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The Crab pulsar and the surrounding nebula powered by the pulsar's rotational energy through the formation and termination of a relativistic electron-positron wind is a bright source of gamma-rays carrying crucial information about this complex conglomerate. We report the detection of $\gamma$-rays with a spectrum showing gradual steepening over three energy decades, from $5\times 10^{-4}$ to $1.1$ petaelectronvolt (PeV). The ultra-high-energy photons exhibit the presence of a PeV electron accelerator (a pevatron) with an acceleration rate exceeding 15% of the absolute theoretical limit. Assuming that unpulsed $\gamma$-rays are produced at the termination of the pulsar's wind, we constrain the pevatron's size, between $0.025$ and $0.1$ pc, and the magnetic field $\approx 110 \mu$G. The production rate of PeV electrons, $2.5 \times 10^{36}$ erg $\rm s^{-1}$, constitutes 0.5% of the pulsar's spin-down luminosity, although we do not exclude a non-negligible contribution of PeV protons to the production of the highest energy $\gamma$-rays., Comment: 43 pages, 13 figures, 2 tables; Published in Science

As a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to cover a large fraction of the northern sky to hunt for gamma-ray sources at energies above 10 TeV. Even though the detector construction is still underway, a half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the pipeline of KM2A data analysis and the first observation on the Crab Nebula, a standard candle in very high energy gamma-ray astronomy. We detect gamma-ray signals from the Crab Nebula in both energy ranges of 10$-$100 TeV and $>$100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance including angular resolution, pointing accuracy and cosmic ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE =(1.13$\pm$0.05$_{stat}$$\pm$0.08$_{sys}$)$\times$10$^{-14}$$\cdot$(E/20TeV)$^{-3.09\pm0.06_{stat}\pm0.02_{sys}}$ cm$^{-2}$ s$^{-1}$ TeV$^{-1}$. It is consistent with previous measurements by other experiments. This opens a new window of gamma-ray astronomy above 0.1 PeV through which ultrahigh-energy gamma-ray new phenomena, such as cosmic PeVatrons, might be discovered., 13 pages, 15 figures,submitted to CPC

A starquake mechanism for pulsar glitches is developed in the solid quark star model. It is found that the general glitch natures (i.e., the glitch amplitudes and the time intervals) could be reproduced if solid quark matter, with high baryon density but low temperature, has properties of shear modulus \mu = 10^{30~34} erg/cm^3 and critical stress \sigma_c = 10^{18~24} erg/cm^3. The post-glitch behavior may represent a kind of damped oscillations., Comment: 11 pages, 4 figures (but Fig.3 is lost), a complete version can be obtained by http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm, a new version to be published on Astroparticle Physics

Magnetars are proposed to be peculiar neutron stars powered by their super strong magnetic field. Observationally, anomalous X-ray pulsars and soft gamma-ray repeaters are believed to be magnetar candidates. While more and more multiwave observations of magnetars are available, unfortunately, we see accumulating failed predictions of the traditional magnetar model. These challenges urge rethinking of magnetar. Wind braking of magnetars is one of the alternative modelings. The release of magnetic energy may generate a particle outflow (i.e., particle wind), that results in both an anomalous X-ray luminosity and significantly high spindown rate. In this wind braking scenario, only strong multipole field is necessary for a magnetar (a strong dipole field is no longer needed). Wind braking of magnetars may help us to understand their multiwave radiation properties, including (1) Non-detection of magnetars in Fermi-LAT observations, (2) The timing behaviors of low magnetic field magnetars, (3) The nature of anti-glitches, (4) The criterion for magnetar's radio emission, etc. In the wind braking model of magentars, timing events of magnetars should always be accompanied by radiative events. It is worth noting that the wind engine should be the central point in the research since other efforts with any reasonable energy mechanism may also reproduce the results., Comment: 6 pages, 1 figure, submitted to conference proceeding of SMFNS2013 (Strong electromagnetic field and neutron stars 2013)

