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

We report the results of a deep (≈65 ks) Chandra exposure of the globular cluster NGC 6540, obtained by combining three observations carried out in 2023–2024 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 (≈300 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 indicate 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-arc-second precision – and of their optical counterparts, we discuss their possible nature and association with 3XMM J180608.9–274553: only one of these newly detected sources is compatible with the position of the flare. We also discuss a few scenarios – such as microlensing-induced amplification and black hole flaring activity analogous to that observed in Sgr A* – to explain the X-ray flare. Although these scenarios are intriguing, current observational evidence makes them both unlikely to be the origin of the XMM–Newton flare.