A NICER thermonuclear burst from the millisecond X-ray pulsar SAX J1808.4-3658

TL;DR

NICER observed the brightest helium-fueled Type I X-ray burst from the millisecond pulsar SAX J1808.4-3658, revealing unique flux evolution, emission lines, and burst oscillations, advancing understanding of neutron star surface phenomena.

Contribution

This study reports the first detection of a highly luminous Type I X-ray burst from SAX J1808.4-3658 with detailed spectral and timing analysis, including emission lines and burst oscillations.

Findings

Brightest NICER X-ray burst observed to date.

Detection of emission lines at 1.0 keV and 6.7 keV.

Observation of burst oscillations at 401 Hz.

Abstract

The Neutron Star Interior Composition Explorer (NICER) has extensively monitored the August 2019 outburst of the 401 Hz millisecond X-ray pulsar SAX J1808.4-3658. In this Letter, we report on the detection of a bright helium-fueled Type I X-ray burst. With a bolometric peak flux of $(2.3\pm0.1)\times 10^{-7}$ erg/cm^2/s, this was the brightest X-ray burst among all bursting sources observed with NICER to date. The burst shows a remarkable two-stage evolution in flux, emission lines at $1.0$ keV and $6.7$ keV, and burst oscillations at the known pulsar spin frequency, with $\approx4$\% fractional sinusoidal amplitude. We interpret the burst flux evolution as the detection of the local Eddington limits associated with the hydrogen and helium layers of the neutron star envelope. The emission lines are likely associated with Fe, due to reprocessing of the burst emission in the accretion disk.