Investigating the nature and properties of MAXI J1810-222 with radio and X-ray observations

TL;DR

This study uses radio and X-ray observations to analyze the peculiar outburst behavior of MAXI J1810-222, suggesting it is a distant black hole X-ray binary with complex spectral state transitions.

Contribution

It provides the first detailed multi-wavelength monitoring of MAXI J1810-222, revealing its unusual spectral evolution and proposing its classification as a distant black hole binary.

Findings

MAXI J1810-222 exhibited a long-lived hard X-ray state.

The source's behavior is consistent with a distant (~6 kpc) black hole binary.

The outburst evolution appears peculiar due to its large distance and detection limits.

Abstract

We present results from radio and X-ray observations of the X-ray transient MAXI J1810-222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array (ATCA), the Neil Gehrels Swift observatory X-ray telescope (XRT), and the Swift Burst Alert Telescope (BAT). We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer (NICER). Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810-222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ~200 days after MAXI J1810-222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (~1.5 years) bright, harder X-ray state. After this hard state, MAXI J1810-222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source's radio behaviour, we argue that the results from this study are most consistent with MAXI J1810-222 being a relatively distant ($\gtrsim$6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky X-ray telescopes.