The 62 Day X-ray Period of the Ultra-luminous X-ray Source M82 X-1 is Likely Super-orbital
Abstract
We have analyzed all archival RXTE/PCA monitoring observations of the intermediate-mass black hole (IMBH, mass range of a few 100-1000 solar masses) candidate and ultraluminous X-ray source (ULX) M82 X-1 in order to study the properties of its previously discovered 62 day X-ray period. The high average X-ray luminosity of M82 X-1 (roughly 5 x 10^40 ergs/sec) suggests that the compact source is very likely a black hole. Given that there are no known black hole binaries with super-orbital periods as short as 62 days (the shortest known black hole super-orbital period is a factor of 3 longer), it has been argued that the observed period is the orbital period of the binary. Using approximately 2000 days of RXTE/PCA monitoring data obtained with an observing cadence of approximately once every 3 days, we studied the stability of this period. We find clear evidence for phase evolution of the light curve that can be modeled with a period derivative of -0.009 sec/sec. This value indicates that the 62 day X-ray period's phase is changing with a timescale (period/period derivative) of ~ 20 years. Such a value is unusually fast for any known orbital evolution phenomenon. Similar, rapidly evolving periods have been previously reported from some accreting compact binaries and have been ascribed to a precessing accretion disk. Furthermore, we extracted the average phase-resolved X-ray energy spectra of the source (3-15 keV) and fit them with a model consisting of a standard thermal accretion disk and a power-law. We find that it is the disk's contribution to the total flux that is responsible for the periodic modulation while the power-law flux remains constant with phase. This is also consistent with a precessing accretion disk as the observed variations in X-ray intensity in this model can be ascribed to changes in the projected area of the accretion disk. We argue that these two lines of evidence suggest that the previously reported 62 day X-ray period may be super-orbital in nature.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1312653R