Showing papers on "Magnetar published in 2004"

A fossil origin for the magnetic field in A stars and white dwarfs

Abstract: Some main-sequence stars of spectral type A are observed to have a strong (0.03–3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as ‘Ap stars’1,2,3,4, such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a ‘fossil’ field5,6. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core7. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

Magnetar spin-down, hyperenergetic supernovae, and gamma-ray bursts

Abstract: The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth of a neutron star in a successful core-collapse supernova, is accompanied by a neutrino-driven wind. For magnetar-strength (~1015 G) large-scale surface magnetic fields, this outflow is magnetically dominated during the entire cooling epoch. Because the strong magnetic field forces the wind to corotate with the proto-neutron star, this outflow can significantly affect the neutron star's early angular momentum evolution, as in analogous models of stellar winds. If the rotational energy is large in comparison with the supernova energy and the spin-down timescale is short with respect to the time required for the supernova shock wave to traverse the stellar progenitor, the energy extracted may modify the supernova shock dynamics significantly. This effect is capable of producing hyperenergetic supernovae and, in some cases, provides conditions favorable for gamma-ray bursts. We estimate spin-down timescales for magnetized, rotating proto-neutron stars and construct steady state models of neutrino-magnetocentrifugally driven winds. We find that if magnetars are born rapidly rotating, with initial spin periods (P) of ~1 ms, then of order 1051-1052 ergs of rotational energy can be extracted in ~10 s. If magnetars are born slowly rotating (P 10 ms), they can spin down to periods of ~1 s on the Kelvin-Helmholtz timescale.

Magnetar Spindown, Hyper-Energetic Supernovae, and Gamma Ray Bursts

Abstract: The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth of a neutron star in a successful core-collapse supernova, is accompanied by a neutrino-driven wind. For magnetar-strength ($\sim10^{15}$ G) large scale surface magnetic fields, this outflow is magnetically-dominated during the entire cooling epoch.Because the strong magnetic field forces the wind to co-rotate with the protoneutron star,this outflow can significantly effect the neutron star's early angular momentum evolution, as in analogous models of stellar winds (e.g. Weber & Davis 1967). If the rotational energy is large in comparison with the supernova energy and the spindown timescale is short with respect to the time required for the supernova shockwave to traverse the stellar progenitor, the energy extracted may modify the supernova shock dynamics significantly. This effect is capable of producing hyper-energetic supernovae and, in some cases, provides conditions favorable for gamma ray bursts. We estimate spindown timescales for magnetized, rotating protoneutron stars and construct steady-state models of neutrino-magnetocentrifugally driven winds. We find that if magnetars are born rapidly rotating, with initial spin periods ($P$) of $\sim1$ millisecond, that of order $10^{51}-10^{52}$ erg of rotational energy can be extracted in $\sim10$ seconds. If magnetars are born slowly rotating ($P\gtrsim10$ ms) they can spin down to periods of $\sim1$ second on the Kelvin-Helmholtz timescale.

Changes in the X-Ray Emission from the Magnetar Candidate 1E 2259+586 during Its 2002 Outburst

Abstract: An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the anomalous X-ray pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile, and spin-down of the underlying X-ray source. We present Rossi X-Ray Timing Explorer and X-Ray Multi-Mirror Mission observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode and identify recovery timescales for each. Specifically, we observe an X-ray flux increase (pulsed and phase-averaged) by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within ~2 days, during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 ? 0.01. The pulsed fraction decreased initially to ~15% rms but recovered rapidly to the preoutburst level of ~23% within the first 3 days. The pulse profile changed significantly during the outburst and recovered almost fully within 2 months of the outburst. A glitch of size ??max/? = (4.24 ? 0.11) ? 10-6 was observed in 1E 2259+586, which preceded the observed burst activity. The glitch could not be well fitted with a simple partial exponential recovery. An exponential rise of ~20% of the frequency jump with a timescale of ~14 days results in a significantly better fit to the data; however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. A fraction of the glitch (~19%) was recovered in a quasi-exponential manner having a recovery timescale of ~16 days. The long-term postglitch spin-down rate decreased in magnitude relative to the preglitch value. The changes in the source properties of 1E 2259+586 during its 2002 outburst are shown to be qualitatively similar to changes seen during or following burst activity in two SGRs, thus further solidifying the common nature of SGRs and AXPs as magnetars. The changes in persistent emission properties of 1E 2259+586 suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior (crustal and possibly core superfluid) and the magnetosphere. Finally, the changes in persistent emission properties coincident with burst activity in 1E 2259+586 enabled us to infer previous burst-active episodes from this and other AXPs. The nondetection of these outbursts by all-sky gamma-ray instruments suggests that the number of active magnetar candidates in our Galaxy is larger than previously thought.

