the Bardeen-Petterson effect, respectively. The accretion flow consists of an inner radiatively inefficient accretion flow and an outer truncated standard thin disc, where the transition radius Rtr ≃ 3x10³Rg for NGC 4258, based on the width and variability of its narrow Fe Kα line. The hybrid jet formation model, as a variant of the Blandford-Znajek model, is used to model the jet power. Therefore, we can estimate the accretion rate and BH spin through the two observed quantities (i.e. X-ray emission and jet power), where the observed jet power is estimated from the low-frequency radio emission. Using this method, we find that the BH of NGC 4258 should be mildly spinning with dimensionless spin parameter a* ≃ 0.7 ± 0.2. The outer thin disc mainly radiates at the near-infrared waveband and the jet contributes predominantly at the radio waveband. Using the above-estimated BH spin and the inferred accretion rate at the region of the maser disc based on the physical existence of the H2O maser, we find that the warp radius is ∼ 8.6x10⁴Rg if it is driven by the Bardeen-Petterson effect. This is very consistent with the observational result.