Eight New Millisecond Pulsars in NGC 6440 and NGC 6441

Abstract

Motivated by the recent discovery of 30 new millisecond pulsars in Terzan 5, made using the Green Bank Telescope's S-band receiver and the Pulsar Spigot spectrometer, we have set out to use the same observing system in a systematic search for pulsars in other globular clusters. Here we report on the discovery of five new pulsars in NGC 6440 and three in NGC 6441; each cluster previously had one known pulsar. Using the most recent distance estimates to these clusters, we conclude that there are as many potentially observable pulsars in NGC 6440 and NGC 6441 as in Terzan 5. We present timing solutions for all of the pulsars in these globular clusters. Four of the new discoveries are in binary systems; one of them, PSR J1748–2021B (NGC 6440B), has a wide ( -->Pb = 20.5 days) and eccentric ( -->e = 0.57) orbit. This allowed a measurement of its rate of advance of periastron, = 0.00391(18)° yr−1$d{ω } = 0.00391(18) ° yr −1$ -->$d{ω } = 0.00391(18) ° yr t SUPgt −1t/SUPgt $ -->. If this is a purely relativistic effect, the total mass of this binary system is -->2.92 ± 0.20 -->M☉ (1 -->σ) implying a median pulsar mass of -->2.74 ± 0.21 -->M☉. There is a 1% probability that the inclination is low enough that pulsar mass is below 2 -->M☉, and 0.10% probability that it is between 1.20 and 1.44 -->M☉. If confirmed, this anomalously large mass would strongly constrain the equation of state for dense matter. The other highly eccentric binary, PSR J1750–37A, has -->e = 0.71, and = 0.0055(3)° yr−1$d{ω } = 0.0055(3) ° yr −1$ -->$d{ω } = 0.0055(3) ° yr t SUPgt −1t/SUPgt $ -->, implying a total system mass of -->1.97 ± 0.15 -->M☉ and, along with the mass function, maximum and median pulsar masses of 1.65 and 1.26 -->M☉, respectively.