A NICER look at the Aql X-1 hard state

Kavli Affiliate: Gregory Prigozhin

| First 5 Authors: Peter Bult, Zaven Arzoumanian, Edward M. Cackett, Deepto Chakrabarty, Keith C. Gendreau

| Summary:

We report on a spectral-timing analysis of the neutron star low-mass X-ray
binary Aql~X-1 with the Neutron Star Interior Composition Explorer (NICER) on
the International Space Station. Aql~X-1 was observed with NICER during a dim
outburst in 2017 July, collecting approximately $50$ ks of good exposure. The
spectral and timing properties of the source correspond to that of an (hard)
extreme island state in the atoll classification. We find that the fractional
amplitude of the low frequency ($<0.3$ Hz) band-limited noise shows a dramatic
turnover as a function of energy: it peaks at 0.5 keV with nearly 5% rms,
drops to $12%$ rms at 2 keV, and rises to $15%$ rms at 10 keV. Through the
analysis of covariance spectra, we demonstrate that band-limited noise exists
in both the soft thermal emission and the power-law emission. Additionally, we
measure hard time lags, indicating the thermal emission at $0.5$ keV leads the
power-law emission at 10 keV on a timescale of $sim100$ ms at $0.3$ Hz to
$sim10$ ms at $3$ Hz. Our results demonstrate that the thermal emission in the
hard state is intrinsically variable, and driving the modulation of the higher
energy power-law. Interpreting the thermal spectrum as disk emission, we find
our results are consistent with the disk propagation model proposed for
accretion onto black holes.

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