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

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