ASASSN-18ey: The Rise of a New Black-Hole X-ray Binary

TL;DR

This paper reports the discovery and detailed multi-wavelength analysis of a new black hole low-mass X-ray binary, ASASSN-18ey, highlighting its optical and X-ray outburst timing, spectral evolution, and variability patterns.

Contribution

It provides the first detailed observational characterization of ASASSN-18ey, including its optical/X-ray outburst sequence and spectroscopic features, advancing understanding of black hole binary behavior.

Findings

Optical outburst preceded X-ray by about 7 days.

Spectra show broad, asymmetric, double-peaked Hα emission.

Variable Bowen blend profiles suggest irradiation effects.

Abstract

We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole low-mass X-ray binary discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient, it was detected as an X-ray transient by MAXI/GCS. Here, we analyze ASAS-SN and Asteroid Terrestrial-impact Last Alert System (ATLAS) pre-outburst optical light curves, finding evidence of intrinsic variability for several years prior to the outburst. While there was no long-term rise leading to outburst, as has been seen in several other systems, the start of the outburst in the optical preceded that in the X-rays by $7.20\pm0.97~\rm days$. We analyze the spectroscopic evolution of ASASSN-18ey from pre-maximum to $> 100~\rm days$ post-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric, double-peaked H$\alpha$ emission. The Bowen blend ($\lambda \approx 4650$\AA) in the post-maximum spectra shows highly variable double-peaked profiles, likely arising from irradiation of the companion by the accretion disk, typical of low-mass X-ray binaries. The optical and X-ray luminosities of ASASSN-18ey are consistent with black hole low-mass X-ray binaries, both in outburst and quiescence.