Discovery of a transient magnetar: XTE J1810-197

Abstract: We report the discovery of a new X-ray pulsar, XTE J1810-197, that was serendipitously discovered on 2003 July 15 by the Rossi X-Ray Timing Explorer (RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a 5.54 s spin period, a soft X-ray spectrum (with a photon index of approx. = 4). and is detectable in earlier RXTE observations back to 2003 January but not before. These show that a transient outburst began between 2002 November 17 and 2003 January 23 and that the source's persistent X-ray flux has been declining since then. The pulsar exhibits a high spin-down rate P approx.= l0(exp -11) s/s with no evidence of Doppler shifts due to a binary companion. The rapid spin-down rate and slow spin period imply a supercritical characteristic magnetic field B approx. = 3 x l0(exp 14) G and a young age tau less than or = 7600 yr. Follow-up Chandra observations provided an accurate position of the source. Within its error radius, the 1.5 m Russian-Turkish Optical Telescope found a limiting magnitude R(sub c) = 21.5. All such properties are strikingly similar to those of anomalous X-ray pulsars ad soft gamma repeaters, providing strong evidence that the source is a new magnetar. However, archival ASCA and ROSAT observations found the source nearly 2 orders of magnitude fainter. This transient behavior and the observed long-term flux variability of the source in absence of an observed SGR-like burst activity make it the first confirmed transient magnetar and suggest that other neutron stars that share the properties of XTE 51810- 197 during its inactive phase may be unidentified transient magnetars awaiting detection via a similar activity. This implies a larger population of magnetars than previously surmised and a possible evolutionary connection between magnetars and other neutron star families. Subject headings: pulsars: general -pulsars: individual (XTE 51810- 197) - stars: magnetic fields -

The Collapse of Rotating Massive Stars in Three Dimensions

Abstract: Most simulations of the core collapse of massive stars have focused on the collapse of spherically symmetric objects. If these stars are rotating, this symmetry is broken, opening up a number of effects that are just now being studied. The list of proposed effects spans a range of extremes: from fragmentation of the collapsed iron core to modifications of the convective instabilities above the core; from the generation of strong magnetic fields that then drive the supernova explosion to the late-time formation of magnetic fields to produce magnetars after the launch of the supernova explosion. The list of the observational effects of rotation ranges from modifications in the gamma-ray line spectra, nucleosynthetic yields, and shape of supernova remnants caused by rotation-induced asymmetric explosions to strong pulsar radiation, the emission of gravitational waves, and altered r-process nucleosynthetic yields caused by rapidly rotating stars. In this paper we present the results of three-dimensional collapse simulations of rotating stars for a range of stellar progenitors. We find that for the most rapidly spinning stars, rotation does indeed modify the convection above the proto-neutron star, but it is not fast enough to cause core fragmentation. Similarly, although strong magnetic fields can be produced once the proto-neutron star cools and contracts, the proto-neutron star does not spin fast enough to generate strong magnetic fields quickly after collapse, and, for our simulations, magnetic fields will not dominate the supernova explosion mechanism. Even so, the resulting pulsars for our most rapidly rotating models may emit enough energy to dominate the total explosion energy of the supernova. However, more recent stellar models predict rotation rates that are much too slow to affect the explosion, but these models are not sophisticated enough to determine whether the most recent or past stellar rotation rates are more likely. Thus, we must rely on observational constraints to determine the true rotation rates of stellar cores just before collapse. We conclude with a discussion of the possible constraints on stellar rotation that we can derive from core-collapse supernovae.