Recent work by Rankin & Deshpande strongly suggests that there exist strong ``micro-storms'' rotating around the magnetic axis of the 1.1s pulsar PSR 0943+10. Such a feature hints that most probably the large-voltage vacuum gap proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap. However, there are severe arguments against the formation of the RS-type gap in pulsars, since the binding energies of both the Fe ions and the electrons in a neutron star's surface layer is too small to prevent thermionic ejection of the particles from the surface. Here we propose that PSR 0943+10 (probably also most of the other ``drifting'' pulsars) might be bare strange stars rather than normal neutron stars, in which the ``binding energy'' at the surface is merely infinity either for the case of ``pulsar'' or ``anti-pulsar''. It is further proposed that identifying a drifting pulsar as an anti-pulsar is the key criterion to distinguish strange stars from neutron stars., 4 pages, no figures, LaTeX, accepted 1999 July 9 by ApJ Letters

The relationship between pulsar-like compact stars and gravitational waves is briefly reviewed. Due to regular spins, pulsars could be useful tools for us to detect ~nano-Hz low-frequency gravitational waves by pulsar-timing array technique; besides, they would also be ~kilo-Hz high-frequency gravitational wave radiators because of their compactness. The wave strain of an isolate pulsar depends on the equation state of cold matter at supra-nuclear densities. Therefore, a real detection of gravitational wave should be very meaningful in gravity physics, micro-theory of elementary strong interaction, and astronomy., 11 pages, 3 figures; in: Gravitation and Astrophysics (Proceedings of the IX Asia-Pacific International Conference, 29 June - 2 July, 2009, Wuhan), eds. J. Luo, Z. B. Zhou, H. C. Yeh, and J. P. Hsu, World Scientific, p.162-172

The atomic nucleus capability of responding by hydromagnetic vibrations, that has been considered long ago by Hannes Alfven, is re-examined in the context of current development of nuclear physics and pulsar astrophysics., Physica Scripta, accepted

In juxtaposition with the standard model of rotation powered pulsar, the model of vibration powered magnetar undergoing quake-induced torsional Alfven vibrations in its own ultra strong magnetic field experiencing decay is considered. The presented line of argument suggests that gradual decrease of frequencies (lengthening of periods) of long-periodic pulsed radiation detected from a set of X-ray sources can be attributed to magnetic-field-decay induced energy conversion from seismic vibrations to magneto-dipole radiation of quaking magnetar., Text of talk presented at "Mini-Workshop on Pulsars", Nov. 12th, 2010; KIAA-PKU, Beijing

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetar candidates, i.e., neutron stars powered by strong magnetic field. If they are indeed magnetars, they will emit high-energy gamma-rays which are detectable by Fermi-LAT according to the outer gap model. However, no significant detection is reported in recent Fermi-LAT observations of all known AXPs and SGRs. Considering the discrepancy between theory and observations, we calculate the theoretical spectra for all AXPs and SGRs with sufficient observational parameters. Our results show that most AXPs and SGRs are high-energy gamma-ray emitters if they are really magnetars. The four AXPs 1E 1547.0-5408, XTE J1810-197, 1E 1048.1-5937, and 4U 0142+61 should have been detected by Fermi-LAT. Then there is conflict between out gap model in the case of magnetars and Fermi observations. Possible explanations in the magnetar model are discussed. On the other hand, if AXPs and SGRs are fallback disk systems, i.e., accretion-powered for the persistent emissions, most of them are not high-energy gamma-ray emitters. Future deep Fermi-LAT observations of AXPs and SGRs will help us make clear whether they are magnetars or fallback disk systems., 15 pages, 3 figures, 1 table, accepted for publication in The Astrophysical Journal

The X-ray dim isolated neutron stars (XDINSs) are peculiar pulsar-like objects, characterized by their very well Planck-like spectrum. In studying their spectral energy distributions, the optical/UV excess is a long standing problem. Recently, Kaplan et al. (2011) have measured the optical/UV excess for all seven sources, which is understandable in the resonant cyclotron scattering (RCS) model previously addressed. The RCS model calculations show that the RCS process can account for the observed optical/UV excess for most sources . The flat spectrum of RX J2143.0+0654 may due to contribution from bremsstrahlung emission of the electron system in addition to the RCS process., Comment: 6 pages, 2 figures, 1 table, accepted for publication in Research in Astronomy and Astrophysics

