Unpulsed optical emission from the Crab pulsar

Based on observations of the Crab pulsar using the TRIFFID high-speed imaging photometer in the UBV bands using the Special Astrophysical Observatory's 6 m telescope in the Russian Caucasus, we report the detection of pronounced emission during the so-called off phase of emission. Following de-extinction, this unpulsed component of emission is shown to be consistent with a power law with an exponent of α = -0.60 ± 0.37, the uncertainty being dominated by the error associated with the inde-pendent CCD photometry used to reference the TRIFFID data. This suggests a steeper power-law form than that reported elsewhere in the literature for the total integrated spectrum, which is essentially flat with α ∼ 0.1, although the difference in this case is only significant at the ≤2 σ level. Deeper reference integrated and TRIFFID phase-resolved photometry, in these bands in conjunction with further observations in the UV and R region, would constrain this fit further.