Comment on ``Discovery of the absorption features in the X-ray spectrum of an isolated neutron star''
We would like to comment on the paper of discovering two absorption lines (Sanwal et al. 2002) that an unequal discussion was taken about those two potential possibilities in generating the absorption features. The authors’ bias, that the features are associated with atomic transitions of once-ionized helium, rejects the possibility that such a pulsar-like star is a bare strange star (Xu 2002); but we are sorry that this conclusion can not be fully accepted. Whereas, we tend to suggest that the absorptions are two cyclotron lines. Our points aimed at the authors’ arguments are as below. 1, The inferred field value, Be = 3 × 10 12 G, based on P and Ṗ is significantly larger than the field Bce ∼ 10 11 G derived by assuming that the 0.7 keV and 0.7 × 2 keV lines are the fundamental and the first harmonics, respectively. However, it is very likely that the star (1E 1207.4-5209) has a debris disk, which is currently conjectured for interpreting the enigmatic sources of anomalous X-ray pulsars and soft γ−ray repeaters in literatures (e.g., Chatterjee et al. 2000), since it is in the center of a supernova remnant (PKS 1209-51/52). In this case, 1E 1207.4-5209 provides an only possibility for us to investigate the disk property by dividing the total braking rate into magnetodipole and disk ones, since the total rotational energy loss (IΩΩ̇, I the moment of inertia) is the sum of the dipole radiation power and the propeller energy loss Ėd = IΩΩ̇d ≃ −GṀM/Rm cm (Francischelli & Wijers 2002), where M is the mass of center star, Ṁ the accretion rate, and the magnetosphere radius Rm ≃ 2.2 × 10 B 4/7 12 Ṁ . It is worth noting that most of the accretion material driven by viscosity in the fossil disk can’t fall onto the stellar surface because of centrifugal forces acting on the matter, but is ejected by the boundary reaction during the propeller phase. The magnetodipole spin down Ω̇m ≃ −8× 10 −16 s for a pulsar with polar field Bce, and then Ω̇d = Ω̇ − Ω̇m ≃ −7× 10 −13 s. The fossil disk torque dominates. Therefore, the accretion rate Ṁ ≃ 9.1× 10 g/s ∼ 6.4× 10M⊙ per year. We find Rm ≃ 1.6× 10 8 cm, the radius of light cylinder RL ≃ 2×10 9 cm, and the corotating radius RC ≃ 9.5×10 7 cm. These radii are consistent with the propeller requirement: RC . Rm < RL. 2, The 0.7 keV line is not much stronger than the 1.4 keV one. It is true that the oscillator strength of the first harmonic is much smaller than that of the fundamental in the weak field limit, but this does not mean that the absorption-like dips would have significant differences. In fact, the spectrum profile should be calculated by modelling the resonant cyclotron radiation transfer. For instance, even for a field of B = 1.7 × 10 G (in this case, the ratio of the oscillator strength could be ∼ 0.04), those two spectrum lines calculated could be similar in depth (Freeman et al. 1999), depending on radiative geometry. Observationally, the cyclotron absorption of the fundamental is not much stronger than that of the first harmonic (e.g., Trümper et al. 1978, with B ∼ 3×10 G). In addition, the observation does show that the integrated photons absorbed by the fundamental transition are much more than those by the first harmonic transition. It is necessary to develop an elaborate model for computing the line formation in order to know the details of the source. 3, The charge density in the star’s magnetosphere can not be large enough to scatter resonantly the photons from the surface. The main reason, which leads the authors to this conclusion, is that the required electron number density, ne ∼ 10 13 cm, is two orders larger than the Goldreich-Julian density, nGJ ∼ 5 × 10 11 cm. However, although pulsar magnetospheres are not known with certain, it is a common point that primary pairs with Lorentz factor γp ∼ 10 6 and with density ∼ nGJ are accelerated in gaps while more secondary pairs with Lorentz factor γs ∼ 10 2−4 are created outside the gaps (e.g., Ruderman & Sutherland 1975). In despite of that the net charge density could be nGJ, the absolute number density should be ∼ 10 times of nGJ, which should be enough to effectively scatter the photons from the stellar surface. Actually, this could be another possibility for cyclotron absorptions in the magnetosphere even if no fossil disk contributes a braking torque. 4, The atomic transition of once-ionized helium may be responsible to these lines, although the authors had not presented a full discussion in this possibility. However, in this interpretation, they assume a general field with super strong strength, B ≃ 1.5×10 G, which is much larger than the derived field Be = 3×10 12 G. It is hard to understand that this “neutron” star with a typical field Be can have such a strong prevalent multi-field on the surface. Strange stars could exist; the exotic surface of a bare strange star might eventually result in the identification of them, especially the most probable one RX J1856 (e.g., Drake et al. 2002, Xu 2002). Although each of the observed phenomena from pulsar-like stars may be interpreted under the regime of traditional neutron star with unusual or artificial physical properties, it could be a natural way to understand the observations by updating “neutron” stars with (bare) strange stars (Xu 2002).

