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Radio flares and X-ray hardening embedded in the long soft state of 4U 1543−475

Zuobin ZhangAstrophysics, Department of Physics, University of Oxford , Keble Road, Oxford OX1 3RH ,Rob FenderDepartment of Astronomy, University of Cape Town , Private Bag X3, Rondebosch 7701 ,Jiachen JiangDepartment of Physics, University of Warwick , Gibbet Hill Road, Coventry CV4 7AL ,Payaswini SaikiaDepartment of Astronomy, Yale University , PO Box 208101, New Haven, CT 06520-8101 ,D. M. RussellCenter for Astrophysics and Space Science (CASS), New York University Abu Dhabi , PO Box 129188, Abu Dhabi ,Andrew HughesAstrophysics, Department of Physics, University of Oxford , Keble Road, Oxford OX1 3RH ,Honghui LiuInstitut für Astronomie und Astrophysik, Eberhard-Karls Universität Tübingen , D-72076 Tübingen ,Francesco CarotenutoINAF – Osservatorio Astronomico di Roma , Via Frascati 33, I-00078 Monte Porzio Catone (RM) ,James F SteinerCenter for Astrophysics, Harvard & Smithsonian , 60 Garden St, Cambridge, MA 02138 ,F. J. CowieAstrophysics, Department of Physics, University of Oxford , Keble Road, Oxford OX1 3RH ,John A TomsickSpace Sciences Laboratory, University of California , 7 Gauss Way, Berkeley, CA 94720-7450 ,Cosimo BambiSchool of Natural Sciences and Humanities, New Uzbekistan University , Tashkent 100000 ,Yimin HuangCenter for Astronomy and Astrophysics, Department of Physics, Fudan University , Shanghai 200438 ,Xian ZhangShanghai Astronomical Observatory, Chinese Academy of Sciences , Shanghai 200030 ,Wenfei YuShanghai Astronomical Observatory, Chinese Academy of Sciences , Shanghai 200030 ,Yuexin ZhangKapteyn Astronomical Institute, University of Groningen , PO BOX 800, NL-9700 AV Groningen ,Rittick RoyAnton Pannekoek Institute for Astronomy, University of Amsterdam , Science Park 904, NL-1098 XH Amsterdam ,

2026en

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ABSTRACT We present a comprehensive multiwavelength study of the black hole X-ray binary 4U 1543−475 during its 2021 outburst, focusing on radio flaring episodes that are commonly interpreted as signatures of episodic jet production and are embedded within states when the X-ray emission was dominated by an accretion disc component. The radio monitoring reveals at least two discrete flares that coincide with periods of enhanced Comptonized X-ray emission. Broad-band spectral modelling shows a significant decrease in the reflection-to-disc flux ratio (by a factor of ${\sim} 3\!-\!4$) during these episodes, consistent with a temporary change in the geometry of the inner accretion flow, although the data do not allow the causal sequence to be firmly established. Optical photometry exhibits variability that broadly tracks the reflection fraction, consistent with changes in the illuminating component. The accompanying spectral hardening indicates that the radio flares were associated with short-lived excursions toward a ‘harder’ state, departing from the soft state. X-ray timing analysis suggests that the radio flares may be associated with changes in the fractional rms variability; however, no consistent or unified pattern can be firmly established across different events. These results provide a multiwavelength observational example of radio flaring activity in a black hole binary and highlight the complex interplay between accretion flow geometry, coronal emission, and jet-related phenomena.

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DOI: 10.1093/mnras/stag1470

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