We report on the SRG/eROSITA detection of ultra-soft (kT=47−5+5​ eV)
X-ray emission (LX​=2.5−0.5+0.6​×1043 erg s−1)
from the tidal disruption event (TDE) candidate AT 2022dsb ∼14 days before
peak optical brightness. As the optical luminosity increases after the eROSITA
detection, then the 0.2--2 keV observed flux decays, decreasing by a factor of
∼39 over the 19 days after the initial X-ray detection. Multi-epoch
optical spectroscopic follow-up observations reveal transient broad Balmer
emission lines and a broad He II 4686A emission complex with respect to the
pre-outburst spectrum. Despite the early drop in the observed X-ray flux, the
He II 4686A complex is still detected for ∼40 days after the optical peak,
suggesting the persistence of an obscured, hard ionising source in the system.
Three outflow signatures are also detected at early times: i) blueshifted
Hα emission lines in a pre-peak optical spectrum, ii) transient radio
emission, and iii) blueshifted Lyα absorption lines. The joint evolution
of this early-time X-ray emission, the He II 4686A complex and these outflow
signatures suggests that the X-ray emitting disc (formed promptly in this TDE)
is still present after optical peak, but may have been enshrouded by optically
thick debris, leading to the X-ray faintness in the months after the
disruption. If the observed early-time properties in this TDE are not unique to
this system, then other TDEs may also be X-ray bright at early times and become
X-ray faint upon being veiled by debris launched shortly after the onset of
circularisation.Comment: Submitted to MNRAS on 2023-08-02. 19 pages, 16 figures and 10 table