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1.
RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N H≈ 1 × 1020 cm−2  and an emitting area with   R ∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ∞≈ 4.3 × 105 K  , gravitational redshift   z g ∼ 0.22  , atmospheric hydrogen column   y H≈ 1 g cm−2  , and magnetic field   B ≈ (3–4) × 1012 G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.  相似文献   

2.
A plasma emission model is presented interpreting the observational properties of RX J1856.5-3754. In particular, on the basis of the Vlasov’s kinetic equation we study the process of the quasi-linear diffusion (QLD) developed by means of the cyclotron instability. This mechanism provides simultaneous generation of optical and X-ray emission on the light cylinder scales, in one location of the pulsar magnetosphere. It is assumed that the observed X-ray spectrum of this source is generated via the synchrotron mechanism. A different approach of the synchrotron theory is considered, giving a spectral energy distribution that is in a good agreement with the observational data.  相似文献   

3.
We present Very Large Telescope (VLT) low-resolution spectroscopy of the neutron star X-ray transient XTE J2123−058 during its quiescent state. Our data reveal the presence of a K7V companion which contributes 77 per cent to the total flux at λ 6300 and orbits the neutron star at     . Contrary to other soft X-ray transients (SXTs), the H α emission is almost exactly in antiphase with the velocity curve of the optical companion. Using the light-centre technique we obtain     and hence     This, combined with a previous determination of the inclination angle     yields     and     . M 2 agrees well with the observed spectral type. Doppler tomography of the H α emission shows a non-symmetric accretion disc distribution mimicking that seen in SW Sex stars. Although we find a large systemic velocity of −     this value is consistent with the galactic rotation velocity at the position of J2123−058, and hence a halo origin. The formation scenario of J2123−058 is still unresolved.  相似文献   

4.
RX J0720.4–3125 has recently been identified as a pulsating soft X-ray source in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well characterized by a blackbody with a temperature of 8 × 105 K. We propose that the radiation from this object is thermal emission from a cooling neutron star. For this blackbody temperature we can obtain a robust estimate of the object's age of ∼ 3 × 105 yr, yielding a polar field ∼ 1014 G for magnetic dipole spin-down and a value of P compatible with current observations.  相似文献   

5.
We present the results of a 22.5 ks pointed ROSAT PSPC observation of the 3.4-h period eclipsing polar MN Hya (RX J0929.1−2404). The X-ray light curve exhibits a 'double-humped' shape, with a secondary minimum occuring at φ∼ 0.45, a morphology consistent with two-pole accretion. Strong aperiodic flaring activity, with flux enhancements of ∼ 6 × the quiescent level, is also observed. A pre-eclipse 'dip' occurs in the phase interval φ= 0.87–0.95 with the X-rays becoming harder, indicative of photoelectric absorption by the pre-shock flow. There is also evidence of a secondary spectrally hard 'dip' near φ = 0.45–0.55, which might be associated with a second accretion stream flowing to the other magnetic pole.   The X-ray spectrum is best represented by a combination of a ∼50 eV blackbody and a thermal bremsstrahlung component of kT 1.6 keV, with a total absorption column of N H  = 2.9 × 1020 cm−2.   The primary maximum (φ∼ 0.65) has a slightly larger column and normalization compared to the secondary maximum. Although there are few photons, the dip spectrum is very flat in comparison to other phases, and is best represented by a single bremsstrahlung component. This is indicative of the spectral hardening seen in the light curves attributed to photoabsorption. The ratio of unabsorbed bremsstrahlung and blackbody luminosities is ∼ 0.1 for the best-fitting average spectral models. This implies a magnetic field strength  30 MG on the basis of the empirical L hard/ L soft −  B relationships, although consideration of the cyclotron flux and aspect effects could allow for an even higher field (55 MG).  相似文献   

6.
We present the results of high-resolution (1–0.4 Å) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I  −  K  > 3.5 ( M bol ≳ 12.8) and those of more massive M dwarfs ( M bol ≈ 7–10). A spectral atlas at high (0.4-Å) resolution for M8–M9+ stars is provided, and the equivalent widths of Cs  I , Rb  I and Hα lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of 'inactive, rapid rotators'. Such objects seem to be common at and below the hydrogen-burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism that heats the chromosphere, or the mechanism that generates magnetic fields, is greatly suppressed.  相似文献   

