NuSTAR and AstroSat observations of thermonuclear X-ray bursts with short-recurrence times in 4U 1636$-$536

TL;DR

This study analyzes NuSTAR and AstroSat observations of 4U 1636-536, revealing the shortest known recurrence time for thermonuclear X-ray bursts and detecting burst oscillations around 581 Hz, providing insights into burst behavior and neutron star properties.

Contribution

First detailed spectro-timing analysis of 4U 1636-536 during hard state with detection of extremely short recurrence times and burst oscillations.

Findings

Shortest recurrence time of 3.8 minutes observed.

Detection of burst oscillations around 581 Hz.

Presence of additional spectral components during some bursts.

Abstract

We report results from the spectro-timing analysis of the atoll source 4U 1636$-$536 observed with NuSTAR and AstroSat during its hard spectral state. In three observations of 207 ks total exposure, we identify 31 thermonuclear X-ray bursts including five doublets and a triplet. Recurrence time as short as 3.8 minutes is seen in one of the doublets. To the best of our knowledge, this is the shortest recurrence time known for this source. Our time-averaged spectroscopy during the bursts indicate the presence of an additional powerlaw or a blackbody component in a few cases, perhaps due to varying temperatures during bursts or plausible deviation from ideal blackbody behavior, however, it is difficult to probe this using the time-resolved spectroscopy owing to limited statistics. Time-resolved bursts are well fit using an absorbed blackbody model with temperatures varying between 1.7 and 2.2 keV. Burst oscillations around 581 Hz are detected with 3$\sigma$ confidence during the decay phase in two of the X-ray bursts. One of the burst oscillations is seen at 582 Hz, a frequency observed during the 2001 superburst in this source.