A NICER look at the Aql X-1 hard state

Abstract

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 $\sim 100$ ms at $0.3$ Hz to $\sim 10$ 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.

arXiv comment7 pages, 4 figures, 2 tables, accepted for publication in ApJL

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• A NICER look at thermonuclear X-ray bursts from Aql X-1
• Dips and eclipses in the X-ray binary Swift J1858.6-0814 observed with NICER
• NICER Detects a Soft X-ray Kilohertz Quasi-periodic Oscillation in 4U 0614+09
• Broadband Spectral and Timing Properties of MAXI J1348-630 using AstroSat and NICER Observations
• On the curious pulsation properties of the accreting millisecond pulsar IGR J17379-3747
• Discovery of Thermonuclear Type-I X-ray Bursts from the X-ray binary MAXI J1807+132
• Testing jet geometries and disk-jet coupling in the neutron star LMXB 4U 0614+091 with the internal shocks model
• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER
• Discovery and Identification of MAXI J1621-501 as a Type I X-ray Burster with a Super-Orbital Period
• Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT
• The stochastic X-ray variability of the accreting millisecond pulsar IGR J17062-6143