NICER / ISS Science Nugget
for May 22, 2025

Multicolor oscillations

For several months in 2023 and 2024, many of the world's telescopes were trained on Swift J1727.8-1613, when this previously unknown black-hole binary system was in outburst. In X-rays, it was one of the brightest objects in the sky for much of this time. A powerful approach to understanding the nature and origin of emissions in such energetic systems is to conduct observations at multiple wavelengths simultaneously; to this end, NICER coordinated the scheduling of some of its observations with optical telescopes on the ground. A peer-reviewed paper by F. Vincentelli (Southampton Univ, UK) and collaborators, recently accepted for publication in the UK journal Monthly Notices of the Royal Astronomical Society, describes results of data collected jointly with NICER and two high-speed optical cameras: ULTRACAM at the 3.58-meter New Technology Telescope in La Silla, Chile, and (on two occasions) HAWK-I at one of the 8.2-m units of the Very Large Telescope in Cerro Paranal, Chile.

Focused specifically on comparing rapid brightness changes - random flickering and any cyclic variations - at different wavelengths, the study identifies quasi-periodic oscillations (QPOs) in the NICER data at 1.4 Hz and 2.2 Hz frequencies on September 9 and 15, 2023, respectively. Such features are well known in the fluctuation power spectra of accreting black-hole binaries. A promising model for their origin involves "wobbling" of both the innermost part of the accretion disk, seen in X-rays, and a collimated "jet" outflow of particles, emitting in optical-IR, where the precession is thought to be a natural consequence of the strong-gravity environment near the black hole. Vincentelli et al. find that weak oscillations, at frequencies matching the X-ray QPOs in both observations, are seen at red and infrared (IR) wavelengths, but not in blue light. Consistent with the precessing jet model, they also show that on Sept. 9, the IR oscillations lag the X-rays by about 70 milliseconds, interpreted as the travel time of fluctuations in the rate of accretion from the disk region to the jet region. On Sept. 15, however, they find no lag at all: the X-ray and IR oscillations line up in phase. The authors suggest that this surprising finding may reflect some inadequacy in the current precessing-jet model - they note that no QPOs faster than 1 Hz have previously been detected at optical-IR wavelengths, and that a new physical feature may be needed to explain behaviors at higher frequencies. Additional simultaneous multi-wavelength coverage, sampled at a higher cadence to track trends without several-day gaps, will be needed to further pin down the structure of matter flows in accreting black-hole systems.