Showing papers on "Magnetar published in 2002"

Electrodynamics of Magnetars: Implications for the Persistent X-Ray Emission and Spin-down of the Soft Gamma Repeaters and Anomalous X-Ray Pulsars

Abstract: We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs), these currents are maintained by magnetic stresses acting deep inside the star, which generate both sudden disruptions (SGR outbursts) and more gradual plastic deformations of the rigid crust. We construct self-similar force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, ∝ r-(2+p), and show that a large-scale twist of field lines softens the radial dependence of the magnetic field to p < 1. The spin-down torque acting on the star is thereby increased in comparison with an orthogonal vacuum dipole. We comment on the strength of the surface magnetic field in the SGR and AXP sources, as inferred from their measured spin-down rates, and the implications of this model for the narrow measured distribution of spin periods. A magnetosphere with a strong twist [B/Bθ = O(1) at the equator] has an optical depth ~1 to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or charge/mass ratio of the scattering charge. When electrons and ions supply the current, the stellar surface is also heated by the impacting charges at a rate comparable to the observed X-ray output of the SGR and AXP sources, if Bdipole ~ 1014 G. Redistribution of the emerging X-ray flux at the cyclotron resonance will strongly modify the emerging pulse profile and, through the Doppler effect, generate a nonthermal tail to the X-ray spectrum. We relate the sudden change in the pulse profile of SGR 1900+14 following the 1998 August 27 giant flare to an enhanced optical depth at the electron cyclotron resonance resulting from a sudden twist imparted to the external magnetic field during the flare. The self-similar structure of the magnetosphere should generate frequency-independent profiles; more complicated pulse profiles may reflect the presence of higher multipoles, ion cyclotron scattering, or possibly nonresonant Compton scattering of O-mode photons by pair-loaded currents.

Magnetar-like X-ray bursts from an anomalous X-ray pulsar

Abstract: Anomalous X-ray pulsars (AXPs) are a class of rare X-ray emitting pulsars whose energy source has been perplexing for some 20 years. Unlike other X-ray emitting pulsars, AXPs cannot be powered by rotational energy or by accretion of matter from a binary companion star, hence the designation 'anomalous'. Many of the rotational and radiative properties of the AXPs are strikingly similar to those of another class of exotic objects, the soft-gamma-ray repeaters (SGRs). But the defining property of the SGRs--their low-energy-gamma-ray and X-ray bursts--has not hitherto been observed for AXPs. Soft-gamma-ray repeaters are thought to be 'magnetars', which are young neutron stars whose emission is powered by the decay of an ultra-high magnetic field; the suggestion that AXPs might also be magnetars has been controversial. Here we report two X-ray bursts, with properties similar to those of SGRs, from the direction of the anomalous X-ray pulsar 1E1048.1 - 5937. These events imply a close relationship (perhaps evolutionary) between AXPs and SGRs, with both being magnetars.

Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20

Abstract: We report evidence for cyclotron resonance features from the Soft Gamma Repeater SCR 1806-20 in outburst, detected with the Rossi X-ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 keV and 17.5 keV respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of approx. 500 eV, and a narrow width of 0.3 Solar Mass/km) that is inconsistent with neutron stars, or requires a low (5 - 7) x 10(exp 11) G magnetic field that is unlikely for SGRs. The line widths are also narrow compared to those of electron cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as ion cyclotron resonances in an ultra-strong magnetic field, which have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0 x 10(exp 15) G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source.

Optical pulsations from the anomalous X-ray pulsar 4U0142+61.

Abstract: Anomalous X-ray pulsars (AXPs) differ from ordinary radio pulsars in that their X-ray luminosity is orders of magnitude greater than their rate of rotational energy loss, and so they require an additional energy source. One possibility is that AXPs are highly magnetized neuron stars—or 'magnetars'—having surface magnetic fields greater than 10^(14) G. This would make them similar to the soft y-ray repeaters (SGRs), but alternative models that do not require extreme magnetic fields also exist. An optical counterpart to the AXP 4U0142+61 was recently discovered, consistent with emission from a magnetar, but also from a magnetized hot white dwarf, or an accreting isolated neutron star. Here we report the detection of optical pulsations from 4U0142+61. The pulsed fraction of optical light (27 per cent) is five to ten times greater than that of soft X-rays, from which we conclude that 4U0142+61 is a magnetar. Although this establishes a direct relationship between AXPs and the soft y-ray repeaters, the evolutionary connection between AXPs, SGRs and radio pulsars remains controversial.

