Qingwen Wu, Hao Yan, Zhu Yi
We model the spectral energy distribution of NGC 4258 with updated broadband observations from radio to X-rays and a coupled accretion-jet model that surrounding a Kerr black hole (BH). The observed radio jet and the warped water maser disk are assumed to be triggered by a spinning BH through Blandford-Znajek mechanism and Bardeen-Petterson effect respectively. The accretion flow is modeled as an inner radiatively inefficient accretion flow (RIAF) and an outer truncated standard thin disk, where the transition radius is ~10^3R_g based on the width and variability of narrow Fe K_alpha line. We find that the RIAF surrounding a mildly spinning BH with dimensionless spin parameter a_*~0.73 can well reproduce the observed X-ray emission and jet power, where the observational jet power is estimated from its low-frequency radio emission. The outer thin disk mainly radiates at near infrared waveband, while the jet contributes predominantly at radio waveband. We propose that the accretion flow initially may consists a two-phase disk that the cold maser disk is sandwiched by substantial hot plasma, since that Bondi radius for the diffused hot plasma is much larger than the size of the observed maser disk. This can naturally explain why the accretion rate of inner RIAF is larger than that of outer maser disk. Using above estimated BH spin and the inferred accretion rate in maser disk based on the physical existence of the water maser, we find that the warp radius of the maser disk that driven by the Bardeen-Petterson effect is ~8.8*10^4 R_g, which is consistent with the observational result very well.
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http://arxiv.org/abs/1305.0072
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