Physics - Journal Articles

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    Spin squeezing in internal bosonic Josephson junctions via enhanced shortcuts to adiabaticity
    (American Physical Society., 2023-11-17) Odelli, Manuel; Stojanović, Vladimir M.; Ruschhaupt, Andreas; Science Foundation Ireland; Deutsche Forschungsgemeinschaft
    We investigate a time-efficient and robust preparation of spin-squeezed states—a class of states of interest for quantum enhanced metrology—in internal bosonic Josephson junctions with a time-dependent nonlinear coupling strength between atoms in two different hyperfine states. We treat this state-preparation problem, which had previously been addressed using shortcuts to adiabaticity (STA), using the recently proposed analytical modification of this class of quantum control protocols that has become known as the enhanced STA (eSTA) method. We characterize the state-preparation process by evaluating the time dependence of the coherent spin-squeezing and number-squeezing parameters and the target-state fidelity. We show that the state-preparation times obtained using the eSTA method compare favorably to those found in previously proposed approaches. We also demonstrate that the increased robustness of the eSTA approach—compared to its STA counterpart—leads to additional advantages for potential experimental realizations of strongly spin-squeezed states in internal bosonic Josephson junctions.
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    Multicolour optical light curves of the companion star to the millisecond pulsar PSR J2051-0827
    (Oxford University Press, 2022-08-25) Dhillon, V. S.; Kennedy, Mark R.; Breton, R. P.; Clark, C. J.; Mata Sánchez, D.; Voisin, G.; Breedt, E.; Brown, A. J.; Dyer, M. J.; Green, M. J.; Kerry, P.; Littlefair, S. P.; Marsh, T. R.; Parsons, S. G.; Pelisoli, I.; Sahman, D. I.; Wild, J. F.; van Kerkwijk, M. H.; Stappers, B. W.; Seventh Framework Programme; European Research Council; Science and Technology Facilities Council; Horizon 2020; Irish Research Council; European Regional Development Fund; Consejería de Educación, Universidades, Cultura y Deportes, Gobierno de Canarias; Science and Technology Facilities Council
    We present simultaneous, multicolour optical light curves of the companion star to the black-widow pulsar PSR J2051−0827, obtained approximately 10 yr apart using ULTRACAM and HiPERCAM, respectively. The ULTRACAM light curves confirm the previously reported asymmetry in which the leading hemisphere of the companion star appears to be brighter than the trailing hemisphere. The HiPERCAM light curves, however, do not show this asymmetry, demonstrating that whatever mechanism is responsible for it varies on time-scales of a decade or less. We fit the symmetrical HiPERCAM light curves with a direct-heating model to derive the system parameters, finding an orbital inclination of 55.9+4.8−4.1 degrees, in good agreement with radio-eclipse constraints. We find that approximately half of the pulsar’s spin-down energy is converted to optical luminosity, resulting in temperatures ranging from approximately 5150+190−190 K on the day side to 2750+130−150 K on the night side of the companion star. The companion star is close to filling its Roche lobe (⁠fRL=0.88+0.02−0.02⁠) and has a mass of 0.039+0.010−0.011 M⊙, giving a mean density of 20.24+0.59−0.44 g cm−3 and an apsidal motion constant in the range 0.0036 < k2 < 0.0047. The companion mass and mean density values are consistent with those of brown dwarfs, but the apsidal motion constant implies a significantly more centrally condensed internal structure than is typical for such objects.
