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 ≃ 4 × 10^-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 $\dot{x}\simeq 2.6\times 10^{-13}\, \text{lt-s}\, \text{s}^{-1}$ and $\dot{P}_{\rm orb}\simeq 4 \times 10^{-10}\, \text{s}\, \text{s}^{-1}$, 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 × 10⁸ G, in line with typical values reported for AMXPs.