NICER Observes a Secondary Peak in the Decay of a Thermonuclear Burst from 4U 1608–52

Abstract

We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608-52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of ≈ 3 keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and re-burning in the cooling tail of the burst.

Discovery of thermonuclear (Type I) X-ray bursts in the X-ray binary Swift J1858.6–0814 observed with NICER and NuSTAR

“…These fits show a fast rise and smooth decay in bolometric flux. The apparent double peak in the flux curve for the single blackbody model is likely due to the poor fit around this time, although double peaks in bolometric luminosity have been seen in other PRE bursts (Jaisawal et al 2019). The comparatively smooth flux profile contrasts with the fast drop in count rate after the peak; the difference being due to the higher temperatures early in the decay producing a lower count rate for a given flux (when convolved with the instrument response, given the NICER effective area curve and the temperatures concerned).…”

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

“…While investigating the cooling phase of X-ray bursts in 4U 1636−53, Zhang et al (2011) reported 12 double-peaked bursts and found that most of them appeared at the vertex of colour-colour diagram. Recently, 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). The double-peaked structures in the burst light curve can be separated into two groups.…”

“…The other group consists of bursts that have a double-peaked profile both in X-rays and the bolometric lightcurve. Most of these bursts have low peak flux, although PRE bursts with double-peaked profiles both in X-rays and bolometric luminosity have been recently observed with NICER (Jaisawal et al 2019;Bult et al 2019).…”

Discovery of thermonuclear Type-I X-ray bursts from the X-ray binary MAXI J1807+132

“…The fa-method has already been used for time-resolved spectroscopy of X-ray bursts using NICER data (see, e.g., Keek et al 2018;Jaisawal et al 2019). Although the famethod has been shown to be a good way to parameterise the effects on the accretion disc, it is likely that fa should be a function of energy rather than a constant (see, e.g., Keek et al 2014;Degenaar et al 2018).…”

“…Double-peaked X-ray bursts from this source have been interpreted in a number of different ways. For example, a second peak may be caused by multi-step thermonuclear energy release, or the ignition of fresh or leftover material (see, e.g., Jaisawal et al 2019, and references therein). Bhattacharyya & Strohmayer (2006a,b) interpreted the double-peaked profile as the result of a temporary stalling in the burning front at the NS's equator.…”