7.
Resonant cyclotron scattering(RCS)in pulsar magnetospheres is considered.The photon diffusion equation(Kompaneets equation)for RCS is derived.The photon system is modeled three dimensionally.Numerical calculations show that there exist not only up scattering but also down scattering of RCS,depending on the parameter space.RCS's possible applications to spectral energy distributions of magnetar candidates and radio quiet isolated neutron stars(INSs)are pointed out.The optical/UV excess of INSs may be caused by the down scattering of RCS.The calculations for RX J1856.5-3754 and RX J0720.4-3125 are presented and compared with their observational data.In our model,the INSs are proposed to be normal neutron stars,although the quark star hypothesis is still possible.The low pulsation amplitude of INSs is a natural consequence in the RCS model.  相似文献   

8.
IGR J18483−0311 was discovered with INTEGRAL in 2003 and later classified as a supergiant fast X-ray transient. It was observed in outburst many times, but its quiescent state is still poorly known. Here, we present the results of XMM–Newton , Swift and Chandra observations of IGR J18483−0311. These data improved the X-ray position of the source, and provided new information on the timing and spectral properties of IGR J18483−0311 in quiescence. We report the detection of pulsations in the quiescent X-ray emission of this source, and give for the first time a measurement of the spin-period derivative of this source. In IGR J18483−0311, the measured spin-period derivative of  −(1.3 ± 0.3) × 10−9 s s−1  likely results from light travel time effects in the binary. We compare the most recent observational results of IGR J18483−0311 and SAX J1818.6−1703, the two supergiant fast X-ray transients for which a similar orbital period has been measured.  相似文献   

9.
The radiation spectra of supersoft X-ray sources based on a model for hydrogen burning on the white dwarf surface are investigated by solving hydrostatic equilibrium, radiative equilibrium, statistical equilibrium and radiative transfer self-consistently for various sets of mass, radius and luminosity. It is found that the radiation spectrum shows many bound–free emission/absorption features and greatly deviates from the blackbody spectrum at the effective temperature. By the effect of incoherent Compton scattering, the bound–free emission/absorption features do not appear in strong emission/absorption edges, as predicted by coherent models without Compton scattering, but appear as weak humps and relatively shallow absorption edges. The difference between the incoherent model and the coherent model is prominent for L 0.5 L Edd. A calculated spectrum is fitted to the ASCA observations of RX J0925.7−4758. It is found that the entire spectrum of RX J0925.7−4758 cannot be reproduced by model atmospheres with any parameter sets. This suggests that the observed spectrum consists of two or more components. In this case, the atmospheric component is explained by the emission from a white dwarf near the Chandrasekhar limit (∼1.4 M⊙) with L ∼0.2 L Edd at a distance of 16–20 kpc.  相似文献   

10.
Optical spectroscopy of CPD −59° 2635, one of the O-type stars in the open cluster Trumpler 16 in the Carina Nebula, reveals this star to be a double-lined binary system. We have obtained the first radial velocity orbit for this system, consisting of a circular solution with a period of 2.2999 d and semi-amplitudes of 208 and 273 km s−1. This results in minimum masses of 15 and 11 M⊙ for the binary components of CPD −59° 2635, which we classified as O8V and O9.5V, although spectral type variations of the order of 1 subclass, which we identify as the Struve–Sahade effect , seem to be present in both components. From ROSAT HRI observations of CPD −59° 2635 we determine a luminosity ratio log( L x/ L bol)≈−7 , which is similar to that observed for other O-type stars in the Carina Nebula region. No evidence of light variations is present in the available optical or X-ray data sets.  相似文献   

