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Magnetars are believed to host the strongest magnetic fields in the present universe ($Bgtrsim10^{14}$ G) and the study of their persistent emission in the X-ray band offers an unprecendented opportunity to gain insight into physical processes in the presence of ultra-strong magnetic fields. Up to now, most of our knowledge about magnetar sources came from spectral analysis, which allowed to test the resonant Compton scattering scenario and to probe the structure of the star magnetosphere. On the other hand, radiation emitted from magnetar surface is expected to be strongly polarized and its observed polarization pattern bears the imprint of both scatterings onto magnetospheric charges and QED effects as it propagates in the magnetized vacuum around the star. X-ray polarimeters scheduled to fly in the next years will finally allow to exploit the wealth of information stored in the polarization observables. Here we revisit the problem of assessing the spectro-polarimetric properties of magnetar persistent emission. At variance with previous investigations, proper account for more physical surface emission models is made by considering either a condensed surface or a magnetized atmosphere. Results are used to simulate polarimetric observations with the forthcoming Imaging X-ray Polarimetry Explorer (IXPE). We find that X-ray polarimetry will allow to detect QED vacuum effects for all the emission models we considered and to discriminate among them.
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Within the magnetar scenario, the twisted magnetosphere model appears very promising in explaining the persistent X-ray emission from the Soft Gamma Repeaters and the Anomalous X-ray Pulsars (SGRs and AXPs). In the first two papers of the series, we have presented a 3D Monte Carlo code for solving radiation transport as soft, thermal photons emitted by the star surface are resonantly upscattered by the magnetospheric particles. A spectral model archive has been generated and implemented in XSPEC. Here we report on the systematic application of our spectral model to different XMM-Newton and Integral observations of SGRs and AXPs. We find that the synthetic spectra provide a very good fit to the data for the nearly all the source (and source states) we have analyzed.
The anomalous X-ray pulsars and soft gamma-repeaters are peculiar high-energy sources believed to host a magnetar, i.e. an ultra-magnetized neutron star. Their persistent, soft X-ray emission (~1-10 keV)is usually modeled by the superposition of a blackbody and a power-law tail. It has been suggested that this spectrum forms as the thermal photons emitted by the star surface traverse the magnetosphere. Magnetar magnetospheres are 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 super-strong interior field. In turn, the magnetosphere will be permeated by currents that can boost primary photons through repeated scatterings. Here we present 3D Monte Carlo simulations of photon propagation in a twisted magnetosphere. Our model is based on a simplified treatment of the charge carriers velocity distribution which, however, accounts for the particle collective motion, in addition to the thermal one. Present treatment is restricted to conservative (Thomson) scattering in the electron rest frame. The code, nonetheless, is completely general and inclusion of the relativistic QED resonant cross section, which is required in the modeling 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. 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 simultaneous radio and X-ray observations of the radio-emitting magnetar 1E1547.0-5408 on 2009 January 25 and February 3, with the 64-m Parkes radio telescope and the Chandra and XMM-Newton X-ray observatories. The magnetar was observed in a period of intense X-ray bursting activity and enhanced X-ray emission. We report here on the detection of two radio bursts from 1E1547.0-5408, reminiscent of Fast Radio Bursts (FRBs). One of the radio bursts was anticipated by ~1s (about half a rotation period of the pulsar) by a bright SGR-like X-ray burst, resulting in a F_radio/F_X ~ 10^-9. Radio pulsations were not detected during the observation showing the FRB-like radio bursts, while they were detected in the previous radio observation. We also found that the two radio bursts are neither aligned with the latter radio pulsations nor with the peak of the X-ray pulse profile (phase shift of ~0.2). Comparing the luminosity of these FRB-like bursts and those reported from SGR1935+2154, we find that the wide range in radio efficiency and/or luminosity of magnetar bursts in the Galaxy may bridge the gap between ordinary pulsar radio bursts and the extragalactic FRB phenomenon.
Magnetars are young, rotating neutron stars that possess larger magnetic fields ($B$ $approx$ $10^{13}$-$10^{15}$ G) and longer rotational periods ($P$ $approx$ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetars 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars.
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 gamma-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 bealready present at B ~5E12 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.
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