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Register a new userSpectral and Timing Properties of Atoll Source 4U 1705-44 : LAXPC/AstroSat Results
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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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For the first time, simultaneous broadband spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8-70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modeling was performed using the full reflection framework and an inner disk radii of 14 Rg was obtained. A hard powerlaw tail was noticed in the soft state and hot component fluxes and varying powerlaw indices point towards a varying corona/sub-keplerian flow. Based on the spectral fits the boundary layer radius and magnetospheric radius were constrained to be $sim$ 14-18 km and $sim$ 9-19 km respectively. Cross- Correlation Function studies were performed between the 0.8-3 keV soft SXT lightcurve and 10-20 keV hard LAXPC lightcurve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6-20 km (b{eta}=0.1). Since the inner disk radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disk. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.
We performed a detailed timing study of the Atoll source 4U 1705-44 in order to understand the accretion disk geometry. Cross correlation function (CCF) studies were performed between soft (3-5 keV) and hard energy (15-30 keV) bands using the AstroSat LAXPC data. We detected hard as well as soft lags of the order of few ten to hundred seconds. A dynamical CCF study was performed in the same energy bands for one of the light curves and we found smaller lags of few tens of seconds ($<$ 50 s) suggesting that the variation is probably originating from the corona. We found a broad noise component around $sim$ 13 Hz in the 3-10 keV band which is absent in 10-20 keV band. We interpret the observed lags as the readjustment timescales of the corona or a boundary layer around the neutron star and constrain the height of this structure to few tens of km. We independently estimated the coronal height to be around 15 km assuming that the 13 Hz feature in the PDS is originating from the oscillation of the viscous shell around the neutron star.
The low-mass X-ray binary 4U1705-44 exhibits dramatic long-term X-ray time variability with a timescale of several hundred days. The All-Sky Monitor (ASM) aboard the Rossi X-ray Timing Explorer (RXTE) and the Japanese Monitor of All-sky X-ray Image (MAXI) aboard the International Space Station together have continuously observed the source from December 1995 through May 2014. The combined ASM-MAXI data provide a continuous time series over fifty times the length of the timescale of interest. Topological analysis can help us identify fingerprints in the phase-space of a system unique to its equations of motion. The Birman-Williams theorem postulates that if such fingerprints are the same between two systems, then their equations of motion must be closely related. The phase-space embedding of the source light curve shows a strong resemblance to the double-welled nonlinear Duffing oscillator. We explore a range of parameters for which the Duffing oscillator closely mirrors the time evolution of 4U1705-44. We extract low period, unstable periodic orbits from the 4U1705-44 and Duffing time series and compare their topological information. The Duffing and 4U1705-44 topological properties are identical, providing strong evidence that they share the same underlying template. This suggests that we can look to the Duffing equation to help guide the development of a physical model to describe the long-term X-ray variability of this and other similarly behaved X-ray binary systems.
4U 1705-44 is one of the most-studied type I X-ray burster and Atoll sources. This source represents a perfect candidate to test different models proposed to self-consistently track the physical changes occurring between different spectral states because it shows clear spectral state transitions. The broadband coverage, the sensitivity and energy resolution of the BeppoSAX satellite offers the opportunity to disentangle the components that form the total X-ray spectrum and to study their changes according to the spectral state. Using two BeppoSAX observations carried out in August and October 2000, respectively, for a total effective exposure time of about 100 ks, we study the spectral evolution of the source from a soft to hard state. Energy spectra are selected according to the source position in the color-color diagram (CCD) Results. We succeeded in modeling the spectra of the source using a physical self-consistent scenario for both the island and banana branches (the double Comptonization scenario). The components observed are the soft Comptonization and hard Comptonization, the blackbody, and a reflection component with a broad iron line. When the source moves from the banana state to the island state, the parameters of the two Comptonization components change significantly and the blackbody component becomes too weak to be detected. We interpret the soft Comptonization component as emission from the hot plasma surrounding the neutron star, hard Comptonization as emission from the disk region, and the blackbody component as emission from the inner accretion disk. The broad feature in the iron line region is compatible with reflection from the inner accretion disk.
integral and sax observations of the neutron-star LMXB 4U~1705--44 have been analysed to deeply investigate the spectral state transitions nature. Its energy spectrum can be described as the sum of one or two blackbody, a 6.4-keV Fe line and a component due to thermal Comptonization. For the first time in this source, we find a strong signature of Compton reflection, presumably due to illumination of the optically-thick accretion disk by the Comptonized spectrum. Detection of two blackbody component in the soft states could originate in the disk and the neutron-star surface, and the Comptonized component arises from a hot inner flow with the seed photons coming from the disk and/or the neutron-star surface. The spectral transitions are shown to be associated with variations in the accretion rate, which changes in turn the temperature of the Comptonizing electrons and the strength of Compton reflection.
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