Discovery of Hard Nonthermal Pulsed X-Ray Emission from the Anomalous X-Ray Pulsar 1E 1841-045

Abstract: We report the discovery of nonthermal pulsed X-ray/soft gamma-ray emission up to ~150 keV from the anomalous 11.8 s X-ray pulsar AXP 1E 1841-045 located near the center of supernova remnant Kes 73 using Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array and High Energy X-Ray Timing Experiment (HEXTE) data. The morphology of the double-peaked pulse profile changes rapidly with energy from 2 keV up to ~8 keV, above which the pulse shape remains more or less stable. The pulsed spectrum is very hard; its shape above 10 keV can be described well by a power law with a photon index of 0.94+/-0.16. 1E 1841-045 is the first AXP for which such very hard pulsed emission has been detected, which points to an origin in the magnetosphere of a magnetar. We have also derived the total emission spectrum from the Kes 73/1E 1841-045 complex for the ~1-270 keV energy range using RXTE HEXTE and XMM-Newton pn data. A comparison of the total emission from the complex with the pulsed+DC emission from 1E 1841-045 as derived from Chandra ACIS CC-mode data (Morii et al. 2003) leaves little room for emission from Kes 73 at energies near 7 keV or above. This suggests that the HEXTE spectrum above ~15 keV, satisfactorily described by a power law with index 1.47+/-0.05, is dominated by emission from 1E 1841-045. In that case the pulsed fraction for energies above 10 keV would increase from about 25% near 10 keV to 100% near 100 keV. The origin of the DC-component extending up to ~100 keV is probably magnetospheric and could be a manifestation of pulsed emission that is ``on'' for all phases.

Magnetorotational Effects on Anisotropic Neutrino Emission and Convection in Core-Collapse Supernovae

Abstract: We perform a series of two-dimensional hydrodynamic simulations of the magnetorotational collapse of a supernova core. We employ a realistic equation of state and take into account electron capture and neutrino transport by the so-called leakage scheme. Recent stellar evolution calculations imply that the magnetic fields of the toroidal components are much stronger than the poloidal ones at the presupernova stage. In this study we systematically investigate the effects of the toroidal magnetic fields on the anisotropic neutrino radiation and convection. Our results show that the shapes of the shock wave and the neutrino spheres generally become more oblate for the models whose profiles of rotation and the magnetic field are shell type and become, in contrast, more prolate for the models whose profiles of rotation and the magnetic field are cylindrical than for the corresponding models without the magnetic fields. Furthermore, we find that magnetorotational instability induced by nonaxisymmetric perturbations is expected to develop within the prompt-shock timescale. Combined with the anisotropic neutrino radiation, which heats matter near the rotational axis preferentially, the growth of the instability may enhance the heating near the axis. This might suggest that magnetar formation is accompanied by a jetlike explosion.

Discovery of hard non-thermal pulsed X-ray emission from the anomalous X-ray pulsar 1E 1841-045

Abstract: We report the discovery of non-thermal pulsed X-ray/soft gamma-ray emission up to about 150 keV from the anomalous X-ray pulsar AXP 1E 1841-045 located near the centre of supernova remnant Kes 73 using RXTE PCA and HEXTE data. The morphology of the double-peaked pulse profile changes rapidly with energy from 2 keV up to about 8 keV, above which the pulse shape remains more or less stable. The pulsed spectrum is very hard, its shape above 10 keV can be described well by a power law with a photon index of 0.94 +/- 0.16. 1E 1841-045 is the first AXP for which such very-hard pulsed emission has been detected, which points to an origin in the magnetosphere of a magnetar.

Astrophysical origins of ultrahigh energy cosmic rays

Abstract: In the first part of this review we discuss the basic observational features at the end of the cosmic ray (CR) energy spectrum. We also present there the main characteristics of each of the experiments involved in the detection of these particles. We then briefly discuss the status of the chemical composition and the distribution of arrival directions of CRs. After that, we examine the energy losses during propagation, introducing the Greisen–Zaptsepin–Kuzmin (GZK) cutoff, and discuss the level of confidence with which each experiment has detected particles beyond the GZK energy limit. In the second part of the review, we discuss the astrophysical environments that are able to accelerate particles up to such high energies, including active galactic nuclei, large scale galactic wind termination shocks, relativistic jets and hot-spots of Fanaroff–Riley radio galaxies, pulsars, magnetars, quasar remnants, starbursts, colliding galaxies, and gamma ray burst fireballs. In the third part of the review we provide a brief summary of scenarios which try to explain the super-GZK events with the help of new physics beyond the standard model. In the last section, we give an overview on neutrino telescopes and existing limits on the energy spectrum and discuss some of the prospects for a new (multi-particle) astronomy. Finally, we outline how extraterrestrial neutrino fluxes can be used to probe new physics beyond the electroweak scale.

