tokens: AO1 Guest Observer program. NuSTAR consists of two co-aligned X-ray telescopes, with the corresponding focal plane modules FPMA and FPMB that provide 18′′ FWHM imaging resolution over a 3–79 keV X-ray band and a characteristic spectral resolution of 400 eV FWHM at 10 keV (Harrison et al. 2013 ). The reconstructed NuSTAR coordinates are accurate to at the 90% confidence level. The nominal timing accuracy of NuSTAR is ∼2 ms rms, after correcting for drift of the onboard clock, with the absolute timescale shown to be better than ms (Mori et al. 2014 ; Madsen et al. 2015 ). Although this is insufficient to resolve the 3.05 ms period of PSR B1821-24, we devise a method, presented herein, to correct the photon arrival times to better than accuracy. NuSTAR data were reprocessed and analyzed using FTOOLS 09May2016_V6.19 ( NUSTARDAS 14Apr16_V1.6.0) with the NuSTAR Calibration Database (CALDB) files of 2016 July 6. The resulting data set provides a total of 140.7 ks and 55 ks of net good exposure time for the two pointings, respectively. For the timing analysis, the nominal NuSTAR clock correction was applied to the photon arrival times which are converted to barycentric dynamical time (TDB) using the DE405 solar system ephemeris and radio timing coordinates of Ray et al. ( 2008 ). For all subsequent analyses we merged data from both FPM detectors. Although the globular cluster M28 hosts at least 11 other pulsars (Bégin 2006 ), they are all over a factor of ∼100 fainter than PSR B1821-24 in X-rays (Bogdanov et al. 2011 ) and thus cannot be studied effectively with NuSTAR . 2.1.2. RXTE PSR B1821-24 has been observed extensively in hard X-rays with RXTE during the mission lifetime, primarily for the purpose of absolute spacecraft time calibration and to measure the X-ray-to-radio phase lags (Rots et al. 1998 ; Ray et al. 2008 ). In anticipation of the NuSTAR observations, we collectively analyzed all available archived RXTE data on PSR B1821-24 for the first time. A total of 180 ks of exposure times were accumulated during three programs: P20159 (1997 February 12–13, 16.9 ks), P90053 (2005 February 28–March 1, 14.9 ks), and P92008 (2006 July 6–8, 48.4 ks; 2006 October 9–10, 51.2 ks; 2007 April 25-26, 48.5 ks). These observations were made with the Proportional Counter Array (PCA; Jahoda et al. 1996 ) on board RXTE . The PCA consists of five collimated xenon proportional counter units (PCUs), each having a front propane anticoincidence layer, with a total effective area of ∼6500 cm −2 over a 1° field of view (FWHM). Each PCU is sensitive to photons in the energy range of 2–60 keV, with an energy resolution of 18% at 6 keV recorded in 256 channels. Data were collected in GoodXenonwithPropane mode with two to five PCU modules in use at any given time, with an average of 3.7 PCUs active overall. In this mode, photon arrival times are recorded with resolution. We analyzed events from the top xenon layers of each PCU only. Including the second layers resulted
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