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

TL;DR

This paper analyzes the spectral and timing properties of the 2018 outburst of the accreting millisecond X-ray pulsar SWIFT J1756.9-2508, revealing its spectral state, orbital evolution, and pulse behavior across multiple observations.

Contribution

It provides the first detailed spectral and timing analysis of the 2018 outburst, updating orbital parameters and studying pulse energy dependence for this source.

Findings

Spectral state is hard with a non-thermal power-law component and a cutoff at ~70 keV.

No evidence of iron lines or reflection features was found.

Estimated neutron star magnetic field is between 3.1E+8 G and 4.5E+8 G.

Abstract

We discuss the spectral and timing properties of the accreting millisecond X-ray pulsar SWIFT J1756.9-2508 observed by XMM-Newton, NICER and NuSTAR during the X-ray outburst occurred in April 2018. The spectral properties of the source are consistent with a hard state dominated at high energies by a non-thermal power-law component with a cut-off at ~70 keV. No evidence of iron emission lines or reflection humps has been found. From the coherent timing analysis of the pulse profiles, we derived an updated set of orbital ephemerides. Combining the parameters measured from the three outbursts shown by the source in the last ~11 years, we investigated the secular evolution of the spin frequency and the orbital period. We estimated a neutron magnetic field 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 dependence of the pulse profile by characterising the behaviour of the pulse fractional amplitude in the energy range 0.3-80 keV. These results are compared with those obtained from the previous outbursts of SWIFT J1756.9-2508 and other previously known accreting millisecond X-ray pulsars.