The optical light curve of the Large Magellanic Cloud pulsar B0540-69 in 2009

This paper reports a detailed analysis of the optical light curve of PSR B0540−69, the second brightest pulsar in the visible band, obtained in 2009 (January 18 and 20, and December 14, 15, 16 and 18) with the very high speed photon-counting photometer Iqueye mounted at the ESO 3.6-m New Technology Telescope in La Silla (Chile). The optical light curve derived by Iqueye shows a double structure in the main peak, with a rising edge steeper than the trailing edge. The double peak can be fitted by two Gaussians with the same height and full width at half-maximum of 13.3 and 15.5ms, respectively. Our new values of spin frequencies allow us to extend by 3.5 yr the time interval over which a reliable estimate of frequency first and second derivatives can be performed. A discussion of implications for the braking index and age of the pulsar is presented. A value of n = 2.087 ± 0.007 for the overall braking index from 1987 to 2009 is derived. The braking index corrected age is confirmed at around 1700 yr.