It is conventionally thought that the state equation of dense matter softens and thus cannot result in high maximum mass if pulsars are quark stars, and that a recently discovered $2M_\odot$ pulsar (PSR J1614-2230) may make quark stars to be unlikely. However, this standard point of view would be revisited and updated if quark clustering could occur in cold quark matter because of the strong coupling between quarks at realistic baryon densities of compact stars, and it is addressed that the state equation of clustering quark matter stiffs to support compact stars with maximum mass $M_{\rm max}>2M_\odot$. In this brief note, it is demonstrated that large parameter spaces are allowed for $M_{\rm max}>2M_\odot$ in a Lennard-Jones model of clustered quark matter, and the newly measured highest mass of PSR J1614-2230 would be meaningful to constrain the number of quarks inside single quark-cluster, to be $N_q, Comment: Minor chances have been made, submitted

Pulsed high energy radiation from pulsars is not yet completely understood. In this paper, we use the 3D self-consistent annular gap model to study light curves for both young and millisecond pulsars observed by the Fermi Gamma-ray Space Telescope. The annular gap can generate high energy emission for short-period pulsars. The annular gap regions are so large that they have enough electric potential drop to accelerate charged particles to produce gamma-ray photons. For young pulsars, the emission region is from the neutron star surface to about half of the light cylinder radius, and the peak emissivity is in the vicinity of the null charge surface. The emission region for the millisecond pulsars is located much lower than that of the young pulsars. The higher energy gamma-ray emission comes from higher altitudes in the magnetosphere. We present the simulated light curves for three young pulsars (the Crab, the Vela, the Geminga) and three millisecond pulsars (PSR J0030+0451, PSR J0218+4232, PSR J0437-3715) using the annular gap model. Our simulations can reproduce the main properties of observed light curves., Comment: 9 pages, 3 figures and 1 table; accepted for publication in MNRAS

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.

Determining radiation location observationally plays a very important role in testing the pulsar radiation models. One-photon pair production in the strong magnetic field, $\gamma-e^{+}e^{1}$, is one of the important physical processes in pulsar radiation mechanisms. Photons near pulsar surface with sufficient energy will be absorbed in the magnetosphere and the absorption optical depth for these GeV $\gamma$-ray photons is usually large. In this paper, we include the aberrational, rotational and general relativistic effects and calculate the $\gamma$-B optical depth for $\gamma$-ray photons. Then we use the derived optical depth to determine the radiation altitude lower bounds for photons with given energies. As a case study, we calculate the lower bounds of radiation altitudes of Crab pulsar for photons with energy from 5 GeV to 1 TeV., Comment: 11 pages, 5 figures,Accepted by MNRAS

We present a theoretical model for the thermal X-ray emission and cooling of isolated pulsars, assuming that pulsars are solid quark stars. We calculate the heat capacity for such a quark star, and the results show that the residual thermal energy cannot sustain the observed thermal X-ray luminosities seen in typical isolated X-ray pulsars. We conclude that other heating mechanisms must be in operation if the pulsars are in fact solid quark stars. Two possible heating mechanisms are explored. Firstly, for pulsars with little magnetospheric activities, accretion from the interstellar medium or from the material in the associated supernova remnants may power the observed thermal emission. In the propeller regime, a disk-accretion rate ${\dot M}\sim$1% of the Eddington rate with an accretion onto the stellar surface at a rate of $\sim 0.1% {\dot M}$ could explain the observed emission luminosities of the dim isolated neutron stars and the central compact objects. Secondly, for pulsars with significant magnetospheric activities, the pulsar spindown luminosities may have been as the sources of the thermal energy via reversing plasma current flows. A phenomenological study between pulsar bolometric X-ray luminosities and the spin energy loss rates presents the probable existence of a 1/2-law or a linear law, i.e. $L_{\rm bol}^{\infty}\propto\dot{E}^{1/2}$ or $L_{\rm bol}^{\infty}\propto\dot{E}$. This result together with the thermal properties of solid quark stars allow us to calculate the thermal evolution of such stars. Thermal evolution curves, or cooling curves, are calculated and compared with the `temperature-age' data obtained from 17 active X-ray pulsars. It is shown that the bolometric X-ray observations of these sources are consistent with the solid quark star pulsar model., Astroparticle Physics Accepted

