During the past week, a Target of Opportunity large observing program with NuSTAR was triggered to monitor the outburst of the black hole binary IGR J17091-3624. This X-ray transient black hole binary candidate was first detected in 2003 and has since exhibited fascinating behavior, including radio jets, X-ray quasi-periodic oscillations, and state transitions characteristic of microquasars. Most peculiar are its X-ray flarings, which resemble a “heartbeat” pattern over time—a phenomenon observed in only one other source, GRS 1915+105. This large observing program, consisting of a total of 500 ks of NuSTAR exposure time coordinated NASA’s NICER mission, was designed to track the full evolution of a transient black hole outburst, capturing rapid changes in X-ray spectra and variability as the system transitions through different accretion states. However, after the initial observations, the outburst appeared to fade rather than fully develop, leading to a pause in the program. This unexpected behavior exemplifies one of the key challenges—and excitements—of time-domain astrophysics: black hole outbursts are not always predictable, and what appears to be the beginning of a dramatic event can sometimes fizzle out. We continue to monitor the source with other observatories to determine whether the NuSTAR program should be resumed. The three observations taken so far remain highly valuable, revealing clear signatures of X-ray reflection off an accretion disk, with spectra showing atomic fluorescence lines distorted by the black hole’s gravity. As new data is acquired, we continue to refine our understanding of the factors that drive complete versus failed outbursts in black hole systems.

Last Wednesday was the due date for NuSTAR General Observer (GO) cycle-11, an annual call soliciting proposals for basic research relevant to the NuSTAR mission. This is the primary opportunity for the scientific community to request observing time with NuSTAR, and includes the possibility of proposing for multi-mission investigations by coordinating with NASA’s NICER and Swift and ESA’s XMM-Newton observatories. This year, the project has received a record number of proposals, 15% higher than previous years. The proposal oversubscription rate of available NuSTAR observing time is the highest for many years, and the interest in proposing for Target of Opportunity investigations continues an increasing trend seen in the last five years. Target of Opportunity investigations are the most oversubscribed and competitive aspect of the NuSTAR GO program, mirroring the community's focus on time-domain astrophysics. These proposals will be peer-reviewed by independent panels of experts in April, with recommendations for selection submitted by the end of that month, in time for cycle-11 observations to begin on June 1st this year.

Authors: Javier Garcia (Senior Scientist, GSFC), Karl Forster (NuSTAR Science Operations Lead, Caltech)