Early Results from GLASS-JWST. I: Confirmation of Lensed $z\geqslant7$ Lyman-Break Galaxies Behind the Abell 2744 Cluster With NIRISS

We present the first search for $z\geqslant7$, continuum-confirmed sources with NIRISS/WFS spectroscopy over Abell 2744 Frontier Fields cluster, as part of GLASS-JWST ERS survey. With $\sim15$ hrs pre-imaging and multi-angle grism exposures in F115W, F150W, F200W filters, we describe general data handling (i.e., reduction, cleaning, modeling, extraction processes) analysis showcase power JWST to peer deep into reionization, when most intergalactic hydrogen is neutral, by confirming two galaxies at $z=8.04\pm0.15$ $z=7.90\pm0.13$ means their Lyman breaks. Fainter continuum spectra are observed both F150W bands, indicative blue ($-1.69$ $-1.33$) UV slopes moderately-bright absolute magnitudes ($-20.37$ $-19.68$ mag). do not detect strong Ly$\alpha$ either galaxy, but observe tentative ($\sim2.7-3.8\sigma$) HeII$\lambda$1640 A, OIII]$\lambda\lambda$1661,1666 NIII]$\lambda\lambda$1747,1749 A line emission one, suggestive low metallicity, star-forming systems possible non-thermal contributions. These novel observations provide a look extraordinary potential JWST/NIRISS representative samples bright $z\geqslant7$ absence lines, gain unprecedented insight contributions towards cosmic reionization.