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

¹ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy
² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
⁴ INAF/IASF Palermo, via Ugo La Malfa 153, I-90146 Palermo, Italy
⁵ Instituto de Astrofísica de Canarias (IAC), Vía Láctea s/n, La Laguna 38205, S/C de Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, La Laguna E-38205, S/C de Tenerife, Spain
⁷ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone (RM), Italy
⁸ European Space Agency (ESA), European Space Astronomy Center (ESAC), Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
⁹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (LC), Italy
¹⁰ Dipartimento di Fisica, Università Degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹¹ Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
¹² Department of Physics, Villanova University, 800 Lancaster Avenue, Villanova, PA 19085, USA
¹³ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica – Emilio Segrè, via Archirafi 36, I-90123 Palermo, Italy
¹⁴ X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
¹⁶ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy

^⋆ Corresponding author: arianna.manca@inaf.it

Received: 15 July 2024
Accepted: 26 May 2025

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 kT_in∼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.