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

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.