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تسجيل مستخدم جديدThe birthplace and age of the isolated neutron star RX J1856.5-3754
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R. P. Mignani
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X-ray observations unveiled various types of radio-silent Isolated Neutron Stars (INSs), phenomenologically very diverse, e.g. the Myr old X-ray Dim INS (XDINSs) and the kyr old magnetars. Although their phenomenology is much diverse, the similar periods (P=2--10 s) and magnetic fields (~10^{14} G) suggest that XDINSs are evolved magnetars, possibly born from similar populations of supermassive stars. One way to test this hypothesis is to identify their parental star clusters by extrapolating backward the neutron star velocity vector in the Galactic potential. By using the information on the age and space velocity of the XDINS RX J1856.5-3754, we computed backwards its orbit in the Galactic potential and searched for its parental stellar cluster by means of a closest approach criterion. We found a very likely association with the Upper Scorpius OB association, for a neutron star age of 0.42+/-0.08 Myr, a radial velocity V_r^NS =67+/- 13$ km s^{-1}, and a present-time parallactic distance d_pi^NS = 123^{+11}_{-15} pc. Our result confirms that the true neutron star age is much lower than the spin-down age (tau_{sd}=3.8 Myrs), and is in good agreement with the cooling age, as computed within standard cooling scenarios. The mismatch between the spin-down and the dynamical/cooling age would require either an anomalously large breaking index (n~20) or a decaying magnetic field with initial value B_0 ~ 10^{14} G. Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB association and the masses of its members we cannot yet draw firm conclusions on the estimated mass of the RX J1856.5-3754 progenitor.
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RX J1856.5-3754 is the X-ray brightest among the nearby isolated neutron stars. Its X-ray spectrum is thermal, and is reproduced remarkably well by a black-body, but its interpretation has remained puzzling. One reason is that the source did not exhi
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ources is expected to be substantially polarised, independently on the mechanism actually responsible for the thermal emission. A large observed polarisation degree is, however, expected only if quantum-electrodynamics (QED) polarisation effects are present in the magnetised vacuum around the star. The detection of a strongly linearly polarised signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarisation measurements in the soft X-rays are not feasible yet, optical polarisation measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes $approx 26$--$28$. Here, we report on the measurement of optical linear polarisation for the prototype, and brightest member, of the class, RX, J1856.5$-$3754 ($Vsim 25.5$), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a polarisation degree $mathrm{P.D.} =16.43% pm5.26%$ and a polarisation position angle $mathrm{P.A.}=145fdg39pm9fdg44$, computed east of the North Celestial Meridian. The $mathrm{P.D.}$ that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED.
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d 63 eV) blackbody model. Fitting ten sets of the data from {sl Suzaku} XIS0, XIS1, XIS3 and {sl XMM-Newton} EPIC-pn with the two-temperature blackbody model, we discover an excess emission, 16--26% in 0.8--1.2,keV. We examine possible causes of this keV-X-ray excess; uncertainty in the background, pile up of the low energy photons and confusion of other sources. None of them succeeds in explaining the keV-X-ray excess observed with different instruments. We thus consider this keV-X-ray excess is most likely originated in RX J1856.5$-$3754. However, it is difficult to constrain the spectral shape of the keV-X-ray excess. The third blackbody component with $kT^infty = 137^{+18}_{-14}$ eV, additional power-law component with a photon index $Gamma = 3.4^{+0.5}_{-0.6}$, or Comptonization of blackbody seed photons into power-law with a photon index $Gamma_c = 4.3^{+0.8}_{-0.8}$ can reproduce the keV-X-ray excess. We also search for the periodicity of 0.8--1.2,keV data, since 7.055 s pulsation is discovered from 0.15--1.2,keV band in the XMM Newton EPIC-pn data ($sim$1.5%). We only obtain the upper limit of pulsed fraction $<$ 3% in the keV-X-ray excess. We shortly discuss the possible origin of the keV-X-ray excess, such as synchrotron radiation and Comptonization of blackbody photons.
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