Deep Upper Limit on the Optical Emission during a Hard X-Ray Burst from the Magnetar SGR J1935+2154

In September 2021 the magnetar SGR J1935+2154 entered a stage of burst/flaring activity in hard X-ray band. On 10, we observed with fiber-fed fast optical photon counter IFI+Iqueye, mounted at 1.22 m Galileo telescope Asiago. During one IFI+Iqueye observing windows burst was detected Fermi Gamma-ray Burst Monitor. We performed search for any significant increase count rate on 1-s, 10-ms and 1-ms binned light curves around time burst. No peak significance above 3$\sigma$ an interval $\pm$90 s Correcting interstellar extinction ($A_V \simeq 5.8$ mag), upper limits to possible from are $V=10.1$ mag, $V=7.2$ mag $V=5.8$ curves, respectively. The corresponding corrected fluence (specific fluence) $3.1 \times 10^{-10}$ erg cm$^{-2}$ (0.35 Jy s), $4.2 10^{-11}$ (4.8 $\cdot$ 10 ms), $1.6 (17.9 orders magnitude deeper than previous simultaneous limit measurement can also place more stringent constraint spectral index J1935+2154, implying spectrum steeper $\nu^{0.64}$. Fast timing observations bursts associated radio emission have then potential yield detection.