We present an analysis of timing residual (noise) of 54 pulsars obtained from 25-m radio telescope at Urumqi Observatory with a time span of 5~8 years, dealing with statistics of the Hurst parameter. The majority of these pulsars were selected to have timing noise that look like white noise rather than smooth curves. The results are compared with artificial series of different constant pairwise covariances. Despite the noise like appearance, many timing residual series showed Hurst parameters significantly deviated from that of independent series. We concluded that Hurst parameter may be capable of detecting dependence in timing residual and of distinguishing chaotic behavior from random processes., Comment: 7 pages, 3 figures, 2 tables, Submitted to MNRAS

Pulsars have been recognized as normal neutron stars, but sometimes argued as quark stars. {\it Sub-millisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars.} We focus on the formation of such sub-millisecond pulsars in this paper. A new approach to form a sub-millisecond pulsar (quark star) via accretion induced collapse (AIC) of a white dwarf is investigated here. Under this AIC process, we found that: (1) almost all the newborn quark stars could have an initial spin period of $\sim 0.1$ ms; (2) the nascent quark stars (even with a low mass) have sufficiently high spin-down luminosity and satisfy the conditions for pair production and sparking process to be as sub-millisecond radio pulsars; (3) in most cases, the timescales of newborn quark stars in the phase of spin period $< 1$ (or $, 10 pages, 4 figures, 4 tables, accepted by MNRAS

We investigate the possibilities that pulsars act as the lens in gravitational microlensing events towards the galactic bulge or a spiral arm. Our estimation is based on expectant survey and observations of FAST (Five hundred meter Aperture Spherical Telescope) and SKA (Square Kilometer Array), and two different models of pulsar distribution are used. We find that the lensing rate is > 1 event/decade, being high enough to search the real events. Therefore, the microlensing observations focusing on pulsars identified by FAST or SKA in the future are meaningful. As an independent determination of pulsar mass, a future detection of microlensing pulsars should be significant in the history of studying pulsars, especially in constraining the state of matter (either hadronic or quark matter) at supra-nuclear densities. The observations of such events by using advanced optical facilities (e.g., the James Webb Space Telescope and the Thirty Meter Telescope) in future are highly suggested., 5pages, 2figures

Determining radiation location observationally plays a very important role in testing the pulsar radiation models. One-photon pair production in the strong magnetic field, $��-e^{+}e^{1}$, is one of the important physical processes in pulsar radiation mechanisms. Photons near pulsar surface with sufficient energy will be absorbed in the magnetosphere and the absorption optical depth for these GeV $��$-ray photons is usually large. In this paper, we include the aberrational, rotational and general relativistic effects and calculate the $��$-B optical depth for $��$-ray photons. Then we use the derived optical depth to determine the radiation altitude lower bounds for photons with given energies. As a case study, we calculate the lower bounds of radiation altitudes of Crab pulsar for photons with energy from 5 GeV to 1 TeV., 11 pages, 5 figures,Accepted by MNRAS

Pulsars have been recognized as normal neutron stars or quark stars. Sub-millisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars. A key question is how sub-millisecond pulsars could form. Both sub-Keplerian (for neutron and quark stars) and super-Keplerian cases (only for quark stars, which are bound additionally by strong interaction) have been discussed in this paper in order to investigate possible ways of forming sub-millisecond pulsars. In the sub-Keplerian case, the equilibrium periods of both neutron and quark stars could be as low as ~0.4 ms when they are spun up through accretion in binary systems. In the super-Keplerian case, pulsars could very likely have an initial period of ~0.1 ms if quark stars with different mass could be formed from accretion-induced collapse (AIC) of white dwarfs. The timescale for a sub-millisecond pulsar to keep its period 10 Myr) for low-mass quark stars could be long enough for us to detect., 12pages, 1 figure, 3 table, this paper has been withdrawn