Diagnosing magnetars with transient cooling

Abstract: Transient X-ray emission, with an approximate t-0.7 decay, was observed from SGR 1900+14 over 40 days following the giant flare of 1998 August 27. We calculate in detail the diffusion of heat to the surface of a neutron star through an intense 1014-1015 G magnetic field, following the release of magnetic energy in its outer layers. We show that the power-law index, the fraction of burst energy in the afterglow, and the return to persistent emission can all be understood if the star is composed of normal baryonic material.

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.

Radio Emission from Magnetars

Abstract: We discuss properties of the expected radio emission from soft gamma-ray repeaters (SGRs) during their bursting activity in the framework of the model of Thompson, Lyutikov, & Kulkarni, in which the high-energy emission is powered by the dissipation of superstrong magnetic fields in the magnetospheres through reconnection-type events. Drawing on analogies with solar flares, we predict that coherent radio emission resembling solar type III radio bursts may be emitted in SGRs during X-ray bursts. The radio emission should have correlated pulse profiles with X-rays, a narrowband-type radio spectrum with Δν ≤ ν, with the typical frequency ν ≥ 10 GHz, and, possibly, a drifting central frequency. We encourage sensitive radio observations of SGRs during the bursting activity.

New Evidence for Proton Cyclotron Resonance in a Magnetar Strength Field from SGR 1806-20

Abstract: A great deal of evidence has recently been gathered in favor of the picture that Soft Gamma Repeaters and Anomalous X-Ray Pulsars are powered by ultra-strong magnetic fields (B > 10^{14} G; i.e. magnetars). Nevertheless, present determination of the magnetic field in such magnetar candidates has been indirect and model dependent. A key prediction concerning magnetars is the detection of ion cyclotron resonance features, which would offer a decisive diagnostic of the field strength. Here we present the detection of a 5 keV absorption feature in a variety of bursts from the Soft Gamma Repeater SGR 1806-20, confirming our initial discovery (Ibrahim et al. 2002) and establishing the presence of the feature in the source's burst spectra. The line feature is well explained as proton cyclotron resonance in an ultra-strong magnetic field, offering a direct measurement of SGR 1806-20's magnetic field (B ~ 10^{15} G) and a clear evidence of a magnetar. Together with the source's spin-down rate, the feature also provides the first measurement of the gravitational redshift, mass and radius of a magnetar.

Chandra High-Resolution Spectrum of the Anomalous X-Ray Pulsar 4U 0142+61

Abstract: We report on a 25 ks observation of the 8.7 s anomalous X-ray pulsar 4U 0142+61 with the High Energy Transmission Grating Spectrometer on the Chandra X-Ray Observatory. The continuum spectrum is consistent with previous measurements and is well fitted by an absorbed power law+blackbody with a photon index Γ = 3.3 ± 0.4 and kT = 0.418 ± 0.013 keV. No evidence was found for emission or absorption lines, with an upper limit of ≈50 eV on the equivalent width of broad features in the 2.5-13 A (0.95-5.0 keV) range and an upper limit of ≈10 eV on the equivalent width of narrow features in the 4.1-17.7 A (0.7-3.0 keV) range. If the source is a magnetar, then the absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. We find no strong features that are indicative of cyclotron absorption for magnetic field strengths of (1.9-9.8) × 1014 G. This is still consistent with the dipole field strength of B = 1.3 × 1014 G (at the polar cap) estimated from the pulsar's spin-down.

Timing analysis of the isolated neutron star RX J0720.4-3125

Abstract: We present a combined analysis of XMM-Newton, Chandra and ROSAT observations of the isolated neutron star RX J0720.4-3125, spanning a total period of similar to7 yr. We develop a maximum likelihood periodogram for our analysis based on the DeltaC statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked a posteriori by folding a further BeppoSAX data set with the period predicted at the time of that observation: the phase is found to be consistent.The study of the spin history and the measure of the spin-down rate are of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of P., here measured for the first time, is approximate to10 (-14) s s(-1). This value cannot be explained in terms of torque from a fossil disc. When interpreted in terms of dipolar losses, it gives a magnetic field of B approximate to10(13) G, making it also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find it unlikely that the field decayed from a much larger value (B approximate to10(15) G, as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of approximate to10(4) yr, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is approximate to10(6) yr old, and its magnetic field has not changed substantially over the lifetime.

