Long-term timing of the relativistic binary PSR J1906+0746

We conducted a timing analysis of over 18 years of data on the young (${\tau }_{\mathrm{c}}=112$ kyr, $P=114$,ms) relativistic binary PSR~J1906+0746, using six radio telescopes: Arecibo, FAST, Green Bank, Lovell, MeerKAT, and Nançay. This pulsar is known to orbit a compact high-mass companion with a period of 3.98,hrs in a mildly eccentric orbit ($e=0.085$). By combining all data and maintaining a coherent timing solution over the full span, we obtained a more precise measurement of the advance of periastron, $\dot{\omega }=7.5841(2)$,$\mathrm{°}$,yr${}^{-1}$, the Einstein delay, $\gamma =4.59(2)\times {10}^{-4}$,s, and the secular change in orbital period, ${\dot{P}}_{\mathrm{b}}=-5.65(2)\times {10}^{-13}$,s,s${}^{-1}$. Assuming the validity of general relativity, we obtain a total mass of 2.6133(1),$M_{\odot }$ and component masses of 1.316(5) $M_{\odot }$ for the pulsar and 1.297(5) $M_{\odot }$ for the companion, consistent with a double neutron star system. However, when fitting for the secular change in the projected semi-major axis we obtain $\dot{x}=-1.8(6)\times {10}^{-13}$,s,s${}^{-1}$, the component masses are shifted by $\sim 3.5\sigma$, which is expected from the correlation of $\dot{x}$ and $\gamma$. The $\dot{x}$ has a similar magnitude to that observed in PSR~J1141$-$6545, which is due to spin-orbit coupling; if confirmed, it would indicate that, as in the latter system, the companion of the PSR~J1906+0746 system is a massive fast-rotating white dwarf formed before the pulsar. Additionally, we report and characterize a large glitch near MJD 56664, with a fractional frequency increase comparable to those observed in the Vela pulsar.