Showing papers on "Magnetar published in 2008"

The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars

Abstract: Two classes of X-ray pulsars, the anomalous X-ray pulsars and the soft gamma-ray repeaters, have been recognized in the last decade as the most promising candidates for being magnetars: isolated neutron stars powered by magnetic energy. I review the observational properties of these objects, focussing on the most recent results, and their interpretation in the magnetar model. Alternative explanations, in particular those based on accretion from residual disks, are also considered. The possible relations between these sources and other classes of neutron stars and astrophysical objects are also discussed.

Short-duration gamma-ray bursts with extended emission from protomagnetar spin-down

Abstract: Evidence is growing for a class of gamma-ray bursts (GRBs) characterized by an initial ∼0.1-1 s spike of hard radiation followed, after a ∼3-10 s lull in emission, by a softer period of extended emission lasting ∼10-100 s. In a few well-studied cases, these 'short GRBs with extended emission' show no evidence for a bright associated supernova (SN). We propose that these events are produced by the formation and early evolution of a highly magnetized, rapidly rotating neutron star (a 'protomagnetar') which is formed from the accretion-induced collapse (AIC) of a white dwarf (WD), the merger and collapse of a WD-WD binary or perhaps, the merger of a double neutron star binary. The initial emission spike is powered by accretion on to the protomagnetar from a small disc that is formed during the AIC or merger event. The extended emission is produced by a relativistic wind that extracts the rotational energy of the protomagnetar on a time-scale ∼ 10-100 s. The ∼ 10 s delay between the prompt and extended emission is the time required for the newly formed protomagnetar to cool sufficiently that the neutrino-heated wind from its surface becomes ultrarelativistic. Because a protomagnetar ejects little or no 56 Ni (< 10 -3 M ⊙ ), these events should not produce a bright SN-like transient. We model the extended emission from GRB060614 using spin-down calculations of a cooling protomagnetar, finding reasonable agreement with observations for a magnetar with an initial rotation period of ∼1 ms and a surface dipole field of ∼3 x 10 15 G. If GRBs are indeed produced by AIC or WD-WD mergers, they should occur within a mixture of both early- and late-type galaxies and should not produce strong gravitational wave emission. An additional consequence of our model is the existence of X-ray flashes unaccompanied by a bright SN and not associated with massive star formation.

Magnetar-like emission from the young pulsar in Kes 75.

Abstract: We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9deg 10(exp 13) gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength.

Are There Magnetars in High-Mass X-Ray Binaries? The Case of Supergiant Fast X-Ray Transients

Abstract: In this paper we survey the theory of wind accretion in high-mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. We concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include inhibition through the centrifugal and magnetic barriers. Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~104 or more on timescales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high-mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a result of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems.

Resonant Cyclotron Scattering in Magnetars’ Emission

Abstract: We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of 10 magnetars, including canonical and transient anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). In this scenario, nonthermal magnetar spectra in the soft X-rays (i.e., below ~10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters as in the commonly used empirical blackbody plus power-law model. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below 10 keV require the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). Although this model in its present form does not explain the hard X-ray emission (i.e., above ~20 keV) of a few of these sources, we took this further component into account in our modeling not to overlook its contribution in the ~4-10 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitude higher than the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities are also in better agreement with more recent estimates. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars shows that the RCS model is capable of catching the main features of the spectra observed below ~10 keV.

Are There Magnetars in High Mass X-ray Binaries? The Case of SuperGiant Fast X-Ray Transients

Abstract: In this paper we survey the theory of wind accretion in high mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. We concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include the inhibition through the centrifugal and magnetic barriers. Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~10^4 or more on time scales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a results of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems.

Relativistic jets and long-duration gamma-ray bursts from the birth of magnetars

Abstract: We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the first � 10 seconds after core collapse. We inject a super-magnetosonic wind with u E = 10 51 ergs s 1 into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a longduration gamma-ray burst (GRB). At � 5 s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii � 10 11 cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on � 0.01 0.1 second timescale. These may contribute to variability in GRB emission (e.g., via internal shocks).

