The study of thermonuclear X-ray bursts in accreting millisecond pulsar MAXI J1816-195 with NuSTAR and NICER

The millisecond pulsar MAXI J1816-195 was recently discovered in an outburst by the Monitor of All-sky X-ray Image (MAXI) in 2022 May. We study different properties of the pulsar using data from the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) observations. The unstable burning of accreted material on the surface of neutron stars induces thermonuclear (Type-I) bursts. Several such thermonuclear bursts have been detected by MAXI J1816-195 during its outburst. We investigate the evolution of the burst profiles with flux and energy using NuSTAR and NICER observations. During the NuSTAR observation, a total of four bursts were detected from the source. The duration of each burst is around ~30 s and the ratio of peak to persistent count rate is ~26 as seen from the NuSTAR data. The burst profiles are modelled using a sharp linear rise and exponential decay function to determine the burst timing parameters. The burst profiles show a relatively long tail at lower energies. The broad-band time-resolved spectra during the burst periods are successfully modelled with a combination of an absorbed blackbody along with a non-thermal component to account for the persistent emission. From our modelling results, we are able to estimate the maximum apparent emitting area of the blackbody of the neutron star to be ~12.5 km during the peak of the outburst and the maximum distance to the object to be 8.7 kpc. Our findings for the mass accretion rate and the α factor indicate the stable burning of hydrogen via a hot CNO cycle during the bursts.