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

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.

💡 Research Summary

The authors report the discovery of a triple‑peaked type‑I X‑ray burst from the low‑mass X‑ray binary 4U 1636‑53 using data from the Rossi X‑ray Timing Explorer (RXTE). This is the first triple‑peaked burst identified with RXTE and only the second ever observed from any source, the first having been recorded by EXOSAT also from 4U 1636‑53. The burst light curve in the 2–60 keV band shows three distinct peaks separated by roughly two seconds of lower flux, and the bolometric (0.1–200 keV) light curve mirrors this structure, confirming that the multi‑peak morphology is intrinsic to the burst emission rather than an instrumental effect.

Time‑resolved spectroscopy was performed with 0.25 s intervals, fitting each segment with an absorbed blackbody model (TBabs*blackbody). The spectral analysis reveals that the burst is not a photospheric‑radius‑expansion (PRE) event; the apparent emitting radius (R_bb) grows steadily from about 10 km at burst onset to roughly 12–13 km, but it exhibits short intervals where the radius remains essentially constant. The blackbody temperature (T_bb) displays a more complex behavior: it rises sharply before the first flux peak, then drops sharply at the first peak, rises again at the second peak, and finally declines after the third peak. Notably, the temperature peaks precede the first and follow the last flux peaks by about half a second, indicating a phase offset between temperature evolution and total luminosity.

A Fourier analysis of the high‑time‑resolution data (2–60 keV) found no significant burst oscillations near the known 580 Hz spin frequency of the neutron star; the 3σ upper limit on the fractional rms amplitude is ≈2 %. The absence of detectable oscillations suggests that the burst emission was largely symmetric, lacking the strong asymmetries that produce oscillations in many type‑I bursts.

The burst occurred when the source occupied a narrow region of the colour‑colour diagram corresponding to a relatively high inferred mass‑accretion rate (≈0.1–0.2 M_Edd). This is the same region where previously reported double‑peaked bursts from 4U 1636‑53 and the earlier EXOSAT triple‑peaked burst were observed, implying that a specific accretion regime may be required for multi‑peaked bursts.

The authors compare the observed properties with existing theoretical frameworks. Models invoking two‑stage nuclear burning (e.g., a helium flash followed by a delayed hydrogen flash) or radiation‑pressure‑driven radius expansion can reproduce double‑peaked light curves but fail to generate three distinct peaks, especially when the radius does not expand. Similarly, models based on a temporary stall of the burning front due to compositional gradients or cooling waves can produce a single pause but not the repeated temperature‑flux offsets seen here. Consequently, none of the current models can simultaneously explain the triple‑peaked light curve, the non‑expanding radius, the phase‑shifted temperature peaks, and the high‑accretion‑rate environment.

The paper concludes that the triple‑peaked burst adds a new, stringent constraint on burst physics. It highlights the need for more sophisticated multi‑dimensional simulations that incorporate detailed fuel composition, variable accretion heating, and possible hydrodynamic instabilities in the neutron‑star envelope. Systematic searches for similar events in other LMXBs, combined with improved theoretical modeling, are essential to uncover the physical mechanism behind these rare multi‑peaked thermonuclear bursts.