AstroSat observation of the magnetar SGR J1830$-$0645 during its first detected X-ray outburst

Abstract

We present here timing and spectral analyses of SGR J1830$-$0645 based on an AstroSat observation carried out on 2020 October 16, about a week after the onset of its first detected X-ray outburst. Using data taken with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC), we have detected 0.9$-$10 keV coherent pulsations at a period of $\sim$10.4 s. The pulse profiles were single-peaked, asymmetric and consisted of minor peaks attributable to hotspots on the neutron star surface. The pulsed fraction evolved significantly with energy, increasing to energies around 5 keV with a steep drop thereafter. The 0.9--25 keV SXT--LAXPC energy spectrum is best described with two thermal components having temperatures $\sim$0.46 and $\sim$1.1 keV (emission radii of $\sim$2.4 and $\sim$0.65 km, respectively, assuming a distance of 4 kpc) along with a power-law component having a photon index of $\sim$0.39. We report the detection of 67 X-ray bursts having an average duration of $\sim$33 ms. The brightest burst lasted for about 90 ms and had a 3--25 keV fluence of $\sim 5 \times 10^{-9}$ erg cm$^{-2}$.