Magnetar

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

Blast Waves from Magnetar Flares and Fast Radio Bursts

Abstract: Magnetars younger than one century are expected to be hyper active. Besides winds powered by rotation they generate frequent magnetic flares, which launch powerful blast waves into the wind. These internal shocks act as masers producing fast (millisecond) radio bursts (FRBs) with the following properties. (1) GHz radio emission occurs at radii $r\sim 10^{14}$ cm and lasts $\lesssim 1$ ms in observer's time. (2) Induced scattering in the surrounding wind does not suppress the radio burst. (3) The emission has linear polarization set by the magnetar rotation axis. (4) The emission drifts to lower frequencies during the burst, and its duration broadens at lower frequencies. (5) Blast waves in inhomogeneous winds may emit variable bursts; periodicity might appear on sub-ms timescales if the magnetar rotates with $\sim 1$ s period. However, the observed FRB structure is likely changed by lensing effects during propagation through the host galaxy. (6) The FRBs from magnetars are expected to repeat, with rare strong bursts (up to $\sim 10^{43}$ erg) or more frequent weak bursts. (7) When a repeating flare strikes the wind bubble in the tail of a previous flare, the FRB turns into a bright optical flash. Its luminosity may approach that of a supernova Ia and last seconds. The rate of these optical flashes in the universe is much lower than the FRB rate, however it may exceed the supernova rate. Locations of hyper-active magnetars in their host galaxies depend on how they form: magnetars created in supernovae explosions will trace star formation regions, and magnetars formed in mergers of compact objects will be offset. The merger magnetars are expected to be most energetic and particularly hyper-active.

Shallow decay phase of GRB X-ray afterglows from relativistic wind bubbles

Abstract: Aims. The postburst object of a GRB is likely to be a highly magnetized, rapidly rotating compact object (e.g., a millisecond magnetar), which could produce an ultrarelativistic electron-positron-pair wind. The interaction of such a wind with an outwardly expanding fireball ejected during the burst leads to a relativistic wind bubble (RWB). We investigate the properties of RWBs and use this model to explain the shallow decay phase of the early X-ray afterglows observed by Swift. Methods. We numerically calculate the dynamics and radiative properties of RWBs. Results. We find that RWBs can fall into two types: forward-shock-dominated and reverse-shock-dominated bubbles. Their radiation during a period of � 10 2 10 5 seconds is dominated by the shocked medium and the shocked wind, respectively, based on different magnetic energy fractions of the shocked materials. For both types, the resulting light curves always have a shallow decay phase, as discovered by Swift. In addition, we provide an example fit to the X-ray afterglows of GRB 060813 and GRB 060814 and show that they could be produced by forward-shock-dominated and reverse-shock-dominated bubbles, respectively. This implies that, for some early afterglows (e.g., GRB 060814), the long-lasting reverse shock emission is strong enough to explain their shallow decay phase.

Discovery of extended VHE \gamma-ray emission from the vicinity of the young massive stellar cluster Westerlund 1

Abstract: Results obtained in very-high-energy (VHE; E > 100 GeV) \gamma-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from \gamma-ray induced particle cascades in the Earth's atmosphere is used to search for VHE \gamma\ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. The detection of the degree-scale extended VHE \gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S. observations performed between 2004 and 2008. The VHE \gamma-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20\sigma. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of \Gamma=2.19 \pm 0.08_{stat} \pm 0.20_{sys} and a flux normalisation at 1 TeV of \Phi_0 = (9.0 \pm 1.4_{stat} \pm 1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2 TeV amounts to (5.2 \pm 0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident with HESS J1646-458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648-4611 could be viable to explain parts of the VHE \gamma-ray emission of HESS J1646-458.

Periodicity in recurrent fast radio bursts and the origin of ultralong period magnetars

Abstract: The recurrent fast radio burst FRB 180916 was recently shown to exhibit a 16 day period (with possible aliasing) in its bursting activity. Given magnetars as widely considered FRB sources, this period has been attributed to precession of the magnetar spin axis or the orbit of a binary companion. Here, we make the simpler connection to a {\it rotational period}, an idea observationally motivated by the 6.7 hour period of the Galactic magnetar candidate, 1E 161348--5055. We explore three physical mechanisms that could lead to the creation of ultra long period magnetars: (i) enhanced spin-down due to episodic mass-loaded charged particle winds (e.g. as may accompany giant flares), (ii) angular momentum kicks from giant flares and (iii) fallback leading to long lasting accretion disks. We show that particle winds and fallback accretion can potentially lead to a sub-set of the magnetar population with ultra long periods, sufficiently long to accommodate FRB 180916 or 1E 161348--5055. If confirmed, such periods implicate magnetars in relatively mature states (ages $1-10$ kyr) and which possessed large internal magnetic fields at birth $B_{\rm int}\gtrsim 10^{16}$ G. In the low-twist magnetar model for FRBs, such long period magnetars may dominate FRB production for repeaters at lower isotropic-equivalent energies and broaden the energy distribution beyond that expected for a canonical population of magnetars which terminate their magnetic activity at shorter periods $P \lesssim 10$ s.

Ionization of the lower ionosphere by γ-rays from a magnetar : Detection of a low energy (3-10 keV) component

Abstract: A gigantic periodic flare from the soft γ repeater SGR 1900+14 produced enhanced ionization at ionospheric altitudes of 30 to 90 km, which was observed as unusually large amplitude and phase changes of very low frequency (VLF) signals propagating in the Earth-ionosphere waveguide. The VLF signals remained perturbed for ∼5 min and exhibited the 5.16 s periodicity of the giant flare detected on the Ulysses spacecraft [Hurley et al., 1999]. Quantitative analysis indicates the presence of an intense initial low energy (3–10 keV) photon component that was not detectable by the Ulysses instrument.