Showing papers on "Magnetar published in 2006"

Physics of strongly magnetized neutron stars

Abstract: There has recently been growing evidence for the existence of neutron stars possessing magnetic fields with strengths that exceed the quantum critical field strength of 4.4 × 1013 G, at which the cyclotron energy equals the electron rest mass. Such evidence has been provided by new discoveries of radio pulsars having very high spin-down rates and by observations of bursting gamma-ray sources termed magnetars. This paper will discuss the exotic physics of this high-field regime, where a new array of processes becomes possible and even dominant and where familiar processes acquire unusual properties. We review the physical processes that are important in neutron star interiors and magnetospheres, including the behaviour of free particles, atoms, molecules, plasma and condensed matter in strong magnetic fields, photon propagation in magnetized plasmas, free-particle radiative processes, the physics of neutron star interiors and field evolution and decay mechanisms. Application of such processes in astrophysical source models, including rotation-powered pulsars, soft gamma-ray repeaters, anomalous x-ray pulsars and accreting x-ray pulsars will also be discussed. Throughout this review, we will highlight the observational signatures of high magnetic field processes, as well as the theoretical issues that remain to be understood.

Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions

Abstract: Over the past decade, long-duration gamma-ray bursts (GRBs)--including the subclass of X-ray flashes (XRFs)--have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10(48) erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

Transient pulsed radio emission from a magnetar

Abstract: Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of >10(14) G, according to the 'magnetar' model. The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810-197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level; a coincident radio source of unknown origin was detected one year later. Here we show that XTE J1810-197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal--and at >20 GHz XTE J1810-197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars.

Producing ultrastrong magnetic fields in neutron star mergers

Abstract: We report an extremely rapid mechanism for magnetic field amplification during the merger of a binary neutron star system. This has implications for the production of the short class of gamma-ray bursts, which recent observations suggest may originate in such mergers. In detailed magnetohydrodynamic simulations of the merger process, the fields are amplified by Kelvin-Helmholtz instabilities beyond magnetar field strength and may therefore represent the strongest magnetic fields in the universe. The amplification occurs in the shear layer that forms between the neutron stars and on a time scale of only 1 millisecond, that is, long before the remnant can collapse into a black hole.

Soft gamma repeaters and anomalous X-ray pulsars: magnetar candidates

Abstract: This article is a review of Soft Gamma Repeaters and Anomalous X-ray Pulsars. It contains a brief historical record of the emergence of these classes of neutron stars, a thorough overview of the observational data, a succinct summary of the magnetar model, and suggested directions for future research in this field.

A neutron star with a massive progenitor in westerlund 1

Abstract: We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2–455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of u P 1M⊙. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2–455216 only 1.6 ′ from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses Mi�35M⊙ and >50 post-main-sequence stars that indicate the cluster is 4±1 Myr old. Therefore, the progenitor to this pulsar had an initial mass Mi>40M⊙. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars. Subject headings: X-rays: stars — neutron stars — open clusters and associations: individual (Westerlund 1)

Supernova remnant energetics and magnetars: no evidence in favour of millisecond proto-neutron stars

Abstract: It is generally accepted that anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetars, i.e. neutron stars with extremely high surface magnetic fields (B > 1014 G). The origin of these high magnetic fields is uncertain, but a popular hypothesis is that magnetars are born with an initial spin period not much exceeding the convective overturn time (∼3 ms), which results in a powerful dynamo action, amplifying the seed magnetic field to ≳1015 G. Part of this rotation energy is then expected to power the supernova through rapid magnetic braking. It is therefore possible that magnetar creation is accompanied by supernovae that are an order of magnitude more energetic than normal supernovae, provided their initial spin period is ∼1 ms. However, we list here evidence that the explosion energies of these supernova remnants associated with AXPs and SGRs – Kes 73 (AXP 1E 1841−045), CTB 109 (AXP 1E2259+586) and N49 (SGR 0526−66) – are close to the canonical supernova explosion energy of 1051 erg, suggesting an initial spin period of P0≳ 5 ms.

The X-ray afterglow flat segment in short GRB 051221A: Energy injection from a millisecond magnetar?