Hydromagnetic instabilities in protoneutron stars

Abstract: The stability properties of newly born neutron stars, or proto--neutron stars, are considered. We take into account dissipative processes, such as neutrino transport and viscosity, in the presence of a magnetic field. In order to find the regions of the star subject to different sorts of instability, we derive the general instability criteria and apply it to evolutionary models of PNSs. The influence of the magnetic field on instabilities is analyzed and the critical magnetic field stabilizing the star is obtained. In the light of our results, we estimate of the maximum poloidal magnetic field that might be present in young pulsars or magnetars.

The spin evolution of nascent neutron stars

Abstract: The loss of angular momentum owing to unstable r-modes in hot young neutron stars has been proposed as a mechanism for achieving the spin rates inferred for young pulsars. One factor that could have a significant effect on the action of the r-mode instability is fallback of supernova remnant material. The associated accretion torque could potentially counteract any gravitational-wave-induced spin-down, and accretion heating could affect the viscous damping rates and hence the instability. We discuss the effects of various external agents on the r-mode instability scenario within a simple model of supernova fallback on to a hot young magnetized neutron star. We find that the outcome depends strongly on the strength of the magnetic field of the star. Our model is capable of generating spin rates for young neutron stars that accord well with initial spin rates inferred from pulsar observations. The combined action of r-mode instability and fallback appears to cause the spin rates of neutron stars born with very different spin rates to converge, on a time-scale of approximately 1 year. The results suggest that stars with magnetic fields ?1013 G could emit a detectable gravitational wave signal for perhaps several years after the supernova event. Stars with higher fields (magnetars) are unlikely to emit a detectable gravitational wave signal via the r-mode instability. The model also suggests that the r-mode instability could be extremely effective in preventing young neutron stars from going dynamically unstable to the bar-mode.

Hydromagnetic instabilities in proto-neutron stars

Abstract: The stability properties of newly born neutron stars, or proto-neutron stars (PNSs), are considered. We take into account dissipative processes, such as neutrino transport and viscosity, in the presence of a magnetic field. In order to find the regions of the star subject to different sorts of instability, we derive the general instability criteria and apply them to evolutionary models of PNSs. The influence of the magnetic field on instabilities is analyzed, and the critical magnetic field stabilizing the star is obtained. In light of our results, we estimate the maximum poloidal magnetic field that might be present in young pulsars or magnetars.

Evolution of neutron star magnetic fields

Abstract: This paper reviews the current status of the theoretical models of the evolution of the magnetic fields of neutron stars other than magnetars. It appears that the magnetic fields of neutron stars decay significantly only if they are in binary systems. Three major physical models for this, namely spindown-induced flux expulsion, ohmic evolution of crustal field and diamagnetic screening of the field by accreted plasma, are reviewed.

X-Ray Timing of the Enigmatic Neutron Star RX J0720.4–3125

Abstract: RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8 s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, and BeppoSAX, we are able to place an upper limit to the period derivative, || < 3.6 × 10-13 s s-1 (3 σ). While our upper limit on allows for the accretion model, this model is increasingly untenable for another similar but better studied neutron star, RX J1856.5-3754, and we therefore consider the accretion model unlikely for RX J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be 1014 G based on cooling models, suggesting that it is not and never was a magnetar but is instead a middle-aged neutron star. We propose that it is either a long-period high magnetic field pulsar with ~ 10-13 s s-1, similar to PSR J1814-1744, or a neutron star born with an initial period of ≈8.3 s and ~ 10-15 s s-1. The proximity of RX J0720.4-3125 is strongly suggestive of a large population of such objects; if so, radio pulsar surveys must have missed many of these sources.

