  The Wide Field Infrared Survey Telescope (WFIRST) will investigate the
origins of cosmic acceleration using weak gravitational lensing at near
infrared wavelengths. Lensing analyses place strict constraints on the
precision of size and ellipticity measurements of the point spread function.
WFIRST will use infrared detector arrays, which must be fully characterized to
inform data reduction and calibration procedures such that unbiased
cosmological results can be achieved. Hirata & Choi 2019 introduces formalism
to connect the cross-correlation signal of different flat field time samples to
non-linear detector behaviors such as the brighter fatter effect (BFE) and
non-linear inter-pixel capacitance (NL-IPC), and this paper applies that
framework to a WFIRST development detector, SCA 18237. We find a residual
correlation signal after accounting for classical non-linearity. This residual
correlation contains a combination of the BFE and NL-IPC; however, further
tests suggest that the BFE is the dominant mechanism. If interpreted as a pure
BFE, it suggests that the effective area of a pixel is increased by
$(2.87\pm0.03)\times 10^{-7}$ (stat.) for every electron in the 4 nearest
neighbors, with a rapid $\sim r^{-5.6\pm0.2}$ fall-off of the effect for more
distant neighbors. We show that the IPC inferred from hot pixels contains the
same large-scale spatial variations as the IPC inferred from auto-correlations,
albeit with an overall offset of $\sim 0.06\%$. The NL-IPC inferred from hot
pixels is too small to explain the cross-correlation measurement, further
supporting the BFE hypothesis. This work presents the first evidence for the
BFE in an H4RG-10 detector, demonstrates some of the useful insights that can
be gleaned from flat field statistics, and represents a significant step
towards calibration of WFIRST data.
