1,721,485 research outputs found
Proton cyclotron features in thermal spectra of ultramagnetized neutron stars
No full textA great deal of interest has been recently raised in connection with the possibility that soft gamma -ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain magnetars, young neutron stars endowed with magnetic fields greater than or similar to 10(14) G. In this paper we calculate thermal spectra from ultramagnetized neutron stars for values of the luminosity and magnetic field believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very close to a blackbody at the star effective temperature and exhibit a distinctive absorption feature at the proton cyclotron energy E-c,E-p similar or equal to 0.63(B/10(14) G) keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad (DeltaE/E similar to0.05-0.2). The detection of the PCFs is well within the capabilities of present X-ray spectrometers, like the HETGS and METGS on board Chandra. Their observation might provide decisive evidence in favor of the existence of magnetars
Bare quark stars or naked neutron stars? The case of RX J1856.5-3754
Full text linkIn a cool neutron star (T less than or similar to 10(6) K) endowed with a rather highmagnetic field (B greater than or similar to 10(13) G), a phase transition may occur in the outermost layers. As a consequence, the neutron star becomes "bare,'' i.e., no gaseous atmosphere sits on the top of the crust. The surface of a cooling, bare neutron star does not necessarily emit a blackbody spectrum because the emissivity is strongly suppressed at energies below the electron plasma frequency, omega(p). Since omega(p) approximate to 1 keV under the conditions typical of the dense electron gas in the condensate, the emission from a T similar to 100 eV bare neutron star will be substantially depressed with respect to that of a perfect Planckian radiator atmost energies. Here we present a detailed analysis of the emission properties of a bare neutron star. In particular, we derive the surface emissivity for an Fe composition in a range of magnetic fields and temperatures representative of cooling isolated neutron stars, like RX J1856.5 - 3754. We find that the emitted spectrum is strongly dependent on the electron conductivity in the solid surface layers. In the cold electron gas approximation ( no electron-lattice interactions), the spectrum turns out to be a featureless depressed blackbody in the 0.1 - 2 keV band with a steeper low-energy distribution. When damping effects due to collisions between electrons and the ion lattice ( mainly due to electron-phonon interactions) are accounted for, the spectrum is more depressed at low energies and spectral features may be present, depending on the magnetic field strength. Details of the emitted spectrum are found, however, to be strongly dependent on the assumed treatment of the transition from the external vacuum to the metallic surface. The implications of our results for RX J1856.5 - 3754 and other isolated neutron stars are discussed
Unveiling the Thermal and Magnetic Map of Neutron Star Surfaces through their X-ray Emission: Method and Lightcurves Analysis
No full textRecent Chandra and XMM-Newton observations of a number of X-ray `dim' pulsating neutron stars have revealed quite unexpected features in the emission from these sources. Their soft thermal spectrum, believed to originate directly from the star surface, shows evidence for a phase-varying absorption line at some hundred eVs. The pulse modulation is relatively large (pulsed fractions in the range ~12-35 per cent), the pulse shape is often non-sinusoidal, and the hard X-ray colour appears to be anticorrelated in phase with the total emission. Moreover, the prototype of this class, RX J0720.4-3125, has been found to undergo rather sensible changes in both its spectral and timing properties over a time-scale of a few years. All these new findings seem difficult to reconcile with the standard picture of a cooling neutron star endowed with a purely dipolar magnetic field, at least if surface emission is produced in an atmosphere on top of the crust. In this paper we explore how a dipolar+quadrupolar star-centred field influences the properties of the observed light curves. The phase-resolved spectrum has been evaluated accounting for both radiative transfer in a magnetized atmosphere and general relativistic ray-bending. We computed over 78000 light curves, varying the quadrupolar components and the viewing geometry. A comparison of the data with our model indicates that higher-order multipoles are required to reproduce the observations
Power-law Tails from Dynamical Comptonization in Converging Flows
No full textThe effects of bulk motion Comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semianalytical, diffusion approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates when photon diffusion holds. Our semianalytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method, we find that although the power-law tail at extremely large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Γ=3 at large accretion rates, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low (~10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Γ switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Γ<3 is produced by the upscattering of primary photons onto infalling electrons if the central object is a black hole
Isolated Neutrons Stars: From the Surface to the Interior
No full textThis book collect the contributions presented at the conference "Isolated Neutron Stars: from the Surface to the Interior", held in London in April 2006. Forty years after the discovery od radio pulsars it presents an up-to-date description of the new vision of isolated neutron stars that has emerged in recent years with the advance of multi-wavelength observations. The great variety of isolated neutron stars, from pulsars to magnetars, some of them discovered very recently and many of them not detectable at radio wavelengths, is amply covered by descriptions of recent observations results and presentations of the latest theoretical efforts
Hot Atmospheres Around Accreting Neutron Stars: A Possible Source For Hard X-ray Emission
No full textX-ray spectra from magnetar candidates. II Resonant cross sections for electron-photon scattering in the relativistic regime
No full textRecent models of spectral formation in magnetars called renewed attention on electron-photon scattering in the presence of ultrastrong magnetic fields. Investigations presented so far mainly focused 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 keV, 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 × 1012 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
Old Isolated Accreting Neutron Stars: The Diffuse X-ray Emission from the Galactic Center
No full textIs RX J1856.5-3754 a naked neutron star ?
No full textRecent Chandra observations have convincingly shown that the soft X-ray emission from the isolated neutron star candidate RX J1856.5-3754 is featureless and best represented by a blackbody spectrum, in apparent contrast with the predictions of current neutron star atmospheric models. Moreover, the star distance (~120-140 pc) implies a radiation radius of at most ~5-6 km, too small for any neutron star equation of state. Proposed explanations include a reduced X-ray emitting region (a heated polar cap), or the presence of a bare quark/strange star. Here we discuss an alternative possibility. Cool neutron stars (T~1013 G) may be left bare of the gaseous atmosphere by a phase transition in the outermost layers. Computed spectra from bare neutron stars with a surface Fe composition are featureless and virtually indistinguishable from a blackbody in the 0.1-2 keV range. Moreover, owing to the reduced surface emissivity, the star only radiates ~30-50% of the blackbody power and this implies a star radius larger than the radiation radius. Our model can potentially account for the observed X-ray properties of RX J1856.5-3754 and predicts a star radius R∞~10-12 km. The optical emission of RX J1856.5-3754 may be explained by the presence a thin gaseous shell on the top of the Fe condensate
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