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Recent Science from NICER on the ISS

An AGN disk powered by X-rays

Fast variability, on timescales of days to weeks, in the multi-wavelength emissions of massive black holes that power Active Galactic Nuclei (AGN) has been suggested to originate in an X-ray "reprocessing" scenario: a central corona of X-ray-emitting electrons illuminates and heats an accretion disk. Variability peaks and troughs in the X-ray light curve, seen with NICER, would then be expected to reappear as "echoes" in ultraviolet light curves after a time lag associated with the light travel time between the corona and the disk. A recent peer-reviewed study of NGC 7469 by E. Partington (Wayne State Univ.) and collaborators, accepted for publication in The Astrophysical Journal, provides a clear example of the X-ray reprocessing scenario at work.

In a campaign lasting from May through December of 2023, the team observed NGC 7469 daily with NICER and NASA's Swift observatory to search for an X-ray/UV relationship. Modeling the NICER X-ray spectrum at each epoch revealed that the fast variability in the X-ray "power law" emission component, at higher photon energies and associated with the corona, matches the variability seen in the X-ray "soft excess" emission at energies below 2 keV. This suggests that the soft excess arises from a reflection of the X-ray corona by the inner accretion disk. In this case, narrow stimulated atomic lines are blurred by the relativistic rotation of the disk to resemble a continuum of soft X-rays in the NICER spectrum

Similar features also arise in the Swift UV light curve, which lags behind the X-ray power law light curve by less than a day. The team showed that the X-ray power law is energetic enough to power the variability seen in both the soft excess and the UV throughout the entire duration of the campaign. This result, combined with other reverberation studies on other AGN that lack the strong X-ray/UV correlation and large X-ray energies indicative of reprocessing, suggests that the relationship between X-rays and accretion disks does not follow a uniform prescription in AGN.


Light curves of NGC 7469 observed by NICER in X-rays (top four panels) and Swift in ultraviolet (bottom panel). The NICER panels show in descending order the total X-ray flux, the flux of the bright power-law feature, the spectral index of the power law, and the flux of the faint soft-excess feature. The narrow panels to the side show the correlation R between the Swift UV light curve and each of the NICER X-ray light curves, when the X-ray light curves are shifted by some time lag Tau between ±5 days (black trace). Histograms indicate the distribution of likely values for Tau. (Credit: Partington et al. 2025) The same format as the panels on the left, showing here the correlation between the X-ray power law and soft excess fluxes with lags ranging between ±5 days. (Credit: Partington et al. 2025)

Left: Light curves of NGC 7469 observed by NICER in X-rays (top four panels) and Swift in ultraviolet (bottom panel). The NICER panels show in descending order the total X-ray flux, the flux of the bright power-law feature, the spectral index of the power law, and the flux of the faint soft-excess feature. The narrow panels to the side show the correlation R between the Swift UV light curve and each of the NICER X-ray light curves, when the X-ray light curves are shifted by some time lag Tau between ±5 days (black trace). Histograms indicate the distribution of likely values for Tau. (Credit: Partington et al. 2025) Right: The same format as the panels on the left, showing here the correlation between the X-ray power law and soft excess fluxes with lags ranging between ±5 days. (Credit: Partington et al. 2025)



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