C. Germanà

University of Padova, Padua, Veneto, Italy

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Publications (18)27.48 Total impact

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    ABSTRACT: Although optical pulsar studies are currently limited a few favoured objects, the observation of pulsars at optical wavelengths provides an opportunity to derive a number of important pulsar characteristics. These parameters will be vital for a comprehensive model of pulsar emission mechanisms. The exploitation of the data available through optical pulsar studies requires that high-time-resolution instruments suitable for the optical domain, and for faint pulsar targets, are developed and thoroughly tested. Observations of the Crab pulsar with the high-time-resolution photon-tagging photometer IquEYE show an optical-radio delay of ˜178 μs. In conjunction with simultaneous Jodrell Bank radio observations, a correlation between giant radio pulses and enhanced optical pulses was detected for this pulsar, which is possible evidence for the reprocessing of radio photons. Comparison with optical linear polarisation measurements indicate a shift between linear and circular polarisation at the radio precursor phase.
    12/2012;
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    Claudio Germanà
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    ABSTRACT: Twin peak high frequency quasi-periodic oscillations (HF QPOs) observed in the power spectra of Low Mass X-ray Binaries (LMXBs), with either a black hole or a neutron star, have central frequencies that are typical of the orbital motion time-scale close to the compact object. Thus, twin HF QPOs might carry the fingerprint of physical effects in a strongly curved space-time. We study the azimuth phase \phi(t) for orbital motion in the Schwarzschild metric and calculate the power spectra to check whether they display the features seen in the observed ones. We show that the timing of \phi(t) on non-closed orbits can account for the observed twin peak HF QPOs. The uppermost couple of peaks in frequency has the lower peak that corresponds to the azimuthal frequency \nu_{\phi}, the upper one to \nu_{\phi}+\nu_r. The azimuth phase temporal behavior \phi(t) on a slightly eccentric orbit in the Schwarzschild metric is described by a linear function of slope \nu_{\phi} plus an oscillating term at the relativistic radial frequency \nu_r. We deduce that the twin peak HF QPOs might originate from a frequency modulated (FM) signal driven by the kinematics of orbital motion in a curved space-time.
    Monthly Notices of the Royal Astronomical Society 11/2012; 430(1). · 5.52 Impact Factor
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    ABSTRACT: 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).
    Astronomy and Astrophysics 10/2012; · 5.08 Impact Factor
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    ABSTRACT: Tidal effects on clumps of material during random non-stationary accretion onto a black hole produce phenomena with distinct temporal characteristics in observed light-curves. During such non-stationary accretion events, the shape of the accreting object evolves in time, and observable quasi-periodic phenomena with variable quasi-periods are produced. A number of characteristic light-curves, obtained with numerical simulations, will be shown. Their relevance to observed phenomena will be briefly discussed.
    04/2012;
  • Proceedings of the MG12 Meeting on General Relativity; 02/2012
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    Claudio Germana
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    ABSTRACT: Recent results from the OPERA experiment reported a neutrino beam traveling faster than light. The experiment measured the neutrino time of flight (TOF) over a baseline from the CERN to the Gran Sasso site. The neutrino beam arrives 60 ns earlier than a light ray would do. Because the result has an enormous impact on science, it might be worth double-checking the time definitions with respect to the non-inertial system in which the neutrino travel time was measured. Potential problems in the OPERA data analysis connected with the definition of the reference frame and time synchronization are emphasized. We aim to investigate the synchronization of non-inertial clocks on Earth by relating this time to the proper time of an inertial observer at Solar System Barycenter(SSB). The Tempo2 software was used to time-stamp events observed on the geoid with respect to the SSB inertial observer time. Neutrino results from OPERA might carry the fingerprint of non-inertial effects. The CERN-Gran Sasso clock synchronization is accomplished by applying corrections that depend on special and general relativistic time dilation effects at the clocks, depending on the position of the clocks in the solar system gravitational well. As a consequence, TOF distributions are centered on values shorter by tens of ns than expected, integrating over a period from April to December, longer if otherwise. It is worth remarking that the OPERA runs have always been carried out from April/May to November. If the analysis by Tempo2 holds for the OPERA experiment, the excellent measurement by the OPERA collaboration will turn into a proof of the General Relativity theory in a weak field approximation. The analysis presented here is falsifiable because it predicts that performing the experiment from January to March/April, the neutrino beam will be detected to arrive 50 ns later than light.
    Astronomy and Astrophysics 01/2012; · 5.08 Impact Factor
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    ABSTRACT: We have built two single photon very high speed photometers (Aqueye for the Asiago 1.8m telescope and Iqueye for the 3.5m ESO NTT) as prototypes of a `quantum' photometer for the European Extremely Large Telescope (E-ELT) The two photometers are the most accurate `time machines' available to optical astronomy. Under the control of a GPS receiver and a Rubidium clock, the arrival time of each detected photon is referenced to UTC with a precision better than 500 picoseconds, continuously for hours of data acquisition. Light curves for three optical pulsars (Crab, B0540-69, Vela) will be reported. Results from simultaneous observations of the Crab pulsar with the Jodrell Bank RadioTelescope will also be reported.
    General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
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    ABSTRACT: 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.
