  IGR J17591-2342 is a recently INTEGRAL discovered accreting millisecond X-ray
pulsar that went into outburst around July 21, 2018. To better understand the
physics acting in these systems during the outburst episode we performed
detailed temporal-, timing- and spectral analyses across the 0.3-300 keV band
using data from NICER, XMM-Newton, NuSTAR and INTEGRAL. The hard X-ray 20-60
keV outburst profile is composed of four flares. During the maximum of the last
flare we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data. We
derived a distance of 7.6+/-0.7 kpc, adopting Eddington luminosity limited
photospheric radius expansion burst emission and assuming anisotropic emission.
In the timing analysis using all NICER 1-10 keV monitoring data we observed a
rather complex behaviour starting with a spin-up period, followed by a
frequency drop, a episode of constant frequency and concluding with irregular
behaviour till the end of the outburst. The 1-50 keV phase distributions of the
pulsed emission, detected up to $\sim$ 120 keV using INTEGRAL ISGRI data, was
decomposed in three Fourier harmonics showing that the pulsed fraction of the
fundamental increases from ~10% to ~17% going from ~1.5 to ~4 keV, while the
harder photons arrive earlier than the soft photons for energies <10 keV. The
total emission spectrum of IGR J17591-2342 across the 0.3-150 keV band could
adequately be fitted in terms of an absorbed compPS model yielding as best fit
parameters a column density of N_H=(2.09+/-0.05) x 10^{22} /cm2, a blackbody
seed photon temperature kT_bb,seed of 0.64+/- 0.02 keV, electron temperature
kT_e=38.8+/-1.2 keV and Thomson optical depth Tau_T=1.59+/-0.04. The fit
normalisation results in an emission area radius of 11.3+/-0.5 km adopting a
distance of 7.6 kpc. Finally, the results are discussed within the framework of
accretion physics- and X-ray thermonuclear burst theory.