title: PSR B1257+12: a quark star with planets?
abstract: A recent observation has shown that PSR B1257+12 could have quite small X‐ray emitting area, only about 2000 m2, which is more than three orders smaller than the canonical polar cap size. We suggest here that PSR B1257+12 could be a low‐mass quark star with radius of R≃0.6 km and mass of M≃3×10−4 M⊙. Such a low‐mass quark star system may form in an accretion induced collapse process or a collision process of two quark stars.

title: The Extremely Long-period X-Ray Source in a Young Supernova Remnant: A Thorne-Żytkow Object Descendant?
abstract: The origin of the 6.67 hr period X-ray source, 1E161348-5055, in the young supernova remnant RCW 103 is puzzling. We propose that it may be the descendant of a Thorne-Żytkow Object (TŻO). A TŻO may at its formation have a rapidly spinning neutron star as a core and a slowly rotating envelope. We found that the core could be braked quickly to an extremely long spin period by the coupling between its magnetic field and the envelope, and that the envelope could be disrupted by some powerful bursts or exhausted via stellar wind. If the envelope is disrupted after the core has spun down, the core will become an extremely long-period compact object, with a slow proper motion speed, surrounded by a supernova-remnant-like shell. These features all agree with the observations of 1E161348-5055. TŻOs are expected to have produced extraordinarily high abundances of lithium and rapid proton process elements that would remain in the remnants and could be used to test this scenario.

title: The state of cold quark matter: a model-independent view
abstract: From a model-independent point of view, we address the possibility that quark clustering could occur in cold quark matter at realistic baryon densities because of the likely strong coupling between quarks in compact stars.

title: H-cluster stars
abstract: The study of dense matter at ultra-high density has a very long history, which is meaningful for us to understand not only cosmic events in extreme circumstances but also fundamental laws of physics. It is well known that the state of cold matter at supra-nuclear density depends on the non-perturbative nature of quantum chromo-dynamics (QCD) and is essential for modeling pulsars. A so-called H-cluster matter is proposed in this paper as the nature of dense matter in reality. 
In compact stars at only a few nuclear densities but low temperature, quarks could be interacting strongly with each other there. That might render quarks grouped in clusters, although the hypothetical quark-clusters in cold dense matter has not been confirmed due to the lack of both theoretical and experimental evidence. Motivated by recent lattice QCD simulations of the H-dibaryons (with structure uuddss), we are therefore considering here a possible kind of quark-clusters, H-clusters, that could emerge inside compact stars during their initial cooling, as the dominant components inside (the degree of freedom could then be H-clusters there).Taking into account the in-medium stiffening effect, we find that at baryon densities of compact stars $H$-cluster matter could be more stable than nuclear matter. We also find that for the H-cluster matter with lattice structure, the equation of state could be so stiff that it would seem to be "superluminal" in most dense region. However, the real sound speed for H-cluster matter is in fact hard to calculate, so at this stage we do not put constraints on our model from the usual requirment of causality.

title: Are AXPs/SGRs magnetars?
abstract: Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters have been generally recognized as neutron stars with super strong magnetic fields, namely "magnetars". The "magnetars" manifest that the luminosity in X-ray band are larger than the rotational energy loss rate, i.e. $L_{X}>\dot {E}_{\rm rot}$, and then the radiation energy is coming from the energy of magnetic field. Here it is argued that magnetars may not really exist. Some X-ray and radio observational results are contradicted with the magnetar model. 
(1) The X-ray luminosity of PSR J1852+0040 is much larger than the rotational energy loss rate ($L_{X}/\dot {E}_{\rm rot}\simeq 18)$, but the magnetic field is just $3.1\times 10^{11}$ G. Does this X-ray radiation energy come from the magnetic field? 
(2) In contrast to the above, the magnetic fields of radio pulsars J1847-0130 and PSR J1718-3718 are higher than that of AXP 1E 2259+586, why is the radiation energy of those two radio pulsars still coming from rotational energy? Furthermore, the magnetic field of the newly discovered SGR 0418+5729 with the lowest magnetic field is 3.0 e13 G, lower than the critical magnetic field $B_{\rm C}=4.414$ e13 G) (Esposito et al. 2010). 
(3) Some "magnetars" also emit normal transient radio pulses, what is the essential difference between radio pulsars and the "magnetars"? 
The observational fact arguments will be presented at first, then we discuss in what situation the conventional method to obtain magnetic field could not be correct.

