Can the 62 Day X-Ray Period of Ulx M82 X-1 Be Due to a Precessing Accretion Disk?

Abstract

We have analyzed all archival Rossi X-Ray Timing Explorer/Proportional Counter Array monitoring observations of the ultraluminous X-ray source M82 X-1 in order to study the properties of its 62 day X-ray period, which was found by Kaaret and Feng in 2007. Based on its high coherence, it has been argued that the observed period is the orbital period of the binary. Utilizing a much longer data set than in previous studies, we find the following. (1) The phase-resolved X-ray (3-15 keV) spectra-modeled with a thermal accretion disk and a power law-suggest that the accretion disk's contribution to the total flux is strongly modulated with phase. (2) Suggestive evidence for a sudden phase shift of approximately 0.4 in phase (25 days) between the first and the second halves of the light curve separated by roughly 1000 days. If confirmed, the implied timescale to change the period is ∼10 yr, which is exceptionally fast for an orbital phenomenon. These two independent pieces of evidence are consistent with the periodicity being due to a precessing accretion disk, similar to the super-orbital periods observed in systems like Her X-1, LMC X-4, and SS433. However, the timing evidence for a change in the period needs to be confirmed with additional observations. This should be possible with further monitoring of M82 with instruments such as the Swift X-Ray Telescope.

A 400-solar-mass black hole in the galaxy M82

“…Moreover, among the plethora of low-frequency quasi-periodic oscillations currently known there is no indication of them preferentially occurring with a 3:2 frequency ratio. Furthermore, the average luminosity of the quasi-periodic oscillations reported here is ≈ 0.03×(the average luminosity of all the sources observed by the PCA in the 3-13 keV band) 21,22 which is ≈ 0.03×5×10 40 ergs s −1 = 1.5×10 39 ergs s −1 . This is comparable to or more than the peak X-ray luminosities of the contaminating sources, except for source 5 33,36 .…”

“…Assuming an rms amplitude of a few percent-similar to that seen in stellar-mass black holes 30 -and using the mean RXTE proportional counter array source and background rates obtained from prior observations 21 , we found that the wealth of publicly available, archival RXTE monitoring data (≈ 1 Megasecond) spread across a timespan of ≈ 6 years would provide a sensitive search for high-frequency quasi-periodic oscillations in M82 X-1. mode.…”

“…2a,b). Combining the average 2-10 keV X-ray luminosity 21,22 of the source of 5×10 40 ergs s −1 with the measured mass suggests that the source is accreting close to the Eddington limit with an accretion efficiency of 0.8±0.2.…”

“…Detailed stellar binary evolution simulations suggest that the long periodicity is best explained by an intermediate-mass black hole with mass in the range of 200-5000 M ⊙ 5,24 . But, a recent study finds that this periodicity may instead be due to a precessing accretion disk in which case a stellar-mass black hole will suffice to explain the apparent long periodicity 21 .…”

A 78 Day X-Ray Period Detected From NGC 5907 Ulx1 by Swift

“…The ∼62 day period observed from the M82 field is well established, generally assumed to arise from M82 X-1 (Kaaret & Feng 2007;Pasham & Strohmayer 2013a; although this origin has recently been questioned; Qiu et al 2015), and a ∼115 day period has been claimed from NGC 5408 X-1 (Pasham & Strohmayer 2013b; although see Grisé et al 2013). Perhaps most famously, the most luminous ULX known to date, ESO 243-49 HLX-1, is seen to outburst every ∼380 days (Godet et al 2014; although recently this behavior has appeared more erratic; Yan et al 2015).…”