Reverberation lags viewed in hard X-rays from an accreting stellar-mass black hole

Abstract

Accreting black holes are thought to swallow matter in the form of a disk and a hot cloud of plasma that glows brightly in X-rays, known as the corona. The X-ray emitting region is far too small to be directly imaged, but rapid variability of the X-ray signal can be used to infer the geometry by measuring time lags caused by material propagating towards the black hole and by coronal X-rays reflecting off the disk to imprint a reverberation lag. Reverberation lags can be recognized by characteristic spectral features, including an iron emission line at $\sim 6.4$ keV and a broad Compton hump peaking at $\sim 30$ keV. These reverberation features have both previously been detected for a few supermassive black holes in active galactic nuclei (AGNs). However, it is much more challenging to detect reverberation lags from stellar-mass black holes because they are more than a million times smaller. Previous reverberation lag measurements for stellar-mass black holes in X-ray binary systems have thus been limited to energies below 10 keV. Here we report on the first detection of the Compton hump reverberation feature from an X-ray binary, achieved by measuring lags in the broad energy range of $\sim 1-150$ keV. The accompanying detection of an iron line feature confirms the scenario of X-ray reverberation and provides strong evidence that the accretion flows in AGNs and X-ray binaries are governed by an ubiquitous process. Reverberation lags are prominent only in the most rapid variability, whereas lags in the slower variability are commonly attributed to propagating mass accretion rate perturbations. Our lag measurements up to the highest energy to date reveal that this lag in the slower variability evolves dramatically on timescales of days.

Figures (10)

• Figure 1: Insight-HXMT lightcurves and corresponding hardness-intensity diagram. (A) Insight-HXMT lightcurves (in units of counts per second) of MAXI J1820+070 in HE (27-250 keV), ME (10-30 keV) and LE (1-10 keV) band, in the rising hard state during the outburst ranging from MJD 58192 to 58286. (B) The corresponding hardness-intensity diagram, defined as the total 1–10 keV count rate (in units of counts per second) versus the ratio of hard (3–10 keV) to soft (1–3 keV) count rates. The central times of the six epochs of interest are shown as colored points, corresponding to MJD = 58192, 58194, 58200, 58217, 58245, and 58286.
• Figure 2: Lag-frequency spectra of MAXI J1820+070. (A) Lag-frequency spectrum for Epoch 1; (B) Lag-frequency spectrum for Epoch 2. In each panel, the blue line shows the lag between the 4--10 keV and 70--150 keV bands, and the red line shows the lag between the 27--70 keV and 70--150 keV bands. A negative lag indicates that the softer band lags behind the harder band. Error bars indicate 1$\sigma$ uncertainties.
• Figure 3: Comparison of high-frequency lag-energy spectra between AGNs and XRB. The left y-axis displays the lag measurements for XRB MAXI J1820+070 during epoch 1, using a reference band of 1-10 keV, within the 2-60 Hz frequency range where significant high-energy soft lags were observed as depicted in Fig. \ref{['lag_f']}. The right y-axis shows the high-frequency lag-energy spectra for three AGNs: MCG–5-23-16, SWIFT J2127.4+5654, and NGC 1365. These AGNs exhibit detected iron K and Compton hump reverberation lags, highlighting the similarities in the reverberation processes across different scales of black hole systems zoghbi2014observationskara2015. The shaded region indicates the 1$\sigma$ uncertainty range.
• Figure 4: Evolution of the lag-energy spectra for the six observation epochs. The frequency range used to produce the lag-energy spectra corresponds to the soft lag region identified in the lag-frequency spectra from the NICER data. The reference band is selected as 27-150 keV. The color of each observation matches that in Fig. \ref{['hxmt_lightcurve']}. Error bars indicate 1$\sigma$ uncertainties.
• Figure 5: Low-frequency lag-energy spectra of MAXI J1820+070.(A) The 0.1–1 Hz lag-energy spectra for the six epochs. The reference band is chosen to be 1-10 keV. The spectra cover the energy range of the ME and HE instruments (10-150 keV). The lags are shifted so that the lowest-energy bin is set to zero. The color of each observation matches the colors used in Fig. \ref{['hxmt_lightcurve']}. (B) The maximum amplitude of the hard lags as a function of the LE count rates. The red dashed line shows the best-fitting linear relation. Error bars indicate 1$\sigma$ uncertainties.