An investigation into the X-ray bursts in Cir X-1

¹ College of Physics and Electronic Engineering, Qilu Normal University, 250200 Jinan, People’s Republic of China
² Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, People’s Republic of China
³ Dongguan Neutron Science Center, 1 Zhongziyuan Road, Dongguan 523808, People’s Republic of China
⁴ School of Astronomy and Space Science and Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, 210023 Nanjing, People’s Republic of China
⁵ University of Chinese Academy of Sciences, Chinese Academy of Sciences, 100049 Beijing, People’s Republic of China

^⋆ Corresponding authors: yuzl@ihep.ac.cn, szhang@ihep.ac.cn

Received: 14 January 2025
Accepted: 25 May 2025

Abstract

We report 155 X-ray bursts from Cir X-1 detected with NICER and 48 with Insight-HXMT, respectively. The K-medoids clustering algorithm was applied to X-ray bursts detected by NICER for classification. These X-ray bursts are classified into two categories, designated as Class A and B. The profile of Class A resembles that of a typical type I X-ray burst. Class B is weaker and its profile is approximately symmetric. In these X-ray bursts, the shortest recurrence time of ∼300 s observed to date in Cir X-1 is detected. Additionally, no burst is detected prior to MJD 58933 in NICER and Insight-HXMT. The underlying reason for this phenomenon remains unclear and requires further investigation.