Study of Type-I X-ray bursts in 4U 1636-536 using XSPECT/XPoSat

Abstract We present the detection and detailed analysis of type-I X-ray bursts from 4U 1636-536 observed using XSPECT on-board XPoSat. Type-I X-ray bursts are the result of unstable thermonuclear burning of accreting matter on the neutron star (NS) surface. A total of nine type-I X-ray bursts are detected during the observations, out of which six bursts are a part of short wait time bursts or doublets with a consistent recurrence time of ~7 minutes. The source was observed in the low flux state and remained in the same state throughout the XSPECT observations. We study the spectral properties of the system during the persistent emission as well as during the detected type-I bursts by means of time-averaged and time-resolved spectroscopy. The persistent emission spectra in the source could be well modeled using a Comptonized multi-color disk model with inner disk temperature, kTin ~ 0.37 keV, and an electron temperature, kTe ~ 2.17 keV. The nature of persistent emission between the doublets (kTin ~ 0.6 keV and kTe ~ 2 keV) is different compared to the overall persistent emission before the bursts. Time-averaged burst spectra are fitted using a blackbody model with temperature ranging between 1.3 − 1.8 keV. We report a clear distinction for the second burst in the doublets in terms of the almost constant recurrence time and decay time for the detected doublets. This observation is valid irrespective of the peak count rate during the burst. The consistently short decay time for these bursts could indicate a different fuel ignition compared to the singlet bursts observed in the source.