NICER / ISS Science Nugget for February 9, 2018

Spectral Timing of Aquila X-1

Low-mass X-ray binaries are systems where a neutron star has a lower-mass companion star orbiting around it. This companion provides a reservoir of matter that steadily flows into the gravitational well of the neutron star, spirals around it as an accretion disc, and finally collects on the neutron star surface.

Accretion disks are bright in X-rays. Their intensity has long been known to vary with energy, revealing the material composition of the disc. But also to vary the with a multitude of periods, revealing geometric structures in the disk. Understanding how, exactly, these spectral and temporal signatures couple is crucial for understanding the dynamics of bulk motion through the highly curved space-time.

NICER, providing excellent capabilities for both timing and spectroscopy, is exceptionally well suited to spectral-timing analysis of the accretion process. Such an analysis of the low-mass X-ray binary system Aquila X-1 is currently underway. Already, this work uncovered a dramatic turnover in the spectral-timing signature: low energies are dominated by slow periodicities coming from the larger radii of the disc, whereas higher energies more evenly probe both small and large radii.

Fractional covarience spectrum of Aquila X-1 in four energy bands
Figure: The fractional covariance spectrum, a measure of the flux variability that is correlated across all energies, is shown for four frequency bands. We see that above 2 keV all frequency bands show increasing variability with energy, a trend that is in line with previous results. Below 2 keV, however, we find that the lowest frequency bands (black & red) diverge from the expected trend: the slow flux variations have increasing large amplitudes toward lower energies.

This result tells us that it is not just the power-law emission at higher energies that is variable, but that the lower energy emission, presumably from the accretion disk, varies too, and dramatically so.

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