arXiv subjects

Ze-Cheng Zou

Publications and source records attributed to Ze-Cheng Zou.

24 records · Page 2Linked to original sources

arXiv2023

Redshift Dependence of the Low-energy Spectral Index of Gamma-Ray Bursts Revisited

A negative correlation was found to exist between the low-energy spectral index and the redshift of gamma-ray bursts (GRBs) by Amati(2002). It was later confirmed by Geng(2013) and Gruber(2014), but the correlation was also found to be quite dispersive when the sample size was significantly expanded. In this study, we have established two even larger samples of gamma-ray bursts to further examine the correlation. One of our sample is consisted of 316 GRBs detected by the Swift satellite, and the other one is consisted of 80 GRBs detected by the Fermi satellite. It is found that there is no correlation between the two parameters for the Swift sample, but there does exist a weak negative correlation for the Fermi sample. The correlation becomes even more significant when the spectral index at the peak flux is considered. It is argued that the absence of the correlation in the Swift sample may be due to the fact that Swift has a very narrow energy response so that it could not measure the low-energy spectral index accurately enough. Further studies based on even larger GRB samples are solicited.

astro-ph.HE↗

arXiv2023

Gluon Condensation Signature in the GeV Gamma-Ray Spectra of Pulsars

The accumulation of gluons inside nucleons, i.e., the gluon condensation, may lead to a characteristic broken power-law gamma-ray spectrum in high-energy nucleon collisions. Here we show that the observed spectra of at least 25 sources in the second Fermi Large Area Telescope Catalog of Gamma-ray Pulsars can be well fitted by such a broken power-law function that has only four free parameters. It strongly indicates that the gamma-ray emission from these pulsars is of hadronic origin, but with gluon condensation inside hadrons. It is well known that the quark-gluon distribution in a free nucleon is different from that in a bound nucleon. This work exposes the nuclear $A$ dependence of the gluon condensation effect, where $A$ refers to the baryon number. Our study reveals the gluon condensation under the condition of $A\to\infty$, which may open a new window for eavesdropping on the structure of compact stars on the subnuclear level.

astro-ph.HE↗

arXiv2023

Gravitational-wave Emission from a Primordial Black Hole Inspiraling inside a Compact Star: a Novel Probe for Dense Matter Equation of State

Primordial black holes of planetary masses captured by compact stars are widely studied to constrain their composition fraction of dark matter. Such a capture may lead to an inspiral process and be detected through gravitational wave signals. In this Letter, we study the post-capture inspiral process by considering two different kinds of compact stars, i.e., strange stars and neutron stars. The dynamical equations are numerically solved and the gravitational wave emission is calculated. It is found that the Advanced LIGO can detect the inspiraling of a $10^{-5}$ solar mass primordial black hole at a distance of 10 kpc, while a Jovian-mass case can even be detected at megaparsecs. Promisingly, the next generation gravitational wave detectors can detect the cases of $10^{-5}$ solar mass primordial black holes up to ${\sim}1$ Mpc, and can detect Jovian-mass cases at several hundred megaparsecs. Moreover, the kilohertz gravitational wave signal shows significant differences for strange stars and neutron stars, potentially making it a novel probe to the dense matter equation of state.

astro-ph.HE↗

arXiv2022

Gravitational Waves from Strange Star Core-Crust Oscillation

According to the strange quark matter hypothesis, pulsars may actually be strange stars composed of self-bound strange quark matter. The normal matter crust of a strange star, unlike that of a normal neutron star, is supported by a strong electric field. A gap is then presented between the crust and the strange quark core. Therefore, peculiar core-crust oscillation may occur in a strange star, which can produce distinctive gravitational waves. In this paper, the waveforms of such gravitational waves are derived using a rigid model. We find that the gravitational waves are extremely weak and undetectable, even for the next-generation detectors. Therefore, the seismology of a strange star is not affected by the core-crust oscillation. Observers will have to search for other effects to diagnose the existence of the crust.

astro-ph.HE↗

arXiv2021

Gamma-Ray Burst in a Binary System

Regardless of their different types of progenitors and central engines, gamma-ray bursts (GRBs) were always assumed to be standalone systems after they formed. Little attention has been paid to the possibility that a stellar companion can still accompany a GRB itself. This paper investigates such a GRB-involved binary system and studies the effects of the stellar companion on the observed GRB emission when it is located inside the jet opening angle. Assuming a typical emission radius of $\sim10^{15}\,$cm, we show that the blockage by a companion star with a radius of $R_\mathrm{c}\sim67\,\mathrm{R_\odot}$ becomes non-negligible when it is located within a typical GRB jet opening angle (e.g., $\sim10$ degrees) and beyond the GRB emission site. In such a case, an on-axis observer will see a GRB with a similar temporal behavior but 25% dimmer. On the other hand, an off-axis observer outside the jet opening angle (hence missed the original GRB) can see a delayed "reflected" GRB, which is much fainter in brightness, much wider in the temporal profile and slightly softer in energy. Our study can naturally explain the origin of some low-luminosity GRBs. Moreover, we also point out that the companion star may be shocked if it is located inside the GRB emission site, which can give rise to an X-ray transient or a GRB followed by a delayed X-ray bump on top of X-ray afterglows.

astro-ph.HE↗

arXiv2020

The Gravitational Wave Emission of Double White Dwarf Coalescences

Type Ia Supernovae (SNe Ia) are widely used as standard candles to probe the Universe. However, how these fierce explosions are produced itself is still a highly debated issue. There are mainly two popular models for SNe Ia, i.e. the double-degenerate scenario and the single-degenerate scenario. The double-degenerate scenario suggests that SNe Ia are produced by the coalescence of two degenerate white dwarfs, while the single-degenerate scenario suggests that the continuous accretion of a single degenerate white dwarf from its normal stellar companion will finally lead to a disastrous explosion when it is over-massive, resulting in an SN Ia. The rapid development of the gravitational wave astronomy sheds new light on the nature of SNe Ia. In this study, we calculate the gravitational wave emissions of double white dwarf coalescences and compare them with the sensitivities of several upcoming detectors. It is found that the gravitational wave emissions from double white dwarf mergers in the locale universe are strong enough to be detected by LISA. We argue that LISA-like gravitational wave detectors sensitive in the frequency range of 0.01 - 0.1 Hz will be a powerful tool to test the double-degenerate model of SNe Ia, and also to probe the Universe.

astro-ph.HE↗