1A 1118-61 reveals a new two-hump X-ray spectrum as its 2026 outburst fades

A new arXiv preprint says the Be/X-ray pulsar 1A 1118-61 developed a distinct two-hump X-ray spectrum as its 2026 outburst faded, adding fresh detail to how the source changes with brightness.

The study combines dense monitoring from SRG/ART-XC and Insight-HXMT with three NuSTAR observations taken across the bright, declining and plateau phases of the outburst. In the authors’ analysis, the source moved from a peak luminosity of about 7 x 10^37 erg s^-1 down to a low-luminosity plateau of roughly 3 x 10^35 to 8 x 10^35 erg s^-1 in the 4-35 keV band.

What the 2026 monitoring showed

As the outburst weakened, the broadband continuum changed from a single high-energy shape into a spectrum with two broad maxima, one near 10 keV and another around 30-40 keV. The new high-energy hump becomes visible during the decline and gives the source a more complex spectral structure than in the brightest phase.

The authors say the transition to the two-hump morphology appears around L_4-35 of roughly 0.8 x 10^36 to 1.8 x 10^36 erg s^-1. They also report a break in the luminosity dependence of the flux ratio between the two humps around L_4-35 of about 10^37 erg s^-1.

That makes the June preprint a luminosity-resolved look at the same outburst that was already attracting attention in May, when a separate arXiv paper described it as the brightest recorded outburst of 1A 1118-61. The earlier paper focused on the peak, pulse-profile changes and a quasi-periodic oscillation, while the new study focuses on how the broadband spectrum itself evolves.

Cyclotron line and magnetic context

The new paper also reports a cyclotron line near 55 keV, located in the high-energy hump. The authors say they do not find a significant luminosity dependence in the line’s centroid energy.

That matters because a broad absorption feature around 55 keV was already reported in a 2010 paper on the 2009 outburst and interpreted as a cyclotron resonance scattering feature. The magnetic-field diagnostic seen in earlier work shows up again here, but the new result is that the surrounding continuum is not static as the source dims.

Why it matters

1A 1118-61 is a Be/X-ray binary pulsar, so its X-ray output is shaped by accretion onto a strongly magnetized neutron star. The new result links changes in spectral shape directly to luminosity over several orders of magnitude, which is the kind of evidence modelers need when testing how the accretion flow and radiative geometry evolve.

The paper’s main scientific stake is the origin of the high-energy hump itself. By showing when the second hump appears, and how its relative strength changes, the study adds a concrete observational constraint on emission or scattering models for magnetized accretion columns.

The broader takeaway is not that the system suddenly became unusual, but that a familiar pulsar offered unusually detailed timing and spectral coverage during a bright outburst. That coverage lets researchers track a transition that would be easy to miss in sparse observations.

The paper is still a preprint, so the interpretation remains model-dependent and open to follow-up. The most immediate next steps are journal review, possible institutional coverage and later papers that may tie the two-hump spectrum to a specific physical mechanism.