Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, neutron stars powered by extreme magnetic fields, B ~ 1014 to 1015 gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 × 1012 gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far

Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 s in 2003 to 1.13 s in 2014. It has an isotropic peak luminosity of ∼1000 times the Eddington limit for a NS at 17.1 Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥1041 erg s −1 ) might harbor NSs.Ultraluminous x-ray sources (ULXs) are observed in off-nucleus regions of nearby galaxiesand have x-ray luminosities in excess of a few 10 39 erg s −1 , which is the Eddington luminosity (L Edd ) for a black hole (BH) of 10 M (1). The L Edd sets an upper limit on the accretion luminosity (L acc ) of a compact object steadily accreting, since for L acc > L Edd accretion will be halted by radiation forces. For spherical accretion of fully ionized hydrogen, the limit can be written as, where σ T is the Thomson scattering cross section, m p is the proton mass, and M/M is the compact object mass in solar masses; for a 1.4 M neutron star (NS), the maximum accreting luminosity is ∼2×10 38 erg s −1 .The high luminosity of ULXs has thus been explained as accretion at or above the Eddington luminosity onto BHs of stellar origin (<80-100 M ), or onto intermediate-mass (10BHs (2, 3). However, if the emission of ULXs were beamed over a fraction b < 1 of the sky, their true luminosity, and thus also the compact object mass required not to exceed L Edd , would be reduced by the same factor. This possibility, together with the recent identification of two accreting NSs associated with the ∼10 40 erg s −1 M82 X-2 (4) and NGC 7793 P13 (5, 6) x-ray sources, have brought support to the view that most low-luminosity ULXs likely host a NS (7) 2 or a stellar-mass BH (8). For the most extreme ULXs with x-ray luminosity exceeding a few ×10 40 erg s −1 , BHs with masses in excess of 100 M are still commonly considered (9, 10).Despite several searches for coherent x-ray pulsations,no other ultraluminous x-ray source has been found to host a NS so far (11).Within the framework of "Exploring the X-ray Transient and variable Sky", EXTraS (12) Fig. 1 and Table 1). In all cases, a strong first period derivative term is present (see Table 1). The pulse shape is nearly sinusoidal, while the pulsed fraction (the semi-amplitude of the sinusoid divided by the average count rate)is energy dependent and increases from about 12% at low energies (<2.5 keV) to ∼20% in the hard band (>7 keV; Fig. 1).To derive constraints on the orbital period (P orb ), we applied a likelihood analysis to the two 2014 NuSTAR observations (see supplementary online text), which have the longest baseline. 3By assuming a circular orbit (as in the case of M...

Magnetars are the strongest magnets in the present universe and the combination of extreme magnetic field, gravity and density makes them unique laboratories to probe current physical theories (from quantum electrodynamics to general relativity) in the strong field limit. Magnetars are observed as peculiar, burst-active x-ray pulsars, the anomalous x-ray pulsars (AXPs) and the soft gamma repeaters (SGRs); the latter emitted also three 'giant flares', extremely powerful events during which luminosities can reach up to 10(47) erg s(-1) for about one second. The last five years have witnessed an explosion in magnetar research which has led, among other things, to the discovery of transient, or 'outbursting', and 'low-field' magnetars. Substantial progress has been made also on the theoretical side. Quite detailed models for explaining the magnetars' persistent x-ray emission, the properties of the bursts, the flux evolution in transient sources have been developed and confronted with observations. New insight on neutron star asteroseismology has been gained through improved models of magnetar oscillations. The long-debated issue of magnetic field decay in neutron stars has been addressed, and its importance recognized in relation to the evolution of magnetars and to the links among magnetars and other families of isolated neutron stars. The aim of this paper is to present a comprehensive overview in which the observational results are discussed in the light of the most up-to-date theoretical models and their implications. This addresses not only the particular case of magnetar sources, but the more fundamental issue of how physics in strong magnetic fields can be constrained by the observations of these unique sources.

Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays 1, 2 . This behaviour can be understood in the magnetar model [3][4][5] , according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties 6-8 of AXPs and SGRs are greater than -or at the high end of the range of -those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters 9 , whereas a strong field has been proposed to reside in the stellar interior 10,11 and in multipole components on the surface 12 . Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star's rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2×10 14 gauss to more than 10 15 gauss.On 2009 June 5 two short bursts of hard X-rays, detected by Fermi and other satellites, revealed the previously unknown source SGR 0418+5729 13 . Subsequent observations with the Rossi X-ray Timing Explorer (RXTE), Swift, Chandra and X-ray Multi-mirror Mission (XMM) Newton satellites found the new SGR to be an X-ray pulsar with a period of ∼9.1 s and a luminosity of ∼1.6 × 10 34 erg s −1 (in the 0.5-10 keV band and for a distance of 2 kpc) 13,14 . During the three years after the onset of the outburst, the spectrum softened and the luminosity declined by three orders of magnitude, but remained still too high to be powered by rotational energy 9, 10, 14 . The measured spin-down rate of 4 × 10 −15 s s −1 translates (under the assumption of rotating magnetic dipole in vacuo) into a magnetic field B = 6 × 10 12 G at the magnetic equator 9 , a value well in the 1 arXiv:1308.4987v1 [astro-ph.HE] 22 Aug 2013 range of normal radio pulsars. However, the presence of high-order multipolar field components of 10 14 G close to the surface has been invoked to interpret the spectrum of the source in the framework of atmosphere models 12 . In any case, a strong crustal magnetic field (> 10 14 G) seems to be required to explain the overall properties of SGR 0418+5729 within the magnetar model 9, 11 .Hints of the presence of an absorption feature at 2 keV in the spectrum of SGR 0418+5729 were found in the phase-resolved analysis of data (with relatively low-count statistics) from the Swift X-ray Telescope (XRT) taken during 2009 July 12-16 14 . Thanks to the large collecting area and good spectral resolution of the European Photon Imaging Camera (EPIC), we were able to perform a more detailed investigation using data collected by XMM-Newton during a 67-ks long observation performed on 2009 August 12, when the source flux was still high (5 × 10 −12 erg cm −2 s −1 in the 2-10 keV band).To examine the spectral va...

We report on the long-term X-ray monitoring with Swift, RXTE, Suzaku, Chandra, and

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.

We report on the discovery of a new member of the magnetar class, SGR J1935+2154, and on its timing and spectral properties measured by an extensive observational campaign carried out between July 2014 and March 2015 with Chandra and XMM-Newton (11 pointings).We discovered the spin period of SGR J1935+2154 through the detection of coherent pulsations at a period of about 3.24 s. The magnetar is slowing-down at a rate oḟ P =1.43(1)×10 −11 s s −1 and with a decreasing trend due to a negativeP of −3.5(7)×10 −19 s s −2 . This implies a surface dipolar magnetic field strength of ∼2.2×10 14 G, a characteristic age of about 3.6 kyr and, a spin-down luminosity L sd ∼1.7×10 34 erg s −1 . The source spectrum is well modelled by a blackbody with temperature of about 500 eV plus a power-law component with photon index of about 2. The source showed a moderate long-term variability, with a flux decay of about 25% during the first four months since its discovery, and a re-brightening of the same amount during the second four months.The X-ray data were also used to study the source environment. In particular, we discovered a diffuse emission extending on spatial scales from about 1 ′′ up to at least 1 ′ around SGR J1935+2154 both in Chandra and XMM-Newton data. This component is constant in flux (at least within uncertainties) and its spectrum is well modelled by a power-law spectrum steeper than that of the pulsar. Though a scattering halo origin seems to be more probable we cannot exclude that part, or all, of the diffuse emission is due to a pulsar wind nebula.

We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of ten magnetars, including canonical and transient anomalous X-ray pulsars (AXPs), and soft gamma repeaters (SGRs). In this scenario, non-thermal 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 than 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, requires 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 of a few of these sources, we took this further component into account in our modeling not to overlook their 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 magnitudes higher than n GJ , the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities, are also in better agreement with more recent estimates inferred from the fit of single X-ray edges. For the entire sample of observations, we find indications for a correlation between the scattering depth and the electron thermal velocity, and the field strength. Moreover, in most transient anomalous X-ray pulsars the outburst state is characterized by a relatively high surface temperature which cools down during the decay, while the properties of the magnetospheric electrons vary in a different way from source to source. Although the treatment of the magnetospheric scattering used here is only approximated, its successful application to all magnetars we considered shows that the RCS model is capable to catch the main features of the spectra observed below ∼ 10 keV.