arXiv subjects

Xiangdong Shi

Publications and source records attributed to Xiangdong Shi.

At least 19 recordsLinked to original sources

arXiv2026

IY Lyr: A Thick-Disk first-overtone RR Lyrae Star with a Possible Neutron Star Companion

IY Lyr, historically misclassified as an eclipsing binary, has been previously identified as a first-overtone RR Lyrae star (RRc star). Using multiband photometry (All-Sky Automated Survey for Supernovae, Zwicky Transient Facility, TESS, and our BVRI data), Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, and Gaia astrometry, we investigate its pulsation, binarity, and Galactic population. From O-C analysis, we detect a long-term period decrease and a light-travel time effect with an orbital period of 3.94 $\pm$ 0.09 years, eccentricity of 0.46 $\pm$ 0.15, and a mass function of 0.65 $\pm$ 0.14 M$_{\odot}$. The companion is independently supported by radial velocity residuals and Gaia proper motions. Combined constraints yield an orbital inclination of 94.2$^{\circ}$ $\pm$ 1.1$^{\circ}$ and a companion mass of 1.37 $\pm$ 0.19 M$_{\odot}$. Chemical abundances ([Fe/H] $\simeq$ -1.0 $\pm$ 0.1, [$\alpha$/Fe] $\simeq$ +0.27 $\pm$ 0.03, Xiang et al. 2019) and dynamics ($L_{\rm z}$ $\simeq$ 1287 $\pm$ 35 kpc km s$^{-1}$, $Z_{\rm max}$ $\simeq$ 1.17 $\pm$ 0.10 kpc) identify IY Lyr as likely an old, high-$\alpha$, thick-disk star. The companion mass lies at the peak of the neutron star mass distribution, and the system's age excludes a main-sequence star; we conclude the companion is most likely a typical neutron star, although a massive white dwarf near the Chandrasekhar limit cannot be ruled out. IY Lyr is among the few RRc binaries with a compact companion supported by multiple methods, and it has important implications for thick-disk binary evolution and neutron star formation.

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arXiv2024

Deep and low mass-ratio contact binaries and their third bodies

Deep and low mass-ratio contact binaries (DLMCBs) are believed to be in the final stage of their contact phase, potentially leading to the formation of fast-rotating single stars such as FK Com-type stars and blue stragglers, as well as luminous red novae. These systems serve as an excellent laboratory for studying stellar coalescence and merging processes. Our search for DLMCBs began in 2004 and has since identified a group of such systems. Together with that collected from the literature, more than 100 DLMCBs have been detected so far. Half of them have had their periods investigated based on O-C curves. Some have shown period increases, while others have exhibited period decreases. Among them, more than half DLMCBs have cyclic variations, suggesting the possibility of the existence of a third body orbiting around the DLMCBs. Furthermore, with more data obtained extending the span of the O-C curve, more cyclic variations could be detected. The high proportion of signs of the presence of third bodies makes them an essential factor to consider when studying the merger of contact binaries.

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arXiv2000

The Increase in the Primordial He-4 Yield in the Two-Doublet Four-Neutrino Mixing Scheme

We assess the effects on Big Bang Nucleosynthesis (BBN) of lepton number generation in the early universe resulting from the two-doublet four-neutrino mass/mixing scheme. It has been argued that this neutrino mass/mixing arrangement gives the most viable fit to the existing data. We study full 4 x 4 mixing matrices and show how possible symmetries in these can affect the BBN He-4 abundance yields. Though there is as yet no consensus on the reliability of BBN calculations with neutrino flavor mixing, we show that, in the case where the sign of the lepton number asymmetry is unpredictable, BBN considerations may pick out specific relationships between mixing angles. In particular, reconciling the observed light element abundances with a \barν_μ<-> \barν_e oscillation interpretation of LSND would allow unique new constraints on the neutrino mixing angles in this model.

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arXiv1999

Transformation-Induced Nonthermal Neutrino Spectra and Primordial Nucleosynthesis

We examine in detail the changes in the production of primordial helium resulting from nonthermal neutrino momentum distributions produced by resonant transformation of electron-type neutrinos to steriles. These transformations, anti-nu_e -> anti-nu_s (nu_e->nu_s), amplify a positive (negative) lepton number asymmetry. We find that the resulting supression relative to a thermal distribution of low energy nu_e reduces n -> p conversion to a greater extent than does the enhancement of n -> p from an identical suppression of anti-nu_e. Thus, equal lepton-number asymmetries of opposite sign have unequal effects on the resulting helium yield in primordial nucleosynthesis.

