G. Anzolin

Overcoming the Rayleigh Criterion Limit with Optical Vortices

We experimentally and numerically tested the separability of two independent equally luminous monochromatic and white light sources at the diffraction limit, using optical vortices (OV). The diffraction pattern of one of the two sources crosses a fork hologram on its center generating the Laguerre-Gaussian (LG) transform of an Airy disk. The second source, crossing the fork hologram in positions different from the optical center, generates nonsymmetric LG patterns. We formulated a criterion, based on the asymmetric intensity distribution of the superposed LG patterns so created, to resolve the two sources at angular distances much below the Rayleigh criterion. Analogous experiments in white light allow angular resolutions which are still one order of magnitude below the Rayleigh criterion. The use of OVs might offer new applications for stellar separation in future space experiments.

Two new intermediate polars with a soft X-ray component

Aims. We analyze the first X-ray observations with XMM-Newton of 1RXS J070407.9+262501 and 1RXS 180340.0+401214, in order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as intermediate polars. Methods. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2–10.0 keV range to characterize the emission properties of both sources. Results. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3–5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 h that is longer than previously estimated. Both systems show complex spectra with a hard (temperature up to 40 keV) optically thin and a soft (kT ∼ 85–100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources. Conclusions. Our observations confirm the two systems as intermediate polars and also add them as new members of the growing group of “soft” systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation.

Method to measure off-axis displacements based on the analysis of the intensity distribution of a vortex beam

We study the properties of the Fraunhofer diffraction patterns produced by Gaussian beams crossing spiral phase plates. We show, both analytically and numerically, that off-axis displacements of the input beam produce asymmetric diffraction patterns. The intensity profile along the direction of maximum asymmetry shows two different peaks. We find that the intensity ratio between these two peaks decreases exponentially with the off-axis displacement of the incident beam, the decay being steeper for higher strengths of the optical singularity of the spiral phase plate. We analyze how this intensity ratio can be used to measure small misalignments of the input beam with a very high precision.

Broad-band properties of the hard X-ray cataclysmic variables IGR J00234+6141 and 1RXS J213344.1+510725

Context. A significant number of cataclysmic variables have been detected as hard X-ray sources in the INTEGRAL survey, most of them of the magnetic intermediate polar type. Aims. We present a detailed X-ray broad-band study of two new sources, IGR J00234+6141 and 1RXS J213344.1+510725, that allow us to classify them as secure members of the intermediate polar class. Methods. Timing and spectral analysis of IGR J00234+6141 are based on an XMM-Newton observation and INTEGRAL publicly available data. For 1RXS J213344.1+510725, we use XMM-Newton and Suzaku observations at different epochs, as well as INTEGRAL publicly available data. Results. We determine a spin period of 561.64 ± 0.56 s for the white dwarf in IGR J00234+6141. The X-ray pulses are observed up to ∼ 2k eV. FromXMM-Newton and Suzaku observations of 1RXS J213344.1+510725, we find a rotational period of 570.862 ± 0.034 s. The observations span three epochs where the pulsation is observed to change at different energies both in amplitude and shape. In both objects, the spectral analysis spanned over a wide energy range, from 0.3 to 100 keV, shows the presence of multiple emission components absorbed by dense material. The X-ray spectrum of IGR J00234+6141 is consistent with a multi-temperature plasma with a maximum temperature of ∼50 keV. In 1RXS J213344.1+510725, multiple optically thin components are inferred, as well as an optically thick (blackbody) soft X-ray emission with a temperature of ∼100 eV. This adds 1RXS J213344.1+510725 to the growing group of soft X-ray intermediate polars. Conclusions. Though showing similar rotational periods and being hard X-ray sources, these two intermediate polars appear different in several respects, among which the presence of a soft X-ray component in 1RXS J213344.1+510725. This source also emits circularly polarized light in the optical band, thus joining the group of soft and polarized intermediate polars as a fifth member. How the hard X-ray and polarized emission are thermalized in these systems is a timely question.

