2015ApJ...811L..11P

Following the discovery of 3:2 resonance quasi-periodic oscillations (QPOs) in M82X-1, we have constructed power density spectra (PDS) of all 15 (sufficiently long) XMM-Newton observations of the ultraluminous X-ray source NGC 1313 X-1 (L_X≈ 2x10⁴⁰ erg/s). We detect a strong QPO at a frequency of 0.29±0.01 Hz in data obtained on 2012 December 16. Subsequent searching of all the remaining observations for a 3:2/2:3 frequency pair revealed a feature at 0.46±0.02 Hz on 2003 December 13 (frequency ratio of 1.59±0.09). The global significance of the 0.29 Hz feature considering all frequencies between 0.1 and 4 Hz is >3.5σ. The significance of the 0.46±0.02 Hz QPO is >3.5σ for a search at 2/3 and 3/2 of 0.29 Hz. We also detect lower-frequency QPOs (32.9±2.6 and 79.7±1.2 mHz). All the QPOs are superimposed on a continuum consisting of flat-topped, band-limited noise, breaking into a power law at a frequency of 16±3 mHz and white noise at ≳0.1 Hz. NGC 1313 X-1's PDS is analogous to stellar-mass black holes' (StMBHs) PDS in the so-called steep power-law state, but with the respective frequencies (both QPOs and break frequencies) scaled down by a factor of ∼1000. Using the inverse mass-to-high-frequency QPO scaling of StMBHs, we estimate NGC 1313 X-1's black hole mass to be 5000±1300 M_☉, consistent with an inference from the scaling of the break frequency. However, the implied Eddington ratio, L_Edd> 0.03±0.01, is significantly lower compared to that of StMBHs in the steep power-law state (L_Edd≳ 0.2).