2015MNRAS.447.1460J

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, but in a 2010 observation had faded to LX ∼ 10³⁸ erg/s. In our 2011 observations, it has rebrightened to LX ∼ 2x10³⁹ erg/s. 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 (<0.7keV) photons. The spectral analysis reveals strong, albeit model-dependent, evidence of emission from highly ionized oxygen (Oviii) when the source is brighter than average.