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 $L_X\sim 7\times 10^{33}\mathrm{erg}{\mathrm{s}}^{-1}$ (assuming a source distance equal to the upper limit of $6.9$ kpc). To place this in context, we analyzed an archival Chandra observation performed in 2019, which yielded a quiescent luminosity of $L_{\mathrm{X},\mathrm{q}}\sim 2\times 10^{32}\mathrm{erg}{\mathrm{s}}^{-1}$ 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 $\sim$42% in the 0.3$-$3 keV band. We also observed a spectral softening compared to the early stages of the outburst. Finally, we report a 3$\sigma$ upper limit of 60 $\mu$Jy beam${}^{-1}$ 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 $L_X\sim 1\times 10^{34}\mathrm{erg}{\mathrm{s}}^{-1}$. We discuss our findings in the context of other accreting millisecond pulsars and draw comparisons with transitional systems in the sub-luminous disk state.

Accepted for publication in Astronomy & Astrophysics

References in corpus (30)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• A Radio Pulsar/X-ray Binary Link
• Spectral and temporal properties of Compton scattering by mildly relativistic thermal electrons
• Pulse profiles of millisecond pulsars and their Fourier amplitudes
• The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
• X-ray states of redback millisecond pulsars
• A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658
• Identification of the High-Energy Gamma-Ray Source 3FGL J1544.6-1125 as a Transitional Millisecond Pulsar Binary in an Accreting State
• Transitional millisecond pulsars
• Disk-jet coupling in low-luminosity accreting neutron stars
• NuSTAR Observations of the State Transition of Millisecond Pulsar Binary PSR J1023+0038
• On the timing properties of SAX J1808.4-3658 during its 2015 outburst
• Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17511-3057
• Prolonged sub-luminous state of the new transitional pulsar candidate CXOU J110926.4-650224
• Timing the pulsations of the accreting millisecond pulsar SAX J1808.4-3658 during its 2019 outburst
• A New Candidate Transitional Millisecond Pulsar in the Sub-luminous Disk State: 4FGL J0407.7--5702
• Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038
• Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4-2807 with NICER, XMM-Newton and NuSTAR
• Investigating the origin of optical and X-ray pulsations of the transitional millisecond pulsar PSR J1023+0038
• Timing analysis of the 2022 outburst of the accreting millisecond X-ray pulsar SAX J1808.4$-$3658: hints of an orbital shrinking
• Spectral Softening Between Outburst and Quiescence In The Neutron Star Low-Mass X-Ray Binary SAX J1750.8-2900
• UV and X-ray pulse amplitude variability in the transitional millisecond pulsar PSR J1023+0038
• The 2015 outburst of the accreting millisecond pulsar IGR J17511-3057 as seen by INTEGRAL, Swift and XMM-Newton
• NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270-4859 in the rotation-powered state
• On the curious pulsation properties of the accreting millisecond pulsar IGR J17379-3747
• On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807
• Quasi-Simultaneous Radio/X-ray Observations of the Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125 During its Low-Luminosity Accretion-Disc State
• Multiwavelength Evidence for Two New Candidate Transitional Millisecond Pulsars in the Sub-luminous Disk State: 4FGL J0639.1--8009 and 4FGL J1824.2+1231
• Polarized multiwavelength emission from pulsar wind - accretion disk interaction in a transitional millisecond pulsar
• Probing multi-band variability and mode switching in the candidate transitional millisecond pulsar 3FGL J1544.6-1125