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

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.

11 pages, 9 figures, 3 tables. Accepted for publication in the special issue of JAA, "Astrophysical jets and observational facilities: National perspective"

References in corpus (11)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• No Time for Dead Time: Timing analysis of bright black hole binaries with NuSTAR
• X-ray time variability across the atoll source states of 4U 1636--53
• Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
• Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I X-ray Bursts
• Discovery of X-ray burst triplets in EXO 0748-676
• Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection
• Evidence of thermonuclear flame spreading on neutron stars from burst rise oscillations
• Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat
• Multiple X-ray bursts and the model of a spreading layer of accreting matter over the neutron star surface
• Evidence of disc reflection in the X-ray spectrum of the neutron star low mass X-ray binary 4U 1636-536