PDF《The absence of a thin disc in M81》

170 eV, no strong Compton re?ection component was detected. Subse- quent observations with BeppoSAX detected a broad com- plex of Fe lines with the same EW,

170 eV, but again no Compton re?ection component. The BeppoSAX spectrum extended to over

100 keV allowing the re?ection fraction R = ?/2π, where ? is the solid angle subtended by the optically thick gas from the point of view of the X-ray con- tinuum source, to be constrained to be R <

0.3 (Pellegrini et al. 2000). More recent observations with XMM-Newton were able to resolve the broad Fe K complex into three sep- arate lines of almost equal strength due to Fe Kα, Fe xxv and Fe xxvi with EWs of 39,

47 and

37 eV, respectively (Page et al. 2004). High spectral resolution observations with the Chandra High Energy Transmission Gratings (HETGS) c

2016 The Authors arXiv:1803.10050v1 [astro-ph.HE]

27 Mar

2018 2 A. J. Young, et al. Observatory Start date Total good time NuSTAR 2015-05-18

209 ks Suzaku 2015-05-18

99 ks Table 1. Observations of M81 taken with NuSTAR and Suzaku showing the start time and total good time (averaged over the active instruments for each observatory). spectrometer showed that the Fe xxvi line is redshifted by ?

2500 km s?1 while the neutral Fe Kα line, with an EW of

47 eV, is consistent with being at rest (Young et al. 2007). The Fe Kα line is unresolved, with a Full Width at Half Max- imum (FWHM) of <

4670 km s?1 . For a disc inclined at 50? this requires rin

104 rg. For a face-on disc, however, the constraint is much weaker and formally ?tting a diskline model (Fabian et al. 1989) only constrains rin >

55rg (Young et al. 2007). The Si Kα line appears to be broadened, with a FWHM ?

1433 km s?1 which corresponds to rin

105 rg in a disc inclined at 50? . This radius is comparable to that of the broad Hα emitting gas. In this truncated disc model the inner accretion ?ow is optically thin, radiatively ine?cient and may produce the ionized Fe xxv and Fe xxvi emission lines, characteristic of emission from plasma at temperatures of

107 ?108 K close to the truncation radius (see Young et al. 2007, for more details of this model). The origin of the Fe Kα line is less straightforward. X-ray monitoring of M81? has shown the continuum power-law photon index to be anti-correlated with X-ray ?ux, so the source is harder when it is brighter (Miller et al. 2010;

Connolly et al. 2016). This is consistent with models in which the continuum is produced by the inner accretion ?ow which Comptonises seed photons f........