Relativistic reverberation in the accretion flow of a tidal disruption event

Abstract

Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating1; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe2. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole3, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet4,5,6,7,8, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation9,10,11,12 arising from gravitationally redshifted iron Kα photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more13. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought5,14.

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Figure 1: Lag–energy spectrum of Swift J1644+57.
Figure 2: The energy spectrum of Swift J1644+57.
Figure 3: Comparison of lags in Swift J1644+57 with other Seyfert galaxies.
Figure 4: Schematic of reverberation off a super-Eddington accretion flow.

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Acknowledgements

E.K. thanks A. Zoghbi, M. C. Miller, F. Tombesi, E. Miller and L. Denby for discussions. E.K. also thanks the Hubble Fellowship Program for support under grant number HST-HF2-51360.001-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. J.M.M. acknowledges N. Schartel and XMM-Newton for executing target-of-opportunity observations of Swift J1644+57. C.R. acknowledges support from NASA under grant number NNX14AF86G. L.D. thanks J. McKinney for discussions. L.D. acknowledges support from NASA/NSF/TCAN (NNX14AB46G), NSF/XSEDE/TACC (TG- PHY120005) and NASA/Pleiades (SMD-14-5451). This work is based on observations made with XMM-Newton, a European Space Agency (ESA) science mission with instruments and contributions directly funded by ESA member states and the US (NASA) and the Suzaku satellite, a collaborative mission between the space agencies of Japan (JAXA) and the US (NASA).

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Contributions

E.K. led the XMM-Newton and Suzaku time lag analysis, simulations, interpretation of the results and manuscript preparation. J.M.M. performed the XMM-Newton spectral analysis and contributed to the interpretation of the results. C.R. developed the analytical toy model for reverberation in a super-Eddington flow and contributed to the interpretation of the results. L.D. had the idea of examining X-ray time lags in a TDE and contributed to the interpretation of the results.

Corresponding author

Correspondence to Erin Kara.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 XMM-Newton lag–frequency spectrum.

The lag–frequency spectrum between rest frame 0.3–1 keV and 1–4 keV (the usual soft and hard bands chosen for reverberation studies9,21). The lag–frequency spectrum shows a negative lag (that is, the soft band lagging behind the hard band) from (2–10) × 10−4 Hz (highlighted by the red hashed region). Because the lag–frequency spectrum shows a soft lag at these frequencies, we examined further the lag–energy spectrum at this particular frequency range (see Fig. 1). Error bars are at the 1σ level.

Extended Data Figure 2 XMM-Newton light curve of Swift J1644+57.

The observed 0.3–10 keV light curve in 200 s bins showing the rapid variability in this 25 ks observation. Note that the y axis does not extend to zero count rate. Error bars are at the 1σ level. Inset, the 4–6 keV (~5.5–8 keV rest frame) light curve (red) and the 6–10 keV (~8–13 keV rest frame) light curve (blue) zoomed in on the most variable part of the light curve from 5,000 s to 15,000 s (indicated by the red line in the main figure). The y axis is in units of count rate divided by the mean of the entire light curve. This illustrates (in the time domain) that on average the 6–10 keV variability leads the 4–6 keV variability. We emphasize that the time delay seen in these coarsely binned light curves places an upper limit on the actual amplitude of the lag. The detailed Fourier analysis (Fig. 1) allows us to obtain a better estimate of the average time lag.

Extended Data Figure 3 Suzaku XIS and HXD spectra.

A ratio plot of the time-integrated energy spectra of the front-illuminated (black) and back-illuminated (red) XIS detectors and the HXD (green) to an absorbed power-law fit from 3–6 keV. The blue line at a ratio of one indicates where the data would be fully described by the absorbed power-law model. Error bars are at the 1σ level. This fit exhibits the potential hardening in the spectrum above ~6 keV that appears to continue up to ~30 keV, beyond which the spectrum turns over. There is no obvious iron line in the XIS spectra.

Extended Data Figure 4 Suzaku lag–energy spectrum.

The lag–energy spectrum of the Suzaku observation taken 10 days before the XMM-Newton observation. An iron K lag at the same frequency and same energy as the XMM-Newton iron K lag is detected in the data. This confirms the presence of an iron K lag with an amplitude of ~120 s in two separate observations, taken with different instruments and analysed using different techniques. Error bars are at the 1σ level.

Extended Data Figure 5 Statistical significance of the XMM-Newton lag–energy spectrum.

a, The observed lag–energy spectrum (black points; same as Fig. 1 except the zero-point lag has not been shifted) compared with the 1σ distribution of 10,000 simulated Monte Carlo light curve pairs in each energy bin with zero lag (blue shaded region). This plot shows that the observed 0.3–10 keV lag energy is inconsistent with zero lag at >99% confidence. See Methods for details of the simulations. b, c, The best Gaussian (b) and asymmetric diskline (c) model fits (red lines) to the 3–10 keV lag–energy spectrum. We compute the Bayes’ factor between the models and find that the diskline is preferred at >98% confidence.

Extended Data Figure 6 Swift J1644+57 lag–energy spectrum compared with Seyfert galaxies.

Overlay of the reverberation signatures in nearby variable Seyfert galaxies compared with Swift J1644+57 (black diamonds). We show IRAS 13224−3809 in green, 1H 0707−495 in blue and Swift J2127.4+5654 in purple. The y axis shows arbitrary time lag units that have been scaled so that all of the sources have the same iron K lag amplitude. We show the scaled lag because all of the sources have different black hole masses (see Fig. 3). There is similarity in the broadband shape, although the iron K line profile of Swift J1644+57 appears to be slightly narrower and peaks at higher energies.

Extended Data Figure 7 Flux–energy and lag–energy spectra of reflection in super-Eddington flow.

Flux–energy (left) and lag–energy (right) spectra derived from our toy model for iron xxvi (rest-frame 6.97 keV) Kα line reverberation from an outflowing funnel wall. The results for terminal velocities of 0, 0.1c, 0.2c, 0.3c, 0.4c, 0.5c and 0.6c are shown (from left to right in peak energies).

Extended Data Table 1 Observations used in this analysis
Extended Data Table 2 Results of the fits to the lag–energy spectrum
Extended Data Table 3 Lags and masses of Seyfert galaxies with reverberation detections

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Kara, E., Miller, J., Reynolds, C. et al. Relativistic reverberation in the accretion flow of a tidal disruption event. Nature 535, 388–390 (2016). https://doi.org/10.1038/nature18007

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