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The extragalactic magnetic field (EGMF) could be probed with $\gamma$-rayobservations of distant sources. Primary very high energy (VHE) $\gamma$-raysfrom these sources absorb on extragalactic background light photons, andsecondary electrons/positrons from the pair production acts create cascade$\gamma$-rays. These cascade $\gamma$-rays could be detected with space$\gamma$-ray telescopes such as Fermi-LAT. The $\gamma$-ray burst GRB 221009Awas an exceptionally bright transient well suited for intergalactic$\gamma$-ray propagation studies. Using publicly-available Fermi-LAT data, weobtain upper limits on the spectrum of delayed emission from GRB 221009A duringthe time windows of 10, 30, and 90 days after the burst, and compare these withmodel spectra calculated for various EGMF strengths $B$, obtaining constraintson $B$. We show that the values of $B$ between 10^{-20} G and 10^{-18} G areexcluded.

First constraints on the strength of the extragalactic magnetic field from $\gamma$-ray observations of GRB 221009A