Quasi-simultaneous Integral, Swift, And Nustar Observations Of The New X-ray Clocked Burster 1rxsj180408.9-342058

TL;DR

This study presents multi-instrument observations of the neutron star X-ray binary 1RXS J180408.9-342058, revealing spectral state transitions, complex corona structures, and evidence of it being a new clocked burster with regular thermonuclear bursts.

Contribution

First detailed broad-band spectral analysis of 1RXS J180408.9-342058 across different states, identifying corona configurations and confirming its status as a new clocked burster.

Findings

Spectral states characterized by thermal Comptonization and reflection.

Detection of regular X-ray bursts with ~7.9 ks and ~4.0 ks intervals.

Evidence of mixed H/He burning triggered by thermally unstable He ignition.

Abstract

We report the quasi-simultaneous INTEGRAL, SWIFT, and NuSTAR observations showing spectral state transitions in the neutron star low mass X-ray binary 1RXS J180408.9-342058 during its 2015 outburst. We present results of the analysis of high-quality broad energy band (0.8-200 keV) data in three different spectral states: high/soft, low/very-hard, and transitional state. The broad band spectra can be described in general as the sum of thermal Comptonization and reflection due to illumination of an optically-thick accretion disc. During the high/soft state, blackbody emission is generated from the accretion disc and the surface of the neutron star. This emission, measured at a temperature of kTbb ~1.2 keV, is then Comptonized by a thick corona with an electron temperature of ~2.5 keV. For the transitional and low/very-hard state, the spectra are successfully explained with emission from a double Comptonizing corona. The first component is described by thermal Comptonization of seed disc/neutron-star photons (kTbb ~1.2 keV) by a cold corona cloud with kT e ~8-10 keV, while the second one originates from lower temperature blackbody photons (kTbb~0.1 keV) Comptonized by a hot corona (kTe~35 keV). Finally, from NuSTAR observations, there is evidence that the source is a new clocked burster. The average time between two successive X-ray bursts corresponds to ~7.9 ks and ~4.0 ks when the persistent emission decreases by a factor ~2, moving from very hard to transitional state. The accretion rate and the decay time of the X-ray bursts longer than ~30 s suggest that the thermonuclear emission is due to mixed H/He burning triggered by thermally unstable He ignition.