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Register a new userTo Understand the UV-optical Excess of RX J1856.5-3754
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Y. L. Yue
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The enigma source, RX J1856.5-3754, is one of the so-called dim thermal neutron stars. Two puzzles of RXJ1856.5-3754 exist: (1) the observational X-ray spectrum is completely featureless; (2) the UV-optical intensity is about seven times larger than that given by the continuation of the blackbody model yielded by the X-ray data. Both the puzzles would not exist anymore if RX J1856.5-3754 is a low mass bare strange quark star, which is in a propeller phase with a low accretion rate. A boundary layer of RX J1856.5-3754 is suggested and modelled, from which the UV-optical emission is radiated. Free-free absorption dominates the opacity of the boundary layer, which results in the opacity to be high in UV-optical but low in X-ray bands. The stars magnetic field, spin period, as well as the accretion rate are constrained by observations.
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RX J1856.5$-$3754 is the brightest and nearest ($sim 120$ pc) source among thermally emitting isolated neutron stars. Its spectra observed with {sl XMM-Newton} and {sl Chandra} satellites are well-fitted with the two-temperature ($kT^infty sim$ 32 and 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.
We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5--3754 using the LABOCA bolometer array on the Atacama Pathfinder Experiment (APEX) Telescope. No cold dust continuum emission peak at the position of RX J1856.5--3754 was detected. The 3 sigma flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new submillimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated-disc model, we obtain a mass accretion limit of dM/dt less than 10^{14} g/s, and a maximum outer disc radius of around 10^{14} cm. By examining the projected proper motion of RX J1856.5--3754, we speculate about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago.
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 exhibit pulsations, and hence a magnetic field strength--vital input to atmosphere models--could not be estimated. Recently, however, very weak pulsations were discovered. Here, we analyze these in detail, using all available data from the XMM-Newton and Chandra X-ray observatories. From frequency measurements, we set a 2-sigma upper limit to the frequency derivative of dot u<1.3e-14 Hz/s. Trying possible phase-connected timing solutions, we find that one solution is far more likely than the others, and we infer a most probable value of dot u=(-5.98+/-0.14)e-16 Hz/s. The inferred magnetic field strength is 1.5e13 G, comparable to what was found for similar neutron stars. From models, the field seems too strong to be consistent with the absence of spectral features for non-condensed atmospheres. It is sufficiently strong, however, that the surface could be condensed, but only if it is consists of heavy elements like iron. Our measurements imply a characteristic age of about 4 Myr. This is longer than the cooling and kinematic ages, as was found for similar objects, but at almost a factor ten, the discrepancy is more extreme. A puzzle raised by our measurement is that the implied rotational energy loss rate of about 3e30 erg/s is orders of magnitude smaller than what was inferred from the H-alpha nebula surrounding the source.
The evolution of magnetic field in isolated neutron stars is one of the most important ingredients in the attempt to build a unified description of these objects. A prediction of field evolution models is the existence of an equilibrium configuration, in which the Hall cascade vanishes. Recent calculations have explored the field structure in this stage, called the Hall attractor. We use X-ray data of near-by, cooling neutron stars to probe this prediction, as these sources are surmised to be close to or at Hall attractor phase. We show that the source RX J1856.5-3754 might be closer to the attractor than other sources of its class. Our modelling indicates that the properties of surface thermal emission, assuming that the star is in the Hall attractor, are in contradiction with the spectral data of RX J1856.5-3754.
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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