Extended Data Fig. 2: UV spectrum and variability of GSN 069 from HST and OM observations.
From: Nine-hour X-ray quasi-periodic eruptions from a low-mass black hole galactic nucleus
a, The 2014/2018 HST/STIS spectra taken quasi-simultaneously with XMM2 (2014) and XMM3 (2018). b, The 2014 STIS spectrum of GSN 069 is compared with that of intermediate-type main-sequence stars (B3 and B6) from the Pickles Atlas, demonstrating that the UV spectrum is strongly contaminated by starlight and possibly dominated by a relatively young nuclear stellar cluster. c, The OM light curve in the UVM2 filter (approximately 231 nm) during the XMM4 observation. The OM light curve shows no variability, with a reduced χ2 of 0.7 when fitted with a constant, despite the simultaneous X-ray QPEs (see Fig. 1b). The STIS spectra as well as the B3 and B6 spectra from the Pickles Atlas are displayed with no uncertainties. Errors in c represent the 1σ confidence intervals.
