Magnetar

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

Large Torque Variations in Two Soft Gamma Repeaters

Abstract: We have monitored the pulse frequencies of the two soft gamma repeaters SGR 1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily Rossi X-Ray Timing Explorer Proportional Counter Array observations. In both sources, we observe large changes in the spin-down torque up to a factor of ~4, which persist for several months. Using long-baseline phase-connected timing solutions as well as the overall frequency histories, we construct torque noise power spectra for each SGR. The power spectrum of each source is very red (power-law slope ~-3.5). The torque noise power levels are consistent with some accreting systems on timescales of ~1 yr, yet the full power spectrum is much steeper in frequency than any known accreting source. To the best of our knowledge, torque noise power spectra with a comparably steep frequency dependence have been seen only in young, glitching radio pulsars (e.g., Vela). The observed changes in spin-down rate do not correlate with burst activity; therefore, the physical mechanisms behind each phenomenon are also likely unrelated. Within the context of the magnetar model, seismic activity cannot account for both the bursts and the long-term torque changes unless the seismically active regions are decoupled from one another.

A Glitch in an Anomalous X-ray Pulsar

Abstract: We report the detection of a sudden spin-up of the 11 s anomalous X-ray pulsar 1RXS J170849.0-400910 in regular timing observations made with the Rossi X-Ray Timing Explorer. The event, which occurred between MJD 51,446 (1999 September 25) and 51,472 (1999 October 21), is well characterized by an increase in the rotational frequency of magnitude, |Δν/ν| = (6.2 ± 0.3) × 10-7, and an increase in the rate of spin down, |Δ/| = (1.38 ± 0.25) × 10-2. These values are very similar to those of glitches observed in the Vela radio pulsar and other young radio pulsars. The event therefore suggests that the internal structure of this anomalous X-ray pulsar is similar to those of the radio pulsars. In particular, it implies that the fractional moment of inertia in neutron superfluid that is not corotating with the crust is ≥1%. The detection of a glitch in this anomalous X-ray pulsar constrains models for the origin of glitches in neutron stars. Most notably, it challenges models that preclude glitches in long-period pulsars and, under the magnetar hypothesis, suggests that large glitches can occur in hot neutron stars. The glitch is consistent with the predictions of the magnetar model for anomalous X-ray pulsars, but accretion-powered scenarios cannot be excluded using our observations alone.

Gravitational wave emission from a magnetically deformed non-barotropic neutron star

Abstract: A strong candidate for a source of gravitational waves is a highly magnetized, rapidly rotating neutron star (magnetar) deformed by internal magnetic stresses. We calculate the mass quadrupole moment by perturbing a zeroth-order hydrostatic equilibrium by an axisymmetric magnetic field with a linked poloidal-toroidal structure. In this work, we do not require the model star to obey a barotropic equation of state (as a realistic neutron star is not barotropic), allowing us to explore the hydromagnetic equilibria with fewer constraints. We derive the relation between the ratio of poloidal to total field energy Λ and ellipticity e, and briefly compare our results to those obtained using the barotropic assumption. Then, we present some examples of how our results can be applied to astrophysical contexts. First, we show how our formulae, in conjunction with current gravitational wave (non-)detections of the Crab pulsar and the Cassiopeia A central compact object (Cas A CCO), can be used to constrain the strength of the internal toroidal fields of those objects. We find that, for the Crab pulsar (whose canonical equatorial dipole field strength, inferred from spin-down, is 4 × 108 T) to emit detectable gravitational radiation, the neutron star must have a strong toroidal field component, with maximum internal toroidal field strength Btm= 7 × 1012 T; for gravitational waves to be detected from the Cas A CCO at 300Hz, Btm∼ 1013 T, whereas detection at 100Hz would require Btm∼ 1014 T. Using our results, we also show how the gravitational wave signal emitted by a magnetar immediately after its birth (assuming it is born rapidly rotating, with Λ≲ 0.2) makes such a newborn magnetar a stronger candidate for gravitational wave detection than, for example, an SGR giant flare. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Abstract: Mergers of neutron stars are known to be associated with short gamma-ray bursts(1-4). If the neutron-star equation of state is sufficiently stiff (that is, the pressure increases sharply as the density increases), at least some such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field(5-8) (that is, a magnetar). Such a magnetar signature may have been observed in the form of the X-ray plateau that follows up to half of observed short gamma-ray bursts(9,10). However, it has been expected that some X-ray transients powered by binary neutron-star mergers may not be associated with a short gamma-ray burst(11,12). A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy, the redshift of which is unknown(13). Its X-ray and host-galaxy properties allow several possible explanations including a short gamma-ray burst seen off-axis, a low-luminosity gamma-ray burst at high redshift, or a tidal disruption event involving an intermediate-mass black hole and a white dwarf(13). Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738 (ref.(14)). The measured light curve is fully consistent with the X-ray transient being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy centre, as short gamma-ray bursts often do(15,16). The estimated event-rate density of similar X-ray transients, when corrected to the local value, is consistent with the event-rate density of binary neutron-star mergers that is robustly inferred from the detection of the gravitational-wave event GW170817.

Evidence for heating of neutron stars by magnetic-field decay.

Abstract: We show the existence of a strong trend between neutron star (NS) surface temperature and the dipolar component of the magnetic field extending through three orders of field magnitude, a range that includes magnetars, radio-quiet isolated neutron stars, and many ordinary radio pulsars. We suggest that this trend can be explained by the decay of currents in the crust over a time scale of approximately 10(6) yr. We estimate the minimum temperature that a NS with a given magnetic field can reach in this interpretation.