PSR J1953+1844 probably being the descendant of an Ultra-compact X-ray binary

PSR J1953+1844 (i.e., M71E) is a millisecond pulsar (MSP)in a 53 minute binary orbit discovered by the Five-hundred-meter Aperture Spherical radio Telescope. The mass function from pulsar timing is $2.3\times10^{-7}$ $M_\odot$. The possible redback origin of this system has been discussed by Pan et al. We discuss here an alternative evolution track for this binary system, namely that PSR J1953+1844 is a descendant of an ultra-compact X-ray binary (UCXB), which has a hydrogen-poor donor accreting onto a neutron star (NS) with an orbital period of $\leq1$ hr. We noticed that some of UCXB systems hold an accreting millisecond X-ray pulsars (AMXPs) and a donor with a mass of about 0.01 M$_\odot$. M71E has a very similar orbit to those of AMXPs, indicating that it might be evolved from a UCXB similar to PSR J1653--0158. The companion star of M71E should be significantly bloated and it most probably has a carbon and oxygen composition, otherwise a low inclination angle of the orbit is required for a helium companion. The discovery of this M71E binary system may shed light on when and how an NS in a UCXBs turns into a radio pulsar.
Comment: Accepted for publication in ApJL. 5 pages, 2 figures