Magnetar

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

A Unified Timing and Spectral Model for the Anomalous X-ray Pulsars XTE J1810-197 and CXOU J164710.2-455216

Abstract: Anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) are two small classes of X-ray sources strongly suspected to host a magnetar, i.e., an ultra-magnetized neutron star with B {approx} 10{sup 14}-10{sup 15} G. Many SGRs/AXPs are known to be variable, and recently the existence of genuinely 'transient' magnetars was discovered. Here, we present a comprehensive study of the pulse profile and spectral evolution of the two transient AXPs (TAXPs) XTE J1810-197 and CXOU J164710.2-455216. Our analysis was carried out in the framework of the twisted magnetosphere model for magnetar emission. Starting from three-dimensional Monte Carlo simulations of the emerging spectrum, we produced a large database of synthetic pulse profiles which was fitted to observed light curves in different spectral bands and at different epochs. This allowed us to derive the physical parameters of the model and their evolution with time, together with the geometry of the two sources, i.e., the inclination of the line of sight and the magnetic axis with respect to the rotation axis. We then fitted the (phase-averaged) spectra of the two TAXPs at different epochs using a model similar to that used to calculate the pulse profiles (ntzang in XSPEC) freezing all parameters to themore » values obtained from the timing analysis and leaving only the normalization free to vary. This provided acceptable fits to XMM-Newton data in all the observations we analyzed. Our results support a picture in which a limited portion of the star surface close to one of the magnetic poles is heated at the outburst onset. The subsequent evolution is driven both by the cooling/varying size of the heated cap and by a progressive untwisting of the magnetosphere.« less

Very fast optical flaring from a possible new Galactic magnetar.

Abstract: Two groups report the observation of optical flares from SWIFT J195509.6+261406, an intriguing X-ray source located in our Galaxy and initially discovered as a γ-ray burst by the orbiting Swift observatory. Stefanescu et al. detected extremely bright and rapid optical flaring, producing optical light-curves similar to the high energy light-curves of soft γ-ray repeaters and anomalous X-ray pulsars, which are thought to be neutron stars with extremely high magnetic fields (magnetars). In a multiwavelength study Castro-Tirado et al. detected more than 40 flaring episodes at optical wavelengths over a time span of three days. They suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths. Highly luminous rapid flares are characteristic of processes around compact objects like white dwarfs, neutron stars and black holes. In the high-energy regime of X-rays and γ-rays, outbursts with variabilities on timescales of seconds or less are routinely observed, for example in γ-ray bursts1 or soft γ-ray repeaters2. At optical wavelengths, flaring activity on such timescales has not been observed, other than from the prompt phase of one exceptional γ-ray burst3. This is mostly due to the fact that outbursts with strong, fast flaring are usually discovered in the high-energy regime; most optical follow-up observations of such transients use instruments with integration times exceeding tens of seconds, which are therefore unable to resolve fast variability. Here we show the observation of extremely bright and rapid optical flaring in the Galactic transient4,5,6,7 SWIFT J195509.6+261406. Our optical light curves are phenomenologically similar to high-energy light curves of soft γ-ray repeaters and anomalous X-ray pulsars8, which are thought to be neutron stars with extremely high magnetic fields (magnetars). This suggests that similar processes are in operation, but with strong emission in the optical, unlike in the case of other known magnetars.

Deformation of a magnetized neutron star

Abstract: Magnetars are compact stars which are observationally determined to have very strong surface magnetic fields of the order of $10^{14}-10^{15}$G. The centre of the star can potentially have a magnetic field several orders of magnitude larger. We study the effect of the field on the mass and shape of such a star. In general, we assume a non-uniform magnetic field inside the star which varies with density. The magnetic energy and pressure as well as the metric are expanded as multipoles in spherical harmonics up to the quadrupole term. Solving the Einstein equations for the gravitational potential, one obtains the correction terms as functions of the magnetic field. Using a nonlinear model for the hadronic EoS the excess mass and change in equatorial radius of the star due to the magnetic field are quite significant if the surface field is $10^{15}$G and the central field is about $10^{18}$ G. For a value of the central magnetic field strength of $1.75\times10^{18}$ G, we find that both the excess mass and the equatorial radius of the star changes by about $3-4\%$ compared to the spherical solution.

Early afterglow detection in the Swift observations of GRB 050801

Abstract: We present results of Swift optical, ultraviolet (UV) and X-ray observations of the afterglow of GRB 050801 The source is visible over the full optical, UV and X-ray energy range of the Swift Ultraviolet and Optical Telescope and X- ray telescope instruments Both optical and X- ray light curves exhibit a broad plateau (Delta t/t similar to 1) during the first few hundred seconds after the gamma-ray event We investigate the multiwavelength spectral and timing properties of the afterglow, and we suggest that the behaviour at early times is compatible with an energy injection by a newly born magnetar with a period of a few tenths of a millisecond, which keeps the forward shock refreshed over this short interval by irradiation Reverse shock emission is not observed Its suppression might be due to GRB ejecta being permeated by high magnetic fields, as expected for outflows powered by a magnetar Finally, the multiwavelength study allows a determination of the burst redshift, z = 156

How to Switch a Gamma-Ray Burst On and Off through a Magnetar

Abstract: One of the most elusive features of gamma-ray bursts (GRBs) is the sporadic emission prior to the main prompt event observed in at least ~15% of cases. These precursors have spectral and temporal properties similar to the main prompt emission, and smaller, but comparable, energetics. They are separated from the main event by a quiescent time that may be extremely long, and, in some cases, more than one precursor has been observed in the same burst. Precursors are still a puzzle: despite many attempts, none of the proposed models can account for all the observed features. Based on the complete sample of bright long GRBs observed by Swift (BAT6), we propose a new scenario for which precursors are explained by assuming that the central GRB engine is a newly born magnetar. In this model the precursor and the prompt emission arise from accretion of matter onto the surface of the magnetar. The accretion process can be halted by the centrifugal drag exerted by the rotating magnetosphere onto the infalling matter, allowing for multiple precursors and very long quiescent times.