Aqueye+: a new ultrafast single photon counter for optical high time resolution astrophysics

Aqueye+ is a new ultrafast optical single photon counter, based on single photon avalanche photodiodes (SPAD) and a 4-fold split-pupil concept. It is a completely revisited version of its predecessor, Aqueye, successfully mounted at the 182 cm Copernicus telescope in Asiago. Here we will present the new technological features implemented on Aqueye+, namely a state of the art timing system, a dedicated and optimized optical train, a high sensitivity and high frame rate field camera and remote control, which will give Aqueye plus much superior performances with respect to its predecessor, unparalleled by any other existing fast photometer. The instrument will host also an optical vorticity module to achieve high performance astronomical coronography and a real time acquisition of atmospheric seeing unit. The present paper describes the instrument and its first performances.

Optical phase coherent timing of the Crab nebula pulsar with Iqueye at the ESO New Technology Telescope

The Crab nebula pulsar was observed in 2009 January and December with a novel very fast optical photon counter, Iqueye, mounted at the ESO 3.5 m New Technology Telescope. Thanks to the exquisite quality of the Iqueye data, we computed accurate phase coherent timing solutions for the two observing runs and over the entire year 2009. Our statistical uncertainty on the determination of the phase of the main pulse and the rotational period of the pulsar for short (a few days) time intervals are $approx 1 , mu$s and ~0.5 ps, respectively. Comparison with the Jodrell Bank radio ephemerides shows that the optical pulse leads the radio one by ~240 $mu$s in January and ~160 $mu$s in December, in agreement with a number of other measurements performed after 1996. A third-order polynomial fit adequately describes the spin-down for the 2009 January plus December optical observations. The phase noise is consistent with being Gaussian distributed with a dispersion $sigma$ of $approx 15 , mu$s in most observations, in agreement with theoretical expectations for photon noise-induced phase variability.

Aqueye optical observations of the Crab Nebula pulsar

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). 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. 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. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a 2 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 are in agreement with those from the Jodrell Bank radio ephemerides archive. We also found evidence of the optical peak leading the radio one by ~230 microseconds. The distribution of phase-residuals of the whole dataset is slightly wider than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape, such as the average count rate and background level are those of the Crab pulsar observed with Aqueye. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in 2 days only. The time of arrival of the optical peak of the Crab pulsar leads the radio one in agreement with what 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).

Searching for the orbital period of the ultraluminous X-ray source NGC 1313 X-2

We analyzed the longest phase-connected photometric dataset available for NGC 1313 X-2, looking for the ~6 day modulation reported by Liu et al. (2009). The folded B band light curve shows a 6 day periodicity with a significance slightly larger than 3 sigma. The low statistical significance of this modulation, along with the lack of detection in the V band, make its identification uncertain.