Hubble Space Telescope Monitoring Reveals a 6.1 Day Period for an Ultraluminous X-Ray Source in NGC 1313

Ultraluminous X-ray sources (ULXs) in nearby galaxies have X-ray luminosities of 10³⁹-10⁴¹ erg s^-1, and are possibly intermediate mass black holes of 10²-10⁵ M _sun. NGC 1313 X-2 is an extreme ULX with a maximum luminosity of 3 × 10⁴⁰ erg s^-1, and a frequent target of X-ray and optical observations. Here we report our Hubble Space Telescope Guest Observer (GO) program to monitor its optical counterpart in a search of its period. The 20 monitoring observations over three weeks exhibited periodic modulations at the 6σ level with a period of P = 6.12 ± 0.16 days and an amplitude of A = 0.102 ± 0.016 mag. The periodicity is understood as from the orbital motion of the gravitationally distorted secondary that is unevenly irradiated by the X-ray radiation from the primary accretion disk. This is the first photometric period ever discovered for a ULX counterpart. The periodicity rules out a radiation mechanism through relativistic beaming effects. Combined with the photometry, the periodicity suggests that the secondary is most likely a giant branch star with a mass of 11.3-21.0 M _sun and a radius of 14.4-18.3R _sun.