AquEYE, a single photon counting photometer for astronomy

This paper describes the results obtained so far with AquEYE, a single photon counting, fixed aperture photometer for the Asiago 182 cm telescope. AquEYE has been conceived as a prototype of a truly ‘quantum’ photometer for future Extremely Large Telescopes of 30–50 m aperture. This prototype is characterized by four independent channels equipped with single photon avalanche diodes (SPADs) as detectors. The counts from the four channels are acquired by a TDC board which has a nominal 25 ps time tagging capability. Taking into account the 35 ps jitter in the SPAD itself, the overall precision of the time tags is of the order of 50 ps. The internal oscillator is locked to an external rubidium clock; a GPS pulse per second is collected by the TDC itself to obtain a UTC reference. The maximum photon count rate which the present system can sustain is 12 MHz.

Fabrication and Testing of l = 2 Optical Vortex phase masks for Coronography

In this paper we present the fabrication process and tests of two different types of l = 2 spiral phase plates (SPPs), designed for an Optical Vortex Coronagraph (OVC) in the visible wavelength regime. Each phase mask is realized dividing the spirals area in sectors respectively of 8 and 512 of levels using lithographic nanofabrication approach. The SPPs produces different optical vortices (OVs) with topological charge l that depends on the number of steps and on the wavelength. We found that the residual light in the central dark region of the OV tends to zero as the number of steps increases.

Wavelet and R/S analysis of the X-ray flickering of cataclysmic variables

Context. Recently, wavelets and R/S analysis have been used as statistical tools to characteriz e the optical flickering of cataclysmic variables. Aims. Here we present the first comprehensive study of the statisti cal properties of X-ray flickering of cataclysmic variables in order to link them with physical parameters. Methods. We analyzed a sample of 97 X-ray light curves of 75 objects of all classes observed with the XMM-Newton space telescope. By using the wavelets analysis, each light curve has been characterized by two parameters, � and �, that describe the energy distribution of flickering on di fferent timescales and the strength at a given timescale, resp ectively. We also used the R/S analysis to determine the Hurst exponent of each light curve and define th eir degree of stochastic memory in time. Results. The X-ray flickering is typically composed of long time scale events (1.5 <� <3), with very similar strengths in all the subtypes of cataclysmic variables (−3 <� <−1.5). The X-ray data are distributed in a much smaller area of the �−� parameter space with respect to those obtained with optical light curves. The tendency of the optical flickering in magnetic systems to sh ow higher � values than the non-magnetic systems is not encountered in the X-rays. The Hurst exponents estimated for all light curves of the sample are larger than those found in the visible, with a peak at 0.82. In particular, we do not obtain values lower than 0.5. The X-ray flickering presents a persistent memory in time, which seems to be stronger in objects containing magnetic white dwarf primaries. Conclusions. The similarity of the X-ray flickering in objects of di fferent classes together with the predominance of a persistent stochastic behavior can be explained it terms of magnetically-driven accretion processes acting in a considerable fra ction of the analyzed objects.

Very fast photon counting photometers for astronomical applications: IquEYE for the ESO 3.5m New Technology Telescope

Almost all astronomical instruments detect and analyze the first order spatial and/or temporal coherence properties of the photon stream coming from celestial sources. Additional information might be hidden in the second and higher order coherence terms, as shown long ago by Hanbury-Brown and Twiss with the Narrabri Intensity Interferometer. The future Extremely Large Telescopes and in particular the 42 m telescope of the European Southern Observatory (ESO) could provide the high photon flux needed to extract this additional information. To put these expectations (which we had already developed at the conceptual level in the QuantEYE study for the 100 m OverWhelmingly Large Telescope to experimental test in the real astronomical environment, we realized a small prototype (Aqueye) for the Asiago 182 cm telescope. This instrument is the fastest photon counting photometer ever built. It has 4 parallel channels operating simultaneously, feeding 4 Single Photon-Avalanche Diodes (SPADs), with the ability to push the time tagging capabilities below the nano-second region for hours of continuous operation. Aqueye has been extensively used to acquire photons from a variety of variable stars, in particular from the pulsar in the Crab Nebula. Following this successful realization, a larger version, named Iqueye, has been built for the 3.5 m New Technology Telescope (NTT) of ESO. Iqueye follows the same optical solution of dividing the telescope pupil in 4 sub-pupils, imaged on new generation SPADs having useful diameters of 100 micrometers, time jitter less than 50 picoseconds and dark-count noise less than 50 counts/second. The spectral efficiency of the system peaks in the visible region of the spectrum. Iqueye operated very successfully at the NTT in January 2009. The present paper describes the main features of the two photometers and present some of the astronomical results already obtained.