Einstein Probe catches the onset of a new outburst from the NS LMXB MXB 1730-335 (Rapid Burster)

ATel #17818; Alessio Marino, Francesco Coti Zelati, Nanda Rea, Yilong Wang, Giulia Illiano (ICE-CSIC), Minghao Zhang (NAO, CAS), Chen Wang (IHEP, CAS), Biao Zhang, Jiapei Feng (USTC), Haiwu Pan (NAO, CAS)
on 27 May 2026; 15:18 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Alessio Marino (marino@ice.csic.es)

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We report on the detection of a new outburst from the neutron star low-mass X-ray binary MXB 1730-335 (a.k.a. the Rapid Burster) with the Wide-field X-ray Telescope (WXT) onboard Einstein Probe (EP). Located in the globular cluster Liller 1, the system is unique among Galactic X-ray bursters in exhibiting both thermonuclear Type-I and accretion-driven Type-II bursts (e.g. Bagnoli et al. 2015). Its previous outburst was reported in September 2025 (ATel #17388).

EP-WXT detected the onset of a new outburst on 2026 May 25, with the 0.5-4 keV observed flux rising from 9.5e-11 erg cm-2 s-1 to 2e-10 erg cm-2 s-1 within about one day. An EP-FXT follow-up observation performed on 2026-05-26 at 12:47:42 UTC confirmed the continued brightening of the source, with a measured 0.5-10 keV observed flux of 1e-09 erg cm-2 s-1.

A 2.5 ks light curve reveals several Type II X-ray bursts, confirming that the peculiar bursting behavior of the source has already resumed.

Multi-wavelength follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.