Polarimetry and photometry of M87: is the jet fading?

Optical linear polarization mapping and photometry of M87 is presented in B and R wavebands. The results indicate significant polarization in the galactic nucleus (7.7±1.5 per cent in B and 2.1±1.5 per cent in R). Polarization in the jet in B is consistent with other recent maps obtained for this area in blue light and no significant variation with wavelength between B and R wavebands is found. The spectral index of the jet radiation is measured to be S=−1.65±0.2 over the wavelength range observed. Comparison of the integrated B magnitude of the jet with previous independent measurements over the period 1934–80 suggests that the jet is variable and has been fading more or less uniformly by about 0.8 mag per decade between 1964 and 1980. The time-scale of this variation is unexpectedly short for an object of this size and possible interpretations are discussed.