A Swift Gaze into the 2006 March 29 Burst Forest of SGR 1900+14

Abstract: In 2006 March the soft gamma-ray repeater SGR 1900+14 resumed its bursting activity after ~2 yr of quiescence. The Swift mission observed the source several times. We report on the intense burst "forest" recorded on March 29, which lasted for ~30 s, when Swift was pointing at the source with the narrow field of view instruments. More than 40 bursts were detected by BAT and XRT, 7 of which were rare intermediate flares (IFs). The BAT data were used to carry out time-resolved spectroscopy in the 14-100 keV range down to 8 ms timescales. BAT and XRT simultaneous data were used to characterize the broadband energy spectra of IFs and verify the results obtained from the BAT-only spectral fits. This unique data set allowed us to test the magnetar model predictions, such as the magnetically trapped fireball and twisted magnetosphere, over an unprecedented range of fluxes and with large statistics. We confirmed that a two-blackbody component adequately fits the time-resolved and integrated spectra of IFs. However, Comptonization models give comparably good reduced χ2. Moreover, we found a change of behavior, around ~1041 erg s−1, above which the softer blackbody shows a sort of saturation, while the harder one still grows to a few times 1041 erg s−1, and a rather sharp correlation between temperature and radii of the blackbodies (R2 ∝ kT−3), which holds for the most luminous parts of the flares (~Ltot ≥ 1041 erg s−1). Within the magnetar model, the majority of these findings are accounted for in terms of thermalized emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15 km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8 × 1014 G (virtually equal to that deduced from the spin-down of SGR 1900+14).

The magnetar 1E 1547.0-5408: radio spectrum, polarimetry, and timing

Abstract: We have investigated the radio emission from the anomalous X-ray pulsar 1E 1547.0–5408 (PSR J1550–5418) using the Parkes telescope and the Australia Telescope Compact Array. The flux density of the pulsar is roughly the same between 1.4 and 45 GHz, but shows time variability. The radiation is nearly 100% linearly polarized between frequencies of 45 and 3.2 GHz, but from 2.3 to 1.4 GHz it becomes increasingly more depolarized. The rotation measure of –1860 rad m−2 is the largest for any known pulsar, and implies an average magnetic field strength along the line of sight of 2.7 μG. The pulse profiles are circularly polarized at all frequencies observed, more so at lower frequencies, at the 15% level. The observed swing of the position angle of linear polarization as a function of pulse phase suggests that in this neutron star the rotation and magnetic axes are nearly aligned, and that its radio emission is only detectable within a small solid angle. Timing measurements indicate that the period derivative of this 2 s pulsar has increased by nearly 40% in a 6 month period. The flat spectrum and variability in flux density and pulse profiles are reminiscent of the properties of XTE J1810–197, the only other known radio-emitting magnetar, and are anomalous by comparison with those of ordinary radio pulsars.

A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini Near-Infrared Spectroscopy

Abstract: We present H- and K-band spectroscopy of OB andWolf-Rayet(WR) members of the Milky Way cluster 1806–20 (G10.0–0.3), to obtain a revised cluster distance of relevance to the 2004 giant flare from the SGR 1806–20 magnetar. From GNIRS spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous midWN and late WC classifications for twoWR stars. Based upon an absolute Ks-band magnitude calibration for B supergiants and WR stars, and near-IR photometry from NIRI at Gemini North plus archival VLT/ISAAC datasets, we obtain a cluster distance modulus of 14.7±0.35 mag. The known stellar content of the 1806–20 cluster suggests an age of 3–5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7±0.7 mag. Together, our results favour a distance modulus of 14.7±0.4 mag (8.7+1.8−1.5 kpc) to the 1806–20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the December 2004 giant flare is reduced by a factor of three to 7 × 1046 ergs−1, such that the contamination of BATSE short gamma ray bursts (GRB’s) from giant flares of extragalactic magnetars is reduced to a few percent. We infer a magnetar progenitor mass of ∼ 48+20−8 M⊙, in close agreement with that obtained recently for the magnetar in Westerlund 1.

