Two Millisecond Pulsars Discovered by the PALFA Survey and a Shapiro Delay Measurement

We present two millisecond pulsars (MSPs) discovered by the PALFA survey, which uses the Arecibo telescope and the seven-beam ALFA receiver to search for pulsars at 1.4 GHz. The PALFA survey is ongoing since 2004 and focuses on low Galactic latitudes (|b| < 5deg) in the portions of the Galactic plane visible from Arecibo. One of the two present discoveries, PSR J1955+25, is an isolated MSP with a period of 4.87 ms. The other new pulsar, PSR J1949+31, has a period of 13.14 ms and is in a 1.9-day binary system with a massive companion. The high inclination of the orbital plane of the binary with respect to the plane of the sky has allowed us to measure the Shapiro delay in the system and derive mass estimates for the pulsar and companion based on the delay in pulse arrival times near superior conjunction, when pulses must pass through the companion's gravitational well before reaching Earth. We estimate the pulsar mass to be 1.6(4) M⊙ and the companion mass to be 0.9(1) M⊙. Continued regular timing observations with the Arecibo telescope will refine these estimates over time.