X-ray variability with spectral state transitions in NS-LMXBs observed with MAXI/GSC and Swift/BAT

Asai, K.; Mihara, T.; Matsuoka, M.; Sugizaki, M.

doi:10.1093/pasj/psv060

Cited by 9 publications

(12 citation statements)

References 47 publications

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“…Figure 14 in Lin et al (2007) shows that the hard-to-soft state transition of Aql X-1 in 2000 took place within ∼ 1 day which is consistent with our result of ∼ 30 ks (∼ 8 hours) and with those of Asai et al (2015) (discussed in section 4.4 in detail).…”

Section: Continuum Modelingsupporting

confidence: 88%

“…As seen in the MAXI light curve (figure 3 of Asai et al 2015), the present transition is clearly associated with a normal outburst in their classification, and the transition time scale we observed, about 30 ks, or ∼ 8 hours, is consistent with those seen in that type of transitions.…”

Section: Transition Time Scalessupporting

confidence: 87%

“…Thus, the present observation fortunately witnessed a very rare occasion of the state transition. For reference, the present episode on MJD 55855, is seen in the long-term MAXI and Swift light curves of figure 3 of Asai et al (2015), as a rapid rise in the 2-10 keV MAXI/GSC intensity and a drop in the 15-50 keV Swift/BAT count rate.…”

Section: Hxd Data Reductionmentioning

confidence: 99%

“…Among these transitions, the best recognized are those between the soft and the hard states (e.g., Remillard & McClintock 2006). Similarly, Neutron-Star (NS) Low-Mass X-ray Binaries (LMXBs), consisting of mass-donating low-mass stars and massaccreting weakly-magnetized NSs, exhibit transitions between their hard and soft states, which are realized when their luminosities are lower and higher than ∼ 4% of their Eddington value, respectively (e.g., Asai et al 2012).…”

Section: Introductionmentioning

confidence: 99%

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A hard-to-soft state transition of Aquila X-1 observed with Suzaku

Ono

Makishima

Sakurai

et al. 2017

Publications of the Astronomical Society of Japan

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The recurrent soft X-ray transient Aquila X-1 was observed with Suzaku for a gross duration of 79.9 ks, on 2011 October 21 when the object was in a rising phase of an outburst. During the observation, the source exhibited a clear spectral transition from the hard state to the soft state, on a time scale of ∼30 ks. Across the transition, the 0.8-10 keV XIS count rate increased by a factor ∼3, that of HXD-PIN in 15-60 keV decreased by a similar factor, and the unabsorbed 0.1-100 keV luminosity increased from 3.5 ×10 37 erg s −1 to 5.1 ×10 37 erg s −1 . The broadband spectral shape changed continuously, from a power-law like one with a high-energy cutoff to a more convex one. Throughout the transition, the 0.8-60 keV spectra were successfully described with a model consisting of a multi-color blackbody and a Comptonized blackbody, which are considered to arise from a standard accretion disk and a closer vicinity of the neu-1 tron star, respectively. All the model parameters were confirmed to change continuously, from those typical in the hard state to those typical of the soft state. More specifically, the inner disk radius decreased from 31 km to 18 km, the effects of Comptonization on the blackbody photons weakened, and the electron temperature of Comptonization decreased from 10 keV to 3 keV. The derived parameters imply that the Comptonizing corona shrinks towards the final soft state, and/or the radial infall velocity of the corona decreases. These results reinforce the view that the soft and hard states of Aql X-1 (and of similar objects) are described by the same "disk plus Comptonized blackbody" model, but with considerably different parameters.

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Section: Continuum Modelingsupporting

confidence: 88%

Section: Transition Time Scalessupporting

confidence: 87%

Section: Hxd Data Reductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A hard-to-soft state transition of Aquila X-1 observed with Suzaku

Ono

Makishima

Sakurai

et al. 2017

Publications of the Astronomical Society of Japan

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“…Aql X-1: The source stayed in the soft state during five outbursts (see figure 1). The HR threshold between the soft and hard states was chosen 0.222 in one-day bin data, which is taken after the analysis in Asai et al (2015).…”

Section: Hardness-luminosity Diagrammentioning

confidence: 99%

Hard-tail emission in the soft state of low-mass X-ray binaries and their relation to the neutron star magnetic field

Asai

Mihara

Mastuoka

et al. 2016

Publications of the Astronomical Society of Japan

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Average hard-tail X-ray emission in the soft state of nine bright Atoll low-mass X-ray binaries containing a neutron star (NS-LMXBs) are investigated by using the light curves of MAXI/GSC and Swift/BAT. Two sources (4U 1820−30 and 4U 1735−44) exhibit large hardness ratio (15-50 keV/2-10 keV: HR > 0.1), while the other sources distribute at HR < ∼ 0.1. In either case, HR does not depend on the 2-10 keV luminosity. Therefore the difference of HR is due to the 15-50 keV luminosity, which is Comptonized emission. The Compton cloud is assumed to be around the neutron star. The size of the Compton cloud would affect the value of HR. Although the magnetic field of NS-LMXB is weak, we could expect a larger Alfvén radius than the innermost stable circular orbit or the neutron star radius in some sources. In such cases, the accretion inflow is stopped at the Alfvén radius and would create relatively large Compton cloud. It would result in the observed larger Comptonized emission. By attributing the difference of the size of Compton cloud to the Alfvén radius, we can estimate the magnetic fields of neutron star. The obtained lower/upper limits are consistent with the previous results.

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AstroSat view of ‘Clocked’ burster GS 1826-238: broad-band spectral nature of persistent and burst emission during soft state

Agrawal

Nandi

Katoch

2022

Monthly Notices of the Royal Astronomical Society

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In this paper, we have carried out a detailed study of the ‘Clocked’ burster GS 1826 − 238 using ∼ 90 ks broad-band (0.7 - 60.0 keV) data obtained with AstroSat observatory. The source was observed during a soft spectral state and traced a ‘banana’ type track in the colour-colour diagram (CCD). We find that a combination of thermal component (multi-colour disc/bbodyrad) and Comptonized component is statistically good description for all the sections of the track in the CCD. The corona becomes optically thick (τ increases from ∼ 5 to 21) and cooler (kTe decreases from ∼ 4.8 to 2.2 keV) as the source moves up in the ‘banana’ branch. Probably cooling is caused by increase in the supply of soft-seed photons from the disc/boundary-layer. Reflection signature is observed at upper ‘banana’ branch of the source. Two type-I X-ray bursts are detected during the AstroSat observations. During the bursts, hard X-rays increased unlike previous observations where a reduction in hard X-rays is observed during the bursts. Decrease in the electron temperature and increase in the optical depth are observed during the bursts. The PSD (Power Spectral Density) of all the sections of the CCD can be represented by a pure power-law component. The strength of this component increases from ∼ 1 per cent to 4.5 per cent as the source moves up in the ‘banana’ track. Search for burst oscillations gave a null result. We discuss the implications of our results in the context of previous findings.

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X-ray variability with spectral state transitions in NS-LMXBs observed with MAXI/GSC and Swift/BAT

Cited by 9 publications

References 47 publications

A hard-to-soft state transition of Aquila X-1 observed with Suzaku

A hard-to-soft state transition of Aquila X-1 observed with Suzaku

Hard-tail emission in the soft state of low-mass X-ray binaries and their relation to the neutron star magnetic field

AstroSat view of ‘Clocked’ burster GS 1826-238: broad-band spectral nature of persistent and burst emission during soft state

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