We report a 5.4 \sigma detection of pulsed gamma rays from PSR B1821 - 24 in the globular cluster M28 using \sim 44 months of Fermi Large Area Telescope ( LAT ) data that have been reprocessed with improved instrument calibration constants .
We constructed a phase-coherent ephemeris , with post-fit residual RMS of 3 \mu s , using radio data spanning \sim 23.2 years , enabling measurements of the multi-wavelength light curve properties of PSR B1821 - 24 at the milliperiod level .
We fold RXTE observations of PSR B1821 - 24 from 1996 to 2007 and discuss implications on the emission zones .
The gamma-ray light curve consists of two peaks , separated by 0.41 \pm 0.02 in phase , with the first gamma-ray peak lagging the first radio peak by 0.05 \pm 0.02 in phase , consistent with the phase of giant radio pulses .
We observe significant emission in the off-peak interval of PSR B1821 - 24 with a best-fit LAT position inconsistent with the core of M28 .
We do not detect significant gamma-ray pulsations at the spin or orbital periods from any other known pulsar in M28 , and we place limits on the number of energetic pulsars in the cluster .
The derived gamma-ray efficiency , \sim 2 % , is typical of other gamma-ray pulsars with comparable spin-down power , suggesting that the measured spin-down rate ( 2.2 \times 10 ^ { 36 } erg s ^ { -1 } ) is not appreciably distorted by acceleration in the cluster potential .
This confirms PSR B1821 - 24 as the second very energetic millisecond pulsar in a globular cluster and raises the question of whether these represent a separate class of objects that only form in regions of very high stellar density .