Joseph, T. D., Maccarone, T. J., Kraft, R. P., and Sivakoff, G. R. 2015. "Deep Chandra observations of the NGC 4472 globular cluster black hole XMMU 122939.7 075333: short-term variability from the first globular cluster black hole binary." Monthly Notices of the Royal Astronomical Society 447:1460-1470. https://doi.org/10.1093/mnras/stu2523
In this paper, we discuss the luminosity modulations and spectral analysis results of the recent deep observations of XMMU 122939.7 075333, the first black hole discovered in a globular cluster. The source has been detected many times, typically with LX > 1039 erg s-1, but in a 2010 observation had faded to LX ˜ 1038 erg s-1. In our 2011 observations, it has rebrightened to LX ˜ 2 × 1039 erg s-1. This significant increase in luminosity over a relatively short time period is not consistent with the idea that the long-term flux modulations displayed by XMMU 122939.7 075333 are caused by the Kozai mechanism alone as had previously been suggested. Instead, given that the source shows `spiky' behaviour in its light curve, it seems likely that the faintness in 2010 was a result of a short observation that did not catch any bright epochs. We also find that when the source is brighter than average, it has an excess of soft (<0.7 keV) photons. The spectral analysis reveals strong, albeit model-dependent, evidence of emission from highly ionized oxygen (O VIII) when the source is brighter than average.