The main idea and procedure are presented for a new synthetical method on constraining 3-D structure of emission regions of pulsars. With this method the emission regions can be synthetically constrained by fitting multi-wavelength observations features, e.g. pulse widths, phase offsets between radio pulse profiles and high energy light curves, and radio polarization properties. The main technique is based on numerically calculating the emission directions along any open field line via including aberration and retardation effects under a certain magnetic field configuration, e.g. vacuum dipole field. It can be easily extended by involving some further effects or magnetic field line configuration. The role of observed linear polarization properties in the method is discussed. The future application to known X/gamma-ray and radio pulsars will be helpful to test and improve current theoretical models on pulsar emission., Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany, May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.134-136

If the binding energy of the pulsar's surface is not so high (the case of a neutron star), both the negative and positive charges will flow out freely from the surface of the star. The annular free flow model for $��$-ray emission of pulsars is suggested in this paper. It is emphasized that: (1). Two kinds of acceleration regions (annular and core) need to be taken into account. The annular acceleration region is defined by the magnetic field lines that cross the null charge surface within the light cylinder. (2). If the potential drop in the annular region of a pulsar is high enough (normally the cases of young pulsars), charges in both the annular and the core regions could be accelerated and produce primary gamma-rays. Secondary pairs are generated in both regions and stream outwards to power the broadband radiations. (3). The potential drop in the annular region grows more rapidly than that in the core region. The annular acceleration process is a key point to produce wide emission beams as observed. (4). The advantages of both the polar cap and outer gap models are retained in this model. The geometric properties of the $��$-ray emission from the annular flow is analogous to that presented in a previous work by Qiao et al., which match the observations well. (5). Since charges with different signs leave the pulsar through the annular and the core regions, respectively, the current closure problem can be partially solved., 11 pages 2 figures, accepted by Chinese Journal of Astronomy and Astrophysics

The consistence of the distributions of pulsar's kick velocities from the model of GRB and from the pulsar observations is tested based on the supernova-GRB ($\gamma$-ray burst) association and under the assumption that the GRB asymmetric explosions produce pulsars. The deduced distribution of kick velocity from the model of GRB and the observed kick distribution of radio pulsars are checked by K-S test. These two distributions are found to come from a same parent population. This result may indicate that GRBs could be really related to supernova, and that the asymmetry of GRB associated with supernova would cause the kick of pulsars., Comment: 4 pages, 1 figure, accepeted by A&A

The pulse profile of pulsar gives geometric information about pulsar's radiation model. After investigating the pulse profiles of PSR B1642-03 and PSR B0950+08, we calculate the ratios of beam width and the emission height between different frequencies. We find that the ratios are almost constants as inclination angle $\alpha$ changes from $0^{\circ}$ to $90^{\circ}$. The ratios can be used to test pulsar's radiation model. In particular it can well constrain the parameter space of the inverse Compton scattering (ICS) model. These constrained parameters indicate some physical implication for the ICS model., Comment: 7 pages, 6 figures, submitted to "Advances in Space Research" (Proceedings of COSPAR 2006)

Three times of supergiant flares from soft $\gamma$-ray repeatres are observed, with typical released energy of $\sim 10^{44-47}$ erg. A conventional model (i.e., the magnetar model) for such events is catastrophic magnetism-powered instability through magnetohydrodynamic process, in which a significant part of short-hard $\gamma$-ray bursts could also be the results of magnetars. Based on various observational features (e.g., precession, glitch, thermal photon emission) and the underlying theory of strong interaction (quantum chromodynamics, QCD), it could not be ruled out yet that pulsar-like stars might be actually solid quark stars. Strain energy develops during a solid star's life, and starquakes could occur when stellar stresses reach a critical value, with huge energy released. An alternative model for supergiant flares of soft $\gamma$-ray repeatres is presented, in which energy release during a star quake of solid quark stars is calculated. Numerical results for spherically asymmetric solid stars show that the released gravitational energy during a giant quake could be as high as $10^{48}$ erg if the tangential pressure is slightly higher than the radial one. Difficulties in magnetar models may be overcome if AXPs/SGRs are accreting solid quark stars with mass $\sim (1-2)M_\odot$., Comment: Extensive discussions are presented, with a new figure added to show the change of mass and radius of accreting quark stars