A Comprehensive Study of the X-Ray Bursts from the Magnetar Candidate 1E 2259+586

Abstract: We present a statistical analysis of the X-ray bursts observed from the 2002 June 18 outburst of the anomalous X-ray pulsar (AXP) 1E 2259+586, observed with the Proportional Counter Array (PCA) aboard the Rossi X-Ray Timing Explorer. We show that the properties of these bursts are similar to those of soft gamma repeaters (SGRs). We find the following similarities: the burst durations follow a lognormal distribution that peaks at 99 ms, the differential burst fluence distribution is well described by a power law of index -1.7, the burst fluences are positively correlated with the burst durations, the distribution of waiting times is well described by a log normal distribution of mean 47 s, and the bursts are generally asymmetric, with shorter rise than fall times. However, we find several quantitative differences between the AXP and SGR bursts. Specifically, the AXP bursts we observed exhibit a wider range of durations, the correlation between burst fluence and duration is flatter than for SGRs, the observed AXP bursts are on average less energetic than observed SGR bursts, and the more energetic AXP bursts have the hardest spectra—the opposite of what is seen for SGRs. Unlike the case of SGRs, we find a correlation of burst phase with pulsed intensity. We conclude that the bursts are sufficiently similar that AXPs and SGRs can be considered united as a source class, yet there are some interesting differences that may help determine what physically differentiates the two closely related manifestations of neutron stars.

The Anomalous X-ray Pulsar 4U 0142+61: Variability in the infrared and a spectral break in the optical

Abstract: We present new optical and infrared observations of the counterpart to the Anomalous X-ray Pulsar (AXP) 4U 0142+61 taken with the Keck I telescope. The counterpart is found to be variable in the infrared. This contrasts with our optical observations, which do not show any evidence for variability. Apart from the variability the AXP shows a remark- able spectral energy distribution. In particular, we find a sudden drop in flux going from V to B, presumably due to a spectral feature. We compare our results to those obtained for the two other securely identified AXP counterparts, to 1E 2259+586 and 1E 1048.1−5937. 4U 0142+61 is very similar to the former source in its X-ray timing and spectral properties, and we find that this similarity extends to the quiescent infrared to X-ray flux ratio. For 1E 1048.1−5937, which has different X-ray properties, the situation is less clear: in one observation, the infrared to X-ray flux ratio was much larger, but another observation gave an upper limit which is consistent with that observed for 4U 0142+61. Assuming the quiescent ratios are all similar, we estimate the optical and infrared brightnesses for the three AXPs that remain to be identified as well as for the four Soft Gamma-ray Repeaters. We also discuss briefly how the observed optical and infrared emission might arise, in particular in the context of the magnetar model.

AN X-RAY SEARCH FOR COMPACT CENTRAL SOURCES IN SUPERNOVA REMNANTS. I. SNRs G093.3+6.9, G315.4-2.3, G084.2+0.8, AND G127.1+0.5

