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تسجيل مستخدم جديدThe Evolution of the Accretion Disk around 4U 1820-30 During a Superburst
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D.R. Ballantyne
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Accretion from a disk onto a collapsed, relativistic star -- a neutron star or black hole -- is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not yet possible to directly observe the evolution or dynamics of the inner parts of the accretion disk where general relativistic effects are dominant. Here, we use the bright X-ray emission from a superburst on the surface of the neutron star 4U 1820-30 as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk to fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. The time-resolved spectral fitting shows that the inner region of the disk is disrupted by the burst, possibly being heated into a thicker, more tenuous flow, before recovering its previous form in ~1000 s. This marks the first instance that the evolution of the inner regions of an accretion disk has been observed in real-time.
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When a thermonuclear X-ray burst ignites on an accreting neutron star, the accretion disk undergoes sudden strong X-ray illumination, which can drive a range of processes in the disk. Observations of superbursts, with durations of several hours, prov
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The ultracompact X-ray binary 4U 1820-30 is well known for its ~170-d superorbital modulation in X-ray flux and spectrum, and the exclusiveness of bursting behavior to the low hard island state. In May-June 2009, there was an exceptionally long 51-d
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We study the rapid X-ray time variability in all public data available from the textit{Rossi X-ray Timing Explorers} Proportional Counter Array on the atoll source 4U 1820--30 in the low-luminosity island state. A total of $sim46$ ks of data were use
d. We compare the frequencies of the variability components of 4U 1820--30 with those in other atolls sources. These frequencies were previously found to follow a universal scheme of correlations. We find that 4U 1820--30 shows correlations that are shifted by factors of $1.13pm0.01$ and $1.21pm0.02$ with respect to those in other atoll sources. These shifts are similar to, but smaller than the shift factor $sim1.45$ previously reported for some accreting millisecond pulsars. Therefore, 4U 1820--30 is the first atoll source which shows no significant pulsations but has a significant shift in the frequency correlations compared with other 3 non-pulsating atoll sources.
The persistently bright ultra-compact neutron star low-mass X-ray binary 4U 1820$-$30 displays a $sim$170 d accretion cycle, evolving between phases of high and low X-ray modes, where the 3 -- 10 keV X-ray flux changes by a factor of up to $approx 8$
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The 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 has been studied with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail above
50 keV in the hard state has been revealed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~ 2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e ~ 6 keV and optical depth of $tau ~ 4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During the observations, the source spent most of the time in the soft state, as previously reported and the $ge$4 keV spectra can be represented by thermal Comptonization with kT_e ~ 3 keV and $tau ~ 6-7.
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