X-ray spectra from magnetar candidates – I. Monte Carlo simulations in the non-relativistic regime

Abstract: The anomalous X-ray pulsars (AXPs) and soft γ -repeaters (SGRs) are peculiar high-energy sources believed to host a magnetar, an ultramagnetized neutron star with surface magnetic field in the petagauss range. Their persistent, soft X-ray emission exhibits a two component spectrum, usually modelled by the superposition of a blackbody and a power-law tail. It has been suggested that the ∼1‐10 keV spectrum of AXPs/SGRs forms as the thermal photons emitted by the cooling star surface traverse the magnetosphere. Magnetar magnetospheres are, in fact, likely different from those of ordinary radio pulsars, since the external magnetic field may acquire a toroidal component as a consequence of the deformation of the star crust induced by the superstrong interior field. In a twisted magnetosphere, the supporting currents can provide a large optical depth to resonant cyclotron scattering. The thermal spectrum emitted by the star surface will be then distorted because primary photons gain energy in the repeated scatterings with the flowing charges, and this may provide a natural explanation for the observed spectra. In this paper we present 3D Monte Carlo simulations of photon propagation in a twisted magnetosphere. Our model is based on a simplified treatment of the charge carrier velocity distribution which however accounts for the particle collective motion, in addition to the thermal one. The present treatment is restricted to conservative (Thomson) scattering in the electron rest frame. The code, none the less, is completely general and inclusion of the relativistic quantum electrodynamical resonant cross-section, which is required in the modelling of the hard (∼20‐200 keV) spectral tails observed in the magnetar candidates, is under way. The properties of emerging spectra have been assessed under different conditions, by exploring the model parameter space, including effects arising from the viewing geometry. Monte Carlo runs have been collected into a spectral archive which has then been implemented in the X-ray fitting package XSPEC. Two tabulated XSPEC spectral models, with and without viewing angles, have been produced and applied to the 0.1‐10 keV XMM‐Newton EPIC-pn spectrum of the AXP CXOU J1647−4552.

A Swift gaze into the 2006 March 29th burst forest of SGR 1900+14

Abstract: We report on the intense burst ``forest'' recorded on 2006 March 29 which lasted for ~30s. More than 40 bursts were detected both by BAT and by XRT, seven of which are rare intermediate flares (IFs): several times 10^{42} ergs were released. The BAT data were used to carry out time-resolved spectroscopy in the 14-100keV range down to 8ms timescales. This unique dataset allowed us to test the magnetar model predictions such as the magnetically trapped fireball and the twisted magnetosphere over an unprecedented range of fluxes and with large statistics (in terms of both photons and IFs). We confirmed that a two blackbody component fits adequately the time-resolved and integrated spectra of IFs. However, Comptonization models give comparable good reduced chi^2. Moreover, we found: i) a change of behavior, around ~10^{41} erg/s, above which the softer blackbody shows a sort of saturation while the harder one still grows to a few times 10^{41} erg/s; ii) a rather sharp correlation between temperature and radii of the blackbodies (R^2 prop kT^{-3}), which holds for the most luminous parts of the flares (approximately for L_{tot} > 10^{41} erg/s). Within the magnetar model, the majority of these findings can be accounted for in terms of thermalised emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8 x 10^{14} G (virtually equal to the one deduced from the spindown of SGR 1900+14).

