The study of multipeaked type-I X-ray bursts in the neutron star low-mass X-ray binary 4U 1636 − 536 with RXTE

Chen Li,Jiancheng Wang,Ming Lyu,Guobao Zhang,Mariano Méndez

doi:10.1093/mnras/staa3539

Abstract

ABSTRACT We have found and analysed 16 multipeaked type-I bursts from the neutron-star low-mass X-ray binary 4U 1636 − 53 with the Rossi X-ray Timing Explorer (RXTE). One of the bursts is a rare quadruple-peaked burst that was not previously reported. All 16 bursts show a multipeaked structure not only in the X-ray light curves but also in the bolometric light curves. Most of the multipeaked bursts appear in observations during the transition from the hard to the soft state in the colour–colour diagram. We find an anticorrelation between the second peak flux and the separation time between two peaks. We also find that in the double-peaked bursts the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra are correlated. This indicates that the double-peaked structure in the light curve of the bursts may be affected by enhanced accretion rate in the disc, or increased temperature of the neutron star.

Highlights

Thermonuclear (Type I) X-ray bursts show a sudden increase in Xray intensity, becoming ∼10−100 times brighter than the persistent level, triggered by unstable ignition of accreted fuel on the surface of an accreting neutron star (NS) in low-mass X-ray binaries (LMXBs; Galloway et al 2008; Galloway & Keek 2017)
We find that (1) in the case of r1,2 < 1, as the peak flux ratio increases, both the rise time of the first peak and the separation in time increase; (2) in the case of r1,2 > 1, the above parameters do not depend upon peak flux ratio
Most of the multipeaked bursts in our sample appear during the transition from the hard to the soft state in the colour– colour diagram (CCD) where photospheric radius expansion (PRE) bursts are present (Watts & Maurer 2007; Zhang et al 2009)

Summary

Introduction

Thermonuclear (Type I) X-ray bursts show a sudden increase in Xray intensity, becoming ∼10−100 times brighter than the persistent level, triggered by unstable ignition of accreted fuel on the surface of an accreting neutron star (NS) in low-mass X-ray binaries (LMXBs; Galloway et al 2008; Galloway & Keek 2017). In a typical X-ray burst, the light curve shows a single-peaked profile with a fast rise (∼1−5 s) and an exponential decay within 10−100 s (Lewin, van Paradijs & Taam 1993; Strohmayer & Bildsten 2006; Galloway et al 2008). With the Rossi X-ray Timing Explorer (RXTE), Watts & Maurer (2007) analysed four double-peaked bursts in 4U 1636 − 53. Bursts with triple-peaked structure have been observed in 4U 1636 − 53 (van Paradijs et al 1986; Zhang et al 2009). There were two new observations of double-peaked burst, one in the soft spectral state in 4U 1608 − 52 (Jaisawal et al 2019), and another one in SAX J1808.4 − 3658 (Bult et al 2019), both using the Neutron Star Interior Composition Explorer (NICER)

Results

Discussion

Conclusion