  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.