A Phase-connected Timing Solution for the Magnetar Candidate 1E 1841–045

Abstract: We report on a long-term monitoring campaign of 1E 1841-045, the 12 s anomalous X-ray pulsar and magnetar candidate at the center of the supernova remnant Kes 73. We have obtained approximately monthly observations of the pulsar with the Rossi X-Ray Timing Explorer (RXTE) spanning over 2 years, during which time 1E 1841-045 is found to be rotating with sufficient stability to derive a phase-connected timing solution. A linear ephemeris is consistent with measurements of the pulse period made over the last 15 years with the Ginga, ASCA, RXTE, and BeppoSAX observatories. Phase residuals suggest the presence of "timing noise," as is typically observed from young radio pulsars. These results confirm a rapid, constant spin-down for the pulsar, which continues to maintain a steady flux; this is inconsistent with most accretion scenarios.

X-ray Timing of the Enigmatic Neutron Star RX J0720.4-3125

Abstract: RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8-s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, and BeppoSAX we are able to place an upper limit to the period derivative, $|\dot{P}| < 3.6\times10^{-13}{s s}^{-1}$ (3-$\sigma$). While our upper limit on $\dot P$ allows for the accretion model, this model is increasingly untenable for another similar but better studied neutron star, RX J1856.5-3754, and we therefore consider the accretion model unlikely for RX J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be $\lsim 10^{14}$ G based on cooling models, suggesting that it is not and never was a magnetar, but is instead middle-aged neutron star. We propose that it is either a long-period high-magnetic field pulsar with $\dot P\sim 10^{-13}{s s}^{-1}$ similar to PSR J1814-1744, or a neutron star born with an initial period of $\approx 8.3$ s and $\dot P\sim 10^{-15}{s s}^{-1}$. The proximity of RX J0720.4-3125 is strongly suggestive of a large population of such objects; if so, radio pulsar surveys must have missed many of these sources.

Astrophysics of isolated neutron stars: radioquiet neutron stars and magnetars

Abstract: In this paper we review present state of theoretical and observational exploration of isolated neutron stars. We mainly discuss objects which do not show usual radiopulsar activity. We include into our review the following types of objects: dim X-ray sources in the Galactic disk and in globular clusters; compact X-ray sources in supernova remnants; anomalous X-ray pulsars; soft gamma repeaters; Geminga. Also we briefly discuss related types of objects: isolated black holes, quark stars and supernovae. We review basic physical processes connected with evolution of isolated neutron stars: cooling; magnetic field decay; period evolution; magnetospheric processes, accretion. We discuss populational synthesis of isolated neutron stars of different types.

Timing Properties of Magnetars

Abstract: We study the pulse morphologies and pulse amplitudes of thermally emitting neutron stars with ultrastrong magnetic fields. The beaming of the radiation emerging from a magnetar was recently shown to be predominantly nonradial, with a small pencil and a broad fan component. We show that the combination of this radiation pattern with the effects of strong lensing in the gravitational field of the neutron star yields pulse profiles that show a qualitatively different behavior compared to that of the radially peaked beaming patterns explored previously. Specifically, we find that (i) the pulse profiles of magnetars with a single hot emission region on their surface exhibit 1-2 peaks, whereas those with an antipodal emission geometry have 1-4 peaks, depending on the neutron star compactness, the observer's viewing angle, and the size of the hot regions; (ii) the energy dependence of the beaming pattern may give rise to weakly or strongly energy-dependent pulse profiles and may introduce phase lags between different energy bands; (iii) the nonradial beaming pattern can give rise to high pulsed fractions even for very relativistic neutron stars; (iv) the pulsed fraction may not vary monotonically with neutron star compactness; (v) the pulsed fraction does not decrease monotonically with the size of the emitting region; (vi) the pulsed fraction from a neutron star with a single hot pole has, in general, a very weak energy dependence, in contrast to the case of an antipodal geometry. Comparison of these results to the observed properties of anomalous X-ray pulsars strongly suggests that they are neutron stars with a single hot region of ultrastrong magnetic field.

Electrical conductivity at the core of a magnetar

Abstract: An expression for the electrical conductivity at the core of a magnetar is derived using Boltzmann kinetic equation with the relaxation time approximation. The rates for the relevant scattering processes, e.g., electron–electron and electron–proton scattering processes are evaluated in presence of strong quantizing magnetic fields using tree level diagrams. It is found that in the presence of a strong quantizing magnetic field, electrical conductivity behaves like a second rank tensor. However, if the zeroth Landau levels are only level occupied by the charged particles, it again behaves like a scalar of a one-dimensional system.

