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arXiv · 2610.17982

A0538-66: A transient ultraluminous X-ray source with a relativistic outflow, 69ms pulsations, and anomalous optical activity

Lorenzo Ducci·Sandro Mereghetti·Ciro Pinto·Matteo Bachetti·Motoki Nakajima·Rahul Sharma·Stefano Covino·Michela Rigoselli·Caterina Tresoldi·Tatehiro Mihara·Mutsumi Sugizaki
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Abstract

A0538-66 is a peculiar Be/X-ray binary in the Large Magellanic Cloud, hosting the fastest spinning neutron star of its class (~69ms) and showing the brightest optical bursts of any system of this type. We report four XMM-Newton observations obtained near periastron in 2025, together with MAXI and REM monitoring, and a reanalysis of archival RXTE data. On 30 September 2025 the source reached L_X~1.5E39 erg/s (0.2-10 keV), the highest level since 1980, and in February, May, and August 2026, MAXI detected similarly bright outbursts. During the XMM-Newton observation taken on 30 September 2025, we detected the pulsations, measuring P_spin=69.25878(2) ms and dP_spin/dt=6.58(9)E-10 s/s; the pulsed fraction was low (~3% in 1-10 keV), increased with energy, and the soft and hard pulse profiles were shifted by half a cycle. A new detection in RXTE data of 3 December 1998 gives a faster spin period of P_spin=68.88813(28) ms. The XMM-Newton/RGS spectra show emission lines from photoionised plasma at rest and blueshifted absorption lines that imply an outflow at ~0.12c. This indicates a highly super-Eddington accretion rate and an intrinsic L_X~1.5E39 erg/s, and it makes A0538-66 a transient ultra-luminous X-ray pulsar, a class of which only a few objects are known so far. Since 2023 the optical bursts observed by REM have become longer and brighter, resembling those of the 1980s, but most of them lack a luminous X-ray counterpart (L_X>1E37 erg/s). We interpret these results in terms of a radiation pressure dominated accretion disc whose geometrically thick inner regions intermittently screen the innermost emitting regions.

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Lorenzo Ducci, Sandro Mereghetti, Ciro Pinto, Matteo Bachetti, Motoki Nakajima, Rahul Sharma, Stefano Covino, Michela Rigoselli, Caterina Tresoldi, Tatehiro Mihara, Mutsumi Sugizaki. 2026-10-13. A0538-66: A transient ultraluminous X-ray source with a relativistic outflow, 69ms pulsations, and anomalous optical activity. https://arxiv.org/abs/2610.17982

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