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    A black widow population dissection through HiPERCAM multiband light-curve modelling
    (Oxford University Press, 2023-01-19) Mata Sánchez, D.; Kennedy, Mark R.; Clark, C. J.; Breton, R. P.; Dhillon, V. S.; Voisin, G.; Camilo, F.; Littlefair, S.; Marsh, T. R.; Horizon 2020; European Regional Development Fund; Consejería de Educación, Universidades, Cultura y Deportes, Gobierno de Canarias; Ministerio de Ciencia e Innovación; Royal Society
    Black widows are extreme millisecond pulsar binaries where the pulsar wind ablates their low-mass companion stars. In the optical range, their light curves vary periodically due to the high irradiation and tidal distortion of the companion, which allows us to infer the binary parameters. We present simultaneous multiband observations obtained with the HIPERCAM instrument at the 10.4-m GTC telescope for six of these systems. The combination of this five-band (us, gs,rs, is, zs) fast photometer with the world’s largest optical telescope enables us to inspect the light curve range near minima. We present the first light curve for PSR J1641+8049, as well as attain a significant increase in signal to noise and cadence compared with previous publications for the remaining five targets: PSR J0023+0923, PSR J0251+2606, PSR J0636+5129, PSR J0952−0607, and PSR J1544+4937. We report on the results of the light-curve modelling with the ICARUS code for all six systems, which reveals some of the hottest and densest companion stars known. We compare the parameters derived with the limited but steadily growing black widow population for which optical modelling is available. We find some expected correlations, such as that between the companion star mean density and the orbital period of the system, which can be attributed to the high number of Roche-lobe filling companions. On the other hand, the positive correlation between the orbital inclination and the irradiation temperature of the companion is puzzling. We propose such a correlation would arise if pulsars with magnetic axis orthogonal to their spin axis are capable of irradiating their companions to a higher degre
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    Neutron star mass estimates from gamma-ray eclipses in spider millisecond pulsar binaries
    (Springer Nature Ltd., 2023-01-26) Clark, C. J.; Kerr, M.; Barr, E. D.; Bhattacharyya, B.; Breton, R. P.; Bruel, P.; Camilo, F.; Chen, W.; Cognard, I.; Cromartie, H. T.; Deneva, J.; Dhillon, V. S.; Guillemot, L.; Kennedy, Mark R.; Kramer, M.; Lyne, A. G.; Mata Sánchez, D.; Nieder, L.; Phillips, C.; Ransom, S. M.; Ray, P. S.; Roberts, M. S. E.; Roy, J.; Smith, D. A.; Spiewak, R.; Stappers, B. W.; Tabassum, S.; Theureau, G.; Voisin, G.; Horizon 2020; Max-Planck-Gesellschaft; Science and Technology Facilities Council; National Aeronautics and Space Administration; National Science Foundation; Irish Research Council; Department of Atomic Energy, Government of India; European Regional Development Fund; Consejería de Educación, Universidades, Cultura y Deportes, Gobierno de Canarias
    Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling.
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    Composition of giant planets: The roles of pebbles and planetesimals
    (EDP Sciences, 2023-10-17) Danti, C.; Bitsch, Bertram; Mah, J.; European Research Council; Deutsche Forschungsgemeinschaft
    One of the current challenges of planet formation theory is to explain the enrichment of observed exoplanetary atmospheres. While past studies have focused on scenarios where either pebbles or planetesimals are the main drivers of heavy element enrichment, here we combine the two approaches to understand whether the composition of a planet can constrain its formation pathway. We study three different formation scenarios: pebble accretion, pebble accretion with planetesimal formation inside the disc, and combined pebble and planetesimal accretion. We used the chemcomp code to perform semi-analytical 1D simulations of protoplanetary discs, including viscous evolution, pebble drift, and simple chemistry to simulate the growth of planets from planetary embryos to gas giants as they migrate through the disc, while simultaneously tracking their composition. Our simulations confirm that the composition of the planetary atmosphere is dominated by the accretion of gas vapour enriched by inward-drifting and evaporating pebbles. Including planetesimal formation hinders this enrichment because many pebbles are locked into planetesimals and cannot evaporate and enrich the disc. This results in a dramatic drop in accreted heavy elements in the cases of planetesimal formation and accretion, demonstrating that planetesimal formation needs to be inefficient in order to explain planets with high heavy element content. On the other hand, accretion of planetesimals enhances the refractory component of the atmosphere, leading to low volatile-to-refractory ratios in the atmosphere, in contrast to the majority of pure pebble simulations. However, low volatile-to-refractory ratios can also be achieved in the pure pebble accretion scenario if the planet migrates all the way into the inner disc and accretes gas that is enriched in evaporated refractories. Distinguishing these two scenarios requires knowledge about the planet’s atmospheric C/H and O/H ratios, which are much higher in the pure pebble scenario compared to the planetesimal formation and accretion scenario. This implies that a detailed knowledge of the composition of planetary atmospheres could help to distinguish between the different formation scenarios.