Probing spectral and temporal evolution of the neutron star low-mass X-ray binary 4U 1724-30 with AstroSat

Abstract

We report the broadband spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724-30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broadband spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or NS surface and a non-thermal emission component possibly originating from the inverse Comptonization of the disc seed photons. We investigate the presence of frequency and energy-dependent variabilities to probe the origin of the disc/coronal fluctuations. We also report the detection of a Type-I X-ray burst displaying a photospheric radius expansion (PRE). During the burst, a hard X-ray shortage in the 30-80 keV energy band and the enhancement of the persistent emission reveal the burst feedback on the overall accretion process. Using the touch-down burst flux $\sim$ $4.25 \times 10^{-8}$ erg s$^{-1}$ cm$^{-2}$, the distance of the source is estimated as $\sim$ 8.4 kpc.