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

¹ INAF–Osservatorio Astronomico di Brera Via Bianchi 46 I-23807 Merate (LC), Italy
² INAF-Osservatorio Astronomico di Roma Via Frascati 33 I-00078 Monte Porzio Catone (RM), Italy
³ Institute of Space Sciences (ICE, CSIC), Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC) E-08860 Castelldefels (Barcelona), Spain
⁵ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Via U. La Malfa 153 I-90146 Palermo, Italy
⁶ ASI – Agenzia Spaziale Italiana Via del Politecnico snc 00133 Roma, Italy
⁷ DTU Space, Technical University of Denmark Elektrovej 327-328 DK-2800 Lyngby, Denmark
⁸ Dipartimento di Fisica, Università degli Studi di Milano Via Celoria 16 I-20133 Milan, Italy
⁹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu KM 0.7 Monserrato 09042, Italy
¹⁰ Australia Telescope National Facility, CSIRO PO Box 76 Epping New South Wales 1710, Australia

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Received: 30 June 2025
Accepted: 8 December 2025

Abstract

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 L_X ∼ 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 L_X, 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 L_X ∼ 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.