The absence of a thin disc in M81

Abstract

We present the results of simultaneous Suzaku and NuSTAR observations of the nearest low-luminosity active galactic nucleus (LLAGN), M81*. The spectrum is well described by a cut-off power law plus narrow emission lines from Fe K α, Fe  xxv, and Fe  xxvi. There is no evidence of Compton reflection from an optically thick disc, and we obtain the strongest constraint on the reflection fraction in M81* to date, with a best-fitting value of R = 0.0 with an upper limit of R < 0.1. The Fe K α line may be produced in optically thin, NH=1×1023cm−2 , gas located in the equatorial plane that could be the broad line region. The ionized iron lines may originate in the hot, inner accretion flow. The X-ray continuum shows significant variability on ∼40 ks time-scales suggesting that the primary X-ray source is ∼100 s of gravitational radii in size. If this X-ray source illuminates any putative optically thick disc, the weakness of reflection implies that such a disc lies outside a few ×103 gravitational radii. An optically thin accretion flow inside a truncated optically thick disc appears to be a common feature of LLAGN that are accreting at only a tiny fraction of the Eddington limit.

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ORCID for Dimitrios Emmanoulopoulos: orcid.org/0000-0002-3335-2446

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