Description
Extra-Galactic fast X-ray transients (FXTs) are X-ray flashes lasting from a few seconds to several hours. Their progenitor mechanism remains largely unclear, but likely comprises a mix of possibilities, including binary neutron star mergers, tidal disruption events involving a white dwarf and an intermediate-mass black hole, supernova (SN) shock breakouts and gamma-ray bursts (GRBs) with cocoon/choked jets or highly redshifted GRBs. Prior to 2024, only about 30 FXTs had been identified in archival Chandra and XMM-Newton data, with just one having a serendipitously observed optical counterpart. For the FXTs without a counterpart, we heavily rely on the information that the environments of the transient can provide us in order to understand the origin of the burst. I will discuss how the properties of the host galaxies, such as redshifts, star formation rates and masses, and the location of the FXTs with respect to the hosts can help us to constrain the properties and progenitor mechanisms of FXTs.
Furthermore, since the launch of Einstein Probe (EP) in January 2024, the number of discovered FXTs has exceeded expectations, with a detection rate surpassing a hundred FXTs per year. So far, the rapid follow-up of these events has led to the identification of the optical and near-infrared counterparts in approximately 55% of the EP-detected FXTs. The discovered transients seem to go beyond the simple classification schemes, as there now seem to be connections between the various classes of high energy transients. Therefore, I will also discuss an intriguing example of an FXT related to a weak GRB, EP240801a, to show how follow-up data of the multi wavelength counterpart helps us to unravel their nature. The event's relatively low isotropic energy and the fluence ratio classify this FXT as the first EP-detected X-ray flash (XRF). Early-time data suggests that its afterglow consists of multiple components, potentially arising from a two-component jet or a jet with energy injection from a central engine. However, with multiple models fitting the data well, it is still uncertain what the best afterglow model for this event is. In this talk, I will present late-time data of the counterpart of EP240801a obtained with the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). I will show how this deep data can further constrain the nature of EP240801a, since combining the early-time data and its afterglow models with the late-time data helps us to constrain the models and look, for example, for SN evidence, shedding light on the association between XRFs and SNe.
| Talk category | NOVA Network 3 |
|---|---|
| Preference for a talk or poster | Talk |
| Talk preference for PhD students | First year PhD |