Abstract: Most astronomers now accept that stars more massive than about 9?M? explode as supernovae and leave stellar remnants, either neutron stars or black holes, with neutron stars being more prevalent. Recent modeling of the explosions suggests a significant diversity in the key natal properties?rotation rate, velocity, and magnetic field strength?of the resulting neutron stars that account for the association of active radio pulsars, pulsar wind nebulae, and magnetars with supernova remnants (SNRs). The discovery of a central X-ray source in Cas?A, the youngest known Galactic SNR, dramatized the expected diversity. However, less than half of the SNRs within 5 kpc have identified central sources, and only three are identified as the remnants of Type?Ia SNe. Here we report a systematic effort to search for compact central sources in the remaining 23 SNRs of this distance limited sample. Our search was inspired, on empirical considerations, by the enigmatic faint X-ray source in Cas?A; motivated, on theoretical grounds, by the expectation that young neutron stars emit cooling X-ray emission; and made possible by the superb angular resolution offered by the Chandra X-ray mission and the sensitivity of the XMM-Newton mission.?????In this first paper we report Chandra observations of four SNRs (G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5). We have undertaken a systematic optical/IR identification program of the X-ray sources detected in the field of each SNR. Foreground (flare stars, active stars) and background (active galactic nuclei) sources have identifiable IR/optical counterparts. In contrast, the counterparts of neutron stars (or black holes) are expected to be very faint. We are able to account for all the well-detected X-ray sources and thus able to state with some confidence that there are no associated central sources down to a level of one-tenth of that of the Cas?A central source, LX 1031 ergs s-1. We compare our limits with cooling curves for neutron stars and find that any putative neutron stars in these SNRs must be cooling faster than expected for traditional 1.35 M? neutron stars and that any putative pulsar must have low spin-down luminosities 1034 ergs s-1. However, our limits are unable to constrain the presence or absence of more unusual options, such as relatively more massive neutron stars with M 1.45 M?, neutron stars with exotic interiors, or quiescent black holes. In subsequent papers, we will report on the X-ray and optical/IR observations of the remaining members of the 5 kpc sample.

Central Compact Objects in Supernova Remnants

Abstract: There are point-like sources in central regions of several supernova remnants which have not been detected outside the X-ray range. The X-ray spectra of these Central Compact Objects (CCOs) have thermal components with blackbody temperatures of 0.2-0.5 keV and characteristic sizes of 0.3-3 km. Most likely, the CCOs are neutron stars born in supernova explosions. We overview their observational properties, emphasizing the Chandra data, and compare them with magnetars.

Magneto-driven shock waves in core-collapse supernovae

Abstract: We perform a series of two-dimensional magnetohydrodynamic simulations of the rotational core collapse of a magnetized massive star. We employ a realistic equation of state and take into account the neutrino cooling by the so-called leakage scheme. In this study we systematically investigate how the strong magnetic field and the rapid rotation affect the propagation of the shock waves. Our results show that in the case of the strong initial poloidal magnetic field, the toroidal magnetic field amplified by the differential rotation becomes strong enough to generate a tightly collimated shock wave along the rotational axis. On the other hand, in the case of the weak initial magnetic field, although the differential rotation amplifies the toroidal magnetic field over the long rotational period, the launched shock wave is weak and the shape of it becomes wider. The former case is expected to be accompanied by the formation of the so-called magnetar. Our models with rapid rotation and strong magnetic field can create a nozzle formed by the collimated shock wave. This might be the analogous situation to the collapsar that is plausible as the central engine of the gamma-ray bursts.

Magneto-driven Shock Waves in Core-Collapse Supernova

Abstract: We perform a series of two-dimensional magnetohydrodynamic simulations of the rotational core-collapse of a magnetized massive star. We employ a realistic equation of state and take into account the neutrino cooling by the so-called leakage scheme. In this study we systematically investigate how the strong magnetic field and the rapid rotation affect the propagation of the shock waves. Our results show that in the case of the strong initial poloidal magnetic field, the toroidal magnetic field amplified by the differential rotation, becomes strong enough to generate a tightly collimated shock wave along the rotational axis. On the other hand, in the case of the weak initial magnetic field, although the differential rotation amplifies toroidal magnetic field over the long rotational period, the launched shock wave is weak and the shape of it becomes wider. The former case is expected to be accompanied by the formation of the so-called magnetar. Our models with rapid rotation and strong magnetic field can create a nozzle formed by the collimated shock wave. This might be the analogous situation of the collapsar that is plausible for the central engine of the Gamma-Ray Bursts.

Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

Abstract: Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission (XMM-Newton) are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant 300 days, but not a power law as predicted in some models of sudden deep crustal heating events. All spectra are well fitted by a blackbody plus a steep power law, a problematic model that is commonly fitted to anomalous X-ray pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.