On the Conditions for Neutron-rich Gamma-Ray Burst Outflows

Abstract: We calculate the structure and neutron content of neutrino-heated magnetohydrodynamic winds driven from the surface of newly formed magnetars (protomagnetars) and from the midplane of hyperaccreting disks, two of the possible central engines for GRBs and hyperenergetic SNe. Both the surface of protomagnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio -->n/p 1). If this substantial free neutron excess is preserved to large radii in ultrarelativistic outflows, several important observational consequences may result. Weak interaction processes, however, can drive -->n/p to ~1 in the nondegenerate regions that obtain just above the surfaces of NDAFs and protomagnetars. Our calculations show that mildly relativistic (Lorentz factor -->? 10) neutron-rich outflows from NDAFs are possible in the presence of a strong poloidal magnetic field. However, neutron-rich winds possess a minimum mass-loss rate that likely precludes simultaneously neutron-rich and ultrarelativistic ( -->? 100) NDAF winds accompanying a substantial accretion power. In contrast, protomagnetars are capable of producing neutron-rich long-duration GRB outflows ~10-30 s following core bounce for submillisecond rotation periods; such outflows would, however, accompany only extremely energetic events, in which the GRB+SN energy budget exceeds ~ -->4 ? 1052 ergs. Neutron-rich highly relativistic outflows may also be produced during some short-duration GRBs by geometrically thick accretion disks formed from compact object mergers. The implications for r-process nucleosynthesis, optical transients due to nonrelativistic neutron-rich winds, and nickel production in protomagnetar and NDAF winds are also briefly discussed.

Variability of the High Magnetic Field X-Ray Pulsar PSR J1846?0258 Associated with the Supernova Remnant Kes 75 as Revealed by the Chandra X-Ray Observatory

Abstract: We present results from the archival Chandra observations of the 0.3 s X-ray pulsar PSR J1846?0258 associated with the supernova remnant (SNR) Kes 75. The pulsar has the highest spin-down luminosity ($d{E}$ -->$d{E}$ -->$d{E}$ --> = 8.3 ? 1036 ergs s -->?1) among all the high magnetic field pulsars (HBPs) and has been classified as a Crab-like pulsar despite its magnetic field (5 ? 1013 G) being above the quantum critical field. It is the only HBP described by a nonthermal Crab-like spectrum, powering a bright pulsar wind nebula (PWN). Our spectroscopic study shows evidence of spectral softening (photon index ? = 1.32 -->+ 0.08?0.09 −0.09+0.08--> to 1.97 -->+ 0.05?0.07 −0.07+0.05-->) and temporal brightening [unabsorbed flux Funabs = (4.3 ? 0.2) ? 10 -->?12 to -->2.7?0.2+0.1 ? 10 -->?11 ergs cm -->?2 s -->?1] of the pulsar by ~6 times from 2000 to 2006. The 0.5-10 keV luminosity of the pulsar at the revised distance of 6 kpc has also increased from LX = (1.85 ? 0.08) ? 1034 to 1.16 -->+ 0.03?0.07 −0.07+0.03--> ? 1035 ergs s -->?1, and the X-ray efficiency increased from 0.2% ? 0.01 -->% to 1.4 -->+ 0.04?0.08% −0.08+0.04-->. The observed X-ray brightening and softening of the pulsar suggests for the first time that this HBP is revealing itself as a magnetar.

Alfvén quasi-periodic oscillations in magnetars

Abstract: We investigate torsional Alfven oscillations of relativistic stars with a global dipole magnetic field, via two-dimensional numerical simulations. We find that (i) there exist two families of quasi-periodic oscillations (QPOs) with harmonics at integer multiples of the fundamental frequency, (ii) the lower-frequency QPO is related to the region of closed field lines, near the equator, while the higher-frequency QPO is generated near the magnetic axis, (iii) the QPOs are long-lived, (iv) for the chosen form of dipolar magnetic field, the frequency ratio of the lower to upper fundamental QPOs is ∼0.6, independent of the equilibrium model or of the strength of the magnetic field, and (v) within a representative sample of equations of state and of various magnetar masses, the Alfven QPO frequencies are given by accurate empirical relations that depend only on the compactness of the star and on the magnetic field strength. The lower and upper QPOs can be interpreted as corresponding to the edges or turning points of an Alfven continuum, according to the model proposed by Levin (2007). Several of the low-frequency QPOs observed in the X-ray tail of SGR 1806-20 can readily be identified with the Alfven QPOs we compute. In particular, one could identify the 18- and 30-Hz observed frequencies with the fundamental lower and upper QPOs, correspondingly, while the observed frequencies of 92 and 150 Hz are then integer multiples of the fundamental upper QPO frequency (three and five times, correspondingly). With this identification, we obtain an upper limit on the strength of the magnetic field of SGR 1806-20 (if is dominated by a dipolar component) between ∼3 and 7 × 10 15 G. Furthermore, we show that an identification of the observed frequency of 26 Hz with the frequency of the fundamental torsional � = 2 oscillation of the magnetar's crust is compatible with a magnetar mass of about from 1.4 to 1.6 Mand an equation of state (EOS) that is very stiff (if the magnetic field strength is near its upper limit) or moderately stiff (for lower values of the magnetic field).

