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

Abstract

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$_X$ > 10$^{39}$ erg s$^{-1}$, but in a 2010 observation had faded to L$_X$ ~ 10$^{38}$ erg s$^{-1}$. In our 2011 observations, it has rebrightened to L$_X$ ~ 2x10$^{39}$ erg s$^{-1}$. This significant increase in luminosity over a 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 ionised oxygen (O VIII) when the source is brighter than average.