Abstract: The flat segment, lasting similar to 10(4) s, in the X-ray afterglow of GRB 051221A represents the first clear case of strong energy injection in the external shock of a short gamma-ray burst (GRB) afterglow. In this work, we show that a millisecond pulsar with a dipole magnetic field similar to 10(14) Gauss could well account for that energy injection. The good quality X-ray flat segment thus suggests that the central engine of this short burst may be a millisecond magnetar.

Magnetar giant flares and afterglows as relativistic magnetized explosions

Abstract: We propose that giant flares on soft γ-ray repeaters produce relativistic, strongly magnetized, weakly baryon-loaded magnetic clouds, somewhat analogous to solar coronal mass ejection (CME) events. The flares are driven by unwinding of the internal non-potential magnetic field which leads to a slow build-up of magnetic energy outside of the neutron star. For large magnetospheric currents, corresponding to a large twist of the external magnetic field, the magnetosphere becomes dynamically unstable on the Alfven crossing time-scale of the inner magnetosphere, t A ∼ R N S/C ∼ 30 μs. The dynamic instability leads to the formation of dissipative current sheets through the development of a tearing mode. The released magnetic energy results in the formation of a strongly magnetized, pair-loaded, quasi-spherically expanding flux rope, topologically connected by the magnetic field to the neutron star during the prompt flare emission. The expansion reaches large Lorentz factors, r ∼ 10-20, at distances r ∼ 1-2 x 10 7 cm, where a leptophotonic load is lost. Beyond this radius plasma is strongly dominated by the magnetic field, though some baryon loading, with M <

Resonant cyclotron scattering and Comptonization in neutron star magnetospheres

Abstract: Resonant cyclotron scattering of the surface radiation in the magnetospheres of neutron stars may considerably modify the emergent spectra and impede efforts to constrain neutron star properties. Resonant cyclotron scattering by a non-relativistic warm plasma in an inhomogeneous magnetic field has a number of unusual characteristics. (i) In the limit of high resonant optical depth, the cyclotron resonant layer is half opaque, in sharp contrast to the case of non-resonant scattering. (ii) The transmitted flux is on average Compton up-scattered by ∼1 + 2βT, where βT is the typical thermal velocity in units of the velocity of light; the reflected flux has on average the initial frequency. (iii) For both the transmitted and reflected fluxes, the dispersion of intensity decreases with increasing optical depth. (iv) The emergent spectrum is appreciably non-Planckian while narrow spectral features produced at the surface may be erased. We derive semi-analytically modification of the surface Planckian emission due to multiple scattering between the resonant layers and apply the model to the anomalous X-ray pulsar 1E 1048.1 − 5937. Our simple model fits just as well as the ‘canonical’ magnetar spectra model of a blackbody plus power law.

Modelling of isolated radio pulsars and magnetars on the fossil field hypothesis

Abstract: We explore the hypothesis that the magnetic fields of neutron stars are of fossil origin. For parametrized models of the distribution of magnetic flux on the main sequence and of the birth spin period of the neutron stars, we calculate the expected properties of isolated radio pulsars in the Galaxy using as our starting point the initial mass function and star formation rate as a function of Galactocentric radius. We then use the 1374-MHz Parkes Multi-Beam Survey of isolated radio pulsars to constrain the parameters in our model and to deduce the required distribution of magnetic fields on the main sequence. We find agreement with observations for a model with a star formation rate that corresponds to a supernova rate of 2 per century in the Galaxy from stars with masses in the range 8-45 M ⊙ and predict 447 000 active pulsars in the Galaxy with luminosities greater than 0.19 mJy kpc 2 . The progenitor OB stars have a field distribution which peaks at ∼46 G with ∼8 per cent of stars having fields in excess of 1000 G. The higher-field progenitors yield a population of 24 neutron stars with fields in excess of 10 14 G, periods ranging from 5 to 12 s, and ages of up to 100 000 yr, which we identify as the dominant component of the magnetars. We also predict that high-field neutron stars (log B > 13.5) originate preferentially from higher-mass progenitors and have a mean mass of 1.6 M ⊙ , which is significantly above the mean mass of 1.4 M ⊙ calculated for the overall population of radio pulsars.

