A new Chandra look at the globular cluster NGC 6540 and its peculiar X-ray flaring source

Abstract

We report the results of a deep ($\approx65$ ks) Chandra observation of the globular cluster NGC 6540, obtained to investigate the nature of the peculiar X-ray source 3XMM J180608.9$-$274553. This source was previously observed with XMM$-$Newton to exhibit a short ($\approx300$ s) and intense X-ray flare whose luminosity and duration are inconsistent with both typical type I X-ray bursts from low mass X-ray binaries and stellar flares. Our new data show the presence of three faint X-ray sources near the position of the flare seen by XMM$-$Newton, only one of which was detected in a previous, much shorter Chandra observation. Based on the properties of these sources, localized at sub-arcsecond precision, and of their optical counterparts, we discuss their possible nature and association with 3XMM J180608.9$-$274553. We also discuss some scenarios to explain the X-ray flare, such as microlensing-induced amplification, black hole flaring activity analogous to that observed from Sgr A$^\ast$. Our results place new constraints on the nature of this unusual transient and highlight the power of high-resolution X-ray observations for disentangling rare phenomena in the dense stellar environments of globular clusters.