Kavli Affiliate: Subo Dong
| First 5 Authors: Luming Sun, Ning Jiang, Liming Dou, Xinwen Shu, Jiazheng Zhu
| Summary:
We report the discovery of a second optical flare that occurred in September
2021 in IRAS F01004-2237, where the first flare occurred in 2010 has been
reported, and present a detailed analysis of multi-band data. The position of
the flare coincides with the galaxy centre with a precision of 650 pc. The
flare peaks in $sim50$ days with an absolute magnitude of $sim-21$ and fades
in two years roughly following $Lpropto t^{-5/3}$. It maintains a nearly
constant blackbody temperature of $sim$22,000 K in the late time. Its optical
and UV spectra show hydrogen and helium broad emission lines with full width at
half maxima of 7,000–21,000 km s$^{-1}$ and He II/H$alpha$ ratio of 0.3–2.3.
It shows weak X-ray emission relative to UV emission, with X-ray flares lasting
for $<2-3$ weeks, during which the spectrum is soft with a power-law index
$Gamma=4.4^{+1.4}_{-1.3}$. These characters are consistent with a tidal
disruption event (TDE), ruling out the possibilities of a supernova or an
active galactic nuclei flare. With a TDE model, we infer a peak UV luminosity
of $3.3pm0.2times10^{44}$ erg s$^{-1}$ and an energy budget of
$4.5pm0.2times10^{51}$ erg. The two optical flares separated by $10.3pm0.3$
years can be interpreted as repeating partial TDEs, double TDEs, or two
independent TDEs. Although no definitive conclusion can be drawn, the partial
TDEs interpretation predicts a third flare around 2033, and the independent
TDEs interpretation predicts a high TDE rate of $gtrsim10^{-2}$ yr$^{-1}$ in
F01004-2237, both of which can be tested by future observations.
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