11.
We report the discovery of WASP-10b, a new transiting extrasolar planet (ESP) discovered by the Wide Angle Search for Planets (WASP) Consortium and confirmed using Nordic Optical Telescope FIbre-fed Echelle Spectrograph and SOPHIE radial velocity data. A 3.09-d period, 29 mmag transit depth and 2.36 h duration are derived for WASP-10b using WASP and high-precision photometric observations. Simultaneous fitting to the photometric and radial velocity data using a Markov Chain Monte Carlo procedure leads to a planet radius of  1.28 R J   , a mass of  2.96 M J   and eccentricity of ≈0.06. WASP-10b is one of the more massive transiting ESPs, and we compare its characteristics to the current sample of transiting ESP, where there is currently little information for masses greater than ≈  2 M J   and non-zero eccentricities. WASP-10's host star, GSC 2752−00114 (USNO-B1.0 1214−0586164) is among the fainter stars in the WASP sample, with   V = 12.7  and a spectral type of K5. This result shows promise for future late-type dwarf star surveys.  相似文献   

12.
We report the discovery of a new AM Herculis binary (polar) as the optical counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT all-sky survey. The magnetic nature of this V  ∼ 17 mag object is confirmed by low-resolution spectroscopy showing strong Balmer and He  II emission lines superimposed on a blue continuum, which is deeply modulated by cyclotron humps. The inferred magnetic field strength is 50 ± 4 MG (or possibly even ≈ 70 MG). Photometric observations spanning ∼ 3 yr reveal a period of 119.9 min, directly below the period gap. The morphology of the optical and X-ray light curves, which do not show eclipses by the secondary star, suggests a self-eclipsing geometry. We derive a lower limit on the distance of d  ≳ 250 pc.  相似文献   

13.
The accretion-induced neutron star (NS) magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar-field strength, and as a result the equatorial field strength increases, which is buried inside the crust on account of the accretion-induced global compression of star crust. The main conclusions of model are as follows: (i) the polar field decays with increase in the accreted mass; (ii) the bottom magnetic field strength of about 108 G can occur when the NS magnetosphere radius approaches the star radius, and it depends on the accretion rate as     ; and (iii) the NS magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting  ∼0.01 M⊙  , which implies that the magnetosphere radii of NSs in low-mass X-ray binaries would be homogeneous if they accreted the comparable masses. As an extension, the physical effects of the possible strong magnetic zone in the X-ray NSs and recycled pulsars are discussed. Moreover, the strong magnetic fields in the binary pulsars PSR 1831−00 and PSR 1718−19 after accreting about  0.5 M⊙  in the binary-accretion phase,  8.7 × 1010  and  1.28 × 1012 G  , respectively, can be explained through considering the incomplete frozen flow in the polar zone. As an expectation of the model, the existence of the low magnetic field  (∼3 × 107 G)  NSs or millisecond pulsars is suggested.  相似文献   

14.
We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is  1.1+2.3−0.7× 10−5 pc−3  . We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as  2 × 10−4 pc−3  (as has been predicted theoretically), the vast majority of CVs must be fainter than   L X≃ 2 × 1029 erg s−1  .  相似文献   

15.
We present an analysis of the secular variability of the longitudinal magnetic field B e in the roAp star γ Equ (HD 201601). Measurements of the stellar magnetic field B e were mostly compiled from the literature, and we appended also our 33 new B e measurements which were obtained with the 1-m optical telescope of the Special Astrophysical Observatory (Russia). All the available data cover the time period of 58 yr, and include both phases of the maximum and minimum B e. We determined that the period of the long-term magnetic B e variations equals  91.1 ± 3.6 yr  , with   B e(max) =+577 ± 31 G  and   B e(min) =−1101 ± 31 G  .  相似文献   

16.
The cooling history of a quark star in the colour superconductive phase is investigated. Here we specifically focus on the two-flavour colour (2SC) phase where the novel process of photon generation via glueball (GLB) decay has already been investigated. The picture we present here can, in principle, be generalized to quark stars entering a superconductive phase where similar photon generation mechanisms are at play. As much as 1045–1047 erg of energy is provided by the GLB decay in the 2SC phase. The generated photons slowly diffuse out of the quark star, keeping it hot and radiating as a blackbody (with possibly a Wien spectrum in gamma-rays) for millions of years. We discuss hot radio-quiet isolated neutron stars in our picture (such as RX J185635–3754 and RX J0720.4–3125) and argue that their nearly blackbody spectra (with a few broad features) and their remarkably tiny hydrogen atmosphere are indications that these might be quark stars in the colour superconductive phase where some sort of photon generation mechanism (reminiscent of the GLB decay) has taken place. Fits to observed data of cooling compact stars favour models with superconductive gaps of  Δ2SC∼ 15–35 MeV  and densities  ρ2SC= (2.5–3.0) ×ρN  (ρN being the nuclear matter saturation density) for quark matter in the 2SC phase. If correct, our model combined with more observations of isolated compact stars could provide vital information to studies of quark matter and its exotic phases.  相似文献   

