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

TL;DR

This paper characterizes the 2025 outburst of the accreting millisecond X-ray pulsar MAXI J1957+032 using multi-wavelength observations, revealing its spin behavior, spectral properties, magnetic field estimates, and optical emission characteristics.

Contribution

It provides the first detailed timing and spectral analysis of the 2025 outburst, including spin-down measurement, spectral modeling, and optical/IR observations, advancing understanding of this ultra-compact pulsar's behavior.

Findings

Detected coherent pulsations at ~313.6 Hz with no frequency derivative.

Measured a long-term spin-down rate of ~-2x10^-14 Hz s^-1.

Estimated the magnetic field strength to be (0.5-3)x10^8 G.

Abstract

MAXI J1957+032 is an accreting millisecond X-ray pulsar that shows brief, recurrent outbursts in an ultra-compact ~1 h orbit. We characterise the 2025 outburst using X-ray timing and spectroscopy from XMM-Newton and Swift (and a late-time NuSTAR observation), together with contemporaneous optical photometry from LCO, and compare the spin frequency with the 2022 outburst. Timing searches detect coherent pulsations at ~313.6 Hz with no measurable frequency derivative during the XMM-Newton exposure. Relative to its 2022 outburst, we measure a long-term spin-down of ~-2x10^-14 Hz s^-1, consistent with magnetic-dipole braking in quiescence. The pulse profile is nearly sinusoidal, with significant power at the fundamental, second, and fifth harmonics; the fractional amplitude decreases with increasing flux and shows soft lags up to a few keV. The 0.5-10 keV spectrum is well described by absorbed thermal Comptonisation (photon index ~2.4) plus a cool blackbody (kT ~0.23 keV) consistent with emission from a surface hotspot; no reflection or Fe-line features are detected. Requiring R_m \leq R_co implies B_s ~(0.5-3)x10^8 G for d=(5 \pm 2) kpc and {\xi}=0.3-0.5, below the upper limit from the secular spin-down (B_p \leq 10^9 G), possibly indicating a mildly leaky propeller. The optical emission lies on the neutron-star branch of the L_OIR-L_X relation, consistent with reprocessing in a compact disc. The optical SEDs are broadly flat, while an early red excess suggests a transient jet contribution during the initial hard X-ray phase; an optical peak delayed relative to the X-rays may trace an outward-propagating heating front and rapid disc evolution in these short-lived outbursts.