A NICER Look at the Aql X-1 Hard State

Peter Bult, Zaven Arzoumanian, Edward M. Cackett, Deepto Chakrabarty, Keith C. Gendreau, Sebastien Guillot, Jeroen Homan, Gaurava K. Jaisawal, Laurens Keek, Steve Kenyon, Frederick K. Lamb, Renee Ludlam, Simin Mahmoodifar, Craig Markwardt, Jon M. Miller, Gregory Prigozhin, Yang Soong, Tod E. Strohmayer, Phil Uttley

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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 ~100 ms at 0.3 Hz 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.
Original languageEnglish
Article numberL1
JournalThe Astrophysical Journal Letters
Issue number1
Number of pages6
Publication statusPublished - 2018


  • Accretion
  • Accretion disks
  • Stars: neutron
  • X-rays: binaries
  • X-rays: individual (Aql X-1)

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