NICER observation list: Target ID 658201

The object 4U~1556-60 was discovered by the Uhuru satellite, but remains surprisingly poorly characterized. It shows no Type I bursts nor pulsations, so it is not clear whether its accretor is a black hole or a neutron star. Its optical spectrum shows hydrogen emission lines, but its optical faintness implies an ultracompact X-ray binary nature. The recent estimation of its distance in Gaia DR2 and EDR3 indicate that it is dramatically nearer and less luminous than had previously been assumed. We propose to search for pulsations in the X-ray emission from the source's soft X-ray emission to see if it is a neutron star. We also propose to search for orbital modulations, and make good measurements of the centroid and profile of the iron line from the source to determine how it is produced.