Averaged power spectrum of 4U 1556–60 of NICER data from stingray. On the left, the PSD shows a very low frequency noise component that remains unbroken to at least near 10⁻³ Hz. This is a very low frequency constraint on the break frequency value compared to other XRBs, an indication of a low intrinsic X-ray luminosity. On the right, the fractional power of 10⁻² is also consistent with low accretion rate hard states in other XRBs, and is much higher than during soft states.