Relativistic models of magnetars: structure and deformations

Abstract: We find numerical solutions of the coupled system of Einstein-Maxwell equations with a linear approach, in which the magnetic field acts as a perturbation of a spherical neutron star. In our study, magnetic fields having both poloidal and toroidal components are considered, and higher order multipoles are also included. We evaluate the deformations induced by different field configurations, paying special attention to those for which the star has a prolate shape. We also explore the dependence of the stellar deformation on the particular choice of the equation of state and on the mass of the star. Our results show that, for neutron stars with mass M = 1.4 M ⊙ and surface magnetic fields of the order of 10 15 G, a quadrupole ellipticity of the order of 10 -6 to 10 -5 should be expected. Low-mass neutron stars are in principle subject to larger deformations (quadrupole ellipticities up to 10 -3 in the most extreme case). The effect of quadrupolar magnetic fields is comparable to that of dipolar components. A magnetic field permeating the whole star is normally needed to obtain negative quadrupole ellipticities, while fields confined to the crust typically produce positive quadrupole ellipticities.

X-Ray and Near-IR Variability of the Anomalous X-Ray Pulsar 1E 1048.1-5937: From Quiescence Back to Activity

Abstract: We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1–5937 performed between 2001 and 2007 with the Rossi X-Ray Telescope Explorer (RXTE), the Chandra X-Ray Observatory, the Swift Gamma-Ray Burst Explorer, the Hubble Space Telescope (HST), and the Very Large Telescope. Monitoring with RXTE revealed that following its ~2001-2004 active period, 1E 1048.1–5937 entered a phase of timing stability; at the same time, simultaneous observations with Chandra and HST in 2006 showed that its X-ray and near-IR radiative properties, all variable prior to 2005, stabilized. Specifically, the 2006 X-ray spectrum is consistent with a two-component blackbody plus power law, with an average -->kt = 0.52 keV and -->Γ = 2.8, at a mean flux level of ~ -->6.5 × 10−12 erg cm−2 s−1 ( -->2–10 keV). The near-IR counterpart in 2005-2006 was detected at -->H ~ 22.7 mag and -->Ks ~ 21.0 mag, considerably fainter than previously measured. In 2007 March, this newfound quiescence was interrupted by sudden X-ray flux, spectral, and pulse morphology changes, simultaneous with a large glitch and near-IR enhancement. Our RXTE observations revealed a factor of ~3 increase in pulsed flux (2-10 keV), while observations with Chandra and Swift saw the total X-ray flux increase much more than the pulsed flux, reaching a peak value of >7 times the quiescent value (2-10 keV). We find a strong anticorrelation between X-ray flux and pulsed fraction, and a correlation between X-ray spectral hardness and flux. Simultaneously with the radiative and timing changes, we observed the X-ray pulse profile change significantly from nearly sinusoidal to having multiple peaks. We compare these remarkable events with other magnetar outbursts and discuss implications in the context of AXP emission models.

X-ray spectra from magnetar candidates – II. Resonant cross-sections for electron–photon scattering in the relativistic regime

Abstract: Recent models of spectral formation in magnetars called renewed attention on electron-photon scattering in the presence of ultra-strong magnetic fields. Investigations presented so far mainly focussed on mildly relativistic particles and magnetic scattering was treated in the non-relativistic (Thomson) limit. This allows for consistent spectral calculations up to a few tens of keVs, but becomes inadequate in modelling the hard tails (. 200 keV) detected by INTEGRAL from magnetar sources. In this paper, the second in a series devoted to model the X-/soft γ-ray persistent spectrum of magnetar candidates, we present explicit, relatively simple expressions for the magnetic Compton cross-section at resonance which account for Landau-Raman scattering up to the second Landau level. No assumption is made on the magnetic field strength. We find that sensible departures from the Thomson regime can be already present at B � 5× 10 12 G. The form of the magnetic cross section we derived can be easily implemented in Monte Carlo transfer codes and a direct application to magnetar spectral calculations will be presented in a forthcoming study.

