NICER / ISS Science Nugget for June 1, 2018

NICER Detects a Soft X-ray kilohertz QPO in 4U 0614+09

A signature feature of accreting neutron stars, kilohertz Quasi-Periodic Oscillations (QPOs) are the fastest phenomena observed in X-ray binaries. Based on their millisecond periods it has long been argued that these signals must somehow be related to the innermost regions of the accretion disk, where matter is orbiting through strongly curved space-time only a few kilometers from the neutron star surface. Many competing theories have been proposed to explain these kHz QPOs, but lacking strong observational constraints the correct interpretation has remained elusive.

For the first time we were able to measure the amplitude of this QPO at photon energies < 2 keV (See figure). This is an important regime where many models make diverging predictions. With this measurement we can now rule out a whole class of models and provide new input in a long standing debate.

This discovery has been accepted in a paper as NICER's 7th peer reviewed paper since launch: Bult et al., Astrophysical Journal Letters, 2018.