Discovery of a New Spectral Transition in Swift J0243.6+6124 in the Sub-Eddington Regime

We conduct a detailed spectral analysis of the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 in its sub-Eddington regime, using Insight-HXMT and NICER observations during multiple outbursts including the 2018 giant outburst. We discover a new transition at $L_{\rm t} \approx 4.5 \times 10^{37}\ {\rm erg\ s^{-1}}$, accompanied by systematic evolution of spectral parameters, in particular a significant turnover in the blackbody normalization. This transition luminosity in the sub-Eddington regime represents the fifth transition identified so far in Swift J0243.6+6124, further highlighting the complexity of its accretion-powered emission. We interpret the transition in terms of a multipolar magnetic-field configuration, where weak ($\sim 2.8 \times 10^{12}\ {\rm G}$) and strong ($\sim 1.6 \times 10^{13}\ {\rm G}$) magnetic poles dominate the emission at different accretion rates. On the magnetospheric scale, this configuration is equivalent to an effective dipole field of $\sim 6.6 \times 10^{12}\ {\rm G}$, while allowing the local surface field to exceed $10^{13}\ {\rm G}$.

💡 Research Summary

The authors present a comprehensive spectral study of the Galactic ultraluminous X‑ray pulsar Swift J0243‑6124 focusing on its sub‑Eddington accretion regime (luminosities between ~10³⁶ and 10³⁸ erg s⁻¹). Using a large set of observations from Insight‑HXMT (LE, ME, HE instruments) and NICER, they cover the decay phases of three outbursts: the 2018 giant outburst and two normal outbursts in 2019 and 2023. Data were reduced with standard pipelines, filtered for good time intervals, and combined on a daily basis to improve signal‑to‑noise at low fluxes. Spectra were modeled in XSPEC with the absorbed blackbody plus cutoff power‑law (tbabs × (bbodyrad + cutoffpl)) and cross‑calibration constants. Parameter uncertainties were derived via Markov Chain Monte Carlo (20 000 iterations, 40 walkers, 2 000 burn‑in steps) and quoted at the 90 % confidence level.

A key discovery is a new spectral transition at a luminosity Lₜ ≈ 4.5 × 10³⁷ erg s⁻¹ (2–150 keV, assuming a distance of 5.2 kpc from Gaia DR3). At this luminosity the blackbody normalization (proportional to the emitting area) shows a pronounced turnover: the inferred radius shrinks from ~2 km to <1 km while the temperature rises from ~0.6 keV to ~0.9 keV. The photon index and cutoff energy of the power‑law component vary only modestly, indicating that the blackbody component drives the transition. This luminosity is distinct from previously reported transitions in the same source: L₁ ≈ 1.5 × 10³⁸ erg s⁻¹ and L₂ ≈ 4.4 × 10³⁸ erg s⁻¹, which have been linked to the critical luminosity where radiation pressure dominates the column. The newly identified Lₜ therefore represents the fifth transition point, now observed in the sub‑critical regime.

To interpret the transition, the authors invoke a multipolar magnetic field geometry. They propose two magnetic poles with markedly different field strengths: a weaker pole of ~2.8 × 10¹² G that dominates at low accretion rates (L < Lₜ) and a stronger pole of ~1.6 × 10¹³ G that takes over at higher rates (L > Lₜ). When averaged over the magnetospheric radius, this configuration mimics an effective dipole field of ~6.6 × 10¹² G, consistent with earlier estimates of the critical luminosity, while still allowing the local surface field to exceed 10¹³ G. This scenario naturally explains why the blackbody emitting area contracts sharply at Lₜ: the accretion column switches from being fed by the larger, weaker pole to a smaller, stronger pole, concentrating the energy release onto a smaller hotspot.

The paper also discusses other physical mechanisms that could contribute to the observed behavior. The Coulomb deceleration luminosity (L_Coul ≈ 7 × 10³⁶ erg s⁻¹) marks the onset of significant Coulomb braking, while the transition from gas‑pressure‑dominated to radiation‑pressure‑dominated accretion disks (GPD→RPD) may affect the outer flow geometry. The authors note that the inclusion of NICER’s low‑energy band (0.7–2 keV) in joint fits raises the derived 2–150 keV luminosities by ~10 % compared with HXMT‑only fits, a systematic effect that does not alter the main conclusions.

Overall, the study provides strong observational evidence that Swift J0243‑6124 cannot be described by a simple dipole magnetic field. Instead, a complex multipolar field leads to multiple, luminosity‑dependent spectral transitions, even in the sub‑Eddington regime. This work advances our understanding of how magnetic topology shapes the accretion physics of ultraluminous X‑ray pulsars and highlights the importance of broadband, multi‑instrument monitoring to capture subtle spectral changes across a wide dynamic range.