NICER / ISS Science Nugget for May 10, 2018

A NICER look at the Aql X-1 hard state

NICER's 5th peer-reviewed paper post-launch was accepted for publication in the Astrophysical Journal Letters this past week. Its title is "A NICER look at the Aql X-1 hard state" by Bult et al. Its abstract is below:

In this paper we report on a spectral-timing analysis of the neutron star low-mass X-ray binary Aql X-1. We find that the slow variability (black & red data in the figure) show an unexpected excess in amplitude below 2 keV. By measuring time delays as a function of energy we found that low energy photons (0.5 keV) systematically arrive earlier than high energy photons (10 keV). These measurements demonstrate that the thermal emission from the disk is intrinsically variable, and drives the slow modulation of the higher energy power-law. These results are consistent with the disk propagation model proposed for accretion onto black holes, and now confirmed in a neutron star system thanks to NICER's excellent sensitivity below 2 keV.