title: Measurement of the D*+- cross-section in two photon collisions at LEP
abstract: The inclusive production of D∗± mesons in photon-photon collisions has been measured by the Aleph experiment at LEP with a beam energy of 45 GeV. The D∗+ are detected in their decay to D0π+ with the D0 observed in three separate decay modes: (1) K−π+, (2) K−π+π0 and (3) K−π+π−π+, and analagously for the D∗− modes. A total of 33 events was observed from an integrated luminosity of 73 pb−1 which corresponds to a cross section for ) of 155 ± 33 ± 21 pb. This result is compatible with both the direct production γγ → c in the Born approximation and with a more complete calculation which includes both radiative QCD corrections and contributions in which one of the photons is first resolved into its quark and gluon constituents. The shapes of distributions for events containing a D∗+ are found to be better described by the latter.

title: The forward-backward asymmetry for charm quarks at the Z pole
abstract: From 1.4 million hadronic Z decays collected by the ALEPH detector at LEP, an enriched sample of Z → cc events is extracted by requiring the presence of a high momentum D∗±. The charm quark forward-backward charge asymmetry at the Z pole is measured to be AFB0.c = (8.0 ± 2.4) % corresponding to an effective electroweak mixing angle of sin2θWeff = 0.2302 ± 0.0054.

title: Study of the B(s)0 anti-B(s)0 oscillation frequency using D(s)- lepton+ combinations in Z decays
abstract: A lower limit on the oscillation frequency of the Bs0s0 system is obtained from approximately four million hadronic Z decays accumulated using the ALEPH detector at LEP from 1991 to 1995. Leptons are combined with opposite sign Ds− candidates reconstructed in seven different decay modes as evidence of semileptonic Bs0 decays. Criteria designed to ensure precise proper time reconstruction select 277Ds−l+ combinations. The initial state of these Bs0 candidates is determined using an algorithm optimized to efficiently utilise the tagging information available for each event. The limit at 95% confidence level on the Bs0s0 oscillation frequency is Δms > 6.6 ps−1. The same data is used to update the measurement of the Bs0 lifetime,τs = 1.54−0.13+0.14 (stat) ± 0.04 (syst) ps.

title: Study of the subjet structure of quark and gluon jets
abstract: Measurements of the subjet structure of quark and gluon jets in hadronic Z decays are presented. The analysis is based on one million hadronic events recorded by the Aleph detector. Roughly symmetric three-jet events are selected with a coarse jet-resolution cut-off, y1. Gluon jets are identified with a purity of 94.6% in those events where evidence of long-lived heavy-flavour hadrons in the other two jets is found. The jets are then analyzed using a smaller cut-off y0 (< y1) so that subjets are resolved. The properties of the jets (subjet multiplicities (Nq), (Ng) and rates Rng(q) for n = 1, 2, 3, 4) are determined and are found to be in good agreement with the expectations of perturbative QCD as long as the subjet resolution parameter y0 is sufficiently large to keep non-perturbative effects small. In particular, the ratio , which to leading order in QCD is given by the ratio of colour factors , is measured to be 1.96 ± 0.15 for y0 = 2 · 10−3, but falls to 1.29 ± 0.03 for y0 = 1.6 · 10−5.

title: A study of Bose-Einstein correlations ine+e− annihilation at 91 GeV
abstract: This paper describes a study of Bose-Einstein correlations made using the ALEPH detector at LEP. The correlations are found to enhance the two particle differential cross section for pairs of identical pions by a factor which can be roughly parametrized byR(Q)=1+λ exp(-Q2σ2), whereQ is the difference in the 3-momenta of the two pions in their centre of mass frame, λ=0.51±0.04±0.11 and σ=3.3±0.2±0.8 GeV−1, which corresponds to a source size of 0.65±0.04±0.16 fm. The large systematic errors on these results reflect their strong dependence on the choice of the reference sample used in the analysis. This problem is believed to occur primarily because of uncertainties in the rates of resonance production and a lack of knowledge about the pion-pion strong interaction. No significant correlations are seen amongst like-charged pion-kaon pairs.