Quantum Astronomy at the University and INAF Astronomical Observatory of Padova, Italy

Twenty years ago, we started to apply quantum optics to the astronomical research carried out inside the Department of Physics and Astronomy and the INAF Astronomical Observatory in Padova, Italy. The initial activities were stimulated by the project of the European Southern Observatory (ESO) to build a 100 m diameter telescope, the Overwhelmingly Large (OWL) telescope. The enormous photon flux expected from such an aperture suggested that quantum optics concepts be utilized in order to obtain novel astrophysical results. Following initial successful attempts to utilize the orbital angular momentum of the light beam to enhance the visibility of faint companions to bright stars, the Padova team concentrated its efforts on very high time resolution, in order to measure and store the arrival time of celestial photons to better than one nanosecond. To obtain observational results, we built two photon counting photometers (AquEye and IquEye) to be used with our telescopes of the Asiago Observatory and with 4 m class telescopes such as the ESO New Technology Telescope (NTT) in Chile. This paper firstly describes these two instruments and then expounds the results obtained on pulsar light curves, lunar occultations and the first photon counting intensity interferometry measurements of the bright star Vega. Indeed, the correlation of photon arrival times on two or more apertures can lead to extremely high angular resolutions, as shown around 1970 by Hanbury Brown and Twiss. Prospects for quantum intensity interferometry with arrays of Cherenkov light telescopes will also be described

Precise optical timing of PSR J1023+0038, the first millisecond pulsar detected with Aqueye+ in Asiago

We report the first detection of an optical millisecond pulsar with the fast photon counter Aqueye+ in Asiago. This is an independent confirmation of the detection of millisecond pulsations from PSR J1023+0038 obtained with SiFAP at the Telescopio Nazionale Galileo. We observed the transitional millisecond pulsar PSR J1023+0038 with Aqueye+ mounted at the Copernicus telescope in January 2018. Highly significant pulsations were detected. The rotational period is in agreement with the value extrapolated from the X-ray ephemeris, while the time of passage at the ascending node is shifted by $11.55 \pm 0.08$ s from the value predicted using the orbital period from the X-rays. An independent optical timing solution is derived over a baseline of a few days, that has an accuracy of $\sim 0.007$ in pulse phase ($\sim 12$ $\mu$s in time). This level of precision is needed to derive an accurate coherent timing solution for the pulsar and to search for possible phase shifts between the optical and X-ray pulses using future simultaneous X-ray and optical observations.Comment: 6 pages, 4 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

Investigating the accuracy achievable in reconstructing the angular sizes of stars through stellar intensity interferometry observations

Context: In recent years, stellar intensity interferometry has seen renewed interest from the astronomical community because it can be efficiently applied to Cherenkov telescope arrays. Aims: We have investigated the accuracy that can be achieved in reconstructing stellar sizes by fitting the visibility curve measured on the ground. The large number of expected available astronomical targets, the limited number of nights in a year, and the likely presence of multiple baselines will require careful planning of the observational strategy to maximise the scientific output. Methods: We studied the trend of the error on the estimated angular size, considering the uniform disk model, by varying several parameters related to the observations, such as the total number of measurements, the integration time, the signal-to-noise ratio, and different positions along the baseline. Results: We found that measuring the value of the zero-baseline correlation is essential to obtain the best possible results. Systems that can measure this value directly or for which it is known in advance will have better sensitivity. We also found that to minimise the integration time, it is sufficient to obtain a second measurement at a baseline half-way between 0 and that corresponding to the first zero of the visibility function. This function does not have to be measured at multiple positions. Finally, we obtained some analytical expressions that can be used under specific conditions to determine the accuracy that can be achieved in reconstructing the angular size of a star in advance. This is useful to optimise the observation schedule.Comment: Accepted for publication by Astronomy & Astrophysics (A&A

Preliminary error budget analysis of the coronagraphic instrument metis for the solar orbiter ESA mission

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is the solar coronagraph foreseen for the ESA Solar Orbiter mission. METIS is conceived to image the solar corona from a near-Sun orbit in three different spectral bands: in the HeII EUV narrow band at 30.4 nm, in the HI UV narrow band at 121.6 nm, and in the polarized visible light band (590 – 650 nm). It also incorporates the capability of multi-slit spectroscopy of the corona in the UV/EUV range at different heliocentric heights. METIS is an externally occulted coronagraph which adopts an “inverted occulted” configuration. The Inverted external occulter (IEO) is a small circular aperture at the METIS entrance; the Sun-disk light is rejected by a spherical mirror M0 through the same aperture, while the coronal light is collected by two annular mirrors M1-M2 realizing a Gregorian telescope. To allocate the spectroscopic part, one portion of the M2 is covered by a grating (i.e. approximately 1/8 of the solar corona will not be imaged). This paper presents the error budget analysis for this newconcept coronagraph configuration, which incorporates 3 different sub-channels: UV and EUV imaging sub-channel, in which the UV and EUV light paths have in common the detector and all of the optical elements but a filter, the polarimetric visible light sub-channel which, after the telescope optics, has a dedicated relay optics and a polarizing unit, and the spectroscopic sub-channel, which shares the filters and the detector with the UV-EUV imaging one, but includes a grating instead of the secondary mirror. The tolerance analysis of such an instrument is quite complex: in fact not only the optical performance for the 3 sub-channels has to be maintained simultaneously, but also the positions of M0 and of the occulters (IEO, internal occulter and Lyot stop), which guarantee the optimal disk light suppression, have to be taken into account as tolerancing parameters. In the aim of assuring the scientific requirements are optimally fulfilled for all the sub-channels, the preliminary results of manufacturing, alignment and stability tolerance analysis for the whole instrument will be described and discussed

