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Register a new userINTEGRAL and BeppoSAX observations of the transient atoll source 4U 1608-522: from quiescent to hard spectral state
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In this paper we report on the spectral evolution of 4U 1608-522 performed as part of the long Galactic Bulge monitoring with INTEGRAL. The data set include the April 2005 outburst. BeppoSAX archival data (two observations, in 1998 and 2000) have been also analysed and compared with the INTEGRAL ones. Three different spectral states have been identified from the hard Color-Intensity diagram derived from INTEGRAL: the canonical Hard and Soft as well as an Intermediate state. The hard state spectrum is well described by a weak black body component plus a Comptonised plasma component with high electron temperature (kTe~60 keV) extending up to 200 keV without any additional cut-off. The soft spectra are characterized by a cold Comptonised plasma (kTe= 2-3 keV, and 7 keV for the intermediate state) and a strong disk black body component. A reflection component, indicating reflection of the X-ray radiation from the accretion disc, is also present in the soft state revealed by BeppoSAX in 1998. The 2000 BeppoSAX observation revealed the source in quiescent state modelled by a neutron star atmosphere (assuming a neutron star with radius 10 km and mass 1.4 Msun) with an effective temperature, kTeff of 0.1 keV plus a power law component with Gamma~3 detected for the first time for this source. This spectrum can also be modelled with a simple black body compatible with emission originating from a small fraction of the NS surface of radius of 0.4 km.
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We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering two orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz QPO, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme only the lower kilohertz QPO and HBO follow identical relations. Using the mutual relations between the frequencies of the variability components we tested several models; the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.
We have studied the spectral and timing behaviour of the atoll source 4U 1608-52. We find that the timing behaviour of 4U 1608-52 is almost identical to that of the atoll sources 4U 0614+09 and 4U 1728-34. Recently Muno, Remillard & Chakrabarty (2002) and Gierlinski & Done (2002) suggested that the atoll sources trace out similar three-branch patterns as the Z sources. The timing behaviour is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually.
To investigate the physics of mass accretion onto weakly-magnetized neutron stars, 95 archival RXTE datasets of an atoll source 4U 1608-522, acquired over 1996-2004 in so-called upper-banana state, were analyzed. The object meantime exhibited 3-30 keV luminosity in the range of <~ 10^35 - 4 x 10^37 erg s^-1, assuming a distance of 3.6 kpc. The 3-30 keV PCA spectra, produced one from each dataset, were represented successfully with a combination of a soft and a hard component, of which the presence was revealed in a model-independent manner by studying spectral variations among the observations. The soft component is expressed by so-called multi-color disk model with a temperature of ~1.8 keV, and is attributed to the emission from an optically-thick standard accretion disk. The hard component is a blackbody emission with a temperature of ~2.7 keV, thought to be emitted from the neutron-star surface. As the total luminosity increases, a continuous decrease was observed in the ratio of the blackbody luminosity to that of the disk component. This property suggests that the matter flowing through the accretion disk gradually becomes difficult to reach the neutron-star surface, presumably forming outflows driven by the increased radiation pressure. On time scales of hours to days, the overall source variability was found to be controlled by two independent variables; the mass accretion rate, and the innermost disk radius which changes both physically and artificially.
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 used. 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.
In this paper, we present the first results of spectral and timing properties of the atoll source 4U 1705-44 using $sim$ 100 ks data obtained with Large Area X-ray Proportional Counter (LAXPC) onboard {it AstroSat}. The source was in the high-soft state during our observations and traced out a {it banana track} in the Hardness Intensity Diagram (HID). We study {bf the} evolution of the Power Density Spectra (PDS) and the energy spectra along the HID. PDS show presence of a broad Lorentzian feature (Peaked Noise or PN) centered at $1-13$ Hz and a very low frequency noise (VLFN). The energy spectra can be described by sum of a thermal Comptonized component, a power-law and a broad iron line. The hard tail seen in the energy spectra is variable and contribute $4-30$% of the total flux. The iron line seen in this source is broad (FWHM $sim$ 2 keV) and strong (EW $sim$ $369-512$ eV). Only relativistic smearing in the accretion disc can not explain the origin of this feature and requires other mechanism such as broadening by Comptonization process in the external part of the `Comptonized Corona. A subtle and systematic evolution of the spectral parameters (optical depth, electron temperature etc.) is seen as the source moves along the HID. We study the correlation between frequency of the PN and the spectral parameters. PN frequency seems to be correlated with the strength of the corona. We discuss the implication of the results in the paper.
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