Magnetars as persistent hard X-ray sources: INTEGRAL discovery of a hard tail in SGR 1900+14

Abstract: Using 2.5 Ms of data obtained by the INTEGRAL satellite in 2003-2004, we discovered persistent hard X-ray emission from the soft gamma-ray repeater SGR 1900+14. Its 20-100 keV spectrum is well described by a steep power law with photon index r = 3.1 ± 0.5 and flux 1.5 x 10 -11 erg cm -2 s -1 . Contrary to SGR 1806-20, the only other soft gamma-ray repeater for which persistent emission above 20 keV was reported, SGR 1900+14 has been detected in the hard X-ray range while it was in a quiescent state (the last bursts from this source were observed in 2002). By comparing the broad band spectra (1-100 keV) of all the magnetars detected by INTEGRAL (the two SGRs and three anomalous X-ray pulsars) we find evidence for a different spectral behaviour of these two classes of sources.

Evolution of the magnetic field in magnetars

Abstract: We use numerical MHD to look at the stability of a possible poloidal field in neutron stars (Flowers & Ruderman 1977, ApJ, 215, 302), and follow its unstable evolution, which leads to the complete decay of the field. We then model a neutron star after the formation of a solid crust of high conductivity. As the initial magnetic field we use the stable "twisted torus" field which was the result of our earlier work (Braithwaite & Nordlund 2006, A&A, 450, 1077), since this field is likely to exist in the interior of the star at the time of crust formation. We follow the evolution of the field under the influence of diffusion, and find that large stresses build up in the crust, which will presumably lead to cracking. We put this forward as a model for outbursts in soft gamma repeaters.

QPOs during magnetar flares are not driven by mechanical normal modes of the crust

Abstract: QPOs have been observed during three powerful magnetar flares, from SGR 0526−66, SGR 1806−20 and SGR 1900+14. These QPOs have been commonly interpreted as being driven by the mechanical modes of the magnetar's solid crust which are excited during the flare. Here we show that this interpretation is in sharp contradiction with the conventional magnetar model. Firstly, we show that a magnetar crustal mode decays on the time-scale of at most 1 s due to the emission of Alfven waves into the neutron star interior. A possible modification is then to assume that the QPOs are associated with the magnetars' global modes. However, we argue that at the frequencies of the observed QPOs, the neutron star core is likely to support a continuum of magnetohydrodynamic normal modes. We demonstrate this on a completely solvable toy model which captures the essential physics of the system. We then show that the frequency of the global mode of the whole star is likely to have a significant imaginary component, and its amplitude is likely to decay on a short time-scale. This is not observed. Thus we conclude that either (i) the origin of the QPO is in the magnetar's magnetosphere, or (ii) the magnetic field has a special configuration: either it is expelled from the magenta's core prior to the flares, or its poloidal component has very small coherence length.

Resonant Cyclotron Scattering in Three Dimensions and the Quiescent Non-thermal X-ray Emission of Magnetars

Abstract: Although the surface of a magnetar is a source of bright thermal X-rays, its spectrum contains substantial non-thermal components. The X-ray emission is pulsed, with pulsed fractions that can be as high as ~ 70%. Several properties of magnetars indicate the presence of persistent, static currents flowing across the stellar surface and closing within the magnetosphere. The charges supporting these currents supply a significant optical depth to resonant cyclotron scattering in the 1-100 keV band. Here we describe a Monte Carlo approach to calculating the redistribution of thermal seed photons in frequency and angle by multiple resonant scattering in the magnetosphere. The calculation includes the full angular dependence of the cyclotron scattering cross section, the relativistic Doppler effect due to the motion of the charges, and allows for an arbitrary particle velocity distribution and magnetic field geometry. We construct synthetic spectra and pulse profiles for arbitrary orientations of the spin axis, magnetic axis, and line of sight, using a self-similar, twisted dipole field geometry, and assuming that the seed photons are supplied by single-temperature black body emission from the stellar surface. Pulse profiles and 1-10 keV spectra typical of AXPs are easily produced by this model, with pulsed fractions of ~ 50%. However, this model cannot reproduce the hard, rising energy spectra that are observed from SGRs during periods of activity, without overproducing the thermal emission peak. This suggests that the 1-100 keV emission of SGRs has a common origin with the hard X-ray emission detected from some AXPs above ~20 keV.

