Quasi-simultaneous INTEGRAL, SWIFT, and NuSTAR Observations of the New X-Ray Clocked Burster 1RXS J180408.9-342058

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 broadband spectra can be described in general as the sum of thermal Comptonization and reflection due to illumination of an optically thick accretion disk. During the high/soft state, blackbody emission is generated from the accretion disk and the surface of the neutron star. This emission, measured at a temperature of kT <SUB>bb</SUB> ∼ 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 disk/neutron star photons (kT <SUB>bb</SUB> ∼ 1.2 keV) by a cold corona cloud with kT <SUB> e </SUB> ∼ 8-10 keV, while the second one originates from lower temperature blackbody photons (kT <SUB>bb</SUB> ≤ 0.1 keV) Comptonized by a hot corona (kT <SUB> e </SUB> ∼ 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 and ∼4.0 ks when the persistent emission decreases by a factor of ∼2, moving from a very hard to transitional state. The accretion rate (∼ 4× {10}<SUP>-9</SUP> {M}<SUB>☉ </SUB> {yr}}<SUP>-1</SUP>) 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.