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We discuss the spectral and timing properties of the accreting millisecondX-ray pulsar SWIFT J1756.9-2508 observed by XMM-Newton, NICER and NuSTAR duringthe X-ray outburst occurred in April 2018. The spectral properties of thesource are consistent with a hard state dominated at high energies by anon-thermal power-law component with a cut-off at ~70 keV. No evidence of ironemission lines or reflection humps has been found. From the coherent timinganalysis of the pulse profiles, we derived an updated set of orbitalephemerides. Combining the parameters measured from the three outbursts shownby the source in the last ~11 years, we investigated the secular evolution ofthe spin frequency and the orbital period. We estimated a neutron magneticfield of 3.1E+8 G < B_pc< 4.5E+8 G and measured an orbital period derivative of-4.1E-12 s/s < P_dot_orb < 7.1E-12 s/s. We also studied the energy dependenceof the pulse profile by characterising the behaviour of the pulse fractionalamplitude in the energy range 0.3-80 keV. These results are compared with thoseobtained from the previous outbursts of SWIFT J1756.9-2508 and other previouslyknown accreting millisecond X-ray pulsars.

SWIFT J1756.9-2508: spectral and timing properties of its 2018 outburst