NICER / ISS Science Nugget
for April 11, 2024

NICER at HEAD 21

The High-Energy Astrophysics Division (HEAD) of the American Astronomical Society is holding its 21st meeting this week in Horseshoe Bay, TX - timed and located to coincide with the total solar eclipse on Monday, April 8. NICER was well represented among oral and poster (see Figure) contributions from U.S. and international attendees, especially early-career scientists.

• M. McCullough (Harvard & Smithsonian CfA) described analysis of time-resolved NICER X-ray spectral measurements for the high-mass black-hole binary Cygnus X-3 in the context of its emissions from radio waves through gamma rays, which exhibit complex variations with the system's 4.5-hour orbital period. N. Ford (McGill Univ.) reported on NICER observations of three X-ray-faint galaxies that are being targeted by the Event Horizon Telescope (EHT) collaboration, responsible for the iconic "ring" images of the black holes in the center of the M87 galaxy and our own Milky Way. As EHT extends its efforts to new targets and to producing "movies" of the accretion and jet emission processes around black holes, coordinated X-ray monitoring will be important for pinpointing the underlying physical mechanisms.
• T. Sextro (Embry-Riddle Aero. Univ.) explore deep fluctuations, revealed by NICER spectra, in the flow of matter between the neutron star and its low-mass companion in the binary system known as GX 1+4. Some of the mass lost in a dusty wind from the companion is accreted by the neutron star, emitting X-rays, while much of the non-accreted matter causes variable obscuration of the accretion stream, providing diagnostics of the physical and chemical properties of the wind.
• J. Chakraborty (MIT) described ongoing tests of a leading model for the phenomenon known as quasi-periodic eruptions (QPEs), seen as brief X-ray flashes from distant galaxies that recur on hours-to-days timescales. A fascinating explanation invokes so-called extreme mass-ratio inspirals (EMRIs) - a stellar-mass object orbiting a supermassive black hole and interacting periodically with the accretion disk around it; EMRIs are a highly anticipated class of targets for the future space-based gravitational-wave mission known as LISA. While several of the X-ray properties and behaviors of QPEs are accounted for by the EMRI model, others fit less well, but a concerted investigation with NICER promises a robust test of the hypothesis.