A very rare triple-peaked type-I X-ray burst in the low-mass X-ray binary 4U 1636-53

TL;DR

This paper reports the first triple-peaked X-ray burst observed from the low-mass X-ray binary 4U 1636-53 using RXTE, highlighting its unique spectral and temporal features and the lack of current models explaining such phenomena.

Contribution

It presents the discovery and detailed analysis of a rare triple-peaked burst from 4U 1636-53, the first observed with RXTE, expanding understanding of burst morphologies in LMXBs.

Findings

First triple-peaked burst observed with RXTE from any LMXB.

The burst's spectral evolution shows no radius expansion.

The temperature profile exhibits three peaks with specific timing relative to light curve peaks.

Abstract

We have discovered a triple-peaked X-ray burst from the low-mass X-ray binary (LMXB) 4U 1636-53 with the Rossi X-ray Timing Explorer (RXTE). This is the first triple-peaked burst reported from any LMXB using RXTE, and it is only the second burst of this kind observed from any source. (The previous one was also from 4U 1636-53, and was observed with EXOSAT.) From fits to time-resolved spectra, we find that this is not a radius-expansion burst, and that the same triple-peaked pattern seen in the X-ray light curve is also present in the bolometric light curve of the burst. Similar to what was previously observed in double-peaked bursts from this source, the radius of the emitting area increases steadily during the burst, with short periods in between during which the radius remains more or less constant. The temperature first increases steeply, and then decreases across the burst also showing three peaks. The first and last peak in the temperature profile occur, respectively, significantly before and significantly after the first and last peaks in the X-ray and bolometric light curves. We found no significant oscillations during this burst. This triple-peaked burst, as well as the one observed with EXOSAT and the double-peak bursts in this source, all took place when 4U 1636-53 occupied a relatively narrow region in the colour-colour diagram, corresponding to a relatively high (inferred) mass-accretion rate. No model presently available is able to explain the multiple-peaked bursts.