Green Bank Telescope Discovery of Two Binary Millisecond Pulsars in the Globular Cluster M30

Abstract

We report the discovery of two binary millisecond pulsars in the core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20 cm. PSR J2140-2310A (M30A) is an eclipsing 11 ms pulsar in a 4 hr circular orbit, and PSR J2140-23B (M30B) is a 13 ms pulsar in an as yet undetermined but most likely highly eccentric (e > 0.5) and relativistic orbit. Timing observations of M30A with a 20 month baseline have provided precise determinations of the pulsar's position (within 4'' of the optical centroid of the cluster) and spin and orbital parameters, which constrain the mass of the companion star to be m2 ≳ 0.1 M☉. The position of M30A is coincident with a possible thermal X-ray point source found in archival Chandra data, which is most likely caused by emission from hot polar caps on the neutron star. In addition, there is a faint (V555 ~ 23.8) star visible in archival Hubble Space Telescope (HST) F555W data that may be the companion to the pulsar. Eclipses of the pulsed radio emission from M30A by the ionized wind from the compact companion star show a frequency-dependent duration (∝ν-α with α ~ 0.4-0.5) and delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms. Future observations of M30 may allow both the measurement of post-Keplerian orbital parameters from M30B and the detection of new pulsars through the effects of strong diffractive scintillation.