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Register a new userMissing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
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Fiamma Capitanio Dr.
Publication date
2015
fields
Physics
and research's language is
English
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4U 1630-472 is a recurrent X-ray transient classified as a black-hole candidate from its spectral and timing properties. One of the peculiarities of this source is the presence of regular outbursts with a recurrence period between 600 and 730 d that has been observed since the discovery of the source in 1969. We report on a comparative study on the spectral and timing behaviour of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years of activity. We show that, in spite of having a similar spectral and timing behaviour in the energy range between 3 and 30 keV, these three outbursts show pronounced differences above 30 keV. In fact, the 2010 outburst extends at high energies without any detectable cut-off until 150-200 keV, while the two previous outbursts that occurred in 2006 and 2008 are not detected at all above 30 keV. Thus, in spite of a very similar accretion disk evolution, these three outbursts exhibit totally different characteristics of the Compton electron corona, showing a softening in their evolution rarely observed before in a low mass X-ray binary hosting a black hole. We argue the possibility that the unknown perturbation that causes the outbursts to be equally spaced in time could be at the origin of this particular behaviour. Finally we describe several possible scenarios that could explain the regularity of the outbursts, identifying the most plausible, such as a third body orbiting around the binary system.
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We re-analyzed SUZAKU data of the black hole candidate 4U 1630-472 being in the high/soft state. We show that the continuum X-ray spectrum of 4U 1630-472 with iron absorption lines can be satisfactorily modeled by the spectrum from an accretion disk atmosphere. Absorption lines of highly ionized iron originating in hot accretion disk atmosphere can be an alternative or complementary explanation to the wind model usually favored for these type of sources. We model continuum and line spectra using a single model. Absorption lines of highly ionized iron can origin in upper parts of the disk atmosphere which is intrinsically hot due to high disk temperature. Iron line profiles computed with natural, thermal and pressure broadenings match very well observations. We showed that the accretion disk atmosphere can effectively produce iron absorption lines observed in 4U 1630-472 spectrum. Absorption line arising in accretion disk atmosphere is the important part of the observed line profile, even if there are also other mechanisms responsible for the absorption features. Nevertheless, the wind theory can be an artifact of the fitting procedure, when the continuum and lines are fitted as separate model components.
We present an in-depth spectral and timing analysis of the Black Hole binary 4U 1630-472 during 2016 and 2018 outbursts as observed by textit{AstroSat} and textit{MAXI}. The extensive coverage of the outbursts with textit{MAXI} is used to obtain the Hardness Intensity Diagram (HID). The source follows a `c-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a `super-outburst and `mini-outbursts. We model the broadband energy spectra ($0.7-20.0$ keV) of textit{AstroSat} observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature ($T_{in}$) $sim$ 1.3 keV and increase in photon index ($Gamma$) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the textit{AstroSat} campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states - the `canonical hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no Quasi-periodic Oscillations (QPOs) are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity and two component flow model to be $3-9$ $M_{odot}$. Finally, we discuss the possible implications of our findings.
The mechanisms that drive disk winds are a window into the physical processes that underlie the disk. Stellar-mass black holes are an ideal setting in which to explore these mechanisms, in part because their outbursts span a broad range in mass accretion rate. We performed a spectral analysis of the disk wind found in six Chandra/HETG observations of the black hole candidate 4U~1630$-$472, covering a range of luminosities over two distinct spectral states. We modeled both wind absorption and extended wind re-emission components using PION, a self-consistent photoionized absorption model. In all but one case, two photoionization zones were required in order to obtain acceptable fits. Two independent constraints on launching radii, obtained via the ionization parameter formalism and the dynamical broadening of the re-emission, helped characterize the geometry of the wind. The innermost wind components ($r simeq {10}^{2-3}$ $GM/{c}^{2}$) tend towards small volume filling factors, high ionization, densities up to $n simeq {10}^{15-16} {text{cm}}^{-3}$, and outflow velocities of $sim 0.003c$. These small launching radii and large densities require magnetic driving, as they are inconsistent with numerical and analytical treatments of thermally driven winds. Outer wind components ($r simeq {10}^{5}$ $GM/{c}^{2}$) are significantly less ionized and have filling factors near unity. Their larger launching radii, lower densities ($n simeq {10}^{12} {text{cm}}^{-3}$), and outflow velocities ($sim 0.0007c$) are nominally consistent with thermally driven winds. The overall wind structure suggests that these components may also be part of a broader MHD outflow and perhaps better described as magneto-thermal hybrid winds.
We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630-47, observed with the AstroSat, Chandra and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is neither detected in hard X-rays (15-50 keV) by the Swift/BAT nor in radio. Such non-detection along with the source behavior in the hardness-intensity and color-color diagrams obtained using MAXI data confirm that both Chandra and AstroSat observations were performed during the high soft spectral state. The High Energy Grating (HEG) spectrum from the Chandra high-energy transmission grating spectrometer (HETGS) shows two strong, moderately blueshifted absorption lines at 6.705$_{-0.002}^{+0.002}$ keV and 6.974$_{-0.003}^{+0.004}$ keV, which are produced by Fe XXV and Fe XXVI in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366$pm$56 km/s. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC and Chandra/HEG + AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk-blackbody model, with the disk flux fraction of $sim 0.97$. Based on the best-fit continuum spectral modeling of Chandra, AstroSat and Chandra+AstroSat joint spectra and using the Markov Chain Monte Carlo simulations, we constrain the spectral hardening factor at 1.56$^{+0.14}_{-0.06}$ and the dimensionless black hole spin parameter at 0.92 $pm$ 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly-spinning black hole in 4U 1630-47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method.
We report on the analysis of the data collected by Swift, INTEGRAL and RXTE of the Black Hole Candidate (BHC) 4U 1630-47 during 3 consecutive outbursts occurred in 2006, 2008 and 2010, respectively. We show that, although a similar spectral and temporal behaviour in the energy range between 2-10 keV, these 3 outbursts present pronounced differences above 20 keV. In fact, the 2010 outburst extends at high energies without any detectable cut-off until 150-200 keV, while the other two previous outbursts, occurred on 2006 and 2008, are not detected at all above 20 keV. Moreover, the 2008 outburst does not show any detectable hard state in its final phases and even during the 2010 outburst, the final hard state shows some peculiarities rarely observed in other BHC. We also investigate on the peculiar huge variation of 4U 1630-47 hydrogen column density (N$_{H}$) reported in the literature using the Swift/XRT data. In fact this instrument is one of the most suitable for this purpose thanks to its lower energy coverage.
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