Discovery of a 115 Day Orbital Period in the Ultraluminous X-ray Source NGC 5408 X-1

Abstract

We report the detection of a 115 day periodicity in SWIFT/XRT monitoring data from the ultraluminous X-ray source (ULX) NGC 5408 X-1. Our ongoing campaign samples its X-ray flux approximately twice weekly and has now achieved a temporal baseline of ~485 days. Periodogram analysis reveals a significant periodicity with a period of 115.5 +- 4 days. The modulation is detected with a significance of 3.2 e-4. The fractional modulation amplitude decreases with increasing energy, ranging from 0.13 above 1 keV to 0.24 below 1 keV. The shape of the profile evolves as well, becoming less sharply peaked at higher energies. The periodogram analysis is consistent with a periodic process, however, continued monitoring is required to confirm the coherent nature of the modulation. Spectral analysis indicates that NGC 5408 X-1 can reach 0.3 - 10 keV luminosities of ~2 e40 ergs/s. We suggest that, like the 62 day period of the ULX in M82 (X41.4+60), the periodicity detected in NGC 5408 X-1 represents the orbital period of the black hole binary containing the ULX. If this is true then the secondary can only be a giant or supergiant star.

Accepted for Publication in the Astrophysical Journal Letters

References in corpus (5)

• An Intermediate-mass Black Hole of Over 500 Solar Masses in the Galaxy ESO 243-49
• A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
• Evidence for an Intermediate Mass Black Hole in NGC 5408 X-1
• A ULX microquasar in NGC 5408?
• Confirmation of the 62 Day X-Ray Periodicity from M82

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