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NICER / ISS Science Nugget
for November 21, 2024



Leading Lags

In one millisecond, light travels 186 miles. Measured by a telescope, time delays at this level imply that we are probing length scales - resolving physical structures - hundreds of miles across for objects many light-years away, a level of detail that can't be achieved with any existing method for acquiring high-resolution images of the sky. Small length scales are especially meaningful for studying the extreme gravity environment around black holes, and the process of accretion - mass transferred from a companion star onto the black hole - provides the X-ray light that makes these regions accessible to study.

A bright outburst in 2017-18 of the previously unknown black-hole binary system MAXI J1820+070 served as an early demonstration of NICER's ability to discern new phenomena on extremely short timescales. Groundbreaking measurements were interpreted with the best available models at the time for the interactions between accretion disks and the energized "corona" of plasma closest to the black hole. Some of those early interpretations (e.g., that the corona is significantly vertically extended) have since been challenged by more extensive analysis and especially by newly available X-ray polarization measurements (with NASA's IXPE mission). A comprehensive new model, published this week by P. Uttley (Univ. of Amsterdam, Netherlands) and J. Malzac (Centre Nat. de Recherche Scientifique, France) in the peer-reviewed UK journal Monthly Notices of the Royal Astronomical Society, was inspired by and revisits NICER's observations of MAXI J1820 in the "luminous hard state", reconciling those measurements of spectral-timing delays with a scenario in which the corona is compact and consistent with our most current understanding of accretion geometry around a black hole.

The new model traces the consequences of perturbations in the accretion flow. Even in its steady state, turbulence in the flow results in density and temperature fluctuations, which manifest observationally as slowly-varying thermal (low-energy) X-ray emission from the disk. Some of the disk photons enter the coronal region and gain energy from the hot plasma, emerging from the system with a low-energy nonthermal ("power law") spectrum. But as the higher-density "clumps" enter the base of the corona, they further seed the plasma, heating it and enhancing its emissions. This results in a higher level of high-energy power-law emission observed directly from the corona, but also an increase in coronal photons that "reverberate" in the disk. The model quantifies these processes and shows that the complex sign-changing time-lags seen in the NICER data can be reproduced with a compact corona and no additional assumptions or inconsistencies with existing observations. Among other implications of the model are the relevant distances, measured in hundreds of kilometers, between the corona and disk interaction regions, where the lags are measured in single-digit milliseconds, yielding a dynamic picture of the environment close to the event horizon of a stellar-mass black hole.


NICER measurements of variability in the broadband X-ray emission of black-hole binary MAXI J1820+070. Data points in the lower panel represent levels of variability at a range of fluctuation frequencies, in three non-overlapping energy bands representing thermal emission (orange), and low/high-energy (green/blue) power-law (PL) emission. The upper panel shows the relative time delays between the thermal and high-energy emission relative to the low-energy PL; the inset captures the negative lags for the thermal emission in a small range of frequencies between 3 and 10 Hz. (Credit: Uttley & Malzac 2024) (Left) The interplay of infalling matter and outgoing radiation, between a disk (seen edge-on in orange) of million-degree gas swirling into a black hole and a compact, hot-plasma corona (blue). (Right) Using the low-energy power-law (PL) signal as a reference, the model accommodates spectral-timing measurements of positive lags (delays) for the high-energy power-law at all fluctuation frequencies (blue dashed curve), but lags that swing from positive to negative at intermediate frequencies relative to the disk (orange solid curve), where reflected coronal emission off the innermost disk regions is delayed. (Credit: Uttley & Malzac 2024)

Left: NICER measurements of variability in the broadband X-ray emission of black-hole binary MAXI J1820+070. Data points in the lower panel represent levels of variability at a range of fluctuation frequencies, in three non-overlapping energy bands representing thermal emission (orange), and low/high-energy (green/blue) power-law (PL) emission. The upper panel shows the relative time delays between the thermal and high-energy emission relative to the low-energy PL; the inset captures the negative lags for the thermal emission in a small range of frequencies between 3 and 10 Hz. (Credit: Uttley & Malzac 2024) Center: The interplay of infalling matter and outgoing radiation, between a disk (seen edge-on in orange) of million-degree gas swirling into a black hole and a compact, hot-plasma corona (blue). Right: Using the low-energy power-law (PL) signal as a reference, the model accommodates spectral-timing measurements of positive lags (delays) for the high-energy power-law at all fluctuation frequencies (blue dashed curve), but lags that swing from positive to negative at intermediate frequencies relative to the disk (orange solid curve), where reflected coronal emission off the innermost disk regions is delayed. (Credit: Uttley & Malzac 2024)



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