Millihertz quasi-periodic oscillations in 4U 1636-53 associated with bursts with positive convexity only

TL;DR

This study shows that in 4U 1636-53, all bursts associated with millihertz QPOs and where QPOs disappear have positive convexity, indicating they originate at the neutron star's equator and clarifying accretion rate discrepancies.

Contribution

It reveals a consistent association between positive convexity bursts and mHz QPOs, suggesting a common ignition site at the neutron star's equator in 4U 1636-53.

Findings

All 39 cases with mHz QPO disappearance have positive convexity.

Bursts with positive convexity are linked to equatorial ignition.

This explains the mismatch between theoretical and observed accretion rates.

Abstract

We investigated the convexity of all type I X-ray bursts with millihertz quasi-periodic oscillations (mHz QPOs) in 4U 1636-53 using archival observations with the Rossi X-ray Timing Explorer. We found that, at a 3.5 $\sigma$ confidence level, in all 39 cases in which the mHz QPOs disappeared at the time of an X-ray burst, the convexity of the burst is positive. The convexity measures the shape of the rising part of the burst light curve and, according to recent models, it is related to the ignition site of bursts on the neutron star surface. This finding suggests that in 4U 1636-53 these 39 bursts and the marginally-stable nuclear burning process responsible for the mHz QPOs take place at the neutron-star equator. This scenario could explain the inconsistency between the high accretion rate required for triggering mHz QPOs in theoretical models and the relatively low accretion rate derived from observations.