Περίληψη :
Fast X-ray Transients (FXTs) are rare and poorly understood, with various potential progenitors. The launch of the Einstein Probe (EP) mission has greatly increased the rate of real-time FXT discoveries and enabled timely multi-wavelength follow-up observations. We have recently begun to do follow-up observations of FXTs with the Three Channel Imager (3KK) on the Fraunhofer Telescope at Wendelstein Observatory (FTW), located in the German Alps and operated by the Ludwig-Maximilians-Universität München.
EP241021a was the first FXT that we observed in this new campaign. The optical and NIR (OIR) counterpart shows a very peculiar panchromatic behavior in its light curve, rebrightening by almost 1.5 mag within ~2 days about 5.5 days after the initial trigger. This event stands out due to its extreme brightness, peaking at an absolute magnitude of approximately -22 mag in the r band after rebrightening, and its consistently red color of J−r≈1mag. While its overall light curve can be described by two decaying power laws ∼t^−1 with a steep rising phase ∼t^4, its brightness and timescale distinguish it from other FXTs, including EP240414a, which exhibited similar behavior but evolved more rapidly and at lower luminosity. These characteristics, along with the absence of a gamma-ray detection, constrain possible origins for this event.
In this talk, I will discuss possible explanations for the extreme rebrightening. Traditional supernova models struggle to account for both the high luminosity and rapid evolution of EP241021a, but alternative scenarios related to the collapse of a massive star may offer a more promising explanation.
