Examinando por Autor "Kajava, J. J. E."

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Acceso Abierto
Observations of a radio-bright, X-ray obscured GRS 1915+105
(Oxford Academics: Oxford University Press, 2021-02-24) Motta, Sara E.; Kajava, J. J. E.; Giustini, M.; Williams, D. R. A.; Del Santo, M.; Fender, R.; Green, D. A.; Heywood, I.; Rhodes, L.; Segreto, A.; Sivakoff, G.; Woudt, P. A.; Science and Technology Facilities Council (STFC); Agencia Estatal de Investigación (AEI); Comunidad de Madrid; Istituto Nazionale di Astrofisica (INAF); European Commission (EC); Motta, S. E. [0000-0002-6154-5843]; Kajava, J. J. E. [0000-0002-3010-8333]; Williams, D. R. A. [0000-0001-7361-0246]; Del Santo, M. [0000-0002-1793-1050]; Green, D. A. [0000-0003-3189-9998]; Woudt, P. A. [0000-0002-6896-1655]
The Galactic black hole transient GRS 1915+105 is famous for its markedly variable X-ray and radio behaviour, and for being the archetypal galactic source of relativistic jets. It entered an X-ray outburst in 1992 and has been active ever since. Since 2018 GRS 1915+105 has declined into an extended low-flux X-ray plateau, occasionally interrupted by multiwavelength flares. Here, we report the radio and X-ray properties of GRS 1915+105 collected in this new phase, and compare the recent data to historic observations. We find that while the X-ray emission remained unprecedentedly low for most of the time following the decline in 2018, the radio emission shows a clear mode change half way through the extended X-ray plateau in 2019 June: from low flux (∼3 mJy) and limited variability, to marked flaring with fluxes two orders of magnitude larger. GRS 1915+105 appears to have entered a low-luminosity canonical hard state, and then transitioned to an unusual accretion phase, characterized by heavy X-ray absorption/obscuration. Hence, we argue that a local absorber hides from the observer the accretion processes feeding the variable jet responsible for the radio flaring. The radio–X-ray correlation suggests that the current low X-ray flux state may be a signature of a super-Eddington state akin to the X-ray binaries SS433 or V404 Cyg.
Acceso Abierto
Rapid late-time X-ray brightening of the tidal disruption event OGLE16aaa
(EDP Sciences, 2020-07-16) Kajava, J. J. E.; Giustini, M.; Saxton, R. D.; Miniutti, G.; Agencia Estatal de Investigación (AEI); Comunidad de Madrid; Kajava, J. J. E. [0000-0002-3010-8333]; Giustini, M. [0000-0002-1329-658X]; Unidad de Excelencia Científica María de Maeztu del Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
Stars that pass too close to a super-massive black hole may be disrupted by strong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) which rapidly brightened and peaked in the optical/UV bands in early 2016 and subsequently decayed over the rest of the year. OGLE16aaa was detected in an XMM-Newton X-ray observation on June 9, 2016 with a flux slightly below the Swift/XRT upper limits obtained during the optical light curve peak. Between June 16–21, 2016, Swift/XRT also detected OGLE16aaa and based on the stacked spectrum, we could infer that the X-ray luminosity had jumped up by more than a factor of ten in just one week. No brightening signal was seen in the simultaneous optical/UV data to cause the X-ray luminosity to exceed the optical/UV one. A further XMM-Newton observation on November 30, 2016 showed that almost a year after the optical/UV peak, the X-ray emission was still at an elevated level, while the optical/UV flux decay had already leveled off to values comparable to those of the host galaxy. In all X-ray observations, the spectra were nicely modeled with a 50–70 eV thermal component with no intrinsic absorption, with a weak X-ray tail seen only in the November 30 XMM-Newton observation. The late-time X-ray behavior of OGLE16aaa strongly resembles the tidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint the time delay between the initial optical TDE onset and the X-ray brightening to 182 ± 5 days, which may possibly represent the timescale between the initial circularization of the disrupted star around the super-massive black hole and the subsequent delayed accretion. Alternatively, the delayed X-ray brightening could be related to a rapid clearing of a thick envelope that covers the central X-ray engine during the first six months.
Acceso Abierto
Rapid spectral transition of the black hole binary V404 Cygni
(EDP Sciences, 2020-02-13) Kajava, J. J. E.; Sánchez Fernández, C.; Alfonso Garzón, J.; Motta, Sara E.; Veledina, A.; Science and Technology Facilities Council (STFC); Academy of Finland (AKA); Agencia Estatal de Investigación (AEI); Ministry of Education and Science of the Russian Federation (Minobrnauka); Kajava, J. J. E. [0000-0002-3010-8333]; Alfonso Garzón, J. [0000-0003-0852-3474]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
During the June 2015 outburst of the black hole binary V404 Cyg, rapid changes in the X-ray brightness and spectra were common. The INTEGRAL monitoring campaign detected spectacular Eddington-limited X-ray flares, but also rapid variations at much lower flux levels. On 2015 June 21 at 20 h 50 min, the 3–10 keV JEM-X data as well as simultaneous optical data started to display a gradual brightening from one of these low-flux states. This was followed 15 min later by an order-of-magnitude increase of flux in the 20–40 keV IBIS/ISGRI light curve in just 15 s. The best-fitting model for both the pre- and post-transition spectra required a Compton-thick partially covering absorber. The absorber parameters remained constant, but the spectral slope varied significantly during the event, with the photon index decreasing from Γ ≈ 3.7 to Γ ≈ 2.3. We propose that the rapid 20–40 keV flux increase was either caused by a spectral state transition that was hidden from our direct view, or that there was a sudden reduction in the amount of Compton down-scattering of the primary X-ray emission in the disk outflow.