The absence of a thin disc in M81

TL;DR

This study uses simultaneous Suzaku and NuSTAR observations to show that M81* lacks a significant optically thick accretion disc, indicating a truncated disc and an optically thin inner flow typical of low-luminosity AGNs.

Contribution

First simultaneous Suzaku and NuSTAR analysis of M81* revealing absence of disc reflection, constraining the accretion flow structure in a LLAGN.

Findings

No evidence of Compton reflection, with R < 0.1.

Fe Kα line likely from optically thin gas in the broad line region.

X-ray variability suggests a primary source within a few hundred gravitational radii.

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$\alpha$, 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-fit value of $R = 0.0$ with an upper limit of $R < 0.1$. The Fe K$\alpha$ line may be produced in optically thin, $N_H = 1 \times 10^{23}$ cm$^{-2}$, 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 $\sim 40$ ks timescales suggesting that the primary X-ray source is $\sim 100$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 $\times 10^3$ 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.