Presentation on theme: "Highly magnetized neutron stars (aka magnetars) recently underwent a phase of renewed interest in high-energy astrophysics. These extreme objects comprise."— Presentation transcript:
Highly magnetized neutron stars (aka magnetars) recently underwent a phase of renewed interest in high-energy astrophysics. These extreme objects comprise the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), two classes of sources observationally very similar in many respects (see Mereghetti et al for a review).They are all slow X-ray pulsars with spin periods clustered in a narrow range (P ~ 2-12 s), relatively large period derivatives (dP/dt ~ s/s), spin-down ages of yr, and magnetic fields, as inferred from the classical magnetic dipole spin-down formula, of G, larger than the electron quantum critical field (B qed ~ 4.4x10 13 G). AXPs and SGRs are strong persistent X-ray emitters, with X-ray luminosities of about erg/s. In the keV energy band, magnetars spectra are relatively soft and usually modelled by an absorbed blackbody (kT~ keV) plus a power-law ( ~ 2-4).Thanks to INTEGRAL and RXTE-HEXTE, hard X-ray emission up to ~200 keV has been recently detected for some sources. This discovery opened a new window in magnetars study, making crucial to revise their classification as soft X-ray sources. Through our knowledge of magnetars greatly increased in the last few years, thanks to the large theoretical effort of several groups around the world, a number of important issues still remain to be clarified. In particular, the basic mechanism responsible for the observed spectral shape in X-ray and gamma-ray is still only glimpsed. Different spectral components have been identified through pulse phase spectroscopy, but this was only possible for the brightest sources. Among the most interesting recent results is the discovery of transient, "outbursting" behaviour from a few AXPs, 1E , 1E , and XTE , that underwent an outburst with a flux increase of a factor of XTEJ , for instance, was a former soft and dim (L~ erg/s) thermal emitting Einstein and ROSAT source, which quiescent properties were not dissimilar from those of hundreds uncatalogued ROSAT X-ray sources. This means that AXPs can undergo several outbursts separated by only a few years. Similarly, just a few weeks ago a former quiescent ROSAT source suddenly emitted a series of short bursts (detected by SWIFT) and has been recognized to be a new SGR, SGR These findings imply that a relatively large number of members of this class has not been discovered yet, and some sources which are now quiescents may manifest themselves in future though a similar phenomenology. The AXPs outbursts has proven to be a unique laboratory to monitor the timing and spectral properties of a cooling/decaying AXPs as a function of flux (varied over two orders of magnitude; Gotthelf & Halpern 2005, Israel et al. 2007). However, so far this has only been possible at relatively bright flux levels. And, again, the proper identification of the varying components in the spectrum requires a detailed modelling of the atmosphere and magnetosphere of the star. Only very recently detailed 3D numerical models of magnetospheric emission have been developed by us and other groups (Fernandez & Thompson 2007; Nobili, Turolla & Zane 2008a,b). In particular, our magnetic Monte Carlo 3D radiative code is the only one available that has been used for quantitative data fitting (being implemented in XSPEC) and properly deals with relativistic quantum effects in the scattering cross section. Our code accounts for resonant cyclotron upscattering of soft thermal photons (emitted by the star surface), by a population of relativistic electrons threated in the magnetosphere. Polarization and QED effects are consistently accounted for, as well different configurations for the magnetosphere. Here we present a summary of its capabilities, in connection with future IXO observations. A Resonant cyclotron scattering model for the X-ray emission of magnetars: spectra and polarization properties. S. Zane (MSSL, UCL, UK), L. Nobili, R. Turolla (Univ of Padua, It) on behalf of a larger collaboration 1) Introduction3) Spectra: a few examples 2) Monte Carlo Code Fig.1. Computed spectra for B= G and different values of the colatitude : 27˚ (long dashed), 64˚ (dashed-dotted-dotted-dotted), 90˚ (dashed-dotted), 116˚ (short dashed) and 153˚ (dotted). The solid line is the seed blackbody, units are arbitrary. Note the lack symmetry between the two hemispheres: as increases, spectra become more and more comptonized. This reflects the preferential direction of the currents flow, in this example assumed to be from north to south. 6) Summary Significant progresses have been obtained in Neutron Star physics by current observatories (such as XMM, Chandra, Rossi-XTE, Swift, Integral) and even much stronger constraints are expected to come from next generation detectors with larger collective area, as IXO. The unprecedented capabilities of IXO will allow for the first time to test detailed and self consistent models of atmospheric and magnetospheric emission against data even at low flux levels, ultimately disentangling the spectral components which are at present beyond reach with XMM-Newton or Chandra. The large collective area of IXO will allow for the first time: - to follow the evolution of transient outbursts up to the faintest levels, and to perform complete observation of the post burst cooling history. This is fundamental for probing the neutron star interior through the way in which the heat is deposited and released in the star crust and envelope. - to perform phase-resolved spectroscopy of the faintest sources, mapping with unprecedented detail the different regions of the surface and magnetosphere of the neutron star. We believe the development of detailed theoretical models, as the one presented here, is therefore timely and fundamental. Polarization studies have already started at low energies (IR), and future X-ray polarimeters as those on board IXO will extend them over a broader spectral band. Polarimetry will add a new and unique dimension to the problem, through the knowledge of polarization degree and swing angle. This, together with phase-resolved spectroscopy, will enable us: - to obtain a direct mapping of the emission regions in the magnetosphere and investigate QED effects in spectral formation and polarization pattern; - to disentangle the amount of polarization emerging from different regions (crust, atmosphere, magnetosphere) of/around the neutron star; - to determine the magnetic field of the source, even in absence of pulsations. 