The X-ray Bursts from the Magnetar Candidate 1E 2259+586

Abstract: We present a statistical analysis of the X-ray bursts observed from the 2002 June 18 outburst of the Anomalous X-ray Pulsar (AXP) 1E 2259+586, observed with the Proportional Counter Array aboard the Rossi X-ray Timing Explorer. We show that the properties of these bursts are similar to those of Soft Gamma-Repeaters (SGRs). The similarities we find are: the burst durations follow a log-normal distribution which peaks at 99 ms, the differential burst fluence distribution is well described by a power law of index -1.7, the burst fluences are positively correlated with the burst durations, the distribution of waiting times is well described by a log-normal distribution of mean 47 s, and the bursts are generally asymmetric with faster rise than fall times. However, we find several quantitative differences between the AXP and SGR bursts. Specifically, there is a correlation of burst phase with pulsed intensity, the AXP bursts we observed exhibit a wider range of durations, the correlation between burst fluence and duration is flatter than for SGRs, the observed AXP bursts are on average less energetic than observed SGR bursts, and the more energetic AXP bursts have the hardest spectra - the opposite of what is seen for SGRs. We conclude that the bursts are sufficiently similar that AXPs and SGRs can be considered united as a source class yet there are some interesting differences that may help determine what physically differentiates the two closely related manifestations of neutron stars.

Time-resolved x-ray spectral modeling of an intermediate burst from sgr 1900+14 observed by hete-2 fregate and wxm

Abstract: We present a detailed analysis of a 3.5 s long burst from SGR 1900+14 that occurred on 2001 July 2. The 2–150 keV time-integrated energy spectrum is well described by the sum of two blackbodies whose temperatures are approximately 4.3 and 9.8 keV. The time-resolved energy spectra are similarly well fitted by the sum of two blackbodies. The higher temperature blackbody evolves with time in a manner consistent with a shrinking emitting surface. The interpretation of these results in the context of the magnetar model suggests that the two-blackbody fit is an approximation of an absorbed, multitemperature spectrum expected on theoretical grounds rather than a physical description of the emission. If this is indeed the case, our data provide further evidence for a strong magnetic field and indicate that the entire neutron star was radiating during most of the burst duration. Subject headingg pulsars: general — stars: individual (SGR 1900+14) — stars: neutron — X-rays: bursts

Nonlinear Electrodynamics and the Surface Redshift of Pulsars

Abstract: It is currently argued that the best method of determining neutron star (NS) fundamental properties is by measuring the gravitational redshift z of spectral lines produced in the stellar photosphere. Measurement of z at the stellar surface provides a unique insight on the NS mass-to-radius relation and thus on its equation of state, which reflects the physics of the strong interaction between the particles making up the star. Evidence for such a measurement has been provided quite recently by Cottam, Paerels, & Mendez and also by Sanwal and coworkers. Here we argue that although the quoted observations are undisputed for canonical pulsars, they could be misidentified if the NS is endowed with a superstrong magnetic field (B) as are the so-called magnetars and strange quark magnetars, e.g., the spectral line discovered by Ibrahim and coworkers. The source of this new "confusion" redshift is related to nonlinear electrodynamics (NLED) effects.

Magnetars: Time Evolution, Superfluid Properties, and Mechanism of Magnetic Field Decay

Abstract: We calculate the coupled thermal evolution and magnetic field decay in relativistic model neutron stars threaded by superstrong magnetic fields (B > 10^{15} G). Our main goal is to evaluate how such ``magnetars'' evolve with time and how field decay modifies the transitions to core superfluidity and cooling dominated by surface X-ray emission. Observations of a thermal X-ray spectral component and fast timing noise place strong constraints on the presence of a superfluid core. We find that the transition to core superfluidity can be significantly delayed by field decay in the age range ~ 10^3-10^5 yrs. The mechanism of Hall drift is related to the stability of the core magnetic field, and to currents flowing outward through the crust. The heating effect is enhanced if it is continuous rather than spasmodic. Condensation of a heavy element layer at the surface is shown to cause only modest changes in the outward conduction of heat.