Discovery of two magnetic massive stars in the Orion Nebula Cluster: a clue to the origin of neutron star magnetic fields?

Abstract: The origin of the magnetic fields in neutron stars, and the physical differences between magnetars and strongly magnetized radio pulsars are still under vigorous debate. It has been suggested that the properties of the progenitors of neutron stars (the massive OB stars), such as rotation, magnetic fields and mass, may play an important role in the outcome of core collapse leading to Type II supernovae. Therefore, knowing the magnetic properties of the progenitor OB stars would be an important asset for constraining models of stellar evolution leading to the birth of a neutron star. We present here the beginning of a broad study with the goal of characterizing the magnetic properties of main-sequence massive OB stars. We report the detection of two new massive magnetic stars in the Orion Nebula Cluster: Par 1772 (HD 36982) and NU Ori (HD 37061), for which the estimated dipole polar strengths, with 1σ error bars, are 1150 +320 −200 and 620 +220 −170 G, respectively.

Variability of the High-Magnetic Field X-ray Pulsar PSR J1846-0258 Associated with the Supernova Remnant Kes 75 as Revealed by the Chandra X-ray Observatory

Abstract: We present results from the archival Chandra observations of the 0.3 s X-ray pulsar PSR J1846-0258 associated with the supernova remnant (SNR) Kes 75. The pulsar has the highest spin-down luminosity (Edot = 8.3e36 erg/s) among all the high magnetic field pulsars (HBPs) and has been classified as a Crab-like pulsar despite its magnetic field (5e13 G) being above the quantum critical field. It is the only HBP described by a non-thermal Crab-like spectrum, powering a bright pulsar wind nebula (PWN). Our spectroscopic study shows evidence of spectral softening (photon index = 1.32 to 1.97) and temporal brightening (unabsorbed flux = 4.3e-12 to 2.7e-11 erg/cm^2/s) of the pulsar by ~6 times from 2000 to 2006. The 0.5-10 keV luminosity of the pulsar at the revised distance of 6 kpc has also increased from 1.85e34 to 1.16e35 erg/s, and the X-ray efficiency increased from 0.2% to 1.4%. The observed X-ray brightening and softening of the pulsar suggests for the first time that this HBP is revealing itself as a magnetar.

Using chandra to unveil the high-energy properties of the high magnetic field radio pulsar j1119-6127

Abstract: PSR J1119–6127 is a high magnetic field (B = 4.1 × 1013 G), young (≤1700 year old), and slow (P = 408 ms) radio pulsar associated with the supernova remnant (SNR) G292.2–0.5. In 2003, Chandra allowed the detection of the X-ray counterpart of the radio pulsar and provided the first evidence for a compact and faint pulsar wind nebula (PWN). We here present new Chandra observations that allowed for the first time an imaging and spectroscopic study of the pulsar and PWN independently of each other. The PWN is only evident in the hard band (above ~2 keV) and consists of jetlike structures extending at least 7'' from the pulsar, with the southern "jet" being longer than the northern "jet." The spectrum of the PWN is described by a power law with a photon index Γ ~ 1.1 for the compact PWN and ~1.4 for the southern long jet (at a column density NH = 1.8 × 1022 cm−2), and a total luminosity LX(0.5–7.0 keV) ~ 4 × 1032 ergs s−1, at a distance of 8.4 kpc. We rule out a single blackbody model for the pulsar and present the first evidence of nonthermal emission that dominates above ~3 keV. A two-component model consisting of a power-law component (with photon index Γ ~ 1.5–2.0) plus a thermal component provides the best fit. The thermal component can be fit by either a blackbody model with a temperature kT ~ 0.21 keV, or a neutron star atmospheric model with a temperature kT ~ 0.14 keV. The efficiency of the pulsar in converting its rotational power, , into nonthermal X-ray emission from the pulsar and PWN is ≈5 × 10−4, comparable to other rotation-powered pulsars with a similar . We discuss our results in the context of the X-ray manifestation of high magnetic field radio pulsars in comparison with rotation-powered pulsars and magnetars.

