Detection of low-luminosity X-ray pulsations from the accreting millisecond pulsar IGR J17511−3057. An ever-thinning thread between bright accretion and subluminous states

After nearly a decade in quiescence, the accreting millisecond pulsar IGR J17511−3057 displayed a new outburst on 2025 February 11, its third since discovery, following previous activity in 2009 and 2015. We report on an XMM-Newton Target of Opportunity observation performed on 2025 March 4, more than twenty days after the outburst onset. From the X-ray spectrum-which is well described by an absorbed Comptonization model-we estimated an unabsorbed 0.5–10 keV luminosity of LX ∼ 7 × 1033 erg s−1 (assuming a source distance equal to the upper limit of 6.9 kpc). To put this into context, we analyzed an archival Chandra observation performed in 2019, which yielded a quiescent luminosity of LX, q ∼ 2 × 1032 erg s−1 in the same energy band. Although this comparison indicates that the source was still well above its quiescent level during the XMM-Newton observation, the estimated low luminosity during the late stage of the 2025 outburst would typically place the source in the propeller regime. Nevertheless, we unexpectedly detected coherent X-ray pulsations with an amplitude peaking at ∼42% in the 0.3–3 keV band. We also observed a spectral softening compared to the early stages of the outburst. Finally, we report a 3σ upper limit of 60 μJy beam−1 on the source flux density at 5.5 GHz from ATCA observations acquired on 2025 April 12, following a decline in the accretion activity, as indicated by our analysis of NICER data from 2025 March 15, which revealed no significant X-ray pulsations at a luminosity level of LX ∼ 1 × 1034 erg s−1. We discuss our findings in the context of other accreting millisecond pulsars and draw comparisons with transitional systems in the subluminous disk state.