NICER Discovery that SRGA J144459.2{\textendash}604207 Is an Accreting Millisecond X-Ray Pulsar

Ng Mason;Ray Paul S.;Sanna Andrea;Strohmayer;Tod E.;Papitto Alessandro;Illiano Giulia;Albayati Arianna C.;Altamirano Diego;and Boztepe Tu(\u(g))ba;G(;Chakrabarty Deepto;Arzoumanian Zaven;Buisson D. ~J. ~K.;Ferrara Elizabeth C.;Gendreau Keith C.;Guillot Sebastien;Hare Jeremy;Jaisawal Gaurava K.;Malacaria Christian;Wolff Michael T.

2024-01-01

---

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 4 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 > 0.1 M ⊙. 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 ≈ 0.9 keV and power-law photon index Γ ≈ 1.9. Time-resolved burst spectroscopy confirmed the thermonuclear nature of the burst, where an additional blackbody component reached a maximum temperature of nearly kT ≈ 3 keV at the peak of the burst. We discuss the nature of the companion as well as the type I X-ray burst.