Radiation from condensed surface of magnetic neutron stars

Abstract: Recent observations show that the thermal X-ray spectra of many isolated neutron stars are featureless and in some cases (e.g., RX J1856.5-3754) well fit by a blackbody. Such a perfect blackbody spectrum is puzzling since radiative transport through typical neutron star atmospheres causes noticeable deviation from blackbody. Previous studies have shown that in a strong magnetic field, the outermost layer of the neutron star may be in a condensed solid or liquid form because of the greatly enhanced cohesive energy of the condensed matter. The critical temperature of condensation increases with the magnetic field strength, and can be as high as 10^6 K (for Fe surface at B \sim 10^{13} G or H surface at B \sim a few times 10^{14} G). Thus the thermal radiation can directly emerge from the degenerate metallic condensed surface, without going through a gaseous atmosphere. Here we calculate the emission properties (spectrum and polarization) of the condensed Fe and H surfaces of magnetic neutron stars in the regimes where such condensation may be possible. For a smooth condensed surface, the overall emission is reduced from the blackbody by less than a factor of 2. The spectrum exhibits modest deviation from blackbody across a wide energy range, and shows mild absorption features associated with the ion cyclotron frequency and the electron plasma frequency in the condensed matter. The roughness of the solid condensate (in the Fe case) tends to decrease the reflectivity of the surface, and make the emission spectrum even closer to blackbody. We discuss the implications of our results for observations of dim, isolated neutron stars and magnetars.

Time-resolved X-ray spectral modeling of an intermediate burst from SGR1900+14 observed by HETE-2/FREGATE and WXM

Abstract: We present a detailed analysis of a 3.5 s long burst from SGR1900+14 which occurred on 2001 July 2. The 2-150 keV time-integrated energy spectrum is well described by the sum of two blackbodies whose temperatures are approximately 4.3 and 9.8 keV. The time-resolved energy spectra are similarly well fit by the sum of two blackbodies. The higher temperature blackbody evolves with time in a manner consistent with a shrinking emitting surface. The interpretation of these results in the context of the magnetar model suggests that the two blackbody fit is an approximation of an absorbed, multi-temperature spectrum expected on theoretical grounds rather than a physical description of the emission. If this is indeed the case, our data provide further evidence for a strong magnetic field, and indicate that the entire neutron was radiating during most of the burst duration.

Magnetars: Time evolution, superfluid properties, and the mechanism of magnetic field decay

Abstract: We calculate the coupled thermal evolution and magnetic field decay in relativistic model neutron stars threaded by superstrong magnetic fields (B > 1015 G). Our main goal is to evaluate how such "magnetars" evolve with time and how field decay modifies the transitions to core superfluidity and cooling dominated by surface X-ray emission. Observations of a thermal X-ray spectral component and fast timing noise place strong constraints on the presence of a superfluid core. We find that the transition to core superfluidity can be significantly delayed by magnetic field decay in the age range ~103-105 yr. The mechanism of Hall drift is related to the stability of the core magnetic field and to currents flowing outward through the crust. The heating effect is enhanced if it is continuous rather than spasmodic. Condensation of a heavy element layer at the surface is shown to cause only modest changes in the outward conduction of heat.

Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

Abstract: Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission (XMM-Newton) are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant 300 days, but not a power law as predicted in some models of sudden deep crustal heating events. All spectra are well fitted by a blackbody plus a steep power law, a problematic model that is commonly fitted to anomalous X-ray pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.

On the infrared, optical, and high-energy emission from the anomalous X-ray pulsar 4U 0142+61

Abstract: We show that the observed pulsed optical emission of the anomalous X-ray pulsar 4U 0142+61 can be accounted for by both the magnetar outer gap models and the disk-star dynamo gap models; therefore, there is no evidence favoring only one of these models as its responsible mechanism. Nevertheless, the estimated high-energy gamma-ray spectra from these models have different power-law indices and can be tested by future observations with the Gamma-Ray Large Area Space Telescope (GLAST). Furthermore, we show by analytical estimations that the expectations of a standard disk model are in agreement with the observed unpulsed optical and infrared luminosities of the AXP 4U 0142+61.

