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 rst black hole discovered in a globular cluster. The source has been detected many times, typically with LX > 10 39 erg s 1 , but in a 2010 observation had faded to LX 10 38 erg s 1 . In our 2011 observations, it has rebrightened to LX 2 10 39 erg s 1 . This signicant increase in luminosity over a a relatively short time period is not consistent with the idea that the long term ux 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 nd 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 ionised oxygen (Oviii) when the source is brighter than average.