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arXiv · 2606.01792

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

A. Sacchi·S. Mereghetti·R. Di Stefano·J. A. Irwin·M. Rigoselli·A. De Luca·N. Sims
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Abstract

We report the results of a deep ($\approx65$ 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 ($\approx300$ 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 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$^\ast$, 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.

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A. Sacchi, S. Mereghetti, R. Di Stefano, J. A. Irwin, M. Rigoselli, A. De Luca, N. Sims. 2026-06-01. A new Chandra look at the globular cluster NGC 6540 and its peculiar X-ray flaring source. https://arxiv.org/abs/2606.01792

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