NICER Discovery that SRGA J144459.2-604207 is an Accreting Millisecond X-ray Pulsar

TL;DR

NICER discovered that SRGA J144459.2-604207 is a 447.9 Hz accreting millisecond X-ray pulsar with a four-week outburst, exhibiting spectral and burst properties characteristic of thermonuclear bursts in a low-mass binary system.

Contribution

This is the first NICER observation confirming SRGA J144459.2-604207 as an accreting millisecond X-ray pulsar with detailed spectral and burst analysis.

Findings

Detected a 447.9 Hz pulsation indicating an AMXP

Observed a type-I X-ray burst with a ~6 s plateau

Spectral analysis consistent with thermonuclear burst characteristics

Abstract

We present the discovery, with the Neutron Star Interior Composition Explorer (NICER), that SRGA J144459.2-604207 is a 447.9 Hz accreting millisecond X-ray pulsar (AMXP), which underwent a four-week long outburst starting on 2024 February 15. The AMXP resides in a 5.22 hr binary, orbiting a low-mass companion donor with $M_d>0.1M_\odot$. We report on the temporal and spectral properties from NICER observations during the early days of the outburst, from 2024 February 21 through 2024 February 23, during which NICER also detected a type-I X-ray burst that exhibited a plateau lasting ~6 s. The spectra of the persistent emission were well described by an absorbed thermal blackbody and power-law model, with blackbody temperature $kT\approx0.9{\rm\,keV}$ and power-law photon index $\Gamma\approx1.9$. Time-resolved burst spectroscopy confirmed the thermonuclear nature of the burst, where an additional blackbody component reached a maximum temperature of nearly $kT\approx3{\rm\,keV}$ at the peak of the burst. We discuss the nature of the companion as well as the type-I X-ray burst.