A rapidly spinning supermassive black hole at the centre of NGC 1365

Abstract

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge of an accretion disk. In this model, line broadening and distortion result from rapid rotation and relativistic effects near the black hole, the line shape being sensitive to its spin. Alternative models in which the distortions result from absorption by intervening structures provide an equally good description of the data, and there has been no general agreement on which is correct. Recent claims that the black hole (2 × 10(6) solar masses) at the centre of the galaxy NGC 1365 is rotating at close to its maximum possible speed rest on the assumption of relativistic reflection. Here we report X-ray observations of NGC 1365 that reveal the relativistic disk features through broadened Fe-line emission and an associated Compton scattering excess of 10-30 kiloelectronvolts. Using temporal and spectral analyses, we disentangle continuum changes due to time-variable absorption from reflection, which we find arises from a region within 2.5 gravitational radii of the rapidly spinning black hole. Absorption-dominated models that do not include relativistic disk reflection can be ruled out both statistically and on physical grounds.

Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658

“…The best-fit baseline-reflection model to the time-averaged Chandra and XMM-Newton spectra (Extended Data Figs. 5,6) suggest that the source is at times reflection dominated, i.e., we find the ratio of the reflected to the illuminating continuum in the Chandra (XMM-Newton) data to be f reflect /f illum = 2.3 ± 1.2 (0.47 ± 0.15) in the 0.1 -10 keV band (local frame; Extended Data Table 2). However, it must be noted that uncertainties in the size of the microlensed regions could affect the absolute value of this ratio.…”

“…Nonetheless, this analysis clearly demonstrates the presence of a significant contribution from a reflection component to the X-ray spectrum of this z = 0.658 quasar. The properties of RX J1131-3 1231 are consistent [11,13,12,14,16] with the known observational characteristics of quasars at 0.5 z 1, and our results suggest that the relativistic reflection component from the large population of unobscured quasars expected in this epoch [17] could significantly contribute in the 20-30 keV band of the Cosmic X-ray background.Although questions have previously been raised over whether reflection is a unique interpretation for the features observed in AGNs, the amassed evidence points towards this theoretical framework [24, 29], and reached culmination with the launch of NuSTAR and the strong confirmation of relativistic disk reflection from a rapidly spinning supermassive black hole at the centre of the nearby galaxy NGC 1365 [6]. Nonetheless, there still remain possible systematic uncertainties, for example, due to the intrinsic assumption that the disk truncates at the innermost stable circular orbit.…”

“…2,4,5), reveal broad residual emission features both at low energies ( 2 keV rest frame, the "soft-excess") and around the iron K energies (3.5-7 keV rest frame), characteristic signatures of relativistic disk reflection [10,23]. To treat these residuals, we consider two template models based on those commonly used to fit the spectra of Seyferts [24,6] and stellar mass black hole binaries [25], and which have also at times been used to model local quasars [26]. The first is a simple phenomenological combination of a power-law, a soft-thermal disk and a relativistic Fe-line component (baseline-simple), and the second employs a self-consistent blurred-reflection model together with a power-law (baseline-reflection).…”

“…Although questions have previously been raised over whether reflection is a unique interpretation for the features observed in AGNs, the amassed evidence points towards this theoretical framework [24, 29], and reached culmination with the launch of NuSTAR and the strong confirmation of relativistic disk reflection from a rapidly spinning supermassive black hole at the centre of the nearby galaxy NGC 1365 [6]. Nonetheless, there still remain possible systematic uncertainties, for example, due to the intrinsic assumption that the disk truncates at the innermost stable circular orbit.…”

Reaching the peak of the quasar spectral energy distribution – I. Observations and models

“…There is currently much interest in constraining spin, which can be estimated if some measure of the radius of innermost stable circular orbit (r isco ) can be made. In practice this is difficult; Fabian et al (2009) and Risaliti et al (2013) claim to make such measures from observations of the broad iron K emission line, but this is still controversial (e.g. Miller & Turner 2013).…”