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arXiv1999

Comments Regarding "On Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial Helium-4 Abundance"

This is a reply to the preprint "On Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial Helium-4 Abundance" by M. V. Chizhov and D. P. Kirilova (hep-ph/9908525), which criticised our recent publication (X. Shi, G. M. Fuller and K. Abazajian Phys. Rev. D 60, 063002 (1999)). Here we point out factual errors in their description of what our paper says. We also show that their criticisms of our work have no merit.

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arXiv1999

Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial $^4$He Abundance

It has been proposed that an asymmetry in the electron neutrino sector may be generated by resonant active-sterile neutrino transformations during Big Bang Nucleosynthesis (BBN). We calculate the change in the primordial $^4$He yield $Y$ resulting from this asymmetry, taking into account both the time evolution of the $ν_e$ and $\barν_e$ distribution function and the spectral distortions in these. We calculate this change in two schemes: (1) a lepton asymmetry directly generated by $ν_e$ mixing with a lighter right-handed sterile neutrino $ν_s$; and (2) a lepton asymmetry generated by a $ν_τ\leftrightarrowν_s$ or $ν_μ\leftrightarrowν_s$ transformation which is subsequently partially converted to an asymmetry in the $ν_e\barν_e$ sector by a matter-enhanced active-active neutrino transformation. In the first scheme, we find that the percentage change in $Y$ is between -1% and 9% (with the sign depending on the sign of the asymmetry), bounded by the Majorana mass limit $m_{ν_e}\la 1$ eV. In the second scheme, the maximal percentage reduction in $Y$ is 2%, if the lepton number asymmetry in neutrinos is positive; Otherwise, the percentage increase in $Y$ is $\la 5%$ for $m^2_{ν_μ,ν_τ}-m^2_{ν_s}\la 10^4$ eV. We conclude that the change in the primordial $^4$He yield induced by a neutrino-mixing-generated lepton number asymmetry can be substantial in the upward direction, but limited in the downward direction.

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arXiv1999

Active-Sterile Neutrino Mixing in the Early Universe and Primordial Nucleosynthesis

We investigate the effects of matter-enhanced (MSW) transformation of neutrinos in the Early Universe on the primordial helium yield (Y_p). We find that Y_p is affected much more by the MSW-induced alterations in the neutrino energy spectra than by the associated change in expansion rate. Specifically, the absence due to transformation of low energy electron neutrinos can significantly affect neutron-proton weak interconversion rates through the lifting of Fermi-blocking of neutron decay at low energies and through halting low-energy neutrino capture on neutrons. We find that the change of Y_p within a causal horizon is -0.005\le δY_p \leq 0.013 for m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \le 10^4 eV^2 in the case of ν_{μ,τ}-ν_s-ν_e mixing, with the lower limit at m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \approx 100 eV^2, and -0.002\le δY_p \leq 0.020 for m_{ν_e}^2 - m_{ν_s}^2 \le 1 eV^2 in the case of ν_e-ν_s mixing.

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arXiv1999

A New Dark Matter Candidate: Non-thermal Sterile Neutrinos

We propose a new and unique dark matter candidate: $\sim 100$ eV to $\sim 10$ keV sterile neutrinos produced via lepton number-driven resonant MSW (Mikheyev-Smirnov-Wolfenstein) conversion of active neutrinos. The requisite lepton number asymmetries in any of the active neutrino flavors range from 10$^{-3}$ to 10$^{-1}$ of the photon number - well within primordial nucleosynthesis bounds. The unique feature here is that the adiabaticity condition of the resonance strongly favors the production of lower energy sterile neutrinos. The resulting non-thermal (cold) energy spectrum can cause these sterile neutrinos to revert to non-relativistic kinematics at an early epoch, so that free-streaming lengths at or below the dwarf galaxy scale are possible. Therefore, the main problem associated with light neutrino dark matter candidates can be circumvented in our model.

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arXiv1999

Leptonic Domains in the Early Universe and Their Implications

We extend a treatment of the causal structure of space-time to active-sterile neutrino transformation-based schemes for lepton number generation in the early universe. We find that these causality considerations necessarily lead to the creation of spatial domains of lepton number with opposite signs. Lepton number gradients at the domain boundaries can open a new channel for MSW resonant production of sterile neutrinos. The enhanced sterile neutrino production via this new channel allows considerable tightening of Big Bang Nucleosynthesis constraints on active-sterile neutrino mixing, including the proposed $ν_μ\to ν_s$ solution for the Super Kamiokande atmospheric $ν_μ$ deficit, and the four-neutrino schemes proposed to simultaneously fit current neutrino experimental results.