Spin-down rate of the transitional millisecond pulsar PSR J1023+0038 in the optical band with Aqueye+

We present a timing analysis of the transitional millisecond pulsar PSR J1023+0038 using observations taken between January 2018 and January 2020 with the high time resolution photon counter Aqueye+ mounted at the 1.82 m Copernicus telescope in Asiago. We report the first measurement of the timing solution and the frequency derivative of PSR J1023+0038 based entirely on optical data. The spin-down rate of the pulsar is $(-2.53 \pm 0.04) \times 10^{-15}$ Hz$^2$, which is $\sim$20% slower than that measured from the X-ray observations taken in 2013-2016 and $\sim$5% faster than that measured in the radio band during the rotation-powered state.Comment: 6 pages, 4 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

Physical Structure of a Coronal Streamer in the Closed-Field Region as Observed from UVCS/SOHO and SXT/Yohkoh

We analyze a coronal helmet streamer observed on 1996 July 25 using instruments aboard two solar spacecraft, the Ultraviolet Coronagraph Spectrometer (UVCS) on board Solar and Heliospheric Observatory (SOHO) and the Soft X-Ray Telescope (SXT) on board Yohkoh. We derive temperatures and electron densities at 1.15 R☉ from SXT/Yohkoh observations. At this height, the streamer temperature is about log T (K) = 6.28 ± 0.05, and the electron density is about log ne(cm-3) = 8.09 ± 0.26, while at 1.5 R☉ a temperature of log T (K) = 6.2 and a density of log ne(cm-3) = 7.1 are obtained by UVCS/SOHO. Within the measurement uncertainty this suggests a constant temperature from the base of the streamer to 1.5 R☉. Electron density measurements suggest that the gas in the streamer core is close to hydrostatic equilibrium. Comparison with potential field models for the magnetic field suggests a plasma β larger than 1 in the closed-field region in the streamer. In deriving electron densities and temperatures from the SXT/Yohkoh data, we include the effects of abundance anomalies on the SXT filter response. We use the elemental abundances derived from the UVCS/SOHO observations to estimate the first ionization potential and gravitational settling effects. We then give the set of abundances for the solar corona, which agrees with our observations. In addition, we analyzed the SXT data from 6 consecutive days. We found that from 1996 July 22 to July 27, the physical properties of the streamer are nearly constant. We conclude that we may be observing the same loop system over 6 days

Bilobate comet morphology and internal structure controlled by shear deformation

Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.Additional co-authors: M. A. Barucci, J.-L. Bertaux, I. Bertini, D. Bodewits, G. Cremonese, V. Da Deppo, S. Debei, M. De Cecco, J. Deller, S. Fornasier, M. Fulle, P. J. Gutiérrez, C. Güttler, W.-H. Ip, H. U. Keller, L. M. Lara, F. La Forgia, M. Lazzarin, A. Lucchetti, J. J. López-Moreno, F. Marzari, M. Massironi, S. Mottola, N. Oklay, M. Pajola, L. Penasa, F. Preusker, H. Rickman, F. Scholten, X. Shi, I. Toth, C. Tubiana & J.-B. Vincen

In-lab characterization of HYPSOS, a novel stereo hyperspectral observing system: first results

HYPSOS (HYPerspectral Stereo Observing System, patented) is a novel remote sensing instrument able to extract the spectral information from the two channels of a pushbroom stereo camera; thus it simultaneously provides 4D information, spatial and spectral, of the observed features. HYPSOS has been designed to be a compact instrument, compatible with small satellite applications, to be suitable both for planetary exploration as well for terrestrial environmental monitoring. An instrument with such global capabilities, both in terms of scientific return and needed resources, is optimal for fully characterizing the observed surface of investigation. HYPSOS optical design couples a pair of folding mirrors to a modified three mirror anastigmat telescope for collecting the light beams from the optical paths of the two stereo channels; then, on the telescope focal plane, there is the entrance slit of an imaging spectrograph, which selects and disperses the light from the two stereo channels on a bidimensional detector. With this optical design, the two stereo channels share the large majority of the optical elements: this allowed to realize a very compact instrument, which needs much less resources than an equivalent system composed by a stereo camera and a spectrometer. To check HYPSOS actual performance, we realized an instrument prototype to be operated in a laboratory environment. The laboratory setup is representative of a possible flight configuration: the light diffused by a surface target is collimated on the HYPSOS channel entrance apertures, and the target is moved with respect to the instrument to reproduce the in- flight pushbroom acquisition mode. Here we describe HYPSOS and the ground support equipment used to characterize the instrument, and show the preliminary results of the instrument alignment activities