Nonlinear electrodynamics and the gravitational redshift of highly magnetised neutron stars

Abstract: The idea that the nonlinear electromagnetic interaction, i. e., light propagation in vacuum, can be geometrized was developed by Novello et al. (2000) and Novello & Salim (2001). Since then a number of physical consequences for the dynamics of a variety of systems have been explored. In a recent paper Mosquera Cuesta & Salim (2003) presented the first astrophysical study where such nonlinear electrodynamics (NLEDs) effects were accounted for in the case of a highly magnetized neutron star or pulsar. In that paper the NLEDs was invoked {\it a la} Euler-Heisenberg, which is an infinite series expansion of which only the first term was used for the analisys. The immediate consequence of that study was an overall modification of the space-time geometry around the pulsar, which is ``perceived'', in principle, only by light propagating out of the star. This translates into an significant change in the surface redshift, as inferred from absorption (emission) lines observed from a super magnetized pulsar. The result proves to be even more dramatic for the so-called magnetars, pulsars endowed with magnetic ($B$) fields higher then the Schafroth quantum electrodynamics critical $B$-field. Here we demonstrate that the same effect still appears if one calls for the NLEDs in the form of the one rigorously derived by Born & Infeld (1934) based on the special relativistic limit for the velocity of approaching of an elementary particle to a pointlike electron [From the mathematical point of view, the Born & Infeld (1934) NLEDs is described by an exact Lagrangean, whose dynamics has been successfully studied in a wide set of physical systems.].

Influence of an Internal Magnetar on Supernova Remnant Expansion

Abstract: Most of the proposed associations between magnetars and supernova remnants suffer from age problems. Usually, supernova remnant ages are determined using some approximation for the Sedov-Taylor supernova phase, which yields a relation between radius and age for a fixed energy of the explosion (generally assumed to be ~1051 ergs). Those ages do not generally agree with the characteristic ages of the (proposed) associated magnetars. We show in this work that a faster expansion results when the energy injected into the supernova remnant by magnetar spin-down is taken into account, thus helping to improve the matches between characteristic ages and supernova remnant ages. However, the magnetar velocities inferred from observations would make some associations inviable if correct. Since characteristic ages may not be good age estimators after all, their influence on the likelihood of the association may not be as important. In this work, we perform simple numerical simulations of supernova remnant expansion with internal magnetars and apply them to the observed sample of objects. A short initial spin period, thought to be important for the very generation of the magnetic field, is shown to be quite relevant to the modified expansion of the remnant. We finally analyze all proposed associations on a case-by-case basis, addressing the likelihood of each one, according to this perspective. We consider a larger explosion energy and reassess the characteristic age issue, and conclude that ~50% of the associations can be real, provided that soft gamma repeaters and anomalous X-ray pulsars are magnetars.

Comparative study of the two large flares from SGR1900+14 with the BeppoSAX Gamma-Ray Burst Monitor

Abstract: We report on spectral and temporal results of the 40-700 keV observations, obtained with the Gamma-Ray Burst Monitor (GRBM) on board BeppoSAX, of the two large flares from the Soft Gamma-ray Repeater SGR 1900+14 occurred on August 27, 1998 and April 18, 2001. From their intensity, fluence and duration, the first one was classified as "giant" and the second as "intermediate".The spectral results have been obtained with an improved response function of the GRBM. We find that the two events have similar spectral properties, but different temporal properties. The major difference concerns the time profiles of the light curves, whereas the lack of evidence in the 2001 flare for the erratic time variability found at high frequencies (10-1000 Hz) in the 1998 flare could be ascribed to lower counting statistics. We discuss these results in the light of the magnetar model proposed for SGR sources.

Radio pulsars as progenitors of anomalous X-ray pulsars and soft gamma-ray repeaters: Magnetic field evolution through pulsar glitches

Abstract: Glitches are common phenomena in pulsars. After each glitch, there is often a permanent increase in the pulsar's spin-down rate. Therefore, a pulsar's present spin-down rate may be much higher than its initial value and the characteristic age of a pulsar based on its present spin-down rate and period may be shorter than its true age. At the same time, the permanent increase of its spin-down rate implies that the pulsar's surface magnetic field is increased after each glitch. Consequently, after many glitches some radio pulsars may evolve into magnetars, i.e., strongly magnetized and slowly rotating neutron stars.