Program: GS-2018A-Q-102

PSR J1640+2224 is a fully-recycled (P=3.16 ms) millisecond pulsar (MSP) in a long-period (175 day) orbit with a white dwarf (WD) companion. Its short spin period and nearly circular orbit both point to it being formed in a low-mass X-ray binary (LMXB), in which case its companion should be a low-mass helium-core (He-core) WD. However, pulsar timing observations and optical observations of the WD have yielded widely varying values for the WD mass. We recently reanalyzed observations of the WD made in 1995 using the Wide Field Planetary Camera 2 on the Hubble Space Telescope and found a WD mass of about 0.7 solar masses, indicating the companion is most likely a high-mass carbon-oxygen (CO) WD. This system challenges binary evolution models because MSPs with CO WD companions formed in intermediate-mass X-ray binaries (IMXBs), but the mass-transfer phase in IMXBs is too short to produce MSPs with spin periods below about 8 ms, especially for wide-orbit systems. We request 3-band photometry (g'r'i' bands) with GMOS-N in order to precisely measure the WD mass. If we determine the companion is a He-core WD, we can test the theoretical relationship between the WD mass and orbital period in the long-period regime where there are currently no observational constraints. If we determine the companion is a CO WD, we may need to completely rethink binary evolution scenarios to explain how this system formed.