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Published July 2025 | Version Published
Journal Article Open

The nature of the X-ray binary transient MAXI J1834–021: Clues from its first observed outburst

Creators

  • 1. ROR icon University of Cagliari
  • 2. ROR icon Institute of Space Sciences
  • 3. ROR icon Institut d'Estudis Espacials de Catalunya
  • 4. INAF/IASF Palermo, via Ugo La Malfa 153, I-90146, Palermo, Italy
  • 5. ROR icon Instituto de Astrofísica de Canarias
  • 6. ROR icon University of La Laguna
  • 7. INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00040, Monte Porzio Catone (RM), Italy
  • 8. ROR icon European Space Astronomy Centre
  • 9. ROR icon Brera Astronomical Observatory
  • 10. ROR icon University of Milan
  • 11. ROR icon Eureka Scientific
  • 12. ROR icon Villanova University
  • 13. ROR icon University of Palermo
  • 14. ROR icon Goddard Space Flight Center
  • 15. ROR icon California Institute of Technology

Abstract

MAXI J1834–021 is a new X-ray transient discovered in February 2023. We analysed the spectral and timing properties of MAXI J1834–021 using NICER, NuSTAR and Swift data collected between March and October 2023. The light curve showed a main peak followed by a second activity phase. Most of the spectra extracted from individual NICER observations could be adequately fit with a Comptonisation component alone, while some required an additional thermal component. The spectral evolution is consistent with a softening trend as the source becomes brighter in X-rays. We also analysed the broadband spectrum by combining data from simultaneous NICER and NuSTAR observations on March 10, 2023. This spectrum can be fitted with a disc component having an inner radius temperature of kTin∼0.4 keV and a Comptonisation component with a power-law photon index of Γ∼1.8. By including a reflection component in the modelling, we obtained a 3σ upper limit for the inner disc radius of 11.4 gravitational radii. We also detected a quasi-periodic oscillation (QPO), whose central frequency varies with time (from 2 Hz to ∼0.9 Hz), and anti-correlates with the hardness ratio. Based on the observed spectral-timing properties, MAXI J1834–021 can be classified as a low-mass X-ray binary in outburst. However, we cannot draw a definitive conclusion about the nature of the accreting compact object, which could currently be a black hole or a neutron star.

Copyright and License

© The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

The authors acknowledge financial contribution from the agreement ASI-INAF n. 2017-14-H.0 and INAF mainstream (PI: A. De Rosa, T. Belloni), from the HERMES project financed by the Italian Space Agency (ASI) Agreement n. 2016/13 U.O and from the ASI-INAF Accordo Attuativo HERMES Technologic Pathfinder n. 2018-10-H.1-2020. We also acknowledge support from the European Union Horizon 2020 Research and Innovation Frame-work Programme under grant agreement HERMES-Scientific Pathfinder n. 821896 and from PRIN-INAF 2019 with the project “Probing the geometry of accretion: from theory to observations” (PI: Belloni). We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for carrying out the observation. We also thank Brad Cenko and the Swift duty scientists and science planners for making the Swift Target of Opportunity observations possible. A. Marino and F.C.Z. are supported by the H2020 ERC Consolidator Grant “MAGNESIA” under grant agreement No. 817661 (PI: Rea) and from grant SGR2021-01269 (PI: Graber/Rea). F.C.Z. is also supported by a Ramón y Cajal fellowship (grant agreement RYC2021-030888-I). A.B. is supported by the Spanish Ministry of Science under the grant EUR2021-122010 (PI: Muñoz-Darias), a L’Oreal–Unesco For Women In Science Fellowship (2023 Italian program) and ESA Fellowship. G.M. acknowledges financial support from the European Union's Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101107057. M.A.P. acknowledges support through the Ramón y Cajal grant RYC2022-035388-I, funded by MCIU/AEI/10.13039/501100011033 and FSE+. T.M.-D. acknowledges support by the Spanish Agencia estatal de investigación via PID2021-124879NB-I00. This work was also partially supported by the program Unidad de Excelencia Maria de Maeztu CEX2020-001058-M. M.C.B. acknowledges support from the INAF-Astrofit fellowship.

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2505.20019 (arXiv)

Funding

Agenzia Spaziale Italiana
2017-14-H.0
National Institute for Astrophysics
Agenzia Spaziale Italiana
2016/13 U.O
Agenzia Spaziale Italiana
2018-10-H.1-2020
European Research Council
821896
European Research Council
MAGNESIA 817661
Institut d'Estudis Espacials de Catalunya
SGR2021-01269
Ministerio de Ciencia, Innovación y Universidades
RYC2021-030888-I
Ministerio de Ciencia, Innovación y Universidades
EUR2021-122010
European Space Agency
European Union
101107057
Ministerio de Ciencia, Innovación y Universidades
RYC2022-035388-I
Agencia Estatal de Investigación
PID2021-124879NB-I00
Ministerio de Ciencia, Innovación y Universidades
CEX2020-001058-M

Dates

Accepted
2025-05-26
Available
2025-07-04
Published online

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Caltech groups
Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published