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

Giulia Illiano; Alessandro Papitto; Sergio Campana; Alessio Marino; Arianna Miraval Zanon; Francesco Carotenuto; Francesco Coti Zelati; Maria Cristina Baglio; Filippo Ambrosino; Christian Malacaria; Caterina Ballocco; Gaurava K. Jaisawal; Marco M. Messa; Emilie Parent; Thomas D. Russell; Andrea Sanna; Anastasios Tzioumis

Paper

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

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 - well described by an absorbed Comptonization model - we estimated an unabsorbed 0.5

10 keV luminosity of

(assuming a source distance equal to the upper limit of

kpc). To place this in context, we analyzed an archival Chandra observation performed in 2019, which yielded a quiescent luminosity of

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 3

upper limit of 60

Jy beam

on the source flux density at 5.5 GHz from ATCA observations acquired on 2025 April 12, following a decline of 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

. We discuss our findings in the context of other accreting millisecond pulsars and draw comparisons with transitional systems in the sub-luminous disk state.