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Literature
Date: 10/15/2024

Arxiv: New JWST redshifts for the host galaxies of CDF-S XT1 and XT2: understanding their nature Published: 10/14/2024 12:26:30 AM Updated: 10/14/2024 12:26:30 AM

Paper abstract: CDF-S XT1 and XT2 are considered two "canonical" extragalactic fast X-raytransients (FXTs). In this work, we report new constraints on both FXTs, basedon recent JWST NIRCam and MIRI photometry, as well as NIRspec spectroscopy forCDF-S XT2 that allow us to improve our understanding of their distances,energetics, and host galaxy properties compared to the pre-JWST era. We use theavailable HST and JWST archival data to determine the host properties andconstrain the energetics of each FXT based on spectral energy distribution(SED) photometric fitting. The host of CDF-S XT1 is now constrained to lie atz_{phot}{=}2.76_{-0.13}^{+0.21}, implying a host absolute magnitudeM_{R}=-19.14 mag, stellar mass M_{*}=2.8e8 M_\odot, and star formationrate SFR=0.62 M_\odot~yr^{-1}. These properties lie at the upper end ofprevious estimates, leaving CDF-S XT1 with a peak X-ray luminosity ofL_{X,peak}=2.8e47 erg s^{-1}. We argue that the best progenitor scenariofor XT1 is a low-luminosity gamma-ray burst (GRB), although we do not fullyrule out a proto-magnetar association or a jetted tidal disruption eventinvolving a white dwarf and an intermediate-massive black hole. In the case ofCDF-S XT2, JWST imaging reveals a new highly obscured component of the hostgalaxy, previously missed by HST, while NIRspec spectroscopy securely placesthe host at z_{spec}{=}3.4598{\pm}0.0022. The new redshift implies a hostwith M_{R}=-21.76 mag, M_{*}=5.5e10 M_\odot, SFR=160 M_\odot yr^{-1},and FXT L_{X,peak}=1.4e47 erg s^{-1}. The revised energetics, similarity toX-ray flash event light curves, small host offset, and high host SFR favor alow-luminosity collapsar progenitor for CDF-S XT2. While these HST and JWSTobservations shed light on the host galaxies of XT1 and XT2, and by extension,on the nature of FXTs, a unique explanation for both sources remains elusive.