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

TL;DR

This study analyzes the spectral and timing properties of the newly discovered X-ray transient MAXI J1834-021 during its first observed outburst, revealing its classification as a low-mass X-ray binary with uncertain compact object nature.

Contribution

It provides the first detailed spectral-timing analysis of MAXI J1834-021, including broadband spectral modeling and QPO detection, offering new insights into its outburst behavior.

Findings

Spectral softening trend observed as the source brightens.

Detection of a variable QPO from 2 Hz to 0.9 Hz.

Upper limit of 11.4 gravitational radii for the inner disc radius.

Abstract

MAXI J1834-021 is a new X-ray transient that was 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. The majority of the spectra extracted from the individual NICER observations could be adequately fitted with a Comptonisation component alone, while a few of them required an additional thermal component. The spectral evolution is consistent with a softening trend as the source gets brighter in X-rays. We also analysed the broadband spectrum combining data from simultaneous NICER and NuStar observations on 2023 March 10. This spectrum can be fitted with a disc component with a temperature at the inner radius of $kT_{\rm in} \sim 0.4$ keV and a Comptonisation component with a power-law photon index of $\Gamma \sim 1.8$. By including a reflection component in the modelling, we obtained a 3$\sigma$ 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 $\sim$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 are not able to draw a definitive conclusion on the nature of the accreting compact object, which at the moment could as well be a black hole or a neutron star.