arXiv : ASASSN-18ey: The Rise of a New Black-Hole X-ray Binary
2018
We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole
low-mass
X-ray binarydiscovered by the
All-Sky Automated Surveyfor
SuperNovae(ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient, it was detected as an
X-ray transientby 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.
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