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

TL;DR

This study presents deep Chandra observations of the first globular cluster black hole, XMMU 122939.7+075333, revealing short-term variability, spectral features, and challenging previous flux modulation hypotheses.

Contribution

It provides new insights into the variability and spectral properties of a globular cluster black hole, including evidence of ionized oxygen emission and short-term flux changes.

Findings

Significant luminosity increase from 2010 to 2011

Detection of soft X-ray excess during brighter states

Evidence of highly ionized oxygen emission

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.