Magnetar

About: Magnetar is a research topic. Over the lifetime, 2905 publications have been published within this topic receiving 106806 citations.

A low-magnetic-field Soft Gamma Repeater

Abstract: Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, neutron stars powered by extreme magnetic fields, B ~ 1014 to 1015 gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 × 1012 gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far.

A fast radio burst associated with a Galactic magnetar.

Abstract: Since their discovery in 20071, much effort has been devoted to uncovering the sources of the extragalactic, millisecond-duration fast radio bursts (FRBs)2. A class of neutron stars known as magnetars is a leading candidate source of FRBs3,4. Magnetars have surface magnetic fields in excess of 1014 gauss, the decay of which powers a range of high-energy phenomena5. Here we report observations of a millisecond-duration radio burst from the Galactic magnetar SGR 1935+2154, with a fluence of 1.5 ± 0.3 megajansky milliseconds. This event, FRB 200428 (ST 200428A), was detected on 28 April 2020 by the STARE2 radio array6 in the 1,281–1,468 megahertz band. The isotropic-equivalent energy released in FRB 200428 is 4 × 103 times greater than that of any radio pulse from the Crab pulsar—previously the source of the brightest Galactic radio bursts observed on similar timescales7. FRB 200428 is just 30 times less energetic than the weakest extragalactic FRB observed so far8, and is drawn from the same population as the observed FRB sample. The coincidence of FRB 200428 with an X-ray burst9–11 favours emission models that describe synchrotron masers or electromagnetic pulses powered by magnetar bursts and giant flares3,4,12,13. The discovery of FRB 200428 implies that active magnetars such as SGR 1935+2154 can produce FRBs at extragalactic distances. Observations of the fast radio burst FRB 200428 coinciding with X-rays from the Galactic magnetar SGR 1935+2154 indicate that active magnetars can produce fast radio bursts at extragalactic distances.

Supergiant pulses from extragalactic neutron stars

Abstract: We consider radio bursts that originate from extragalactic neutron stars (NSs) by addressing three questions about source distances. What are the physical limitations on coherent radiation at GHz frequencies? Do they permit detection at cosmological distances? How many bursts per NS are needed to produce the inferred burst rate 10 3 -10 4 sky 1 day 1 ? The burst rate is comparable to the NS formation rate in a Hubble volume, requiring only one per NS if they are bright enough. However, radiation physics causes us to favor a closer population. More bursts per NS are then required but repeats in 10 to 100 yr could still be negligible. Bursts are modeled as sub-ns, coherent shot pulses superposed incoherently to produce msduration 1 Jy amplitudes; each shot-pulse can be much weaker than the burst amplitude, placing less restrictive requirements on the emission process. Nonetheless, single shot pulses are similar to the extreme, unresolved (< 0:4 ns) MJy shot pulse seen from the Crab pulsar, which is consistent with coherent curvature radiation emitted near the light cylinder by an almost neutral clump with net charge 10 21 e and total energy & 10 23 ergs. Bursts from Gpc distances require incoherent superposition of 10 12 d 2 shot pulses or a total energy & 10 35 d 2 erg. The energy reservoir near the light cylinder limits the detection distance to . few 100 Mpc for a fluence 1 Jy ms unless conditions are more extreme than for the Crab pulsar. Similarly, extreme single pulses from ordinary pulsars and magnetars could be detectable from throughout the Local Group and perhaps farther. Contributions to dispersion measures from galaxy clusters will be significant for some of the bursts. We discuss tests for the signatures of bursts associated with extragalactic NSs.

Producing ultrastrong magnetic fields in neutron star mergers

Abstract: We report an extremely rapid mechanism for magnetic field amplification during the merger of a binary neutron star system. This has implications for the production of the short class of gamma-ray bursts, which recent observations suggest may originate in such mergers. In detailed magnetohydrodynamic simulations of the merger process, the fields are amplified by Kelvin-Helmholtz instabilities beyond magnetar field strength and may therefore represent the strongest magnetic fields in the universe. The amplification occurs in the shear layer that forms between the neutron stars and on a time scale of only 1 millisecond, that is, long before the remnant can collapse into a black hole.

The Giant Flare of 1998 August 27 from SGR 1900+14. II. Radiative Mechanism and Physical Constraints on the Source

Abstract: The extraordinary 1998 August 27 giant flare places strong constraints on the physical properties of its source, SGR 1900+14. We make detailed comparisons of the published data with the magnetar model, which identifies the soft gamma repeaters as neutron stars endowed with ~1015 G magnetic fields. The giant flare evolved through three stages, whose radiative mechanisms we address in turn. The extreme peak luminosity L > 106LEdd, hard spectrum, and rapid variability of the initial ~0.5 s spike emission all point to an expanding pair fireball with very low baryon contamination. We argue that this energy must have been deposited directly through shearing and reconnection of a magnetar-strength external magnetic field. Low-order torsional oscillations of the star fail to transmit energy rapidly enough to the exterior, if the surface field is much weaker. A triggering mechanism is proposed, whereby a helical distortion of the core magnetic field induces large-scale fracturing in the crust and a twisting deformation of the crust and exterior magnetic field. After the initial spike (whose ~0.4 s duration can be related to the Alfven crossing time of the core), very hot (T 1 MeV) plasma rich in electron-positron pairs remains confined close to the star on closed magnetic field lines. The envelope of the August 27 flare can be accurately fitted, after ~40 s, by the contracting surface of such a "trapped fireball." The form of this fit gives evidence that the temperature of the trapped pair plasma decreases outward from its center. We quantify the effects of direct neutrino pair emission on the X-ray light curve, thereby deducing a lower bound μmin ~ 1 × 1032 G cm3 to the magnetic moment of the confining field, comparable to the strongest fields measured in radio pulsars. The radiative flux during the intermediate ~40 s of the burst appears to exceed the trapped fireball fit. The lack of strong rotational modulation and intermediate hardness of this smooth tail are consistent with the emission from an extended pair corona, in which O-mode photons are heated by Compton scattering. This feature could represent residual seismic activity within the star and accounts for ~10% of the total flare fluence. We consider in detail the critical luminosity, below which a stable balance can be maintained between heating and radiative cooling in a confined, magnetized pair plasma, but above which the confined plasma runs away to a trapped fireball in LTE. The emergence of large-amplitude pulsations at ~40 s probably represents a transition to a pair-depleted photosphere whose main source of opacity is electrons (and ions) ablated from the heated neutron star surface. The best-fit temperature of the blackbody component of the spectrum equilibrates at a value that agrees well with the regulating effect of photon splitting. The remarkable four-peaked substructure within each 5.16 s pulse, as well as the corresponding collimation of the X-ray flux, has a simple explanation based on the strong inequality between the scattering cross sections of the two photon polarization modes. The width of each X-ray "jet" is directly related to the amount of matter advected outward by the high cross section ordinary mode.