Constraints on the Low-frequency Radio Emission of the Galactic FRB Source SGR 1935+2154

Abstract: We present a search for radio pulses from the Galactic magnetar SGR 1935+2154, a well-known source of fast radio bursts (FRBs), at $\sim$110 MHz using the Large Phased Array (LPA) of the Pushchino Radio Astronomy Observatory. Data from two active periods in 2020 (March -- May and September -- November, with $\sim 3.5$ minutes of daily coverage) were analyzed with new methods tailored to both FRB-like single pulses and pulsar-like periodic signals. No significant FRB-like pulses were found. Using Monte Carlo simulations, $3σ$ upper limits were derived for the burst rate: for a log-normal energy distribution the limit is $\sim${10}^{1.5}~{\rm{d}}^{-1}</sup></sup>forameanofaveragemonochromaticisotropicluminosity</sup></sup> for a mean of average monochromatic isotropic luminosity L_{ν{\rm ,mean}}\sim1.3\times{10}^{29}~{\rm{erg~s^{-1}~ {Hz}^{-1}}}</sup>andanaturallog−spacescatterof</sup> and a natural log-space scatter of σ\sim0.85;whileforapower−lawdistributionitis; while for a power-law distribution it is \sim10<sup>1.8 d<sup>−1{10}<sup>{1.8}~{\rm{d}}<sup>{-1} for an index β≲3.0β\lesssim3.0 and a minimum average monochromatic isotropic luminosity Lν,min≲0.7×10<sup>25 erg s<sup>−1 Hz<sup>−1L_{ν{\rm{,min}}}\lesssim0.7\times{10}<sup>{25}~{\rm{erg~s<sup>{-1}~{Hz}<sup>{-1}}}. When folded at the known 3.24781628 s period of SGR 1935+2154, a weak pulse was noted (S/N $&lt;$ 3.16), but the significance is insufficient for a secure detection of the pulsar-like emission signal. A conservative upper limit on the average monochromatic isotropic luminosity of any possible periodic emission is $$2.08\times{10}<sup>{19}~{\rm{erg~s<sup>{-1}~{Hz}<sup>{-1}}}$$. Our results offer meaningful low-frequency upper limits on the burst rate of SGR 1935+2154, and hint for very faint pulsar-like radiation at meter wavelengths.