The 2024 outburst of 1A 1744–361 observed by NICER and Insight-HXMT: Type-I X-ray burst and persistent emission

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Tübingen 72076, Germany
² Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China
³ School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai 519082, PR China
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁵ School of Physics and Optoelectronic Engineering, Shandong University of Technology Zibo 255000, PR China

^⋆ Corresponding authors: pengju.wang@mnf.uni-tuebingen.de, chenyp@ihep.ac.cn

Received: 26 March 2025
Accepted: 22 July 2025

Abstract

We present a detailed report on the 2024 outburst observations of 1A 1744−361, including the evolution of persistent emission and a type-I X-ray burst. Observations covered the rising phase of the outburst, a phase rarely reported in previous studies of this source. During the island state in the rising phase, we detected a ∼1 Hz quasiperiodic oscillation (QPO) phenomenon characteristic of dipping sources. The QPO frequency evolved from 0.98 Hz to 2.18 Hz as the outburst flux increased, and the QPO disappeared after the source transitioned into the banana state. Based on the spectral and timing analysis, we suggest that the ∼1 Hz QPO arose from the periodic occultation of the central emission by inner disk structures undergoing Lense-Thirring precession. The ∼1 Hz QPO was also detected during the type-I X-ray burst, with an rms amplitude of 10.53%, a level similar to that observed in the persistent emission. Furthermore, spectral measurements of the type-I X-ray burst indicate a relatively small blackbody radiation radius compared to a typical neutron star radius, which may suggest that type-I X-ray bursts associated with the ∼1 Hz QPO originate from a smaller emission area and exhibit a reduced vertical extent.