4) Application to XMM data 5) Looking at magnetars with Polaroid glasses We follow the idea, proposed by Thompson, Lyutikov and Kulkarni (2002), that most of the magnetars phenomenology can be explained by the onset of a long-lasting magnetospheric twist. Basically, in magnetars the strong toroidal component of the internal magnetic field stresses the star crust inducing a deformation of the surface layers and twisting up the external field. Twisted magnetospheres are permeated by self- induced electric fields that in turn can lift particles from the stellar surface, maintain currents and eventually initiate avalanches of pair cascades. The charge carriers can provide a large optical depth to resonant (cyclotron) scattering and hence reprocess the thermal photons originating from the star surface, giving rise to the typical observed blackbody plus power law spectral shape. In order to test quantitatively this model against X-ray data, we developed a self-consistent physical model of resonant cyclotron scattering, by performing detailed Monte Carlo simulations. We refer for all details to Nobili, Turolla & Zane 2008a,b. In order to model the surface emission, the star surface is divided into patches by an angular grid. Each patch has its own temperature and beaming prescription to reproduce different thermal maps (tests shown here refer to blackbody, isotropic emission). We assume that magnetospheric charges move along the field lines and are characterized by a bulk velocity, bulk, and by a velocity spread. The electron velocity distribution is then a 1D relativistic Maxwellian at a temperature T e and centered at bulk (along the field direction ) plus and a set of Landau levels in transverse direction. In the XSPEC implementation, in order to minimize the number of free parameters, T e has been related to bulk via equi- partition. In all models e - /e + pairs are neglected. We implemented in XSPEC two model tables. The first one is angle- averaged, and spectra depend on 4 parameters: - kT: the BB temperature of the seed surface photons; - bulk : the bulk velocity of the magnetospheric currents - : the twist angle of the magnetosphere (zero for untwisted dipole) - K: a normalization constant. In the second model we introduced two additional parameters, i.e. two viewing angles,,, which account for a disalignement between magnetic field/spin axis with respect to the line of sight. This second model is particularly useful when we need to disentangle details in the light curve or in phase resolved spectra. Just as an example, here we present a few spectral results. Ordinary seed photonsExtraordinary seed photons = 116˚ bulk Fig.2. Left: Computed spectra for B= G and bulk : 0.3, 0.5, 0.7, 0.9. The solid line is the seed blackbody Note the shift in the peak: if electrons are more relativistic Comptonization starts to saturate, and photons fill the Wien peak of the Bose-Einstein distribution the spectrum is not peaked at ~kT, but at ~kTe. Right: This means that for some parameters values we expect double hamped spectra. The first peak is related to the neutron star surface temperature, while the second peak, at higher energy, gives a direct information on the energy of the magnetospheric electrons. This has never been measured so far, possibly due to the scarcity of data taken simultaneously at soft/hard energies. Here B = G, = 0.2. Solid line: KT = 0.1 keV, bulk = 0.7; dashed line: KT = 0.6 keV, bulk = 0.6 Fig.3. Testing QED. Two spectra computed for the same set of parameters, but with (right) and without (left) QED effects and electron recoils accounted for in the scattering cross section (the seed BB is shown for comparison). When QED and relativistic effects are accounted for, the spectrum exhibits a typical break. The break energy moves down (eventually entering the soft X-ray range) as electrons become more and more relativistic. Figs 4: Fits of the XMM- Newton EPIC-pn spectra of three different AXPs, taken ad different epochs (courtesy of G.L. Israel and N. Rea). The model has been already preliminary applied to XMM- Newton data, allowing to probe the properties of the magnetospheric currents. The typical charge densities is found to be ~10 3 times larger than the stanrdard Goldreich-Julian value. This is a strong probe for the presence of magnetic field configurations much more complex than simple dipolar ones. 1E E Since we properly deal with polarized radiative transfer in the magnetosphere, our code will provide not only models for spectra and light curves, but also quantitative predictions for the amount of X-ray polarization. Fig. 5 show the amount of polarization expected as a function of some of the model parameters. We found that, would the seed photons be unpolarised, then the amount of polarization gained by radiation as due to magnetospheric effects is of order of a few tens of percents. If future X-ray polarimeters will measure an excess over this value, this has to be attributed to the radiation emerging from the crust/atmosphere (see Fig. 6) CXOU J Fig.6. Amount of polarization expected from a thin atmospheric layer in radiative equilibrium, computed using the atmospheric models in Zane at al, Different curves are for different values of magnetic field and effective temperature. The fraction of polarization strongly depends on the energy band and shows a variety of different behaviors. Its sign is determined by the competition between plasma and vacuum properties in the photospheric layers. However, independently of the model parameters, the degree of polarization crosses zero at the very vicinity of the proton cyclotron energy, where the mode absorption coefficients of the two modes (O-X) cross each other. This means that measures of polarization can correctly identify a proton cyclotron line (against an electron cyclotron one), ultimately providing a powerful tool for determining the magnetic field of the source, even in the absence of pulsations. Fig.5. Degree of polarization expected for B= G and KT = 0.5 keV, as a function of bulk and of the twist angle. The polarization fraction is energy-integrated, and is integrated over all viewing angles at infinity. Solid line: ordinary seed photons; dotted line: extraordinary seed photons; dashed line: unpolarized seed photons.