An unusual type-I X-ray burst from the neutron star X-ray binary IGR J17591–2342: A possible double-photospheric radius-expansion burst

Context. The type-I X-ray bursts observed from neutron stars originate from the intermittent unstable thermonuclear burning of accreted matter on these stars. Such bursts, particularly those reaching the Eddington luminosity and having a temporary photospheric radius expansion due to radiation pressure, provide a testbed to study nuclear fusion processes in intense radiation, gravity, and magnetic fields. Aims. Here, we investigate time-resolved spectroscopic properties of a type-I burst from the accretion-powered millisecond X-ray pulsar IGR J17591–2342. Methods. Our basic spectral model includes an absorbed blackbody to describe the burst emission and an absorbed power law to represent the non-burst emission. Results. The blackbody normalisation shows two consecutive humps aligned with blackbody temperature dips during the burst. Such an unusual behaviour could imply two consecutive photospheric radius-expansion events during the same burst or a systematic metallicity evolution in the neutron star atmosphere. However, our spectral analysis suggests the latter option is less likely to be happening for IGR J17591–2342. Conclusions. The novel former option implies that sufficient fuel survived after the first photospheric radius-expansion event to power a second similar event a few seconds later and thus challenges current theoretical understanding. If confirmed, the double-photospheric radius-expansion event observed in IGR J17591–2342 suggests the possibility of avoiding photospheric expansion at luminosities exceeding Eddington. Mechanisms such as temporary enhancement of the magnetic field by convection and confinement of the plasma could be invoked to explain the peculiar behaviour of the source.