Accretion Geometry of the New Galactic Black Hole Candidate AT2019wey in the Hard State

TL;DR

This study analyzes the accretion geometry and spectral properties of the Galactic black hole candidate AT2019wey in the hard state, revealing a truncated disk, dominant Comptonization, and evidence of disk variability and reverberation effects.

Contribution

It provides detailed broadband spectral modeling of AT2019wey, identifying two Comptonizing regions and confirming disk truncation through independent methods, advancing understanding of accretion in low-mass black holes.

Findings

Disk truncated at 16-56 gravitational radii

Dominant Comptonizing region accounts for over 80% of flux

Detection of thermal reverberation effects in lag-energy spectrum

Abstract

We perform broadband spectral and timing studies of the Galactic low-mass black hole candidate AT2019wey using quasi-simultaneous NICER, Swift, and NuSTAR observations obtained in 2022. The long-term MAXI light curve, along with the hardness-intensity diagram (HID), indicates that the source remained in the hard state and did not switch to the soft state. Spectral modeling using two different model combinations reveals that the broadband spectrum is best described by two distinct Comptonizing regions, associated reflection components, and thermal emission from the disk. The harder Comptonizing region dominates ($\gtrsim80\%$) the total flux and is primarily responsible for the observed reflection features from the distant part of the disk. We find that the accretion disk is truncated at a radius of $\sim16-56~r_{\rm{g}}$, while the luminosity is $\sim1.9\%$ of the Eddington limit, assuming a black hole mass of $10 ~ M_\odot$ and distance of 8 kpc. Our spectral results also show consistency in the estimated inner disk radius obtained through two independent methods: modeling the disk continuum and the reflection spectrum. The variability studies imply the presence of intrinsic disk variability, likely originating from an instability in the disk. We also detect hard time lags at low frequencies, possibly arising from the inward propagation of mass accretion rate fluctuations from the outer to the inner regions of the accretion disk. Moreover, an observed deviation of the lag-energy spectrum from the log-linear trend at $\lesssim 0.7$ keV is most likely attributed to thermal reverberation, arising from the reprocessing of hard coronal photons in the accretion disk.