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

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 L_X > 10³⁹ erg s^-1, but in a 2010 observation had faded to L_X ∼ 10³⁸ erg s^-1. In our 2011 observations, it has rebrightened to L_X ∼ 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.