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In this work we try to search for signals generated by ultra-heavy dark matter at the Large High Altitude Air Shower Observatory (LHAASO) data. We look for possible gamma-ray by dark matter annihilation or decay from 16 dwarf spheroidal galaxies in the field of view of LHAASO. Dwarf spheroidal galaxies are among the most promising targets for indirect detection of dark matter which have low fluxes of astrophysical $\gamma$-ray background while large amount of dark matter. By analyzing more than 700 days observational data at LHAASO, no significant dark matter signal from 1 TeV to 1 EeV is detected. Accordingly we derive the most stringent constraints on the ultra-heavy dark matter annihilation cross-section up to EeV. The constraints on the lifetime of dark matter in decay mode are also derived., Comment: 17 pages, 12 figures, accepted by PRL

We present the first catalog of very-high energy and ultra-high energy gamma-ray sources detected by the Large High Altitude Air Shower Observatory (LHAASO). The catalog was compiled using 508 days of data collected by the Water Cherenkov Detector Array (WCDA) from March 2021 to September 2022 and 933 days of data recorded by the Kilometer Squared Array (KM2A) from January 2020 to September 2022. This catalog represents the main result from the most sensitive large coverage gamma-ray survey of the sky above 1 TeV, covering declination from $-$20$^{\circ}$ to 80$^{\circ}$. In total, the catalog contains 90 sources with an extended size smaller than $2^\circ$ and a significance of detection at $> 5\sigma$. Based on our source association criteria, 32 new TeV sources are proposed in this study. Among the 90 sources, 43 sources are detected with ultra-high energy ($E > 100$ TeV) emission at $> 4\sigma$ significance level. We provide the position, extension, and spectral characteristics of all the sources in this catalog., Comment: 40 pages, 13 figures, 4 tables

The state of super-dense matter is essential for us to understand the nature of pulsars, but the non- perturbative quantum chromodynamics (QCD) makes it very difficult for direct calculations of the state of cold matter at realistic baryon number densities inside compact stars. Nevertheless, from an observational point of view, it is conjectured that pulsars could be made up of quark clusters since the strong coupling between quarks might render quarks grouped in clusters. We are trying an effort to find an equation of state of condensed quark-cluster matter in a phenomenological way. Supposing that the quark-clusters could be analogized to inert gases, we apply here the corresponding-state approach to derive the equation of state of quark-cluster matter, as was similarly demonstrated for nuclear and neutron-star matter in 1970s. According to the calculations presented, the quark-cluster stars, which are composed of quark-cluster matter, could then have high maximum mass that is consistent with observations and, in turn, further observations of pulsar mass would also put constraints to the properties of quark-cluster matter. Moreover, the melting heat during solid-liquid phase conversion and the related astrophysical consequences are also briefly discussed., Comment: 10 pages, 8 figures, 1 table

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are enigmatic pulsar-like objects. The energy budget is the fundamental problem in their studies. In the magnetar model, they are supposed to be powered by the extremely strong magnetic fields (>~ 10^14 G) of neutron stars. Observations for and against the magnetar model are both summarized. Considering the difficulties encountered by the magnetar model to comfortably understand more and more observations, one may doubt that AXPs and SGRs are really magnetars. If they are not magnetar candidates (including magnetar-based models), then they must be "quark star/fallback disk" systems., Comment: 10 pages, 1 figure; talk at SMFN2011

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., Comment: 9 pages and 7 figures. MNRAS, in press

The quark matter may have great implications in astrophysical studies, which could appear in the early Universe, in compact stars, and/or as cosmic rays. After a general review of astrophysical quark matter, the density-dominated quark matter is focused., Comment: Talk presented in the 5th micro-QSO Workshop in June 11, 2004. Tsinghua Univ., Beijing, 6 pages, 2 eps-figs

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

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

Both rotation- and accretion-powered low-mass bare strange stars are studied, the astrophysical appearances of which are especially focused. It is suggested that low-mass bare strange stars, with weaker ferromagnetic fields than that of normal pulsars, could result from accretion-induced collapses (AIC) of white dwarfs. According to its peculiar timing behavior, we propose that the radio-quiet object, 1E 1207.4-5209, could be a low-mass bare strange star with polar surface magnetic field ~ 6 x 10^10 G and a few kilometers in radius. The low-mass bare strange star idea is helpful to distinguish neutron and strange stars, and is testable by imaging pulsar-like stars with the future Constellation-X telescope., Comment: 13 pages, 4 figures, 1 table; changed significantly, a section "quark matter phenomenology" added; accepted by MNRAS

