Examinando por Autor "Goldoni, P."

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    1RXS J180408.9-342058: An ultra compact X-ray binary candidate with a transient jet
    (EDP SCIENCES, 2016-03) Baglio, M.C.; D’Avanzo, P.; Campana, S.; Goldoni, P.; Masetti, N.; Muñoz-Darias, T.; Patiño-Álvarez, V.; Chavushyan, V.
    Aims. We present a detailed near-infrared/optical/UV study of the transient low-mass X-ray binary 1RXS J180408.9-342058 performed during its 2015 outburst, which is aimed at determining the nature of its companion star. Methods. We obtained three optical spectra (R similar to 1000) at the 2.1 m San Pedro Martir Observatory telescope (Mexico). We performed optical and NIR photometric observations with both the REM telescope and the New Technology Telescope (NTT) in La Silla. We obtained optical and UV observations from the Swift archive. Finally, we performed optical polarimetry of the source using the EFOSC2 instrument mounted on the NTT. Results. The optical spectrum of the source is almost featureless since the hydrogen and He I emissions lines, typically observed in LMXBs, are not detected. Similarly, carbon and oxygen lines are not observed either. We marginally detect the He II 4686 angstrom emission line, suggesting the presence of helium in the accretion disc. No significant optical polarisation level was observed. Conclusions. The lack of hydrogen and He I emission lines in the spectrum implies that the companion is likely not a main-sequence star. Driven by the tentative detection of the He II 4686 angstrom emission line, we suggest that the system could harbour a helium white dwarf. If this is the case, 1RXS J180408.9-342058 would be an ultra-compact X-ray binary. By combining an estimate of the mass accretion rate together with evolutionary tracks for a He white dwarf, we obtain a tentative orbital period of similar to 40 min. We also built the NIR-optical-UV spectral energy distribution (SED) of the source at two different epochs. One SED was gathered when the source was in the soft X-ray state and this SED is consistent with the presence of a single thermal component. The second SED, obtained when the source was in the hard X-ray state, shows a thermal component along with a tail in the NIR, which likely indicates the presence of a (transient) jet.
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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.