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Register a new userThe properties of cross-correlation and spectra of the low-mass X-ray binary 4U 1608-52
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With {it RXTE} data, we analyzed the cross-correlation function between the soft and hard X-rays of the transient atoll source 4U 1608-52. We found anti-correlations in three outbursts occurred in 1998, 2002 and 2010, and significant time lags of several hundreds of seconds in the latter two outbursts. Our results show no correlation between the soft and hard X-rays in the island state, and a dominated positive correlation in the lower banana state. Anti-correlations are presented at the upper banana state for the outburst of 2010 and at the lower left banana states for the other two outbursts. So far for atoll sources the cross-correlation has been studied statistically only for 4U 1735-44, where anti-correlations showed up in the upper banana state. Here our investigation upon 4U 1608-52 provides a similar result in its 2010 outburst. In addition, we notice that the luminosities in the upper banana of 1998 and 2002 outbursts are about 1.5 times that of 2010 outburst whose luminosity in the upper banana is close to that of 4U 1735-44. The results suggest that the states in color-color diagram of a source could be correlated with the luminosity of the source. A further spectral analysis shows that, during the 2010 outburst, although an anti-correlation presents at the highest fluxes, the contemporary spectrum is not the softest one along the outburst evolution. This suggests that the observed anti-correlation may be relevant to the transition between the hard and soft states, which is consistent with the previous results on 4U 1735-44 and several black hole X-ray binaries that anti-correlations are observed during the transition states.
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(abridged) We studied the energy and frequency dependence of the Fourier time lags and intrinsic coherence of the kHz QPOs in the NS LMXBs 4U 1608-52 and 4U 1636-53 using RXTE data. In both sources we confirmed energy-dependent soft lags of 10-100 mu s for the lower kHz QPO. We also found that the time lags of the upper kHz QPO are independent of energy and inconsistent with the soft lags of the lower kHz QPO. The intrinsic coherence of the lower kHz QPO remains constant at 0.6 from 5 to 12 keV, and then drops to zero, while for the upper kHz QPO the intrinsic coherence is consistent with zero across the full energy range. The intrinsic coherence of the upper kHz QPO is consistent with zero over the full frequency range of the QPO, except in 4U 1636-53 at ~780 Hz where it increases to 0.13. In 4U 1636-53, for the lower kHz QPO the 4-12 keV photons lag the 12-20 keV ones by 25 mu s in the QPO frequency range 500-850 Hz, with the lags decreasing to 15 mu s at higher frequencies. In 4U 1608-52 the soft lags of the lower kHz QPO remain constant at 40 mu s. In 4U 1636-53, for the upper kHz QPO the 12-20 keV photons lag the 4-12 keV ones by 11 +/- 3 mu s, independent of QPO frequency; we found consistent results for the time lags of the upper kHz QPO in 4U 1608-52. The intrinsic coherence of the lower kHz QPO increases from ~0-0.4 at 600 Hz to 1 and 0.6 at 800 Hz in 4U 1636-53 and 4U 1608-52, respectively. In 4U 1636-53 it decreases to 0.5 at 920 Hz, while in 4U 1608-52 we do not have data above 800 Hz. We discuss our results in the context of scenarios in which the soft lags are either due to reflection off the accretion disc or up-/down-scattering in a hot medium close to the neutron star. We finally explore the connection between, on one hand the time lags and the intrinsic coherence of the kHz QPOs, and on the other the QPOs amplitude and quality factor in these two sources.
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.
It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below $sim 10^{-2}$ of the Eddington limit ($L_{rm Edd}$). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608-52, to study changes in the reflection spectrum. We find that the broad Fe-K$alpha$ line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is $sim 10^{37}$ erg/s ($sim 0.05$ $L_{rm Edd}$), disappear during the decay of the outburst when the source luminosity drops to $sim 4.5 times 10^{35}$ erg/s ($sim 0.002$ $L_{rm Edd}$). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disk radius, disk ionisation, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disk ionisation ($log xi geq 4.1$) or a much decreased reflection fraction. A changing disk truncation alone, however, cannot account for the lack of reprocessed Fe-K$alpha$ emission. The required increase in ionisation parameter could occur if the inner accretion flow evaporates from a thin disk into a geometrically thicker flow, such as the commonly assumed formation of an radiatively inefficient accretion flow at lower mass accretion rates.
We detect millihertz quasi-periodic oscillations (mHz QPOs) using the Rossi X-ray Time Explorer (RXTE) from the atoll neutron-star (NS) low-mass X-ray binaries 4U 1608--52 and Aql X--1. From the analysis of all RXTE observations of 4U 1608--52 and Aql X--1, we find mHz QPOs with a significance level $>3sigma$ in 49 and 47 observations, respectively. The QPO frequency is constrained between $sim$ 4.2 and 13.4 mHz. These types of mHz QPOs have been interpreted as being the result of marginally stable nuclear burning of He on the NS surface. We also report the discovery of a downward frequency drift in three observations of 4U 1608--52, making it the third source that shows this behaviour. We only find strong evidence of frequency drift in one occasion in Aql X--1, probably because the observations were too short to measure a significant drift. Finally, the mHz QPOs are mainly detected when both sources are in the soft or intermediate states; the cases that show frequency drift only occur when the sources are in intermediate states. Our results are consistent with the phenomenology observed for the NS systems 4U 1636--53 and EXO 0748--676, suggesting that all four sources can reach the conditions for marginally stable burning of He on the NS surface. These conditions depend on the source state in the same manner in all four systems.
The evidences for the influence of thermonuclear (type-I) X-ray bursts upon the surrounding environments in neutron star low-mass X-ray binaries (LMXB) were detected previously via spectral and timing analyses. Benefitting from a broad energy coverage of Insight-HXMT, we analyze one photospheric radius expansion (PRE) burst, and find an emission excess at soft X-rays. Our spectral analysis shows that, such an excess is not likely relevant to the disk reflection induced by the burst emission and can be attributed to an enhanced pre-burst/persistent emission. We find that the burst and enhanced persistent emissions sum up to exceed Eddington luminosity by $sim$ 40 percentages. We speculate that the enhanced emission is from a region beyond the PRE radius, or through the Comptonization of the corona.
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