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

Abstract

We discuss the spectral and timing properties of the accreting millisecond\nX-ray pulsar SWIFT J1756.9-2508 observed by XMM-Newton, NICER and NuSTAR during\nthe X-ray outburst occurred in April 2018. The spectral properties of the\nsource are consistent with a hard state dominated at high energies by a\nnon-thermal power-law component with a cut-off at ~70 keV. No evidence of iron\nemission lines or reflection humps has been found. From the coherent timing\nanalysis of the pulse profiles, we derived an updated set of orbital\nephemerides. Combining the parameters measured from the three outbursts shown\nby the source in the last ~11 years, we investigated the secular evolution of\nthe spin frequency and the orbital period. We estimated a neutron magnetic\nfield of 3.1E+8 G < B_pc< 4.5E+8 G and measured an orbital period derivative of\n-4.1E-12 s/s < P_dot_orb < 7.1E-12 s/s. We also studied the energy dependence\nof the pulse profile by characterising the behaviour of the pulse fractional\namplitude in the energy range 0.3-80 keV. These results are compared with those\nobtained from the previous outbursts of SWIFT J1756.9-2508 and other previously\nknown accreting millisecond X-ray pulsars.\n