New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more

TL;DR

This comprehensive X-ray study of the LMXB 1RXS J180408.9-342058 reveals an intermediate accretion state, detailed spectral evolution, and type-I X-ray bursts, providing new insights into its accretion processes and neutron star environment.

Contribution

First detailed analysis of the intermediate state in 1RXS J180408.9-342058, including spectral evolution and burst properties, expanding understanding of accretion regimes in neutron star binaries.

Findings

Identification of an intermediate accretion state with disk emission and corona cooling.

Detection of a hard tail above 30 keV in the intermediate state.

Type-I X-ray bursts indicating H/He accreted material, ruling out a helium dwarf companion.

Abstract

1RXS J180408.9--342058 is a low mass X-ray binary hosting a neutron star, which shows X-ray activity at very different mass-accretion regimes, from very faint to almost the Eddington luminosity. In this work, we present a comprehensive X-ray study of this source using data from the Neil Gehrels Swift Observatory, NuSTAR and INTEGRAL/JEM-X. In order to follow the spectral evolution, we analysed the 2015 outburst using Swift data and three Nustar observations. Besides the canonical hard and soft spectral states, we identified the rarely observed intermediate state. This was witnessed by the appeareance of the accretion disk emission in the spectrum (at $kT_{\rm disk}$ $\sim$0.7 keV) and the simultaneous cooling of the hot corona. In addition, we also unveiled a hard tail above 30 keV in this state. In the hard state, a thermal Comptonization model with two seed photons populations ($kT_{\rm s,1}\sim 1.5$ keV and $kT_{\rm s,2}\sim 0.4$ keV, respectively) and a hot Comptonising plasma, represents the physically best motivated scenario to describe the data. We also estimated a reflection fraction below 20% in all states, while no constraints on the inclination and only lower limits on the inner disk radius could be inferred. Finally, we studied a number of type-I X-ray bursts displayed from the source, one of them at the Eddington limit (observed with JEM-X). Their characteristics, combined with the clocked behaviour observed during the intermediate state, point out H/He composition for the accreted material, which makes unlikely the helium dwarf nature for the companion.