Speaker
Description
The origins of fast X-ray transients (FXTs), which are bursts of soft X-ray photons (~0.3-10 keV), have so far been amongst the most elusive of transients. Their origins in binary neutron star mergers which lead to the formation of a millisecond pulsars whose rapid spin down power the bursts; tidal disruption events (TDE) involving white dwarfs disrupted by intermediate-mass black holes; shock breakouts after compact progenitor supernovae; or jet breakouts following long-GRBs through cocoon-like emission are actively being investigated through photometric and spectroscopic observations of a possible optical counterpart taken within the days to weeks after the FXT.
The recent rapid progress in identifying the origins of FXTs has become possible through timely discoveries and localisations with the Einstein Probe (EP) mission. Before the launch of the EP mission in January 2024, FXTs were discovered from intensive archival searches of Chandra and XMM-Newton data. The discovery of an optical counterpart was only possible in the case of SN2008D because of the short delay between the burst and the optical follow-up. For most of the archival FXTs, the optical counterpart was no longer available and the association with a host galaxy provided the only means to infer some of the properties of the burst. However, the EP mission now promptly localizes FXTs, which presents the opportunity to follow up FXTs shortly after their detection. Early investigations of their origin through photometric and spectroscopic observations indicate that FXTs may frequently, but not always, be associated with gamma-ray bursts (GRBs).
In this talk, I will discuss the multi-wavelength counterpart of FXT EP240414a, specifically focusing on the photometric and spectroscopic follow-up campaign described in van Dalen et al., 2024. The optical light curve is peculiar and complex as it shows at least three distinct emission episodes with timescales of ~ 1, 4 and 15 days, where the second peak is the brightest. The spectroscopy evolves from initially blue and inconsistent with afterglow emission, to flat and featureless, and finally reveals a broad-lined Type Ic supernova at late times. While such supernovae are often associated with GRBs, no gamma-rays were detected for EP240414a. Additionally, the transient has an unusual environment with its significant relative offset of ~25 kpc from a host galaxy as compared to other collapsars and is located in an uncommonly large host galaxy at redshift 0.4.
I will describe the detailed investigation of the FXT and its host galaxy. Through light curve and spectral comparisons of the transient as well as modelling of the complex optical and infrared light curves, the origin of the transient is identified as a jet-forming supernova inside a dense envelope. The interaction of jet and supernova shock waves can explain the light curve before the appearance of the long-GRB related supernova. This shock interaction has been suggested for some Fast Blue Optical Transients (LFBOTs), through which I will make a causal link between the progenitors of long-GRBs, FXTs and Fast Blue Optical Transients LFBOTs.
| Talk category | NOVA Network 3 |
|---|---|
| Preference for a talk or poster | Talk |
| Talk preference for PhD students | second year PhD |
