The Prolific Thermonuclear X-Ray Bursts from the Outburst of the Newly Discovered Millisecond Pulsar MAXI J1816–195 Observed by Insight-HXMT and NICER

Abstract

MAXI J1816–195 is a newly discovered accreting millisecond pulsar with prolific thermonuclear bursts, detected during its outburst in 2022 June by Insight-HXMT and NICER. During the outburst, Insight-HXMT detected 73 bursts in its peak and decay phase, serving as a prolific burst system found in the accreting millisecond pulsars. By analyzing one burst that was simultaneously detected by Insight-HXMT and NICER, we find a mild deviation from the conventional blackbody model. By stacking the Insight-HXMT light curves of 66 bursts that have similar profiles and intensities, a hard X-ray shortage is detected with a significance of 15.7σ in 30–100 keV. The shortage is about 30% of the persistent flux, which is low compared with other bursters. The shortage fraction is energy-dependent: larger in a higher energy band. These findings make the newly discovered millisecond MAXI J1816–195 a rather peculiar system compared with other millisecond pulsars and atoll bursters. In addition, based on the brightest burst, we derive an upper limit of the distance as 6.3 kpc, and therefore estimate the upper limit of the inner-disk radius of the accretion disk to be ∼40 km. Assuming the radius as the magnetospheric radius, the derived magnetic field strength is about 7.1 × 108 G.