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1.
RX J1856.5−3754 is one of the brightest, nearby isolated neutron stars (NSs), and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere match well the entire spectrum, from X-rays to optical (with best-fitting NS radius   R ≈ 14  km, gravitational redshift   z g∼ 0.2  , and magnetic field   B ≈ 4 × 1012  G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM–Newton observations that uncovered 7-s pulsations at the     level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5−3754, with one angle <6° and the other angle     , though the solutions are not definitive, given the observational and model uncertainties. These angles indicate a close alignment between the rotation and the magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5−3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS .  相似文献   

2.
We discuss the evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of  >5 × 1010 cm−3  . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches  1.5( n /5 × 1010 cm−3)1/2 mG  , while if the core is rotationally dominated the angular velocity approaches  2.57 × 10−3 ( n /5 × 1010 cm−3)1/2 yr−1  , where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds  5 × 1010 cm−3  . Fragmentation of the very dense core and emergence of outflows from fragments will be shown in the subsequent paper.  相似文献   

3.
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   

4.
Theoretical electron density sensitive line ratios   R 1– R 6  of Si  x soft X-ray emission lines are presented. We found that these line ratios are sensitive to electron density n e, and the ratio R 1 is insensitive to electron temperature T e. For reliable determination of the electron density of laboratory and astrophysical plasmas, atomic data, such as electron impact excitation rates, are very important. Our results reveal that the discrepancy of the line ratios from different atomic data calculated with the distorted wave (DW) approximation and the R-matrix method is up to 19 per cent at   n e= 2 × 108 cm−3  . We applied the theoretical intensity ratio R 1 to the Low Energy Transmission Grating Spectrometer (LETGS) spectrum of the solar-like star Procyon. By comparing the observed value (1.29) with the theoretical calculation, the derived electron density n e is  2.6 × 108 cm−3  , which is consistent with that derived from  (C  v < 8.3 × 108 cm−3)  . When the temperature structure of the Procyon corona is taken into account, the derived electron density increases from   n e= 2.6 × 108  to  2.8 × 108 cm−3  .  相似文献   

5.
We present ROSAT High Resolution Imager (HRI) and ASCA observations of the well-known ultraluminous infrared galaxy (ULIRG) IRAS 19254−7245 (the 'Superantennae' ). The object is not detected by ROSAT , implying a 3 σ upper limit of X-ray luminosity L X∼8×1041 erg s−1 in the 0.1–2 keV band. However, we obtain a clear detection by ASCA , yielding a luminosity in the 2–10 keV band of 2×1042 erg s−1. The X-ray spectrum of IRAS 19254−7245 is very hard, equivalent to a photon index of Γ=1.0±0.35. We therefore attempt to model the X-ray data using a 'scatterer' model, in which the intrinsic X-ray emission along our line of sight is obscured by an absorbing screen while some fraction, f , is scattered into our line of sight by an ionized medium; this is the standard model for the X-ray emission in obscured (but non Compton-thick) Seyfert galaxies. We obtain an absorbing column density of N H=2×1023 cm−2 for a power-law photon index of Γ=1.9, an order of magnitude above the column estimated on the basis of optical observations; the percentage of the scattered emission is high (∼20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus ( N H>1024 cm−2) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90 per cent) to the equivalent width of the 6.4 keV line is high (∼3 keV). Overall , the results suggest that most of the X-ray emission originates in a highly obscured Seyfert 2 nucleus.  相似文献   

6.
We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), carried out in a broad X-ray band covering 0.1–300 keV only 15 d after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments which measured a flux F x≃1.8×10−11 erg cm−2 s−1 in the 0.1–10 keV range and a 2 σ upper limit F x<6.7×10−12 erg cm−2 s−1 in the 15–60 keV range. We attribute the emission to shocked nebular ejecta at a plasma temperature kT ≃6 keV . At six months no bright component emerged in the 15–60 keV range, but a bright central supersoft X-ray source appeared. The hot nebular component previously detected had cooled to a plasma temperature kT <1 keV . There was strong intrinsic absorption of the ejecta in the first observation and not in the second, because the column density of neutral hydrogen decreased from N (H)≃1.7×1023 to N (H)≃1021 cm−2 (close to the interstellar value). The unabsorbed X-ray flux also decreased from F x=4.3×10−11 to F x≃10−12 erg cm−2 s−1 .  相似文献   

7.
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   

8.
We have searched for molecular absorption lines at millimetre wavelengths in 11 gravitational lens systems discovered in the JVAS/CLASS surveys of flat spectrum radio sources. Spectra of only one source 1030+074 were obtained in the 3-, 2- and 1.3-mm bands at the frequencies corresponding to common molecular transitions of CO and HCO+ as continuum emission was not found in any of the other sources. We calculated upper limits to the column density in molecular absorption for 1030+074, using an excitation temperature of 15 K, to be N CO<6.3×1013 cm−2 and N HCO+<1.3×1011 cm−2 , equivalent to hydrogen column density of the order N H<1018 cm−2 , assuming standard molecular abundances. We also present the best upper limits of the continuum at the lower frequency for the other 10 gravitational lenses.  相似文献   

