Examinando por Autor "Israel, G."

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    Broad-band characteristics of seven new hard X-ray selected cataclysmic variables
    (Oxford University Press, 2017-10) Bernardini, F.; de Martino, D.; Mukai, K.; Russell, D.M.; Falanga, M.; Masetti, N.; Ferrigno, C.; Israel, G.
    We present timing and spectral analysis of a sample of seven hard X-ray selected cataclysmic variable candidates based on simultaneous X-ray and optical observations collected with XMM–Newton, complemented with Swift/BAT and INTEGRAL /IBIS hard X-ray data and ground-based optical photometry. For six sources, X-ray pulsations are detected for the first time in the range of ∼296–6098 s, identifying them as members of the magnetic class. Swift J0927.7−6945, Swift J0958.0−4208, Swift J1701.3−4304, Swift J2113.5+5422 and possibly PBC J0801.2−4625 are intermediate polars (IPs), while Swift J0706.8+0325 is a short (1.7 h) orbital period polar, the 11th hard X-ray-selected identified so far. X-ray orbital modulation is also observed in Swift J0927.7−6945 (5.2 h) and Swift J2113.5+5422 (4.1 h). Swift J1701.3−4304 is discovered as the longest orbital period (12.8 h) deep eclipsing IP. The spectra of the magnetic systems reveal optically thin multitemperature emission between 0.2 and 60 keV. Energy-dependent spin pulses and the orbital modulation in Swift J0927.7−6945 and Swift J2113.5+5422 are due to intervening local high-density absorbing material (NH ∼ 1022 − 23 cm−2). In Swift J0958.0−4208 and Swift J1701.3−4304, a soft X-ray blackbody (kT ∼ 50 and ∼80 eV) is detected, adding them to the growing group of ‘soft’ IPs. White dwarf masses are determined in the range of ∼0.58–1.18 M, indicating massive accreting primaries in five of them. Most sources accrete at rates lower than the expected secular value for their orbital period. Formerly proposed as a long-period (9.4 h) nova-like CV, Swift J0746.3−1608 shows peculiar spectrum and light curves suggesting either an atypical low-luminosity CV or a low-mass X-ray binary.
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    GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226
    (Oxford University Press, 2018-02) Brocato, E.; Branchesi, M.; Cappellaro, E.; Covino, S.; Grado, A.; Greco, G.; Limatola, L.; Stratta, G.; Yang, S.; Campana, S.; D'Avanzo, P.; Getman, F.; Melandri, A.; Nicastro, L.; Palazzi, E.; Pian, E.; Piranomonte, S.; Pulone, L.; Rossi, A.; Tomasella, L.; Amati, L.; Antonelli, L.A.; Ascenzi, S.; Benetti, S.; Bulgarelli, A.; Capaccioli, M.; Cella, G.; Dadina, M.; De Cesare, G.; D'Elia, V.; Ghirlanda, G.; Ghisellini, G.; Giuffrida, G.; Iannicola, G.; Israel, G.; Lisi, M.; Longo, F.; Mapelli, M.; Marinoni, S.; Marrese, P.; Masetti, N.; Patricelli, B.; Possenti, A.; Radovich, M.; Razzano, M.; Salvaterra, R.; Schipani, P.; Spera, M.; Stamerra, A.; Stella, L.; Tagliaferri, G.; Testa, V.
    We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitationalWave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of 90 and 72 deg 2 for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the r band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB150827A detected by Fermi-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational wave events is evaluated for the present and future follow-up surveys. © 2017 The Author(s).
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    Swift J0525.6+2416 and IGR J04571+4527: Two new hard X-ray-selected magnetic cataclysmic variables identified with XMM-Newton
    (Oxford University Press, 2015-11) Bernardini, F.; de Martino, D.; Mukai, K.; Israel, G.; Falanga, M.; Ramsay, G.; Masetti, N.
    IGR J04571+4527 and Swift J0525.6+2416 are two hard X-ray sources detected in the Swift/BAT and INTEGRAL/IBIS surveys. They were proposed to be magnetic cataclysmic variables of the intermediate polar (IP) type, based on optical spectroscopy. IGR J04571+4527 also showed a 1218 s optical periodicity, suggestive of the rotational period of a white dwarf, further pointing towards an IP classification. We here present detailed X-ray (0.3–10 keV) timing and spectral analysis performed with XMM–Newton, complemented with hard X-ray coverage (15–70 keV) from Swift/BAT. These are the first high-S/N observations in the soft X-ray domain for both sources, allowing us to identify the white dwarf X-ray spin period of Swift J0525.6+2416 (226.28 s), and IGR J04571+4527 (1222.6 s). A model consisting of multitemperature optically thin emission with complex absorption adequately fits the broad-band spectrum of both sources. We estimate a white dwarf mass of about 1.1 and 1.0 M⊙ for IGR J04571+4527 and Swift J0525.6+2416, respectively. The above characteristics allow us to unambiguously classify both sources as IPs, confirming the high incidence of this subclass among hard X-ray emitting cataclysmic variables.