NICER / ISS Science Nugget
for October 17, 2024

Dissecting a heartbeat

Of the hundreds of X-ray emitting binary systems known, only two are seen to exhibit the phenomenon known as "heartbeats": deep, repeating brightness fluctuations that follow patterns reminiscent of electrocardiogram traces from a human heart. The first known heartbeat source, the well-studied binary GRS 1915+105, historically has demonstrated 14 different-shaped heartbeat modes; unfortunately, GRS 1915 has been in an unprecedented dim state since approximately a year into the NICER mission (and the community eagerly awaits its return). The second system, IGR J17091-3624, was last seen in outburst in 2022, during which observations with NICER and NASA's NuSTAR telescope (sensitive to higher-energy X-rays) detected a previously unknown heartbeat mode, one of nine now categorized. Dubbed Class X, these oscillations change the overall system brightness by a factor of 2, recur with an approximately 70-second period, and are more regular and symmetric than other modes.

Both GRS 1915 and IGR 1791 are accreting black-hole binaries. Heartbeat oscillations are thought to arise as a result of a "limit cycle" phenomenon, in which matter flowing onto the black hole at a high rate produces radiation near the event horizon - at the inner edge of an accretion disk - so powerful that these emissions push back on the infalling matter, temporarily lowering the flow rate. As the energy available to the radiation dies out, the relentless flow eventually overpowers it and the cycle begins anew.

In a peer-reviewed paper recently published in The Astrophysical Journal, Qing-Cang Shui (Chinese Academy of Sciences, PRC) and collaborators describe analysis of the NICER and NuSTAR observations of the Class X heartbeat in IGR J17091. Using a numerical approach designed to deal with not-strictly-periodic phenomena, they examine how the X-ray emission varies with the heartbeat cycle in both low-energy (NICER) and high-energy (NuSTAR) X-rays. Specifically, they adopt models for the X-ray spectrum arising from thermal accretion-disk emission as well as nonthermal emission from a "corona" of hot plasma, in 10 time-slices across heartbeat phase. Their primary finding is that, at least for the Class X mode, the brightness oscillation is principally driven by the temperature of the gas in the innermost parts of the accretion disk, consistent with the limit-cycle hypothesis, but that some as-yet-unresolved process also alters the properties, to a lesser extent, of the corona, with the most notable consequence being a deficit of emission in the hardest X-rays visible with NuSTAR. Such time-resolved spectroscopy is a powerful tool for discerning, in real time, the physics of dynamic systems such as accreting black hole binaries.