    Monthly Notices of the Royal Astronomical Society 02/2011; 412(4):2689 - 2694. · 5.52 Impact Factor
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    ABSTRACT: We are developing fast photon-counter instruments to study the rapid variability of astrophysical sources by time tagging photon arrival times with unprecedented accuracy, making use of a Rubidium clock and GPS receiver. The first realization of such optical photon-counters, dubbed AquEYE (the Asiago Quantum Eye), was mounted in 2008 at the 182 cm Copernicus Observatory in Asiago. AquEYE observed the Crab pulsar several times and collected data of extraordinary quality that allowed us to perform accurate optical timing of the Crab pulsar and to study the pulse shape stability on a timescale from days to years with an excellent definition. Our results reinforce the evidence for decadal stability of the inclination angle between the spin and magnetic axis of the Crab pulsar. Future realizations of our instrument will make use of the Galileo Global Navigation Satellite System (GNSS) time signal.
    Advances in Space Research 01/2011; · 1.18 Impact Factor
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    ABSTRACT: 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 (Jan. 18 and 20, and Dec. 14, 15, 16, 18) with the very high speed photon counting photometer Iqueye mounted at the ESO 3.6-m NTT in La Silla (Chile). The optical light curve derived by Iqueye shows a double structure in the main peak, with a raising edge steeper than the trailing edge. The double peak can be fitted by two Gaussians with the same height and FWHM of 13.3 and 15.5 ms respectively. Our new values of spin frequencies allow to extend by 3.5 years the time interval over which a reliable estimate of frequency first and second derivatives can be performed. A discussion of implications on the braking index and age of the pulsar is carried out. 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 around 1700 years. Comment: Accepted for publication in MNRAS
    12/2010;
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    ABSTRACT: Iqueye is a single photon counting very high speed photometer built for the ESO 3.5m New Technology Telescope (NTT) in La Silla (Chile) as prototype of a 'quantum' photometer for the 42m European Extremely Large Telescope (E-ELT). The optics of Iqueye splits the telescope pupil into four portions, each feeding a Single Photon Avalanche Diode (SPAD) operated in Geiger mode. The SPADs sensitive area has a diameter of 100 μm, with a quantum efficiency better than 55% at 500 nm, and a dark count less than 50 Hz. The quenching circuit and temperature control are integrated in each module. A time-to-digital converter (TDC) board, controlled by a rubidium oscillator plus a GPS receiver, time tags the pulses from the 4 channels. The individual times are stored in a 2 TeraByte memory. Iqueye can run continuously for hours, handling count rates up to 8 MHz, with a final absolute accuracy of each time tag better that 0.5 ns. A first very successful run was performed in Jan 2009; both very faint and very bright stars were observed, demonstrating the high photometric quality of the instrument. The first run allowed also to identify some opto-mechanical improvements, which have been implemented for a second run performed in Dec 2009. The present paper will describe the first version, the improvements implemented in the second one, and some of the obtained astronomical results.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    04/2010;
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    ABSTRACT: Iqueye is a single photon counting very high speed photometer built for the ESO 3.5m New Technology Telescope (NTT) in La Silla (Chile) as prototype of a 'quantum' photometer for the 42m European Extremely Large Telescope (E-ELT). The optics of Iqueye splits the telescope pupil into four portions, each feeding a Single Photon Avalanche Diode (SPAD) operated in Geiger mode. The SPADs sensitive area has a diameter of 100 mum, with a quantum efficiency better than 55% at 500 nm, and a dark count less than 50 Hz. The quenching circuit and temperature control are integrated in each module. A time-to-digital converter (TDC) board, controlled by a rubidium oscillator plus a GPS receiver, time tags the pulses from the 4 channels. The individual times are stored in a 2 TeraByte memory. Iqueye can run continuously for hours, handling count rates up to 8 MHz, with a final absolute accuracy of each time tag better that 0.5 ns. A first very successful run was performed in Jan 2009; both very faint and very bright stars were observed, demonstrating the high photometric quality of the instrument. The first run allowed also to identify some opto-mechanical improvements, which have been implemented for a second run performed in Dec 2009. The present paper will describe the first version, the improvements implemented in the second one, and some of the obtained astronomical results.
    Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 04/2010
  • 01/2010;
  • 01/2010;
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    ABSTRACT: As is well known, the Crab pulsar displays a light curve with a characteristic double peak profile having a period of 33 milliseconds. Its pulse profile has been extensively monitored by several authors; it is also used for testing high-speed photon counters. We have studied the optical light curve of the Crab pulsar by means of a novel photon counting instrument, that has the capability to provide data with exceptionally high temporal resolution and time tagging accuracy of each incoming photon.
    05/2009;
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    C. Germana, U. Kostic, A. Cadez, M. Calvani
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    ABSTRACT: Low Mass X-ray Binaries (LMXBs) 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 r_g 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 a black hole. Our numerical 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. Comment: 3 pages, 3 figures, to appear in the proceedings of the Symposium Simbol-X 2008 "Focusing on the hard X-ray universe"
    02/2009;
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    ABSTRACT: 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.
    Astronomy and Astrophysics 01/2009; · 5.08 Impact Factor
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    01/2008;