Short-living Supermassive Magnetar Model for the Early X-ray Flares Following Short GRBs

Abstract: We suggest a short-lived supermassive magnetar model to account for the X-ray flares following short γ-ray bursts. In this model the central engine of the short γ-ray bursts is a supermassive millisecond magnetar, formed in coalescence of double neutron stars. The X-ray flares are powered by the dipole radiation of the magnetar. When the magnetar has lost a significant part of its angular momentum, it collapses to a black hole and the X-ray flares cease abruptly.

A long-period, violently variable X-ray source in a young supernova remnant.

Abstract: Observations with the Newton X-ray Multimirror Mission satellite show a strong periodic modulation at 6.67 ± 0.03 hours of the x-ray source at the center of the 2000-year-old supernova remnant RCW 103. No fast pulsations are visible. If genetically tied to the supernova remnant, the source could either be an x-ray binary, composed of a compact object and a low-mass star in an eccentric orbit, or an isolated neutron star. In the latter case, the combination of its age and period would indicate that it is a peculiar magnetar, dramatically slowed down, possibly by a supernova debris disc. Both scenarios require nonstandard assumptions about the formation and evolution of compact objects in supernova explosions.

Stellar Explosions by Magnetic Towers

Abstract: We propose a magnetic mechanism for the collimated explosion of massive stars relevant for long-duration gamma-ray bursts (GRBs), X-ray flashes (XRFs), and asymmetric core collapse supernovae. In particular, we apply Lynden-Bell's magnetic tower scenario to the interior of a massive rotating star after the core has collapsed to form a collapsar with a black hole accretion disk or a millisecond magnetar as the central engine. Of key importance, the toroidal magnetic field, continuously generated by differential rotation of the central engine, drives a rapid expansion, which becomes vertically collimated after lateral force balance with the surrounding gas pressure is reached. The collimation naturally occurs because hoop stress concentrates magnetic field toward the rotation axis and inhibits lateral expansion without affecting vertical expansion. This leads to the growth of a self-collimated magnetic structure that Lynden-Bell termed a magnetic tower. When embedded in a massive star, the supersonic expansion of the tower drives a strong bow shock, behind which an overpressured cocoon of shocked stellar material forms, as observed in hydrodynamic simulations of collapsar jets. The cocoon confines the tower by supplying collimating pressure support. Because the tower consists of closed field lines starting and ending on the central engine, mixing of baryons from the cocoon into the tower is suppressed. The channel cleared by the growing tower is thus plausibly free of baryons and allows the escape of magnetic energy from the central engine through the star. While propagating down the stellar density gradient, the expansion of the tower accelerates and becomes relativistic. Eventually, fast collisionless reconnection becomes possible, with the resulting dissipation of magnetic energy into accelerated particles being responsible for GRB prompt emission.

Elastic or magnetic? A toy model for global magnetar oscillations with implications for quasi-periodic oscillations during flares

Abstract: We use a simple toy-model to discuss global magnetohydrodynamic modes of a neutron star, taking into account the magnetic coupling between the elastic crust and the fluid core. Our results suggest that the notion of pure torsional crust modes is not useful for the coupled system. All modes excite Alfven waves in the core. However, we also show that the modes that are most likely to be excited by a fractured crust, e.g. during a magnetar flare, are such that the crust and the core oscillate in concert. For our simple model, the frequencies of these modes are similar to the ‘pure crustal’ frequencies. In addition, our model provides a natural explanation for the presence of lower frequency (<30 Hz) quasi-periodic oscillations seen in the 2004 December giant flare of SGR 1806−20.

