Search

The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May, which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here, we report on the detailed properties of light curves during the outburst and the post-outburst active phase. We reveal that rapid optical variations of similar to 100 s became more prominent when a thermal flare weakened and the optical spectrum flattened in the I-c, R-c, and V-band regions. In conjunction with the flat spectrum in the radio ran-e, this strongly indicates that the origin of rapid variations is not thermal emission, but synchrotron emission.Just after the outburst, we detected repeated flares at optical and X-ray wavelengths. The optical and X-ray light curves exhibited a strong correlation with the X-rays lagging by about 7 min. The X-ray lag can be understood in terms of a hot region propagating into the inner region of the accretion flow. The short X-ray lag, however, requires modifications of this simple scenario to account for the short propagation time. We also detected high-amplitude rapid variations 50 days after the outburst, which we call optical flashes. During the most prominent optical flash, the object brightened by 1.2 mag only within 30 s. The released energy indicates that the emission source should be at the innermost region of the accretion flow.

The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May, which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here, we report on the detailed properties of light curves during the outburst and the post-outburst active phase. We reveal that rapid optical variations of similar to 100 s became more prominent when a thermal flare weakened and the optical spectrum flattened in the I-c, R-c, and V-band regions. In conjunction with the flat spectrum in the radio ran-e, this strongly indicates that the origin of rapid variations is not thermal emission, but synchrotron emission.Just after the outburst, we detected repeated flares at optical and X-ray wavelengths. The optical and X-ray light curves exhibited a strong correlation with the X-rays lagging by about 7 min. The X-ray lag can be understood in terms of a hot region propagating into the inner region of the accretion flow. The short X-ray lag, however, requires modifications of this simple scenario to account for the short propagation time. We also detected high-amplitude rapid variations 50 days after the outburst, which we call optical flashes. During the most prominent optical flash, the object brightened by 1.2 mag only within 30 s. The released energy indicates that the emission source should be at the innermost region of the accretion flow.

We present a spectral analysis of the black hole candidate and X-ray transient source Swift J1753.5 0127 making use of simultaneous observations of XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was in outburst. The aim of this paper is to test whether a thermal component due to the accretion disc is present in the X-ray spectrum. We fit the data with a range of spectral models, and we find that for all of these models the fits to the X-ray energy spectra significantly require the addition of the disc black-body component. We also find a broad iron emission line at around 6.5 keV, most likely due to iron in the accretion disc. Our results confirm the existence of a cool inner disc extending near or close to the innermost circular orbit (ISCO).We further discovered broad emission lines of NVII and OVIII at ~ 0.52 keV and 0.65 keV, respectively in the RGS spectrum of Swift J1753.5-0127., Comment: 17 pages, 6 figures, 4 tables, Accepted 2013 February 19 for publication in Monthly Notices of the Royal Astronomical Society Main Journal

We apply the global mode analysis, which has been recently developed for the modeling of kHz quasi-periodic oscillations (QPOs) from neutron stars, to the inner region of an accretion disk around a rotating black hole. Within a pseudo-Newtonian approach that keeps the ratio of the radial epicyclic frequency kappa to the orbital frequency Omega the same as the corresponding ratio for a Kerr black hole, we determine the innermost disk region where the hydrodynamic modes grow in amplitude. We find that the radiation flux emerging from the inner disk has the highest values within the same region. Using the flux-weighted averages of the frequency bands over this region we identify the growing modes with highest frequency branches Omega+kappa and Omega to be the plausible candidates for the high-frequency QPO pairs observed in black hole systems. The observed frequency ratio around 1.5 can therefore be understood naturally in terms of the global free oscillations in the innermost region of a viscous accretion disk around a black hole without invoking a particular resonance to produce black hole QPOs. Although the frequency ratio / is found to be not sensitive to the black hole's spin which is good for explaining the high-frequency QPOs, it may work as a limited diagnostic of the spin parameter to distinguish black holes with very large spin from the slowly rotating ones. Within our model we estimate the frequency ratio of a high-frequency QPO pair to be greater than 1.5 if the black hole is a slow rotator. For fast rotating black holes, we expect the same ratio to be less than 1.5.

The black hole X-ray binary V4641 Sgr experienced an outburst in 2002 May which was detected at X-ray, optical, and radio wavelengths. The outburst lasted for only 6 days, but the object remained active for the next several months. Here we report on the detailed properties of light curves during the outburst and the post-outburst active phase. We reveal that rapid optical variations of ~100 s became more prominent when a thermal flare weakened and the optical spectrum flattened in the Ic, Rc, and V-band region. In conjunction with the flat spectrum in the radio range, this strongly indicates that the origin of rapid variations is not thermal emission, but synchrotron emission. Just after the outburst, we detected repeated flares at optical and X-ray wavelengths. The optical and X-ray light curves exhibited a strong correlation, with the X-rays, lagging by about 7 min. The X-ray lag can be understood in terms of a hot region propagating into the inner region of the accretion flow. The short X-ray lag, however, requires modifications of this simple scenario to account for the short propagation time. We also detected rapid optical variations with surprisingly high amplitude 50 days after the outburst, which we call optical flashes. During the most prominent optical flash, the object brightened by 1.2 mag only within 30 s. The released energy indicates that the emission source should be at the innermost region of the accretion flow., Comment: 17 pages, 11 figures, PASJ accepted