Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 859, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/2041-8213/aac2e2
Keywords
accretion, accretion disks; stars: neutron; X-rays: binaries; X-rays: individual (Aql X-1)
Categories
Funding
- NASA through the NICER mission
- Astrophysics Explorers Program
- NPP fellowship at NASA Goddard Space Flight Center
- National Science Foundation [AST-1351222]
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We report on a spectral-timing analysis of the neutron star low-mass X-ray binary (LMXB) Aql X-1 with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS). 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 a (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 25% 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 similar to 100 ms at 0.3 Hz to similar to 10 ms at 3 Hz. Our results demonstrate that the thermal emission in the hard state is intrinsically variable, and is driving the modulation of the higher energy power-law. Interpreting the thermal spectrum as disk emission, we find that our results are consistent with the disk propagation model proposed for accretion onto black holes.
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