The reason, we need three terms of `strange', `bare', and `solid' before quark stars, is presented concisely though some fundamental issues are not certain. Observations favoring these stars are introduced., Comment: 4 page and no figure, IAU Symposium No.218, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_C.htm

Within the realms of the possibility of solid quark matter, we fitted the 500ks Chandra LETG/HRC data for RX J1856.5-3754 with a phenomenological spectral model, and found that electric conductivity of quark matter on the stellar surface is about > 1.2 x 10^{18} s^{-1}., Comment: 2 page and no figure, IAU Symposium No.218, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_C.htm

It is found that 1E 1207.4-5209 could be a low-mass bare strange star if its small radius or low altitude cyclotron formation can be identified. The age problems of five sources could be solved by a fossil-disk-assisted torque. The magnetic dipole radiation dominates the evolution of PSR B1757-24 at present, and the others are in propeller (or tracking) phases., Comment: ApJL accepted, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

It is conjectured that cold quark matter with much high baryon density could be in a solid state, and strange stars with low temperatures should thus be solid stars. The speculation could be close to the truth if no peculiar polarization of thermal X-ray emission (in, e.g., RXJ1856), or no gravitational wave in post-glitch phases, is detected in future advanced facilities, or if spin frequencies beyond the critical ones limited by r-mode instability are discovered. The shear modulus of solid quark matter could be ~ 10^{32} erg/cm^3 if the kHz QPOs observed are relevant to the eigenvalues of the center star oscillations., Comment: Revised significantly, ApJL accepted, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

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., Comment: 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

A pedagogical overview of strange quark matter and strange stars is presented. After a historical notation of the research and an introduction to quark matter, a major part is devoted to the physics and astrophysics of strange stars, with attention being paid to the possible ways by which neutron stars and strange stars can be distinguished in astrophysics. Recent possible evidence for bare strange stars is also discussed., Comment: 8 page and no figure, IAU Symposium No.214, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

Motivated by trying to understand the absence of spectral lines in the thermal components of the X-ray compact sources observed recently by Chandra or XMM, we propose that these sources could be simply bare strange stars. The formation, cooling, and thermal photon radiation of bare strange stars have been investigated. It is suggested that thermal featureless spectrum could be a new probe for identifying strange stars., Comment: 4 pages, revised version to be published in ApJL, or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

The surface electric characteristics of bare strange stars are investigated with the inclusion of boundary effects. The thickness of the electron layer where pairs can be spontaneously created is calculated as a function of the bag parameters. We find that previous estimates are representative for bag parameters within a rather wide range, and therefore our results favor the thermal radiation mechanism of bare strange stars advanced by V. V. Usov., Comment: 5 pages, revised version for A&A (accepted), or at http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm

According to the observational limits on the radius and mass, the fastest rotating pulsar (PSR 1937+21) is probably a strange star, or at least some neutron star equations of state should be ruled out, if we suggest that a dipole magnetic field is relevant to its radio emission. We presume that the millisecond pulsar is a strange star with {\em much} low mass, small radius, and weak magnetic moment., Comment: 9 pages, 2 figures, 1 table, to be published in Chinese Physics Letters (2001), http://vega.bac.pku.edu.cn/~rxxu

The possibility and the implications of the idea, that pulsars are born as strange stars, are explored. Strange stars are very likely to have atmospheres with typical mass of $\sim 5\times 10^{-15}M_\odot$ but bare polar caps almost throughout their lifetimes, if they are produced during supernova explosions. A direct consequence of the bare polar cap is that the binding energies of both positively and negatively charged particles at the bare quark surface are nearly infinity, so that the vacuum polar gap sparking scenario as proposed by Ruderman & Sutherland should operate above the cap, regardless of the sense of the magnetic pole with respect to the rotational pole. Heat can not accumulate on the polar cap region due to the large thermal conductivity on the bare quark surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the present broad band emission data of pulsars, and propose several possible criteria to distinguish BPCSSs from neutron stars., Comment: 31 pages in Latex. Accepted by AstroParticle Physics