HIGH-SPEED PHOTOMETER OBSERVATIONS OF THE LMC-PULSAR B0540-69

The High-Speed Photometer (HSP) on board the Hubble Space Telescope (H ST) observed the 50 ms pulsar B0540-69 in the Large Magellanic Cloud o n 1993 August 26 for 1 hr in a passband extending from 1600 Angstrom t o 7000 Angstrom. The pulse profile resembles both that in the optical region alone and in the X-ray region. The pulsed fraction in the UV-pl us-visible passband is 0.16 +/- 0.02, similar to that measured in the X-ray region. The pulsar frequency on MJD 49225.2557 (start time of th e observation) was 19.83448 +/- 0.00002 Hz. When we combine our observ ed frequency with other published X-ray and optical observations and a ssume no discontinuities (glitches) in the spin-down rate since 1979, we derive a braking index n = 2.28 +/- 0.02.