NICER observation list: Target ID 261601

The unique ultracompact X-ray binary (UCXB) 4U 1626-67 is the only known UCXB hosting a strongly-magnetized, slowly-rotating accreting neutron star. This system has a plethora of interesting properties, with a short (~42 min) orbital period, strong and disk-broadened emission lines of neon and oxygen, an unusually weak and possibly variable iron fluorescence line, a cyclotron resonance scattering feature (CRSF), and variability on short and long timescales. The X-ray variability in particular is essential both for proper interpretation of the spectral lines and for constraining the binary orbit. We are requesting a 40 ks NICER observation, which will give us the opportunity to carry out the most detailed study of variability to date in this source.