As a result of observational difficulties, braking indices of only six rotation-powered pulsars are obtained with certainty, all of which are remarkably smaller than the value ($n=3$) expected for pure magnetodipole radiation model. This is still a real fundamental question not being well answered after nearly forty years of the discovery of pulsar. The main problem is that we are shamefully not sure about the dominant mechanisms that result in pulsars' spin-down. Based on the previous works, the braking index is re-examined, with a conclusion of suggesting a constant gap potential drop for pulsars with magnetospheric activities. New constrains on model parameters from observed braking indices are presented., Comment: 10 pages, 4 figures, submitted to "Advances in Space Research" (Proceedings of COSPAR 2006)

The nature of AXPs/SGRs (anomalous X-ray pulsars/soft $\gamma$-ray repeaters) and high field radio pulsars is still unclear even in the magnetar and/or accretion models. The detection of radio emission from AXP XTE J1810$-$197 and the discovery of a debris disk around AXP 4U 0142+61 might shed light on the problem. We propose that AXPs/SGRs could be pulsars that have magnetic field $B\lesssim 10^{13} \mathrm{G}$ as normal pulsars, but in accretion environments. We investigate these issues under the accretion model and find that two of the AXPs/SGRs might be low mass quark stars if all AXPs and SGRs are likely grouped together., Comment: 6 pages, 1 figures, submitted to "Advances in Space Research" (Proceedings of COSPAR 2006)

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 star's magnetic field, spin period, as well as the accretion rate are constrained by observations., Comment: 12 pages, 3 figures, or at: http://vega.bac.pku.edu.cn/rxxu/publications/index_P.htm

After a brief introduction to the necessary of quark stars in modelling pulsars, I present a qualitative analysis of the solidification of quark matter with low-temperature but high-density. The reason, that a solid neutron star could not be possible, is also given., Comment: 3 pages, no figure. Proceedings of CAS Symposium No.365/The 18th Guo Shoujing Summer School on Astrophysics: "Beyond the Milky Way: Progress in Extragalactic X-Ray Astronomy with Chandra and XMM-Newton" (September 21-24, 2003), Eds: Fuzhen Cheng, Junhan You, and K. Makishima

Although pulsars can radiate electromagnetic wave from radio to gamma ray bands, we still have no a united model to understand the multi-band emission. In this paper the effort for a joint model is presented. The inverse Compton scattering (ICS) and a second acceleration process near the null charge surface are involved to account for the radio and the gama-ray emission, respectively. Various kind of pulse profiles and other observational properties can be reproduced., Comment: 5 pages, 2 figure, IAU Symposium No.214 on "High Energy Processes and Phenomena in Astrophysics", August, 2002, Suzhou, China, in press

The fastest millisecond pulsar PSR B1937+21 presents an interpulse separated from the main pulse by nealy 180^o at radio frequencies. Recently, the ASCA observations (Takahashi et al. 2001) detected pulsed X-ray emission from this pulsar. Only a single narrow X-ray pulse is observed, which is coincident with the radio interpulse in phase. We investigate the possible origin of the pulsed X-rays from the polar cap (PC) accelerators or the outer gap (OG) accelerators in the frame of PC model and OG model, respectively, by assuming a dipolar magnetic field structure and the same radio emission pattern from its poles for the pulsar. The OG model can naturally explain the main observational facts. For the PC model, the coincidence between the X-ray pulse and the radio interpulse can not be reproduced in the assumed case. However when considering possible deviation from our assumption, PC model may still be valid for this pulsar in some cases., 7 pages, to appear in Astrophysical Journal, or at http://vega.bac.pku.edu.cn/~rxxu/publications/P/wxq02.ps.gz

