Flashing fast: Characterising the 2025 outburst of MAXI J1957+032

Context. MAXI J1957+032 is an accreting millisecond X-ray pulsar that shows brief, recurrent outbursts within an ultra-compact ≈1 h orbit. Aims. We characterised the X-ray timings and spectral and optical properties during the 2025 outburst and measured the long-term spin evolution relative to its previous 2022 outburst. Methods. We analysed X-ray observations from XMM-Newton, Swift, and NuSTAR, together with contemporaneous optical photometry obtained with LCO during the 2025 outburst. X-ray timing analysis included standard epoch-folding and coherent searches, while energy-resolved pulse profiles were studied through harmonic decomposition. Spectral fits used absorbed thermal–Comptonisation models complemented with a soft blackbody component, whose emission radius suggests that the blackbody emission originates from a hotspot on the neutron star surface. Results. Coherent pulsations were detected at ν ≈ 313.6 Hz, with no measurable frequency derivative within the XMM-Newton exposure. Via comparisons with the 2022 outburst, we find a long-term spin-down of ⟨ν˙⟩ ∼ −2 × 10−14 Hz s−1, consistent with magnetic dipole braking during quiescence. The pulse shape is almost sinusoidal, showing significant power at the fundamental, second, and fifth harmonics. The fractional amplitude decreases with increasing flux and exhibits soft lags extending to a few keV. The X-ray spectrum between 0.5 and 10 keV is well reproduced by a thermal–Comptonised continuum with photon index Γ ≈ 2.4, plus a cool blackbody with kT ≈ 0.23 keV. No reflection or Fe-line features are detected. Assuming Rm ≲ Rco, the magnetic field is limited to Bs ≈ (0.5–3)×108 G (for d = (5 ± 2) kpc and truncation factor ξ = 0.3–0.5), lower than the upper limit implied by the secular spin-down (Bp ≲ 109 G), possibly indicating a mildly leaky propeller regime. The optical and infrared (OIR) emission follows the neutron-star branch of the LOIR–LX relation, consistent with X-ray reprocessing in a compact accretion disc. The optical spectral energy distributions are broadly flat, supporting irradiation-dominated disc emission, and an early red excess suggests a jet contribution during the initial hard X-ray phase. A delayed optical peak relative to the X-rays may reflect the outward propagation of a heating front through the disc, consistent with rapid disc evolution in short-lived outbursts.