Elastic or magnetic? A toy model for global magnetar oscillations with implications for QPOs during flares

Abstract: We use a simple toy-model to discuss global MHD modes of a neutron star, taking into account the magnetic coupling between the elastic crust and the fluid core. Our results suggest that the notion of pure torsional crust modes is not useful for the coupled system, all modes excite Alfven waves in the core. However, we also show that the modes that are most likely to be excited by a fractured crust, eg. during a magnetar flare, are such that the crust and the core oscillate in concert. For our simple model, the frequencies of these modes are similar to the ``pure crustal'' frequencies. In addition, our model provides a natural explanation for the presence of lower frequency ($< 30$~Hz) QPOs seen in the December 2004 giant flare of SGR 1806-20.

The superflares of soft γ-ray repeaters: giant quakes in solid quark stars?

Abstract: Three supergiant flares from soft γ -ray repeaters are observed, with typical released energy of ∼10 44‐47 erg. A conventional model (i.e. the magnetar model) for such events is catastrophic magnetism-powered instability through a magnetohydrodynamic process, in which a significant part of the short‐hard γ -ray bursts could also be the result of magnetars. Based on various observational features (e.g. precession, glitches, thermal photon emission) and the underlying theory of strong interaction (quantum chromodynamics), it cannot yet be ruled out that pulsar-like stars might be actually solid quark stars. Strain energy develops during the life of a solid star, and starquakes could occur when stellar stresses reach a critical value, with a huge amount of energy released. An alternative model for supergiant flares of soft γ -ray repeaters is presented, in which the energy release during a starquake of a solid quark star is calculated. Numerical results for spherically asymmetric solid stars show that the gravitational energy released during a giant quake could be as high as 10 48 erg if the tangential pressure is slightly higher than the radial one. Difficulties in magnetar models may be overcome if anomalous X-ray pulsars/soft γ -ray repeaters are accreting solid quark stars with mass ∼1‐2 M� .

Protoneutron star dynamos: pulsars, magnetars, and radio-silent x-ray emitting neutron stars

Abstract: We discuss the mean-field dynamo action in protoneutron stars that are subject to instabilities during the early evolutionary phase. The mean field is generated in the neutron-finger unstable region where the Rossby number is ∼ 1 and mean-field dynamo is efficient. Depending on the rotation rate, the mean-field dynamo can lead to the formation of three different types of pulsars. If the initial period of the protoneutron star is short, then the generated large-scale field is very strong (${>} 3 \times 10^{13}$ G) and exceeds the small-scale field at the neutron star surface. If rotation is moderate, then the pulsars are formed with more or less standard dipole fields (${<} 3 \times 10^{13}$ G) but with surface small-scale magnetic fields stronger than the dipole field. If rotation is very slow, then the mean-field dynamo does not operate, and the neutron star has no global field. Nevertheless, strong small-scale fields are generated in such pulsars, and they can manifest themselves as objects with very low spin-down rate but with a strong magnetic field inferred from the spectral features.

Magnetars as cooling neutron stars with internal heating

Abstract: We study thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in a spherical internal layer. We explore the location of this layer as well as the heating rate that could explain high observable thermal luminosities of magnetars and would be consistent with the energy budget of neutron stars. We conclude that the heat source should be located in an outer magnetar’s crust, at densities ρ 5 × 10 11 gc m −3 , and should have the heat intensity of ∼10 20 erg cm −3 s −1 . Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the surface.

The superflares of soft Gamma-ray repeatres: giant quakes in solid quark stars?

Abstract: Three times of supergiant flares from soft $\gamma$-ray repeatres are observed, with typical released energy of $\sim 10^{44-47}$ erg. A conventional model (i.e., the magnetar model) for such events is catastrophic magnetism-powered instability through magnetohydrodynamic process, in which a significant part of short-hard $\gamma$-ray bursts could also be the results of magnetars. Based on various observational features (e.g., precession, glitch, thermal photon emission) and the underlying theory of strong interaction (quantum chromodynamics, QCD), it could not be ruled out yet that pulsar-like stars might be actually solid quark stars. Strain energy develops during a solid star's life, and starquakes could occur when stellar stresses reach a critical value, with huge energy released. An alternative model for supergiant flares of soft $\gamma$-ray repeatres is presented, in which energy release during a star quake of solid quark stars is calculated. Numerical results for spherically asymmetric solid stars show that the released gravitational energy during a giant quake could be as high as $10^{48}$ erg if the tangential pressure is slightly higher than the radial one. Difficulties in magnetar models may be overcome if AXPs/SGRs are accreting solid quark stars with mass $\sim (1-2)M_\odot$.