The conjecture that ultra high energy cosmic rays (UHECRs) are actually strangelets is discussed. Besides the reason that strangelets can do as cosmic rays beyond the GZK-cutoff, another argument to support the conjecture is addressed in this letter via the study of formation of TeV-scale microscopic black holes when UHECRs bombarding bare strange stars. It is proposed that the exotic quark surface of a bare strange star could be an effective astro-laboratory in the investigations of the extra dimensions and of the detection of ultra-high energy neutrino fluxes. The flux of neutrinos (and other point-like particles) with energy >2.3 x 10^{20} eV could be expected to be smaller than 10^{-26} cm^{-2}$ s^{-1} if there are two extra spatial dimensions., accepted by Chin. Phys. Lett., or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

Recent achievements of bare strange stars are briefly reviewed. A nascent protostrange star should be bare because of strong mass ejection and high temperature after the supernova detonation flame, and a crust can also hardly form except for a super-Eddington accretion. The magnetosphere of a bare strange star is composed mainly of electron-positron pair plasma, where both inner and outer vacuum gaps work for radio as well as high energy nonthermal emission. A featureless thermal spectrum is expected since no ion is above the quark surface, whilst electron cyclotron lines could appear in some bare strange stars with suitable magnetic fields. Various astrophysical implications of bare strange stars are discussed., Comment: 9 page and no figure, The 6th pacific rim conference on steller astrophysics -- a tribute to Helmut A. Abt, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_C.htm

Extremely relativistic particles play an important role in various radiative mechanisms of pulsar radio as well as high-energy emission. It is thus very essential to estimate the Lorentz factor of these particles in the radiation regions both observationally and theoretically. In this paper, according to the frequency dependence of component separation and considering some radiation processes, we obtain the Lorentz factor and its variation in the radiation region observationally for seven multi-frequency observed pulsars. It is found that the Lorentz factor decays substantially in the emission region, and the decay can not be well understood by all known energy loss processes. This result hints that there are some unknown processes which are responsible to loosing energy effectively., 14 pages, 3 figures, Submitted to Chinese Journal of Astronomy and Astrophysics

Some characteristics of the inverse Compton scattering (ICS) model are reviewed. At least the following properties of radio pulsars can be reproduced in the model: core or central emission beam, one or two hollow emission cones, different emission heights of these components, diverse pulse profiles at various frequencies, linear and circular polarization features of core and cones., 5 pages, no figures, LaTeX, a proceeding paper for Pacific Rim Conference on Stellar Astrophysics, Aug. 1999, HongKong, China

It is suggested that the inner energetic engine of Gamma ray burst (GRB) may be the result of the transition of normal hadron to quark-gluon plasma (QGP) in rapidly-rotating and spin-down newborn neutron star. When such a nascent neutron star slows down through dipole electromagnetic and quadruple gravitational radiation, the increasing center density may reach the QCD transition density, i.e., 5-10 nuclear density. Such kind of energy release from the phase transition would be responsible for GRB and its possible beaming effect. The relative dense gaseous environment of GRB location and the iron line observed in the X-ray afterglow support this idea. Some predictions in this model are given. keywords: Gamma-ray Bursts, Supernova, Quark-gluon plasma, 7 pages, no figures, LaTeX, presented in 1999 Pacific Rim Conference of Stellar Astrophysics in Hong Kong University

Many theoretical efforts were made to understand the core and conal emission identified from observations by Rankin (1983) and Lyne and Manchester (1988). One of them, named as inverse Compton scattering (ICS) model (Qiao & Lin 1998), has been proposed. It is found in the model that: there are central or `core' emission beam, and one or two hollow conical emission beams; the different emission components are emitted at different heights; owing to different radiation components emitted from different height, the observed emission beams can be shifted from each other due to retardation and aberration effects; the sizes of emission components change with frequencies. Recent developments of the model include: simulations of pulse profiles at different frequencies; studying the basic polarization properties of inverse Compton scattering in strong magnetic fields; computing the polarizations and spectrum of core and cones. A new classification system was also proposed. The main results calculated from the model are consistent with the observations., 4 pages, no figures, LaTeX