tokens: in the range 0.4%–0.5%. For the dwells with mHz detections, the symbol size is proportional to the fractional amplitude (rms). 3. Properties of mHz Oscillations Since the power spectra (Figure 3 ) from the two 2017 September dwells with strong mHz oscillations are consistent with rather coherent oscillations during these intervals, we computed phases for all events in each of these dwells separately using the frequencies corresponding to the peak Fourier power obtained from each dwell. To combine the two dwells we first found a constant phase offset for the second dwell that maximized the signal power of the sum of both dwells (Buccheri et al. 1983 ). The resulting phase-folded pulse profile in the 0.4–7.5 keV band is shown in Figure 8 . We fit a model of the form and find an amplitude B / A = 1.12 ± 0.05%. The fit is good with a minimum χ 2 = 11.0 for 13 degrees of freedom, and the best-fitting model is also plotted in Figure 8 (red curve). To explore the energy dependence we also computed profiles for different energy bands and then fit them with the same model as above. Figure 9 shows the derived amplitudes, B / A , and phases, φ 0 , versus energy. The amplitude shows a modest increase from about 0.7% at 1 keV to ≈2% at 7 keV. There is no strong evidence for a variation in φ 0 with energy. Figure 8. Average, phase-folded mHz oscillation profile from the two NICER dwells shown in Figure 2 . The profile includes all events in the 0.4–7.5 keV band. The best-fitting model, , is also shown (red curve). Phase ranges used to extract spectra are indicated by the vertical lines. See Section 3 for additional discussion. Figure 9. Fractional pulsed amplitude (top) and reference phase, φ 0 (bottom), obtained from fits to the phase-folded mHz oscillation profile are shown as a function of energy. The dashed line in the bottom panel marks the mean reference phase value. This constant value provides an acceptable fit to the distribution of values, with a χ 2 = 6.9 for 5 degrees of freedom. To explore the pulse-phase spectrum we extracted two spectra, from regions around the pulse minimum and maximum. The phase ranges used for these extractions are indicated by the dashed and dashed–dotted vertical lines in Figure 8 , respectively. In each case, we used a range of pulse phase equal to 0.25 cycles, so that each spectrum has an exposure of 0.25 × (2(2200)) = 1100 s. We used XSPEC version 12.9.1 with version 1.02 of the NICER instrument response, and we fit the spectra in the range 0.6–9 keV. We use a model comprising thermal comptonization ( comptt in XSPEC) for the persistent accretion-driven emission and a blackbody ( bbodyrad in XSPEC) to capture the neutron star thermal emission (Thompson et al. 2008 ; Chenevez et al. 2016 ). To model Galactic absorption we employ the tbabs model with abundances from Wilms et al. ( 2000 ), and we fix (freeze) the value of n H = 0.4 × 10 22 cm −2 based on prior measurements, since the source is in the same (soft) state
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