X-ray spectra and polarization from magnetar candidates

Taverna, Roberto; Turolla, R.; Suleimanov, V.; Potekhin, A. Y.; Zane, S.

doi:10.1093/mnras/staa204

Cited by 33 publications

(67 citation statements)

References 115 publications

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“…These in-teresting physics elements can be probed using polarimetric measurements of magnetars in the soft X-ray band, which are expected to be provided by future missions like IXPE 1 (Weisskopf et al 2016) and eXTP (Zhang et al 2016). Such polarimetry introduces an extra dimension to diagnostics of source physical properties that complement spectroscopy, and should permit the detection of the QED vacuum effect from magnetar atmospheres (see Taverna et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Polarized radiation transfer in neutron star surface layers

Barchas

Baring

2020

Monthly Notices of the Royal Astronomical Society

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The study of polarized radiation transfer in the highly-magnetized surface locales of neutron stars is of great interest to the understanding of accreting X-ray pulsars, rotation-powered pulsars and magnetars. This paper explores scattering transport in the classical magnetic Thomson domain that is of broad applicability to these neutron star classes. The development of a Monte Carlo simulation for the polarized radiative transfer is detailed: it employs an electric field vector formalism to enable a breadth of utility in relating linear, circular and elliptical polarizations. The simulation can be applied to any neutron star surface locale, and is adaptable to accretion column and magnetospheric problems. Validation of the code for both intensity and Stokes parameter determination is illustrated in a variety of ways. Representative results for emergent polarization signals from surface layers are presented for both polar and equatorial magnetic locales, exhibiting contrasting signatures between the two regions. There is also a strong dependence of these characteristics on the ratio of the frequency ω of a photon to the cyclotron frequency ωB =eB/mc. Polarization signatures for high opacity domains are presented, highlighting compact analytic approximations for the Stokes parameters and anisotropy relative to the local field direction for an extended range of frequencies. These are very useful in defining injection conditions deep in the simulation slab geometries, expediting the generation of emission signals from highly opaque stellar atmospheres. The results are interpreted throughout using the polarization characteristics of the magnetic Thomson differential cross section.

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Section: Introductionmentioning

confidence: 99%

Polarized radiation transfer in neutron star surface layers

Barchas

Baring

2020

Monthly Notices of the Royal Astronomical Society

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“…These satellites will be especially effective in magnetar observations. Complex modern models, such as the one presented in [220], can include details of the surface emission (different atmospheric composition and surface temperature distribution), emission propagation (light-bending), degree of the initial polarisation, different combinations of angles related to orientation of the emitting system with respect to the observer, and finally, many effects related to the magnetic field, including twisted topology, global or local. Some of the ingredients can be determined from analysis of pulse profiles, spectral data, etc.…”

Section: Polarisationmentioning

confidence: 99%

Evolution of Neutron Star Magnetic Fields

2021

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Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects, its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for the interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review, we first briefly describe theoretical models of the formation and evolution of the magnetic field of neutron stars, paying special attention to field decay processes. Then, we present important observational results related to the field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, and sources in binary systems. After that, we discuss which observations can shed light on the obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems.

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“…The polarimet-ric observations will probe the emission mechanism, and will provide new independent constraints on the geometry of the source, removing degeneracies which affect spectral analyses. Furthermore, the observations will allow us to observe the effect of vacuum birefringence, a strong-field QED prediction (Heyl et al 2003;Taverna et al 2015;González Caniulef et al 2016;Taverna et al 2020).…”

Section: Introductionmentioning

confidence: 97%

“…While Xray spectral observations can test some predictions of these models, the spectral information alone is not sufficient to identify the emission model, as they tend to predict similar thermal energy spectra. The models however predict very different polarization properties (see e.g., Suleimanov et al 2009;Potekhin et al 2012;Taverna et al 2020). As a consequence, X-ray polarimetry can definitely probe the nature of magnetar surface layers.…”

Section: Introductionmentioning

confidence: 99%

“…The source is one of the brightest (persistent) magnetar sources (see the McGill online magnetar catalog 1 , Olausen & Kaspi 2014) and is among the highest-priority targets to be observed in the first year of the IXPE science mission. We first compare the energy spectra predicted by the magnetar emission models of Taverna et al (2020) with archival XMM-Newton observations of this source (see Rea et al 2008;Zane et al 2009). We perform for the first time not only a phaseaveraged analysis, but also a phase-resolved analysis.…”

Section: Introductionmentioning

confidence: 99%

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Fitting XMM-Newton Observations of the AXP 1RXS J170849.0-400910 with four magnetar surface emission models, and predictions for X-ray polarization observations with IXPE

Krawczynski,

Taverna,

Turolla

et al. 2021

Preprint

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Context. Phase-resolved spectral and spectropolarimetric X-ray observations of magnetars present us with the opportunity to test models of the origin of the X-ray emission from these objects, and to constrain the properties of the neutron star surface and atmosphere. Aims. Our first aim is to use archival XMM-Newton observations of the magnetar 1RXS J170849.0−400910 to ascertain how well four emission models describe the phase-resolved XMM-Newton energy spectra. Our second aim is to evaluate the scientific potential of future spectropolarimetric observations of 1RXS J170849.0−400910 with the Imaging X-ray Polarimetry Explorer (IXPE) scheduled for launch in late 2021. The most salient questions are whether IXPE is able to distinguish between the different emission models, and whether IXPE can unambiguously detect the signatures of quantum electrodynamics (QED) effects in strong magnetic fields. Methods. We used numerical radiation transport calculations for a large number of different system parameters to predict the X-ray flux and polarization energy spectra of the source 1RXS J170849.0−400910. Based on the numerical results, we developed a new model to fit phase-resolved and phase-averaged X-ray spectral (i.e., XMM-Newton and IXPE) and spectropolarimetric (IXPE) data. In order to test the sensitivity of IXPE to strong-field QED effects, we fit a simulated IXPE observation with two versions of the model, i.e., with and without QED effects accounted for. Results. The fixed-ions condensed surface model gives the best description of the phase-resolved XMM-Newton spectra, followed by the blackbody and free-ions condensed surface models. The magnetized atmosphere model gives a poor description of the data and seems to be largely excluded. Simulations show that the IXPE observations of sources such as 1RXS J170849.0−400910 will allow us to cleanly distinguish between high-polarization (blackbody, magnetized atmosphere) and low-polarization (condensed surface) models. If the blackbody or magnetized atmosphere models apply, IXPE can easily prove QED effects based on ∼200 ksec observations as studied here; longer IXPE observation times will be needed for a clear detection in the case of the condensed surface models. Conclusions. The XMM-Newton data have such a good signal-to-noise ratio that they reveal some limitations of the theoretical models. Notwithstanding this caveat, the fits clearly favor the fixed-ions condensed surface and blackbody models over the free-ions condensed surface and magnetized atmosphere models. The IXPE polarization information will greatly help us to figure out how to improve the models. The first detection of strong-field QED effects in the signal from astrophysical sources seems possible if an adequate amount of time is dedicated to the observations.

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X-ray spectra and polarization from magnetar candidates

Cited by 33 publications

References 115 publications

Polarized radiation transfer in neutron star surface layers

Polarized radiation transfer in neutron star surface layers

Evolution of Neutron Star Magnetic Fields

Fitting XMM-Newton Observations of the AXP 1RXS J170849.0-400910 with four magnetar surface emission models, and predictions for X-ray polarization observations with IXPE

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