Origin of neutron star magnetic fields

Abstract: Possible origins of the magnetic fields of neutron stars include inheritance from the main sequence progenitor and dynamo action at some stage of evolution of progenitor. Inheritance is not sufficient to explain the fields of magnetars. Energetic considerations point to differential rotation in the final stages of core collapse process as the most likely source of field generation, at least for magnetars. A runaway phase of exponential growth is needed to achieve sufficient field amplification during relevant phase of core collapse; it can probably be provided by a some form of magnetorotational instability. Once formed in core collapse, the field is in danger of decaying again by magnetic instabilities. The evolution of a magnetic field in a newly formed neutron star is discussed, with emphasis on the existence of stable equilibrium configurations as end products of this evolution, and the role of magnetic helicity in their existence.

Radio spectrum of the AXP J1810 197 and of its profile components

Abstract: As part of a European Pulsar Network (EPN) multitelescope observing campaign, we performed simultaneous multifrequency observations at 1.4, 4.9 and 8.4 GHz during 2006 July and quasi-simultaneous multifrequency observations from 2006 December until 2007 July at 2.7, 4.9, 8.4, 14.6 and 32 GHz, in order to obtain flux density measurements and spectral features of the 5.5 s radio-emitting magnetar AXP J1810−197. We monitored the spectral evolution of its pulse shape which consists of a main pulse (MP) and an interpulse (IP). We present the flux density spectrum of the average profile and of the separate pulse components of this first-known radio-emitting transient anomalous X-ray pulsar. We observe a decrease in the flux density by a factor of 10 within 8 m and follow the disappearance of one of the two main components. Although the spectrum is generally flat, we observe large fluctuations of the spectral index with time. For that reason, we have made some measurements of modulation indices for individual pulses in order to investigate the origin of these fluctuations.

Constraints on the Emission and Viewing Geometry of the Transient Anomalous X-ray Pulsar XTE J1810-197

Abstract: The temporal decay of the flux components of the transient anomalous X-ray pulsar XTE J1 810-197 following its 2002 outburst presents a unique opportunity to probe the emission geometry of a magnetar. Toward this goal, we model the magnitude of the pulsar's modulation in narrow spectral bands over time. Following previous work, we assume that the postoutburst flux is produced in two distinct thermal components arising from a hot spot and a warm concentric ring. We include general relativistic effects on the blackbody spectra due to gravitational redshift and light bending near the stellar surface, which strongly depend on radius. This affects the model fits for the temperature and size of the emission regions. For the hot spot, the observed temporal and energy-dependent pulse modulation is found to require an anisotropic, pencil-beamed radiation pattern. We are able to constrain an allowed range for the angles that the line of sight (psi) and the hot spot pole (xi) make with respect to the spin axis. Within errors, this is defined by the locus of points in the xi-psi plane that lie along the line [xi + beta(R)] [psi + [beta(R)] = const, where beta(R) is a function of the radius R of the star. For a canonical value of R = 12 km, the viewing parameters range from psi = xi = 37deg to (psi, xi) = (85deg, 15deg). We discuss our results in the context of magnetar emission models.

The Long-Term Radiative Evolution of Anomalous X-Ray Pulsar 1E 2259+586 After Its 2002 Outburst

Abstract: We present an analysis of five XMM-Newton observations of the anomalous X-ray pulsar (AXP) 1E 2259+586 taken in 2004 and 2005 during its relaxation following its 2002 outburst. We compare these data with those of five previous XMM-Newton observations taken in 2002 and 2003, and find the observed flux decay is well described by a power law of index –0.69 ± 0.03. As of mid-2005, the source may still have been brighter than preoutburst, and was certainly hotter. We find a strong correlation between hardness and flux, as seen in other AXPs. We discuss the implications of these results for the magnetar model.

