We observed the young ( \sim 1700 yrs ) pulsar PSR B0540 - 69Â in the near-ultraviolet ( UV ) for the first time with the Space Telescope Imaging Spectrograph ( STIS ) aboard the Hubble Space Telescope .
Imaging observations with the NUV- and FUV-MAMA detectors in TIME-TAG mode allowed us to clearly detect the pulsar in two bands around 2350Ã Â and 1590Ã , with magnitudes m _ { NUV } = 21.45 \pm 0.02 and m _ { FUV } = 21.83 \pm 0.10 .
We also detected the pulsar-wind nebula ( PWN ) in the NUV-MAMA image , with a morphology similar to that observed in the optical and near-infrared ( IR ) .
The extinction-corrected NUV and FUV pulsar fluxes are compatible with a very steep power law spectrum F _ { \nu } \propto \nu ^ { - \alpha } with spectral index \alpha _ { UV } \sim 3 , non compatible with a Rayleigh Jeans spectrum , indicating a non-thermal origin of the emission .
The comparison with the optical/near-IR power-law spectrum ( spectral index \alpha _ { O,nIR } \sim 0.7 ) , indicates an abrupt turnover at wavelengths below 2500 Ã , not yet observed in other pulsars .
We detected pulsations in both the NUV and FUV data at the 50 ms pulsar period .
In both cases , the folded light curve features a broad pulse with two peaks closely spaced in phase , as observed in the optical and X-ray light curves .
The NUV/FUV peaks are also aligned in phase with those observed in the radio ( 1.4 GHz ) , optical , X , and \gamma -ray light curves , like in the Crab pulsar , implying a similar beaming geometry across all wavelengths .
PSR B0540 - 69Â is now the fifth isolated pulsar , together with Crab , Vela , PSR B0656+14 , and the radio-quiet Geminga , detected in the optical , near-UV , near-IR , X-rays and \gamma -rays , and seen to pulsate in at least four of these energy bands .