During the last two months, NuSTAR has responded to the unusual behavior of Hercules X-1, one the most enigmatic binary X-ray pulsars, through a series of short observations. High-energy, all-sky monitoring instruments, including NASA's Swift and JAXA's MAXI missions, had observed the X-ray flux of Her X-1 decrease to a very low level, indicating the source had entered a so-called "anomalous low" state. Such a behavior has been observed only a few times previously, with durations ranging from 70 to 602 days. In response, a small group of Her X-1 enthusiasts succeeded in organizing a coordinated observing campaign, triggering proposed Target of Opportunity observations and obtaining Director's Discretionary Time using several satellites, including NASA's NuSTAR, JAXA's XRISM, and ESA's XMM-Newton observatories. The goal is to understand in detail what is physically happening in Her X-1, and initial analysis of the data indicates that the X-ray source has not actually turned off. The accretion of material from the companion star onto the highly magnetized neutron star continues, but the low X-ray energy radiation is now substantially absorbed by the accretion disk itself, not just at certain phases of the generally observed 35-day modulation, but continuously. It may be possible that precessing accretion disk has either changed its inclination with respect to the binary orbital plane, or has swelled up to a substantially larger thickness. Less certain is whether the rotation of the neutron star itself precesses freely with a period very close to that of the accretion disk, though analysis of the NuSTAR data shows that the neutron star pulsation period of 1.24 seconds appears normal. It is possible that the neutron star and accretion precessional movements are generally locked to each other by physical interactions, which might change at times or even temporarily break down. Monitoring of Her X-1 will continue into the next month.

Author: Ruediger Staubert (Professor of Astrophysics, University of Tuebingen, Germany)