A tale of two populations: rotating radio transients and x-ray dim isolated neutron stars

Abstract: We highlight similarities between recently discovered Rotating Radio Transients and X-ray Dim Isolated Neutron Stars. In particular, it is shown that X-ray Dim Isolated Neutron Stars have a birthrate comparable to that of Rotating Radio Transients. On the contrary, magnetars have too low a formation rate to account for the bulk of the radio transient population. The consequences of the recent detection of a thermal X-ray source associated with one of the Rotating Radio Transients on the proposed scenarios for these sources are also discussed.

Gravitational wave background from magnetars

Abstract: We investigate the gravitational wave background produced by magnetars. The statistical properties of these highly magnetized stars were derived by population synthesis methods and assumed to be also representative of extragalactic objects. The adopted ellipticity was calculated from relativistic models using equations of state and assumptions concerning the distribution of currents in the neutron star interior. The maximum amplitude occurs around 1.2 kHz, corresponding to $\Omega_{gw} \sim 10^{-9}$ for a type I superconducting neutron star model. The expected signal is a continuous background that could mask the cosmological contribution produced in the early stage of the Universe.

Two years of INTEGRAL monitoring of the soft gamma-ray repeater SGR 1806-20: from quiescence to frenzy

Abstract: SGR 180620 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27, 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Gotz et al. (2004a, A&A, 417, L45). Our sample includes a very bright outburst that occurred on October 5, 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of 3 × 1042 erg.We present a detailed analysis of it and discuss our results in the framework of the magnetar model.

Multiwavelength Variability of the Magnetar 4U 0142+61

Abstract: We have collected data spanning seven years of observations of the magnetar 4U 0142+61 in the infrared, optical, and soft X-rays. These combine our own observations and analysis of archival data. We find that the source is variable in the optical, in contrast to what had been previously reported, that the K-band flux can vary by over a magnitude on the timescale of days, and that the X-ray pulsed flux is not obviously correlated with either the total X-ray flux or infrared and optical fluxes. Furthermore, from multicolor photometry of the source within single nights, we conclude that there are two separate components to the infrared emission. The overall picture is unclear and prompts the need for further, more frequent observations.

Could electron-positron annihilation lines in the Galactic center result from pulsar winds?

Abstract: Observations of a strong and extended positron-electron annihilation line emission in the Galactic center (GC) region by the Spectrometer on the International Gamma-Ray Astrophysical Laboratory (SPI/INTEGRAL) are challenging to the existing models of positron sources in the Galaxy. In this paper, we study the possibility that pulsar winds in the GC produce the 511 keV line. We propose that three possible scenarios of pulsar winds may exist as the positron sources: normal pulsars, rapidly spinning strongly magnetized neutron stars (magnetars) in gamma-ray burst (GRB) progenitors, a population of millisecond pulsars in the Galactic center. These e ± pairs could be trapped in the region by the magnetic field in the Galactic center, and cool through synchrotron radiation and Coulomb interactions with the medium, thereby becoming non-relativistic particles. The cooling timescales are shorter than the diffuse timescale of positrons, so low-energy positrons could annihilate directly with electrons into 511 keV photons or could form positronium before annihilation. We find that normal pulsars cannot be a significant contributor to the positron sources. Although magnetars in the GC could be potential sources of positrons, their birth rate and birth locations may pose some problems for this scenario. We believe that the most likely candidates for positron sources in the GC may be a population of millisecond pulsars in the GC. Our preliminary estimations predict that the e ± annihilation rate in the GC is >5 x 10 42 s -1 , which is consistent with the present observational constraints. Therefore, the e ± pairs from pulsars winds can contribute significantly to the positron sources in the Galactic center region. Furthermore, since the diffusion length of positrons is short, we predict that the intensity distribution of the annihilation line should follow the distribution of millisecond pulsars, which should then correlate to the mass distribution in the GC.