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arXiv1999

A Possible Explanation for the Radio ``Flare'' in the Early Afterglow of GRB990123

We suggest that the deceleration of the relativistic shock by a denser part of the interstellar medium off line-of-sight produced the observed radio ``flare'' in the early afterglow of GRB990123. We find that this scenario is consistent with observations if the particle number density of this denser part of the medium is between $\sim 200$ and $\sim 2\times 10^4$ cm$^{-3}$. Because of the premature deceleration of part of the shock, the later stage of the afterglow should decay modestly faster than the powerlaw expected from an isotropic shock propagation.

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arXiv1998

Observing the Birth of Supermassive Black Holes with the Planned ICECUBE Neutrino Detector

It has been suggested that the supermassive black holes, at the centers of galaxies and quasars, may initially form in single collapses of relativistic star clusters or supermassive stars built-up during the evolution of dense star clusters. We show that it may be possible for ICECUBE (a planned 1 km^3 neutrino detector in Antarctica) to detect the neutrino bursts associated with those collapses at redshift $z\la 0.2$ with a rate of $\sim$ 0.1 to 1 burst per year. Such detections could give new insights into the formation of structure in the universe, especially when correlated with gravitational wave signatures or even gamma-ray bursts.

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arXiv1998

Big Bang Nucleosynthesis and Active-Sterile Neutrino Mixing: Evidence for Maximal Mu Neutrino <-> Tau Neutrino Mixing in Super Kamiokande?

We discuss Big Bang Nucleosynthesis constraints on maximal $ν_μ\leftrightarrowν_s$ mixing. Vacuum $ν_μ\leftrightarrowν_s$ oscillation has been proposed as one possible explanation of the Super Kamiokande atmospheric neutrino data. Based on the most recent primordial abundance measurements, we find that the effective number of neutrino species for Big Bang Nucleosynthesis (BBN) is $N_ν\la 3.3$. Assuming that all three active neutrinos are light (with masses $\ll 1$ MeV), we examine BBN constraints on $ν_μ\leftrightarrowν_s$ mixing in two scenarios: (1) a negligible lepton asymmetry (the standard picture); (2) the presence of a large lepton asymmetry which has resulted from an amplification by $ν_τ\leftrightarrowν_{s'}$ mixing ($ν_{s'}$ being $ν_s$ or another sterile neutrino species). The latter scenario has been proposed recently to reconcile the BBN constraints and large-angle $ν_μ\leftrightarrowν_s$ mixing. We find that the large-angle $ν_μ\leftrightarrowν_s$ mixing in the first scenario, which would yield $N_ν\approx 4$, is ruled out as an explanation of the Super Kamiokande data. It is conceivably possible for the $ν_μ\leftrightarrowν_s$ solution to evade BBN bounds in the second scenario, but only if 200 eV$^2\la m^2_{ν_τ}-m^2_{ν_{s'}}\la 10^4$ eV$^2$ is satisfied, and if $ν_τ$ decays non-radiatively with a lifetime $\la 10^3$ years. This mass-squared difference implies 15 eV$\la m_{ν_τ}\la 100$ eV if $ν_{s'}$ is much lighter than $ν_τ$. We conclude that maximal (or near maximal) $ν_μ\leftrightarrowν_τ$ mixing is a more likely explanation of the Super Kamiokande data.

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arXiv1998

A Reply to "Comment on 'Big Bang Nucleosynthesis and Active-Sterile Neutrino Mixing: Evidence for Maximal $ν_μ\leftrightarrowν_τ$ Mixing in Super Kamiokande?'"

In the paper "Big Bang Nucleosynthesis and Active-Sterile Neutrino Mixing: Evidence for Maximal Muon-Neutrino/Sterile-Neutrino Mixing in Super Kamiokande" (astro-ph/9810075), we suggested that to evade the Big Bang Nucleosynthesis exclusion of the muon neutrino to sterile neutrino oscillation explanation of the Super Kamiokande data, the tau neutrino must have a mass over about 15 eV and it must mix with a lighter sterile neutrino. A stable tau neutrino with this mass is inconsistent with cosmological structure formation. In a comment on our paper (astro-ph/9811067), Foot and Volkas argued that our result is incorrect and that the required tau neutrino mass should be much lower. Here we back up our original result with a more detailed calculation. We show that the argument of Foot and Volkas is invalid, most likely due to an insufficient energy resolution in the low energy part of the neutrino spectrum.

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arXiv1998

Are Gamma-Ray Bursts Signals of Supermassive Black Hole Formation?

