NICER / ISS Science Nugget
for April 10, 2025
Transitioning AGN
Traditional models suggest that the flow of matter onto a black hole forms a geometrically thin disk, generally predicted to be stable on long timescales (hundreds of years). The emissions of most active galactic nuclei (AGN) in visible through X-ray light, powered by black-hole accretion at the centers of other galaxies, do indeed show modest (~ 10%) variations with time. But there are a handful of exceptions, loosely called "changing look" AGN: in addition to brightness variations, they usually exhibit dramatic changes in their spectra - how emission is distributed across a range of wavelengths or energies of light - as well.
The AGN Markarian 590 appears to be an outlier among the changing-look outliers: its steady, unremarkable behavior in the 1980s and 1990s changed abruptly in 2010, when accretion appeared to turn off, only to resume in 2017. Since then, it has exhibited flares in UV-optical and X-rays once or twice each year; but while its luminosity changes by 5-10x in these flares, its spectrum changes hardly at all. One component of this spectrum, an excess in low-energy X-rays, is both unexpected and present both during and between flares. Intriguingly, the X-ray brightness variations come first, followed 3 days later by identical changes in the UV-optical brightness.
This behavior, and the soft X-ray excess, are explored in a paper by D. Lawther (Univ. of Arizona) and collaborators, recently accepted for peer-reviewed publication in the U.K. journal Monthly Notices of the Royal Astronomical Society, based on observations with NICER, NASA's Swift and NuSTAR observatories, and ESA's XMM-Newton telescope. Based on X-ray spectroscopy, the authors find little evidence for the commonly seen "reflection" of X-rays - emission stimulated in a thin accretion disk at small radii by photons generated in a hot plasma close to the black hole. Instead, both the soft X-ray excess and the 3-day-delayed UV emission appear to be the result of reprocessing of the same high-energy coronal X-rays by different structures: a thick accretion disk "atmosphere" and a distant, warm-gas region, respectively. The results suggest that a variety of physical components and mechanisms can be responsible for variability in AGN, and demonstrate the promise of spectral-timing studies to elucidate their arrangements and roles.
Left: A sample NICER spectrum (black crosses) for the active galactic nucleus (AGN) Markarian 590, acquired in a 1,740-sec exposure on 16 July 2022. A model that jointly accounts for the AGN's X-ray emission as a function of photon energy (yellow trace) as well as non-astrophysical "background" detected by NICER (pink) sums to the total best-fit spectrum (red). The background model, named SCORPEON, comprises three components in this example: noise in NICER's detectors (green solid trace), non-X-ray background (ambient particle and gamma-ray radiation, "nxb" dash-dot), and cosmic X-rays (e.g., diffuse sky glow) unrelated to Mrk 590 ("xrb" dotted). (Credit: Lawther et al. 2025)
Right: Schematic depiction of the accretion elements and emitting regions around the black hole at the heart of Mrk 590, based on analysis of X-ray and UV spectrum and timing measurements. Energetic X-rays from a hot "corona" plasma (left) illuminate a disk of accreting material; its intrinsic glow, together with "reflected" corona X-rays, is modified by an atmosphere (yellow) that sandwiches the disk, resulting in excess low-energy ("soft") emission in X-rays. Some X-rays from the vicinity of the black hole reach a region that emits in UV, the brightness of which tracks observed X-rays with a 3-day delay. (Credit: Lawther et al. 2025)
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