9.
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   

10.
We have investigated multiband optical photometric variability and stability of the Hα line profile of the transient X-ray binary IGR J01583+6713. We set an upper limit of 0.05 mag on photometric variations in the V band over a time-scale of three months. The Hα line is found to consist of non-Gaussian profile and quite stable for a duration of two months. We have identified the spectral type of the companion star to be B2 IVe while the distance to the source is estimated to be ∼4.0 kpc. Along with the optical observations, we have also carried out analysis of X-ray data from three short observations of the source, two with the Swift –XRT and one with the RXTE –PCA. We have detected a variation in the absorption column density, from a value of  22.0 × 1022 cm−2  immediately after the outburst down to  2.6 × 1022 cm−2  four months afterwards. In the quiescent state, the X-ray absorption is consistent with the optical reddening measurement of   E ( B − V ) = 1.46  mag. From one of the Swift observations, during which the X-ray intensity was higher, we have a possible pulse detection with a period of 469.2 s. For a Be X-ray binary, this indicates an orbital period in the range of 216–561 d for this binary system.  相似文献   

11.
Coulomb corrections to the equation of state of degenerate matter are usually neglected in high-temperature regimes, owing to the inverse dependence of the plasma coupling constant, Γ, on temperature. However, nuclear statistical equilibrium matter is characterized by a large abundance by mass of large- Z (iron group) nuclei. It is found that Coulomb corrections to the ion ideal gas equation of state of matter in nuclear statistical equilibrium are important at temperatures T ≲5–10×109 K and densities ρ ≳108 g cm−3. At a temperature T =8.5×109 K and a density ρ =8×109 g cm−3, the neutronization rate is larger by ≳28 per cent when Coulomb corrections are included. However, the conductive velocity of a thermonuclear deflagration wave in C–O drops by ∼16 per cent when Coulomb corrections to the heat capacity are taken into account. The implications for SNIa models and nucleosynthesis, and also for the accretion-induced collapse of white dwarfs, are discussed. Particularly relevant is the result that the minimum density for collapse of a white dwarf to a neutron star is shifted down to 5.5–6×109 g cm−3, a value substantially lower than previously thought.  相似文献   

12.
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   

13.
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   

14.
We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for   J ≤ 8  rotational levels, and for the vibrational levels v = 0, 1, 2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of the kinetic temperatures, T k, from 102 to  2 × 104 K  and the number densities, n H, from 1 to  108 cm−3  . We find that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For   n H≳ 105  and   T k∼ 104 K  the cooling function becomes more than an order of magnitude higher than previously reported. We give also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 276 K (redshifts of 10, 30 and 100). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal_Pages/anton/DATA/HD_cooling/HD_cool.html . We discuss the relevance to explore the effects of including our results into models and simulations of galaxy formation, especially in the regime when gas cools down from temperatures above ∼3000 K.  相似文献   

15.
We present a set of gas-phase Planck mean and Rosseland mean opacity tables applicable for simulations of star and planet formation, stellar evolution and disc modelling at various metallicities in hydrogen-rich environments. The tables are calculated for gas temperatures between 1000 and 10 000 K and total hydrogen number densities between 102 and  1017 cm−3  . The carbon-to-oxygen ratio is varied from 0.43 to well above 2.0, the nitrogen-to-oxygen ratio between 0.14 and 100.0. The tables are calculated for a range of metallicities down to  [M/H]'= log  N M/ N H=−7.0  . We demonstrate how the mean opacities and the abundances of the opacity species vary with C/O, N/O and  [M/H]'  . We use the element abundances from Grevesse et al., and we provide additional tables for the oxygen abundance value from Caffau et al. All tables will be available online at http://star-www.st-and.ac.uk/~ch80/datasources.html .  相似文献   

16.
The single glitch observed in PSR B1821−24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller  (Δν/ν∼ 10−11)  than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with   B ≳ 1011 G  and are young, whereas PSR B1821−24 is an old recycled pulsar with a field strength of  2.25 × 109 G  . We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.  相似文献   

17.
We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H2, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H2 and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s−1), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H2 emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H2 v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 103 and 105 cm−3, and stopping column densities N H=1022–1024 cm−2. Assuming azimuthal symmetry, at least 2.3×108 M⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼1010 M⊙. The latter plausibly accounts for 109 yr of mass deposition from the cluster cooling flow, for which within this radius.  相似文献   

18.
We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10−13 erg cm−2 s−1]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 1042 to  1045 erg s−1  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N H > 1022 cm−2)  and unobscured  ( N H < 1022 cm−2)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.  相似文献   

19.
We present XMM–Newton observations of the Seyfert 1 active galactic nucleus (AGN)  H 0557 − 385  . We have conducted a study into the warm absorber present in this source, and using high-resolution Reflection Grating Spectrometer (RGS) data we find that the absorption can be characterized by two phases: a phase with log ionization parameter ξ of 0.50 (where ξ is in units of erg cm s−1) and a column of  0.2 × 1021 cm−2  , and a phase with log ξ of 1.62 and a column of  1.3 × 1022 cm−2  . An iron Kα line is detected. Neutral absorption is also present in the source, and we discuss possible origins for this. On the assumption that the ionized absorbers originate as an outflow from the inner edge of the torus, we use a new method for finding the volume filling factor. Both phases of  H 0557 − 385  have small volume filling factors (≤1 per cent). We also derive the volume filling factors for a sample of 23 AGN using this assumption and for the absorbers with  log ξ > 0.7  , we find reasonable agreement with the filling factors obtained through the alternative method of equating the momentum flow of the absorbers to the momentum loss of the radiation field. By comparing the filling factors obtained by the two methods, we infer that some absorbers with  log ξ < 0.7  occur at significantly larger distances from the nucleus than the inner edge of the torus.  相似文献   

20.
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   

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