The 2025 outburst of IGR J17511−3057: Timing and spectral insights from NICER and NuSTAR

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy
² DTU Space, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark
³ Astrophysics Science Division and Joint Space-Science Institute, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
⁵ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
⁶ Dipartimento di Fisica e Chimica – Emilio Segrè, Università di Palermo, Via Archirafi 36 – 90123, Palermo, Italy
⁷ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy
⁸ Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA
⁹ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, 8800 Greenbelt Road, Greenbelt, MD, USA
¹⁰ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹¹ Department of Physics, McGill University, 3600 rue University, Montréal QC H3A 2T8, Canada
¹² Trottier Space Institute at McGill University, 3550 rue University, Montréal QC H3A 2A7, Canada
¹³ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁴ Istanbul University, Graduate School of Sciences, Department of Astronomy and Space Sciences, Beyazıt, 34119 İstanbul, Türkiye

^⋆ Corresponding author: andrea.sanna@dsf.unica.it

Received: 30 May 2025
Accepted: 18 September 2025

Abstract

Context. IGR J17511−3057 is an accreting millisecond X-ray pulsar and a known type-I burster. The source was observed in outburst for the first time in 2009 and again in 2015, followed by a decade-long quiescence phase.

Aims. The source was observed in a new outburst phase starting in February 2025 and lasting at least nine days. We investigated the spectral and temporal properties of IGR J17511−3057, aiming to characterize its current status and highlight possible long-term evolution of its properties.

Methods. We analyzed the available NICER and NuSTAR observations performed during the latest outburst of the source. We updated the ephemerides of the neutron star and compared them to previous outbursts to investigate its long-term evolution. We also performed a spectral analysis of the broadband energy spectrum in different outburst phases and investigated the time-resolved spectrum of the type-I X-ray burst event observed with NuSTAR.

Results. We detected X-ray pulsations at the frequency of ∼245 Hz. The long-term evolution of the neutron star ephemerides suggests a spin-down derivative of ∼ − 2.3 × 10⁻¹⁵ Hz/s, compatible with a rotation-powered phase while in quiescence. Moreover, the evolution of the orbital period and the time of the ascending node suggests a fast orbital shrinkage, which challenges the standard evolution scenario for this class of pulsars involving angular momentum loss via gravitational wave emission. The spectral analysis revealed a dominant power law-like Comptonization component, along with a thermal blackbody component, consistent with a hard state. Weak broad emission residuals around 6.6 keV suggest the presence of a Kα transition of neutral or He-like Fe originating from the inner region of the accretion disk. A set of self-consistent reflection models confirmed the moderate ionization of the disk truncated at around (82–370) km from the neutron star. Finally, the study of the type-I X-ray burst revealed no signature of photospheric radius expansion. We found marginally significant burst oscillations during the rise and decay of the event, consistent with the neutron star spin frequency.