17.
We present 5-, 8-, and 15-GHz total intensity and polarimetric observations of the radio source PKS 2322−123 taken with the Very Large Array (VLA). This small (11 kpc) source is located at the centre of the cooling-core cluster Abell 2597. The inner X-ray structure, the radio morphology and the steep spectral index  (α=−1.8)  in the lobes all suggest that the radio emission is confined by the ambient X-ray gas. We detect a small region of polarized flux in the southern lobe and are able to calculate a Faraday rotation measure (RM) of 3620 rad m−2 over this region. Based on these observations and Chandra X-ray data, we suggest that the southern lobe has been deflected from its original south-western orientation to the south and into our line of sight. Using the observed rotation measures (RMs) and our calculated electron density profiles, and assuming both a uniform and tangled magnetic field topology, we estimate a lower limit of the line-of-sight cluster magnetic field,   B ∥= 2.1  μG  .  相似文献   

18.
We present timing measurements, astrometry, and high-resolution spectra of a number of nearby, thermally emitting, isolated neutron stars. We use these to infer magnetic field strengths and distances, but also encounter a number of puzzles. We discuss three specific ones in detail: (i) For RX J0720.4-3125 and RX J1308.6+2127, the characteristic ages are in excess of 1 Myr, while their temperatures and kinematic ages indicate that they are much younger; (ii) For RX J1856.5-3754, the brightness temperature for the optical emission is in excess of that measured at X-ray wavelengths for reasonable neutron-star radii; (iii) For RX J0720.4-3125, the spectrum changed from an initially featureless state to one with an absorption feature, yet there was only a relatively small change in T eff. Furthermore, we attempt to see whether the spectra of all seven sources, in six of which absorption features have now been found, can be understood in the context of strongly magnetised hydrogen atmospheres. We find that the energies of the absorption features can be reproduced, but that it remains puzzling that, for J0720.4-3125 specifically, the spectrum was featureless in one state, and that, generally, the spectra do not have high-energy tails that are harder than the Wien-like ones obseved.   相似文献   

19.
The timing properties of the 4.45 s pulsar in the Be X-ray binary system GRO J1750−27 are examined using hard X-ray data from INTEGRAL and Swift during a type II outburst observed during 2008. The orbital parameters of the system are measured and agree well with those found during the last known outburst of the system in 1995. Correcting the effects of the Doppler shifting of the period, due to the orbital motion of the pulsar, leads to the detection of an intrinsic spin-up that is well described by a simple model including     and     terms of  −7.5 × 10−10 s s−1  and  1 × 10−16 s s−2  , respectively. The model is then used to compare the time-resolved variation of the X-ray flux and intrinsic spin-up against the accretion torque model of Ghosh & Lamb; this finds that GRO J1750−27 is likely located 12–22 kpc distant and that the surface magnetic field of the neutron star is  ∼2 × 1012  G. The shape of the pulse and the pulsed fraction shows different behaviour above and below 20 keV, indicating that the observed pulsations are the convolution of many complex components.  相似文献   

20.
RX J1856.5–3754 is one of the brightest nearby isolated neutron stars, and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5–3754, within the uncertainties. In our simplest model, the best-fit parameters are an interstellar column density N H≈1×1020 cm?2 and an emitting area with R ∞≈17 km (assuming a distance to RX J1856.5–3754 of 140 pc), temperature T ∞≈4.3×105 K, gravitational redshift z g ～0.22, atmospheric hydrogen column y H≈1 g cm?2, and magnetic field B≈(3–4)×1012 G; the values for the temperature and magnetic field indicate an effective average over the surface.  相似文献   

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