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Register a new userMagnetars: a short review and some sparse considerations
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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 DallOsso (2011); Rea and Esposito (2011); Turolla and Esposito (2013); Mereghetti et al. (2015); Turolla et al. (2015); Kaspi and Beloborodov (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.
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I present a brief up-to-date review of the current understanding of Supergiant Fast X-ray Transients, with an emphasis on the observational point of view. After more than a decade since their discovery, a remarkable progress has been made in getting the picture of their phenomenology at X-ray energies. However, a similar in-depth investigation of the properties of the supergiant companions is needed, but has started more recently. A multifrequency approach is the key to fully understand the physical mechanism driving the SFXT behaviour, still under debate.
Magnetars are young and highly magnetized neutron stars which display a wide array of X-ray activity including short bursts, large outbursts, giant flares and quasi-periodic oscillations, often coupled with interesting timing behavior including enhanced spin-down, glitches and anti-glitches. The bulk of this activity is explained by the evolution and decay of an ultrastrong magnetic field, stressing and breaking the neutron star crust, which in turn drives twists of the external magnetosphere and powerful magnetospheric currents. The population of detected magnetars has grown to about 30 objects and shows unambiguous phenomenological connection with very highly magnetized radio pulsars. Recent progress in magnetar theory includes explanation of the hard X-ray component in the magnetar spectrum and development of surface heating models, explaining the sources remarkable radiative output.
Basic aspects of phononless resonant capture of monoenergetic electron antineutrinos (Moessbauer antineutrinos) emitted in boundstate beta-decay in the 3H - 3He system are considered. It is shown that stochastic magnetic relaxation phenomena as well as the direct influence of solid-state effects on the energy of the electron antineutrino will cause line broadening by a factor of more than 10^(13). Lattice expansion and contraction after the transformation of the nucleus will drastically reduce the probability for phononless transitions. Thus, the observation of Moessbauer electron antineutrinos of the 3H - 3He system will most probably be unsuccessful. As a possible alternative, the Rare-Earth system 163Ho - 163Dy is briefly discussed.
Be/X-ray binary systems provide an excellent opportunity to study the physics around neutron stars through the study of the behaviour of matter around them. Intermediate and low-luminosity type outbursts are interesting because they provide relatively clean environments around neutron stars. In these conditions the physics of the magnetosphere around the neutron star can be better studied without being very disturbed by other phenomena regarding the transfer of matter between the two components of the Be/X-ray binary system. A recent study presents the optical longterm evolution of the Be/X-ray binary V 0332+53 plus the X-ray emission mainly during the intermediate-luminosity outburst on 2008. In this paper we comment on the context of these observations and on the properties that can be derived through the analysis of them.
Two classes of X-ray/$gamma$-ray sources, the Soft Gamma Repeaters and the Anomalous X-ray Pulsars have been identified with isolated, slowly spinning magnetars, neutron stars whose emission draws energy from their extremely strong magnetic field ($sim 10^{15}-10^{16}$ G). Magnetars are believed to form with millisecond spin period and to represent an important fraction of the whole population of young neutron stars. Newborn magnetars can convert very quickly their rotational energy into electromagnetic and/or gravitational waves, by virtue of their strong magnetic fields and fast spins. This chapter provides a brief summary of astrophysical problems and scenarios in which millisecond magnetars are believed to play a key role: these include Gamma Ray Bursts, Supernovae, Gravitational Wave events and Fast Radio Bursts.
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