Context. A new extremely high speed photon-counting photometer, Iqueye, has been installed and tested at the New Technology Telescope, in La Silla. Aims. This instrument is the second prototype of a "quantum" photometer being developed for future Extremely Large Telescopes of 30-50 m aperture. Methods. Iqueye divides the telescope aperture into four portions, each feeding a single photon avalanche diode. The counts from the four channels are collected by a time-to-digital converter board, where each photon is appropriately time-tagged. Owing to a rubidium oscillator and a GPS receiver, an absolute rms timing accuracy better than 0.5 ns during one-hour observations is achieved. The system can sustain a count rate of up to 8 MHz uninterruptedly for an entire night of observation. Results. During five nights of observations, the system performed smoothly, and the observations of optical pulsar calibration targets provided excellent results.

Low Mass X-ray Binaries (LMXB s) with either a black hole or a neutron star show power spectra characterised by Quasi Periodic Oscillations (QPOs). Twin peak high frequency QPOs are characterised by frequencies that are typical for matter orbiting within 10 fg from the compact object. We consider clumps of material orbiting a Schwarzschild black hole, that are deformed by tidal interaction. We present some preliminary calculations of corresponding light curves and power spectra. We were able to fit the simulated power spectra with the high frequency part of the power spectra observed in the LMXB XTE J1550-564 containing ablackhole. Ournumerical simulations reproduce the twin high frequency QPOs and the power-law. The lower peak corresponds to the Keplerian frequency, the upper one to the sum of the Keplerian and the radial frequency.

Context. We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago -Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye), which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Aims. Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. Methods. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. Results. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a two day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative agree with those from the Jodrell Bank radio ephemerides archive. We also found evidence that the optical peak precedes the radio peak by ∼230 μs. The distribution of phase residuals of the whole dataset is slightly more scattered than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape. Conclusions. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in two days only. The time of arrival of the optical peak of the Crab pulsar precedes the radio peak in agreement with what was recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).

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 15.5 ms, 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.