INTEGRAL search for magnetar giant flares from the Virgo Cluster and in nearby galaxies with high star formation rate

Abstract

Giant flares from magnetars can reach, for a fraction of a second, luminosities greater than 10$^{47}$ erg s$^{-1}$ in the hard X-ray/soft $\gamma$-ray range. This makes them visible at distances of several megaparsecs. However, at extragalactic distances (farther than the Magellanic Clouds), they are difficult to distinguish from the short $\gamma$-ray bursts, which occur much more frequently. Since magnetars are young neutron stars, nearby galaxies with a high rate of star formation are optimal targets to search for magnetar giant flares (MGFs). Here, we report the results of a search for MGFs in observations of the Virgo cluster and in a small sample of nearby galaxies obtained with the IBIS instrument on the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite. From the currently known MGF sample, we find that their energy distribution is well described by a power law with slope $\gamma$ = 2 (with 90 per cent c.l. interval [1.7-2.2]). From the lack of detections in this extensive data set (besides 231115A in M82), we derive a 90 per cent c.l. upper limit on the rate of MGF with $E > 3 \times 10^{45}$ erg of ${\sim}2\times 10^{-3}\,{\rm yr}^{-1}$ per magnetar and a lower limit on $R(>E) $ of ${\sim} 4\times 10^{-4}\,{\rm yr}^{-1}$ magnetar$^{-1}$ for $E < 10^{45}$ erg.