Examinando por Autor "Russell, D.M."

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    A Misfired Outburst in the Neutron Star X-Ray Binary Centaurus X-4
    (Institute of Physics, 2022-05-01) Baglio, M.C.; Saikia, P.; Russell, D.M.; Homan, J.; Waterval, S.; Bramich, D.M.; Campana, S.; Lewis, F.; Eijnden, J. Van Den; Alabarta, K.; Covino, S.; D'Avanzo, P.; Goldoni, P.; Masetti, N.; Muñoz-Darias, T.
    We report on a long-Term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 yr. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disk; we detect short-duration flares, likely originating also in the disk, superimposed with a small-Amplitude (<0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-Term (∼1/42300 days) downward trend, followed by a shorter (∼1/41000 days) upward one, is observed in the disk light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disk instability model. For Cen X-4, the rise of the optical flux proceeded for ∼1/43 yr, and culminated in a flux increase at all wavelengths (optical-UV-X-rays) at the end of 2020. This increase faded after ∼1/42 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disk. The propagation might have stalled soon after the ignition due to the increasing surface density in the disk that the front encountered while propagating outward. The stall was likely eased by the low-level irradiation of the outer regions of the large accretion disk, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission. © 2022. The Author(s). Published by the American Astronomical Society.
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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.