On the Nature of Quasi-periodic Oscillations in the Tail of Soft Gamma Repeater Giant Flares

Abstract: A model is presented for the quasi-periodic component of magnetar emission during the tail phase of giant flares. The model invokes modulation of the particle number density in the magnetosphere. The magnetospheric currents are modulated by torsional motion of the surface, and we calculate that the amplitude of neutron star (NS) surface oscillation should be ~1% of the NS radius in order to produce the observed features in the power spectrum. Using an axisymmetric analytical model for structure of the magnetosphere of an oscillating NS, we calculate the angular distribution of the optical depth to the resonant Compton scattering. The anisotropy of the optical depth may be why quasi-periodic oscillations are observed only at particular rotational phases.

Simultaneous multi-frequency single-pulse properties of AXP XTE J1810-197

Abstract: We have used the 76-m Lovell, 94-m equivalent WSRT and 100-m Effelsberg radio telescopes to investigate the simultaneous single-pulse properties of the radio emitting magnetar AXP XTE J1810-197 at frequencies of 1.4, 4.8 and 8.35 GHz during May and July 2006. We study the magnetar's pulse-energy distributions which are found to be very peculiar as they are changing on time-scales of days and cannot be fit by a single statistical model. The magnetar exhibits strong spiky single giant-pulse-like subpulses, but they do not fit the definition of the giant pulse or giant micropulse phenomena. Measurements of the longitude-resolved modulation index reveal a high degree of intensity fluctuations on day-to-day time-scales and dramatic changes across pulse phase. We find the frequency evolution of the modulation index values differs significantly from what is observed in normal radio pulsars. We find that no regular drifting subpulse phenomenon is present at any of the observed frequencies at any observing epoch. However, we find a quasi-periodicity of the subpulses present in the majority of the observing sessions. A correlation analysis indicates a relationship between components from different frequencies. We discuss the results of our analysis in light of the emission properties of normal radio pulsars and a recently proposed model which takes radio emission from magnetars into consideration.

Very fast optical flaring from a possible new Galactic magnetar.

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

Origin and evolution of magnetars

Abstract: We present a population synthesis study of the observed properties of the magnetars investigating the hypothesis that they are drawn from a population of progenitors that are more massive than those of the normal radio pulsars. We assume that the anomalous X-ray emission is caused by the decay of a toroidal or tangled up field that does not take part in the spin-down of the star. Our model assumes that the magnetic flux of the neutron star is distributed as a Gaussian in the logarithm about a mean value that is described by a power law , where Mp is the mass of the progenitor. We find that we can explain the observed properties of the magnetars for a model with Φ0= 2 × 1025 G cm2 and γ= 5 if we suitably parametrize the time evolution of the anomalous X-ray luminosity as an exponentially decaying function of time. Our modelling suggests that magnetars arise from stars in the high-mass end (20 M⊙≤Mp≤ 45 M⊙) of this distribution. The lower mass progenitors are assumed to give rise to the radio pulsars. The high value of γ can be interpreted in one of two ways. It may indicate that the magnetic flux distribution on the main sequence is a strong function of mass and that this is reflected in the magnetic fluxes of the neutron stars that form from this mass range (the fossil field hypothesis). The recent evidence for magnetic fluxes similar to those of the magnetars in a high fraction (∼25 per cent) of massive O-type stars lends support to such a hypothesis. Another possibility is that the spin of the neutron star is a strong function of the progenitor mass, and it is only for stars that are more massive than ∼20 M⊙ that magnetar-type fields can be generated by the α−ω dynamo mechanism (the convective dynamo hypothesis). In either interpretation, it has to be assumed that all or a subset of stars in the mass range ∼20–45 M⊙, which on standard stellar evolution models lead to black holes via the formation of a fall-back disc, must give rise to magnetars. Unlike with the radio pulsars, the magnetars only weakly constrain the birth spin period, due to their rapid spin-down. Our model predicts a birthrate of ∼1.5–3 × 10−3 yr−1 for the magnetars.