Gamma-ray bursts from extragalactic Magnetar Flares

Abstract: The prototype for events that we call MFs—“March Fifth” events or “Magnetar Flares”—was observed on March 5, 1979. There is evidence that MFs are powered by catastrophic magnetic instabilities in ultra-magnetized neutron stars. These events begin with brief (Δt∼0.1–1 s), intense, hard spikes of gamma rays, probably emitted in concurrence with relativistic outflows; followed by long (t∼100 s) softer tails of hard X-rays, modulated on the stellar rotation period. Prototypical MFs could have been detected by BATSE out to ∼13 Mpc, nearly reaching the Virgo cluster. The likely number of isotropic, standard-candle MFs detected by the BATSE experiment is ∼12. These short-duration, fast-rising gamma-ray bursts could in principle be identified by their positional coincidences with nearby galaxies and the Supergalactic Plane. The ensuing soft tail emission would not have been detected by BATSE for sources more distant than the Andromeda Galaxy. Bayes’ Theorem implies that there is a ∼99% chance for at least 1 isotr...

Electromagnetic activity of a pulsating paramagnetic neutron star

Abstract: The fact that neutron star matter possesses the capability of maintaining a highly intense magnetic field has been and still is among the most debatable issues in pulsar astrophysics. Over the years, there were several independent suggestions that the dominant source of pulsar magnetism is either the field-induced or the spontaneous magnetic polarization of the baryon material. The Pauli paramagnetism of degenerate neutron matter is one of the plausible and comprehensive mechanisms of the magnetic ordering of neutron magnetic moments, promoted by a seed magnetic field inherited by the neutron star from a massive progenitor and amplified by its implosive contraction due to the magnetic flux conservation. Adhering to this attitude and based on the equations of magnetoelastic dynamics underlying continuum mechanics of single-axis magnetic insulators, we investigate electrodynamics of a paramagnetic neutron star undergoing nonradial pulsations. We show that the suggested approach regains a recent finding of Akhiezer et al. [1] that the spin-polarized neutron matter can transmit perturbations by low-frequency transverse magnetoelastic waves. We found that nonradial torsional magnetoelastic pulsations of a paramagnetic neutron star can serve as a powerful generator of a highly intense electric field producing the magnetospheric polarization charge whose acceleration along the open magnetic field lines leads to the synchrotron and curvature radiation. Analytic and numerical estimates for periods of non-radial torsional magnetoelastic modes are presented and are followed by a discussion of their possible manifestation in currently monitored activity of pulsars and magnetars.

Isolated neutron stars discovered by ROSAT

Abstract: ROSAT has discovered a new group of isolated neutron stars characterized by soft black-body like spectra (kT∼50–120 eV), apparent absence of radio emission and no association with supernovae remnants. So far only six such sources are known. A small fraction of these stars exhibit X-ray pulsations with relatively long periods of the order of 10 sec. Two very different mechanisms may be envisaged to explain their properties. The neutron stars may be old and re-heated by accretion from the ISM in which case their population properties could provide information on past stellar formation and secular magnetic field decay. Alternatively, this group may at least partly be made of relatively young cooling neutron stars possibly descendant from magnetars. We review the last observational results and show how they can shed light on the evolutionary path of these new objects within the whole class of isolated neutron stars.

On Fossil Disk Models of Anomalous X-Ray Pulsars

Abstract: Currently, two competing models are invoked in order to explain the observable properties of Anomalous X-ray Pulsars (AXPs). One model assumes that AXP emission is powered by a strongly magnetized neutron star - i.e., a magnetar. Other groups have postulated that the unusually long spin periods associated with AXPs could, instead, be due to accretion. As there are severe observational constraints on any binary accretion model, fossil disk models have been suggested as a plausible alternative. Here we analyze fossil disk models of AXPs in some detail, and point out some of their inherent inconsistencies. For example, we find that, unless it has an exceptionally high magnetic field strength, a neutron star in a fossil disk cannot be observed as an AXP if the disk opacity is dominated by Kramers' law. However, standard alpha-disk models show that a Kramers opacity must dominate for the case log B > 12, making it unlikely that a fossil disk scenario can successfully produce AXPs. Additionally, we find that in order to sufficiently spin down a neutron star in a fossil disk, an unusually efficient propeller torque must be used. Such torques are inconsistent with observations of other accreting sytems - particularly High Mass X-ray Binaries. Thus, our analysis lends credence to the magnetar model of AXPs.