The formation of supermassive black holes through the gravitational collapse of supermassive objects ($M \ga 10^4 M_\odot$) has been proposed as a source of cosmological gamma-ray bursts. The major advantage of this model is that such collapses are far more energetic than stellar-remnant mergers. The major drawback of this idea is the severe baryon loading problem in one-dimensional models. We can show that the observed log N - log P (number vs. peak flux) distribution for gamma-ray bursts in the BATSE database is not inconsistent with an identification of supermassive object collapse as the origin of the gamma-ray bursts. This conclusion is valid for a range of plausible cosmological and gamma-ray burst spectral parameters.

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arXiv1998

Supermassive Objects as Gamma-Ray Bursters

We propose that the gravitational collapse of supermassive objects ($ M\ga 10^4 M_\odot$), either as relativistic star clusters or as single supermassive stars (which may result from stellar mergers in dense star clusters), could be a cosmological source of $γ$-ray bursts. These events could provide the seeds of the supermassive black holes observed at the center of many galaxies. Collapsing supermassive objects will release a fraction of their huge gravitational binding energy as thermal neutrino pairs. We show that the accompanying neutrino/antineutrino annihilation-induced heating could drive electron/positron ``fireball'' formation, relativistic expansion, and associated $γ$-ray emission. The major advantage of this model is its energetics: supermassive object collapses are far more energetic than solar mass-scale compact object mergers; therefore, the conversion of gravitational energy to fireball kinetic energy in the supermassive object scenario need not be highly efficient, nor is it necessary to invoke directional beaming. The major weakness of this model is difficulty in avoiding a baryon loading problem for one dimensional collapse scenarios.

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arXiv1998

Testing Cold Dark Matter Models Using Hubble Flow Variations

COBE-normalized flat (matter plus cosmological constant) and open Cold Dark Matter (CDM) models are tested by comparing their expected Hubble flow variations and the observed variations in a Type Ia supernova sample and a Tully Fisher cluster sample. The test provides a probe of the CDM power spectrum on scales of $0.02h$ Mpc$^{-1}\la k\la 0.2h$ Mpc$^{-1}$, free of the bias factor $b$. The results favor a low matter content universe, or a flat matter-dominated universe with a very low Hubble constant and/or a very small spectral index $n_{ps}$, with the best fits having $Ω_0\sim 0.3$ to 0.4. The test is found to be more discriminative to the open CDM models than to the flat CDM models. For example, the test results are found to be compatible with those from the X-ray cluster abundance measurements at smaller length scales, and consistent with the galaxy and cluster correlation analysis of Peacock and Dodds (1994) at similar length scales, if our universe is flat; but the results are marginally incompatible with the X-ray cluster abundance measurements if our universe is open. The open CDM results are consistent with that of Peacock and Dodds only if the matter density of the universe is less than about 60% of the critical density. The shortcoming of the test is discussed, so are ways to minimize it.

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arXiv1998

Neutrinos and Supermassive Stars: Prospects for Neutrino Emission and Detection

We calculate the luminosity and energy spectrum of the neutrino emission from electron-positron pair annihilation during the collapse of a supermassive star (${M} \ga 5\times10^4 {M_\odot}$). We then estimate the cumulative flux and energy spectrum of the resulting neutrino background as a function of the abundance and redshift of supermassive stars and the efficiency of these objects in converting gravitational energy into neutrino energy. We estimate the expected signal in some of the new generation of astrophysical neutrino detectors from both a cumulative background of supermassive stars and single collapse events associated with these objects.

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arXiv1997

Peculiar Hubble Flows in Our Local Universe

A formalism that simultaneously searches for the monopolar and dipolar peculiar velocities is presented. The formalism is applied to (1) the Mark III catalogue, (2) Lauer and Postman's Abell cluster catalogue, and (3) Riess et al.'s Type Ia supernova sample. The emphasis is drawn to the monopolar peculiar velocities, i.e., peculiar Hubble flows, within these samples. The samples show inconsistent peculiar Hubble flows within a depth of $\sim 60h^{-1}$ Mpc. Beyond a depth of $\sim 80h^{-1}$ Mpc, the Hubble flows of all samples converge to the global Hubble flow to better than 10% at the $2σ$ level. The results are compared with theoretical predictions. They at face value disfavor models predicting smaller peculiar velocities such as the tilted Cold Dark Matter model. Limitations of the catalogues are discussed, so are ways to improve the catalogues so that an accurate map of Hubble flows in our local universe can be drawn and be compared with theoretical predictions.

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