The 2025 outburst of IGR J17511-3057: timing and spectral insights from NICER and NuSTAR

TL;DR

The 2025 outburst of IGR J17511-3057 was studied with NICER and NuSTAR to map timing, spectral, and long-term evolution. The timing analysis reveals coherent pulsations near 245 Hz and updated ephemerides, including a secular spin-down of about $\dot{\nu}_{sd} \approx -2.3\times 10^{-15}$ Hz s$^{-1}$ and a fast orbital contraction with $\dot{P}_{orb} \approx -2.6\times 10^{-11}$ s s$^{-1}$, challenging purely gravitational-wave-driven evolution. The spectrum is hard-dominated with a Comptonized continuum and a thermal component, along with weak Fe Kα features; reflection modelling indicates a moderately ionised, disc-truncated inner edge at $R_{in} \sim (82-370)$ km and an inclination $i \gtrsim 37^{\circ}$. A NuSTAR burst shows no photospheric radius expansion but exhibits burst oscillations at the spin frequency, with a peak blackbody temperature near 3 keV and a fluence of about $5.1\times10^{-7}$ erg cm$^{-2}$. Collectively, these results imply intricate angular momentum exchange involving accretion-driven spin-up during outbursts and rotation-powered spin-down in quiescence, along with an orbit evolving on relatively short timescales; further outbursts are needed to confirm the orbital evolution and test non-conservative mass-transfer scenarios.

Abstract

IGR J17511-3057 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 characterise its current status and highlight possible long-term evolution of its properties. We analysed 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 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. We detected X-ray pulsations at a frequency of around 245 Hz. The long-term evolution of the neutron star ephemerides suggests a spin-down derivative of about -2.3e-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 Comptonisation 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 K-alpha transition of neutral or He-like Fe originating from the inner region of the accretion disc. Self-consistent reflection models confirmed a moderate ionisation of the disc 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.

Figures (11)

• Figure 1: Top panel - NICER 0.3–10 keV light curve of IGR J17511$-$3057 during the latest outburst starting from February 11, 2025 (MJD 60717.7). The count rate is estimated by collecting 50 s of exposure time. Middle panel - Temporal evolution of the fractional amplitude of the sinusoidal component used to model the source pulse profiles. Bottom panel - Pulse phase residuals in units of phase cycles relative to the best-fitting solution.
• Figure 2: Top panel - NuSTAR Background-subtracted light curve of the latest outburst of IGR J17511$-$3057 as observed combining data collected by NuSTAR FPMA and FPMB (ObsID. 91101304002) starting from February 19, 2025 (MJD 60725.6). A type-I X-ray burst is detected at around MJD 60725.98. Middle panel - Temporal evolution of the fractional amplitude of the sinusoidal component used to model the source pulse profiles. Bottom panel - Pulse phase residuals in units of phase cycles relative to the best-fitting solution.
• Figure 3: NICER 0.3-10 keV average pulse profiles (top panel) and 3-40 keV average pulse profiles (bottom panel) of IGR J17511$-$3057 generated by folding at the best-fit timing solution reported in Table \ref{['tab:solution']}. For both profiles, the best-fitting models (red solid lines) are the superposition of up to three harmonically related sinusoidal components represented in light blue, green, and purple from smaller to higher order, respectively. Two cycles of the pulse profile are shown for clarity.
• Figure 4: Evolution of the NICER and NuSTAR pulse profiles of IGR J17511$-$3057 as a function of energy. Pulse profiles are generated by folding at different energy ranges after correcting the photon time-of-arrival for the most updated binary ephemerides reported in Table \ref{['tab:solution']}. The best-fitting models (red solid lines) are the superposition of up to two harmonically related sinusoidal components. Two cycles of the pulse profile are shown for clarity. Colour coding follows the convention described in Fig. \ref{['fig:average_prof']}.
• Figure 5: The 0.4-10 keV NICER spectrum fitted with an absorbed blackbody with a Comptonization model with two Gaussian emission line components (top panel). The corresponding spectral residuals are shown in the bottom panel.