PDF《The absence of a thin disc in M81》

MNRAS 000, 1C7 (2016) Preprint

9 November

2018 Compiled using MNRAS L A TEX style ?le v3.

0 The absence of a thin disc in M81 A. J. Young1 , I. McHardy2 , D. Emmanoulopoulos2 and S. Connolly2 1H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL. 2Department of Physics and Astronomy, University of Southampton, University Road, Southampton SO17 1BJ.

9 November

2018 ABSTRACT We present the results of simultaneous Suzaku and NuSTAR observations of the near- est Low-Luminosity Active Galactic Nucleus (LLAGN), M81? . The spectrum is well described by a cut-o? power law plus narrow emission lines from Fe Kα, Fe xxv and Fe xxvi. There is no evidence of Compton re?ection from an optically thick disc, and we obtain the strongest constraint on the re?ection fraction in M81? to date, with a best-?t value of R = 0.0 with an upper limit of R <

0.1. The Fe Kα line may be produced in optically thin, NH =

1 *

1023 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 ?ow. The X-ray continuum shows signi?cant variability on ?

40 ks timescales suggesting that the primary X-ray source is ? 100s of gravitational radii in size. If this X-ray source illuminates any putative optically thick disc, the weakness of re?ection implies that such a disc lies outside a few *

103 gravitational radii. An optically thin accretion ?ow 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. Key words: accretion, accretion discs C galaxies: active C galaxies: individual: M81 C X-rays: galaxies

1 INTRODUCTION M81, also known as NGC 3031, is a spiral galaxy hosting the closest Low-Luminosity Active Galactic Nucleus (LLAGN), M81? , at a distance of 3.63 Mpc (Freedman et al. 1994). Its central black hole has a mass of M =

7 *

107 M (De- vereux et al. 2003). The bolometric luminosity of M81? is 9.3 *

1040 erg s?1 (Ho et al. 1996), which corresponds to 1.1 * 10?5 LEdd, where LEdd is the Eddington luminosity. Thus, logarithmically speaking, M81? is midway between ultra-low Eddington fraction accreters such as Sgr A* at the centre of the Galaxy with LSgr A* ?

3 * 10?10 LEdd (Melia &

Falcke 2001) and a typical high-e?ciency AGN with LAGN ? 0.1 ? 1LEdd. M81? is particularly interesting to study because it is nearby and bright enough to allow de- tailed spectroscopy, while being in the sub-Eddington regime in which the accretion ?ow is thought to be qualitatively dif- ferent to that of an e?ciently radiating, high accretion rate AGN. Orbiting the black hole in M81? at large, ?

10 pc, scales is a disc inclined at 14? to the line of sight (i.e., the angle between the normal to the disc and the observer is 14? ) as measured from its narrow optical emission lines (Devereux et al. 2003). On smaller scales, there is a 0.8 pc radius (2.5* E-mail: Andy.Young@bristol.ac.uk

105 rg, where rg = GM/c2 ) disc inclined at 50? to the line of sight, the presence of which is inferred from its broad Hα line (Devereux &

Shearer 2007). The inclination angle of this inner disc is consistent with that of the short, 0.017 pc long, radio jet from the nucleus which is also inclined at 50? (i.e., the jet is perpendicular to the inner disc;

Bietenholz et al. 2000). These observations suggest that there is a warp between the pc and

10 pc-scale discs. The thin disc does not extend all the way into the event horizon of the black hole. While X-ray observations with ASCA (Ishisaki et al. 1996) revealed a possibly broadened Fe K line with an Equivalent Width (EW) of