NICER / ISS Science Nugget for May 3, 2018

PhD thesis discovers QPO in NICER data of MAXI J1535-571

According to Einstein's theory of general relativity, black holes from collapsed massive stars are dense enough to significantly warp space and time. By studying X-ray emission from regions very close to a black hole, we can decipher the effects of strong gravity on physical processes and test general relativity in the strong-field limit. One type of rapid sub-second variability seen in the brightness "light curves" of accreting black holes is the enigmatic quasi-periodic oscillation (QPO). As part of her PhD research, Abigail Stevens developed a new technique to analyze the energy spectrum of a QPO as a function of the brightness oscillation's phase. This technique can be used to constrain the origin of the QPO signal and look for signatures of general relativity in NICER data.

In Chapter 5 of her thesis (the first PhD to be awarded based in part on NICER data!), Dr. Stevens presented the discovery of a weak "Type B" QPO in NICER observations of the newly-identified black hole transient MAXI J1535-571 from autumn 2017. NICER data track the spectral variations with QPO phase in low-energy X-rays, a band that was previously unobservable on such short timescales. The QPO-phase-resolved spectra (Figure 1) show that the low-energy X-rays from the accretion disk lag in time behind the high-energy X-rays by about a quarter of an oscillation period.

Cross-correlation function of the 5.7 Hz QPO in MAXI J1535-571
Figure 1: The energy-dependent cross-correlation function of the 5.7 Hz QPO, showing relative time on the horizontal axis and X-ray energy on the vertical axis. We can see that the high-energy X-ray lead the low-energy X-rays by about a quarter of a period. (Image credit: A.L. Stevens, 2018)

This could be explained by an effect of general relativity known as "frame dragging," which causes the hot inner flow of gas to precess around the spin axis of the black hole like a wobbling top, illuminating adjacent regions of the accretion disk (Figure 2). The completed spectral calibration of NICER will now allow us to fit detailed models to the QPO-phase-resolved spectra for publication soon in The Astrophysical Journal.

Graphic of precessing accretion flow
Figure 2: An illustration of a black hole with an accretion disk and hot inner gas flow. The hot inner gass flow is precessin around the spin axis of the black hole like a wobbling top and illuminating adjacent regions of the accretion disk.
(Image credit: ESA/ATG Medialab/A. Ingram)

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