High energy neutrinos from magnetars

Abstract: Magnetars can accelerate cosmic rays to high energies through the unipolar effect, and are also copious soft photon emitters. We show that young, fast-rotating magnetars whose spin and magnetic moment point in opposite directions emit high energy neutrinos from their polar caps through photomeson interactions. We identify a neutrino cut-off band in the magnetar period-magnetic field strength phase diagram, corresponding to the photomeson interaction threshold. Within uncertainties, we point out four possible neutrino emission candidates among the currently known magnetars, the brightest of which may be detectable for a chance on-beam alignment. Young magnetars in the universe would also contribute to a weak diffuse neutrino background, whose detectability is marginal, depending on the typical neutrino energy.

Chandra High-Resolution Spectrum of the Anomalous X-ray Pulsar 4U 0142+61

Abstract: We report on a 25 ks observation of the 8.7 s anomalous X-ray pulsar 4U~0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody with photon index Gamma=3.3+/-0.4 and kT=0.418+/-0.013 keV. No evidence was found for emission or absorption lines, with an upper limit of ~50 eV on the equivalent width of broad features in the 2.5-13 A (0.95-5.0 keV) range and an upper limit of ~10 eV on the equivalent width of narrow features in the 4.1-17.7 A (0.7-3.0 keV) range. If the source is a magnetar, then the absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. We find no strong features that are indicative of cyclotron absorption for magnetic field strengths of (1.9-9.8)x10^{14} G. This is still consistent with the dipole field strength of B=1.3x10^{14} G (at the polar cap) estimated from the pulsar's spindown.

Core-Collapse Supernovae and Neutron Star Kicks

Abstract: Recent observations have revealed many new puzzles related to core-collapse supernovae, including the formation of magnetars and black holes and their possible GRB connections. We review our current understanding of the origin of pulsar kicks and supernova asymmetry. It is argued that neutron star kicks are intimately connected to the other fundamental parameters of young neutron stars, such as the initial spin and magnetic field strength.

Optical pulse-phased observations of faint pulsars with a phase-binning CCD camera

Abstract: We have constructed a phase-binning CCD camera optimized for optical observations of faint pulsars. The phase-binning CCD camera combines the high quantum efficiency of a CCD with a pulse-phased time resolution capable of observing pulsars as fast as 10 ms, with no read noise penalty. The phase-binning CCD can also operate as a two-channel imaging polarimeter, obtaining pulse-phased linear photopolarimetric observations. We have used this phase-binning CCD to make the first measurements of optical pulsations from an anomalous X-ray pulsar. We measured the optical pulse profile of 4U 0142+61, finding a pulsed fraction of 27%, many times larger than the pulsed fraction in X-rays. From this observation, we concluded that 4U 0142+61 must be a magnetar, an ultramagnetized neutron star (B > 10^14 G). The optical pulse is double-peaked, similar to the soft X-ray pulse profile. We also used the phase-binning CCD to obtain the photometric and polarimetric pulse profiles of PSR B0656+14, a middle-aged isolated rotation-powered pulsar. The optical pulse profile we measured significantly disagrees with the low signal-to-noise profile previously published for this pulsar. Our results show that the optical flux is entirely pulsed, with optical peaks at phases 0.2 and 0.8 with respect to the radio peak, and a bridge of emission between the peaks. The significance of the detection of pulsed polarized flux is low, but the position angles match the extrapolation of the radio polarization profile. The optical data, both photometric and polarimetric, are consistent with the polar cap model of pulsar magnetospheric emission. The fit of the optical data with the competing emission model, the outer gap model, has not yet been determined. We have developed a number of statistical tools, both to estimate the errors in our measurements and to identify systematic errors present in the pulse profiles. The statistical tools, when applied to the data presented here, show that the systematic errors are negligible, bolstering the claims of significance of these results.