A1 Refereed original research article in a scientific journal
NICER detection of a new candidate cyclotron line in the bursting X-ray pulsar GRO J1744–28; 
Authors: Liu, Qi; Kong, Lingda; Ji, Long; Ducci, Lorenzo; Dai, Xiaohang; Santangelo, Andrea; Cai, Ce; Cui, Shuwang; Ge, Mingyu; Zhang, Shu; Zhang, Shuang-Nan; Tao Lian; Feng Hua; Wang Wei; Suleimanov Valery; Tsygankov Sergey; Poutanen Juri; Mushtukov Alexander
Publisher: EDP Sciences
Publication year: 2026
Journal: Astronomy and Astrophysics
Article number: A101
Volume: 711
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202659195
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202659195
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/527013710
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
We report the detection of cyclotron resonant scattering features (CRSFs) in the spectrum of the unique bursting pulsar GRO J1744–28, observed during its recent outburst in 2021 with the Neutron Star Interior Composition Explorer (NICER). Clear pulsations at a frequency of 2.141128 Hz as well as Type II X-ray bursts were observed. The pulse profile exhibits a single-peaked shape in all energy bands, with the pulse fraction showing a positive correlation with energy. We find that the persistent X-ray continuum of the accreting pulsar is well described by typical phenomenological models, and we confirm the presence of the cyclotron line at ∼5 keV as reported in previous studies. In addition, we detect a candidate absorption feature with a centroid energy of 2 keV. If confirmed, this feature could be interpreted as a CRSF, which would correspond to a magnetic field of ∼1.8 × 1011 G. Pulse-phase-resolved analysis also reveals this absorption line around the peak pulse phases. These NICER observations provide tentative evidence for the cyclotron line candidate, establishing GRO J1744–28 as a key laboratory for studying accretion physics in an intermediate-strength magnetic field.
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Funding information in the publication:
This work is supported by the Science Foundation of Hebei Normal University (No. L2026B49), Top-notch Talent Project (Overseas Platform) under the Hebei Yanzhao Golden Terrace Talent Gathering Program, the NSFC (12133007, 12173103 and 12261141691), the National Key Research and Development Program of China (Grant No. 2021YFA0718503), the Fundamental Research Funds for the Central Universities (010-63263126), and Science Research Project of Hebei Education Department (BJ2026091). LD acknowledges funding from German Research Foundation (DFG), Projektnummer 549824807. ST and JP acknowledge support by the Research Council of Finland, the Centre of Excellence in Neutron-Star Physics (project 374064). This work has made use of archival data provided by the High Energy Astrophysics Science Archive Research Center (HEASARC).
