Magnetars: A Short Review and Some Sparse Considerations

We currently know about 30 magnetars: seemingly isolated neutron stars whose properties can be (in part) comprehended only acknowledging that they are endowed with magnetic fields of complex morphology and exceptional intensity—at least in some components of the field structure. Although magnetars represent only a small percentage of the known isolated neutron stars, there are almost certainly many more of them, since most magnetars were discovered in transitory phases called outbursts, during which they are particularly noticeable. In outburst, in fact, a magnetar can be brighter in X-rays by orders of magnitude and usually emit powerful bursts of hard-X/soft-gamma-ray photons that can be detected almost everywhere in the Galaxy with all-sky monitors such as those on board the Fermi satellite or the Neil Gehrels Swift Observatory. Magnetars command great attention because the large progress that has been made in their understanding is proving fundamental to fathom the whole population of isolated neutron stars, and because, due to their extreme properties, they are relevant for a vast range of different astrophysical topics, from the study of gamma-ray bursts and superluminous supernovae, to ultraluminous X-ray sources, fast radio bursts, and even to sources of gravitational waves. Several excellent reviews with different focuses were published on magnetars in the last few years: among others, Israel and Dall’Osso, Bursts and flares from highly magnetic pulsars, in Proceedings of the First Session of the Sant Cugat Forum on Astrophysics High-Energy Emission from Pulsars and Their Systems, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science Proceedings (Springer, Heidelberg, 2011), pp. 279–298, Rea and Esposito, Magnetar outbursts: an observational review, in High-Energy Emission from Pulsars and Their Systems. Proceedings of the First Session of the Sant Cugat Forum on Astrophysics, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science Proceedings (Springer, Heidelberg, 2011), pp. 247–273, Turolla and Esposito Int J Mod Phys D 22:1330024-163, 2013, Mereghetti, et al. Space Sci Rev 191:315–338, 2015, Turolla et al., 78:116901, 2015; Kaspi and Beloborodov Annu Rev Astron Astrophys 55:261–301, 2017. Here, we quickly recall the history of these sources and travel through the main observational facts, trying to touch some recent and sometimes little-discussed ramifications of magnetars.