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NICER / ISS Science Nugget
for November 14, 2024



By the light of a warm corona

The X-ray emissions of many active galactic nuclei (AGN), the luminous cores of distant galaxies thought to harbor black holes millions of times the mass of our Sun, are known to include a poorly understood excess at low photon energies, precisely where NICER is designed to be most sensitive. In a peer-reviewed paper recently accepted for publication in The Astrophysical Journal, E. Partington (Wayne State Univ.) and collaborators describe an effective approach to understanding the origin of this soft excess, using "reverberation" - light-travel-time delays between cause-and-effect emissions arising from distinct geometric structures - to map out the flows of matter onto a black hole, within regions too small, too distant, and often too obscured by surrounding matter to image with any existing telescope.

The team acquired data concurrently with NICER and NASA's Swift observatory to sample, respectively, the X-ray and ultraviolet (UV) variations of the AGN Fairall 9 across nearly 1,000 days, with observations approximately every other day. In high-energy X-rays, emission from a hot (billion-degree) plasma dominates; in the standard model for AGN, this plasma exists in the near vicinity of the black hole, likely vertically offset (a "hot corona") from the swirling disk of cooler accreting matter (radiating thermally at UV wavelengths). At lower X-ray photon energies, the soft excess can be isolated through modeling and decomposition of the spectrum. Spectrally, it is consistent with million-degree gas, which suggests a region of emission intermediate between the hot corona and the cool accretion disk. Analysis of the time-variations in these three bands - high-energy X-rays, low-energy X-rays, and UV - provides the crucial insight: brightness changes in the UV are strongly correlated with only the soft excess, with the latter leading by a few days. The clear implication of the UV fluctuations following the soft X-ray fluctuations is that the soft-excess emission powers some of the UV emission. The team's proposed scenario for the accretion environment around the black hole captures these elements: the hot corona around the black hole is seen both directly (in hard "power-law" X-rays) and through its reflection within the innermost parts of the accretion disk, perhaps stimulating there the puffing up of a "warm corona," which is itself seen both directly (the soft-excess X-rays) and in its reflection within a cooler part of the disk, where it stimulates part of the UV emission.

Similar work on other AGN aims to confirm and further refine this emerging picture of the accretion environment around supermassive black holes, and how their emissions drive the long-term evolution of the host galaxy.


Approximately 1,000 days of observations of Fairall 9 with NICER and the ultraviolet telescope on NASA's Swift observatory (bottom panel); the top three panels capture total X-ray brightness and the inferred contributions from two components used to model it, a power law dominating at high energies and a soft excess at lower energies. The narrow panels at right show the results of cross-correlation analysis using the UV variations as the reference band: the total X-ray flux and power-law are weakly correlated, while the soft excess is significantly correlation (highest R value, black trace). Histograms represent estimates of the peak (colors) and centroid (gray) time lags between the X-ray components and UV, with the soft excess appearing to lead the UV variations by a few days. (Credit: Partington et al. 2024) A sketch of the modeled accretion structures and emission for the Fairall 9 active galactic nucleus. The black hole at right is surrounded by a hot corona (purple ring), responsible for observed non-thermal power law emission at higher X-ray energies (purple arrow), a portion of which is gravitationally pulled back down toward the innermost parts of an extended accretion disk (red to light-blue gradient representing increasing temperature from left to right). That energetic input stimulates reflected fluorescent emission from the disk, supplementing an observed excess of low-energy (soft) X-ray photons arising from a warm corona (dark blue crescent and arrow) that extends out to the radius RWC. The warm corona in turn stimulates emission from parts of the disk still further out (radius RUVW2), producing ultraviolet emission observed with Swift (light blue arrow). (Credit: Partington et al. 2024)

Left: Approximately 1,000 days of observations of Fairall 9 with NICER and the ultraviolet telescope on NASA's Swift observatory (bottom panel); the top three panels capture total X-ray brightness and the inferred contributions from two components used to model it, a "power law" dominating at high energies and a "soft excess" at lower energies. The narrow panels at right show the results of cross-correlation analysis using the UV variations as the reference band: the total X-ray flux and power-law are weakly correlated, while the soft excess is significantly correlation (highest R value, black trace). Histograms represent estimates of the peak (colors) and centroid (gray) time lags between the X-ray components and UV, with the soft excess appearing to lead the UV variations by a few days. (Credit: Partington et al. 2024) Right: A sketch of the modeled accretion structures and emission for the Fairall 9 active galactic nucleus. The black hole at right is surrounded by a hot corona (purple ring), responsible for observed non-thermal "power law" emission at higher X-ray energies (purple arrow), a portion of which is gravitationally pulled back down toward the innermost parts of an extended accretion disk (red to light-blue gradient representing increasing temperature from left to right). That energetic input stimulates "reflected" fluorescent emission from the disk, supplementing an observed excess of low-energy (soft) X-ray photons arising from a warm corona (dark blue crescent and arrow) that extends out to the radius RWC. The warm corona in turn stimulates emission from parts of the disk still further out (radius RUVW2), producing ultraviolet emission observed with Swift (light blue arrow). (Credit: Partington et al. 2024)



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