Unveiling the nature of compact object in the LMXB MAXI J1957+032 using Swift-xrt

Abstract

MAXI J1957+032 is a transient Low-mass X-ray binary (LMXB) that underwent four short outbursts in 1.5 years since its discovery in 2015. The nature of the compact object in MAXI J1957+032 is not clear, but it was proposed to be a neutron star based on the short-duration of its outbursts. Here, we report the results obtained after performing spectral analysis using data obtained with the X-ray telescope aboard the Neil Gehrels Swift satellite. When describing the spectrum with an absorbed power law, we found that the spectra softens (the power-law index increases from ∼ 1.8 to 2.5) as the luminosity decreases. Near the end of its outbursts the observed value of power-law index (Γ) is ∼ 2.5. To identify the nature of the compact object in this system, we used Γ as a tracer of the spectral evolution with luminosity. We found that for the distance of 4 kpc, our results suggest that the source harbours a neutron star.

The advection-dominated accretion flow for the anti-correlation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries

“…Three sources, i.e., 1RXS J180408.9-342058, EXO 1745-248 and IGR J17361-4441 are from Parikh et al (2017). MAXI J1957+032 is from Beri et al (2019). Twelve sources in the sample (except for IGR J17494-3030, XTE J1719-291 and IGR J17361-4441 in the sample) are confirmed as NS binaries since the Type I X-ray bursts have been observed.…”

“…By comparing with a sample of non-pulsating NS-LMXBs with well measured Γ (obtained by fitting the observed X-ray spectrum between 0.5 and 10 keV with a single power law) in the range of L 0.5−10keV ∼ 10 34 − 10 36 erg s −1 , we find that indeed only a small value of 0.003 f th 0.1 is needed to match the observed anti-correlation between Γ and L 0.5−10keV . The data in the sample are searched from literatures, which are summarized at Table 1 of Wijnands et al (2015), Appendix of Parikh et al (2017) and Beri et al (2019) respectively. Theoretically, in our model, there are two components, i.e., a thermal component and a power-law component, both of which can contribute a fraction of the X-ray luminosity between 0.5 and 10 keV respectively.…”

MAXI J1957+032: a new accreting millisecond X-ray pulsar in an ultra-compact binary

“…Swift/XRT observations on June 20, 2022, suggested spectral properties consistent with the 2016 outburst of the source (see, e.g. Beri et al 2019). Moreover, Swift/UVOT detected the UV counterpart of the source, with a magnitude UVW2 = 20.37 ± 0.08 (Beri et al 2022).…”

“…Instead, it is suggested to be a possible triple system, with the main-sequence counterpart likely being in a wide orbit around a compact interacting binary. Moreover, the spectral evolution investigated by combining the Swift observations collected during the four outbursts suggested that, for a distance of the order of 4 kpc, a neutron star (NS) might be hosted in the binary system (Beri et al 2019).…”

The radiative efficiency of neutron stars at low-level accretion

“…Further, in Qiao & Liu (2020b), based on the sample of nonpulsating NS-LMXBs in Wijnands et al (2015), and adding some more non-pulsating NS-LMXBs from Parikh et al (2017) and Beri et al (2019), we explain the anticorrelation between the Xray photon index Γ (obtained by fitting the X-ray spectra between 0.5 and 10 keV with a single power law) and L 0.5−10keV , i.e., the softening of the X-ray spectra with decreasing L 0.5−10keV , in the range of L 0.5−10keV ∼ 10 34 − 10 36 erg s −1 by adjusting the value of f th . Moreover, it is shown that a fraction of the sources in Qiao & Liu (2020b) are once reported to be fitted with the twocomponent model (with XMM − Newton X-ray data), i.e., a thermal soft X-ray component plus a power-law component.…”