On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807

We present the pulsar timing analysis of the accreting millisecond X-ray pulsar SWIFT J1749.4 - 2807 monitored by NICER and XMM-Newton during its latest outburst after almost 11 yr of quiescence. From the coherent timing analysis of the pulse profiles, we updated the orbital ephemerides of the system. Large phase jumps of the fundamental frequency phase of the signal are visible during the outburst, consistent with what was observed during the previous outburst. Moreover, we report on the marginally significant evidence for non-zero eccentricity (e similar or equal to 4 x10(-5)) obtained independently from the analysis of both the 2021 and 2010 outbursts and we discuss possible compatible scenarios. Long-term orbital evolution of SWIFT J1749.4 - 2807 suggests a fast expansion of both the NS projected semimajor axis (x), and the orbital period (P-orb), at a rate of ((x) over dot) similar or equal to 2.6 x10(-13) lt-s s(-1) and (P) over dot)(orb) similar or equal to 4 x10(-10) s s(-1), respectively. SWIFT J1749.4 - 2807 is the only accreting millisecond X-ray pulsar, so far, from which the orbital period derivative has been directly measured from appreciable changes on the observed orbital period. Finally, no significant secular deceleration of the spin frequency of the compact object is detected, which allowed us to set a constraint on the magnetic field strength at the polar caps of B PC < 1.3 x10(8) G, in line with typical values reported for AMXPs.

Automated summary

The study focused on the pulsar timing analysis of the accreting millisecond X-ray pulsar SWIFT J1749.4-2807 during its latest outburst, updating the orbital ephemerides and identifying large phase jumps in the signal. It also reported marginally significant evidence for non-zero eccentricity and discussed possible scenarios, as well as showed long-term orbital evolution with a fast expansion rate. Additionally, it highlighted SWIFT J1749.4-2807 as the only accreting millisecond X-ray pulsar where the orbital period derivative has been directly measured, and set a constraint on the magnetic field strength at the polar caps.