arXiv subjects

L. J. Wang

Publications and source records attributed to L. J. Wang.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Energy Sources of Superluminous Supernovae

Supernovae (SNe) are the most brilliant optical stellar-class explosions. Over the past two decades, several optical transient survey projects discovered more than $\sim 100$ so-called superluminous supernovae (SLSNe) whose peak luminosities and radiated energy are $\gtrsim 7\times 10^{43}$ erg s$^{-1}$ and $\gtrsim 10^{51}$ erg, at least an order of magnitude larger than that of normal SNe. According to their optical spectra features, SLSNe have been split into two broad categories of type I that are hydrogen-deficient and type II that are hydrogen-rich. Investigating and determining the energy sources of SLSNe would be of outstanding importance for understanding the stellar evolution and explosion mechanisms. The energy sources of SLSNe can be determined by analyzing their light curves (LCs) and spectra. The most prevailing models accounting for the SLSN LCs are the $^{56}$Ni cascade decay model, the magnetar spin-down model, the ejecta-CSM interaction model, and the jet-ejecta interaction model. In this \textit{review}, we present several energy-source models and their different combinations.

astro-ph.HE↗

arXiv2018

Generator-coordinate reference states for spectra and $0νββ$ decay in the in-medium similarity renormalization group

We use a reference state based on symmetry-restored states from deformed mean-field or generator-coordinate-method (GCM) calculations in conjunction with the in-medium similarity-renormalization group (IMSRG) to compute spectra and matrix elements for neutrinoless double-beta ($0νββ$) decay. Because the decay involves ground states from two nuclei, we use evolved operators from the IMSRG in one nucleus in a subsequent GCM calculation in the other. We benchmark the resulting IMSRG+GCM method against complete shell-model diagonalization for both the energies of low-lying states in $^{48}$Ca and $^{48}$Ti and the $0νββ$ matrix element for the decay of $^{48}$Ca, all in a single valence shell. Our approach produces better spectra than either the IMSRG with a spherical-mean-field reference or GCM calculations with unevolved operators. For the $0νββ$ matrix element the improvement is slight, but we expect more significant effects in full ab-initio calculations.

nucl-th↗

arXiv2018

Neutron star cooling with a dynamic stellar structure

The observations combined with theory of neutron star (NS) cooling play a crucial role in achieving the intriguing information of the stellar interior, such as the equation of state (EOS), composition and superfluidity of dense matter. The traditional NS cooling theory is based on the assumption that the stellar structure does not change with time. The validity of such a static description has not yet been confirmed. We generalize the theory to a dynamic treatment; that is, continuous change of the NS structure (rearrangement of the stellar density distribution with the total baryon number fixed) as the decrease of temperature during the thermal evolution, is taken into account. It is found that the practical thermal energy used for the cooling is slightly lower than that is estimated in static situation, and hence the cooling of NSs is accelerated correspondingly but the effect is rather weak. Therefore, the static treatment is a good approximation in the calculations of NS cooling.

nucl-th↗

arXiv2018

The correlation of co-located hydrogen masers

The correlation of co-located hydrogen masers (H-masers) is difficult to measure because their common-mode noise induced by the environment will be cancelled out during the comparison measurement. With the development of fibre-based high-precision time and frequency transfer technique, the correlation of co-located hydrogen masers can be directly measured with the help of remote H-masers. Recently, a fiber-based frequency synchronization network was constructed in the Beijing region by connecting 5 H-masers from 4 institutions. The correlation coefficient of atomic clocks is defined and the correlation between two co-located H-masers is measured using both experimental and simulative methods. The results show that the correlation is not prominent until the averaging time is larger than $\sim10^3$s; then, the coefficient grows rapidly for averaging times ranging from $\sim10^3$s to $\sim10^5$s and decreases beyond $\sim10^5$s up to 5 days.

physics.ins-det↗

arXiv2018

Generalized isobaric multiplet mass equation and its application to the Nolen-Schiffer anomaly

The Wigner Isobaric Multiplet Mass Equation (IMME) is the most fundamental prediction in nuclear physics with the concept of isospin. However, it was deduced based on the Wigner-Eckart theorem with the assumption that all charge-violating interactions can be written as tensors of rank two. In the present work, the charge-symmetry breaking (CSB) and charge-independent breaking (CIB) components of the nucleon-nucleon force, which contribute to the effective interaction in nuclear medium, are established in the framework of Brueckner theory with AV18 and AV14 bare interactions. Because such charge-violating components can no longer be expressed as an irreducible tensor due to density dependence, its matrix element cannot be analytically reduced by the Wigner-Eckart theorem. With an alternative approach, we derive a generalized IMME (GIMME) that modifies the coefficients of the original IMME. As the first application of GIMME, we study the long-standing question for the origin of the Nolen-Schiffer anomaly found in the Coulomb displacement energy of mirror nuclei. We find that the naturally-emerged CSB term in GIMME is largely responsible for explaining the Nolen-Schiffer anomaly.

nucl-th↗

arXiv2017

Suppression of Dick Effect in the Ramsey-CPT atomic clock by Interleaving Lock

For most passive atomic clocks, the Dick effect is one of the main limits to reach its frequency stability limitation due to quantum projection noise. In this paper, we demonstrate that the minimization of the Dick effect for the Ramsey-CPT atomic clock can be accomplished by interleaving lock. By optimizing the duty circle of laser pulses, averaging time during detection and optical intensity of laser beam, the Dick effect induced Allan deviation can be reduced to the level of 10^-14.

physics.ins-det↗

arXiv2017

Evidence for magnetar formation in broad-lined type Ic supernovae 1998bw and 2002ap

Broad-lined type Ic supernovae (SNe Ic-BL) are peculiar stellar explosions that distinguish themselves from ordinary SNe. Some SNe Ic-BL are associated with long-duration (\gtrsim 2 s) gamma-ray bursts (GRBs). Black holes and magnetars are two types of compact objects that are hypothesized to be central engines of GRBs. In spite of decades of investigations, no direct evidence for the formation of black holes or magnetars has been found for GRBs so far. Here we report the finding that the early peak (t \lesssim 50 days) and late-time (t \gtrsim 300 days) slow decay displayed in the light curves of both SNe 1998bw (associated with GRB 980425) and 2002ap (not GRB-associated) can be attributed to magnetar spin-down with initial rotation period P0 \sim 20 ms, while the intermediate-time (50 \lesssim t \lesssim 300 days) linear decline is caused by radioactive decay of 56Ni. The connection between the early peak and late-time slow decline in the light curves is unexpected in alternative models. We thus suggest that GRB 980425 and SN 2002ap were powered by magnetars.

astro-ph.HE↗

arXiv2016

A Triple-Energy-Source Model for Superluminous Supernova iPTF13ehe

Almost all superluminous supernovae (SLSNe) whose peak magnitudes are $\lesssim -21$ mag can be explained by the $^{56}$Ni-powered model, magnetar-powered (highly magnetized pulsar) model or ejecta-circumstellar medium (CSM) interaction model. Recently, iPTF13ehe challenges these energy-source models, because the spectral analysis shows that $\sim 2.5M_\odot$ of $^{56}$Ni have been synthesized but are inadequate to power the peak bolometric emission of iPTF13ehe, while the rebrightening of the late-time light-curve (LC) and the H$α$ emission lines indicate that the ejecta-CSM interaction must play a key role in powering the late-time LC. Here we propose a triple-energy-source model, in which a magnetar together with some amount ($\lesssim 2.5M_\odot$) of $^{56}$Ni may power the early LC of iPTF13ehe while the late-time rebrightening can be quantitatively explained by an ejecta-CSM interaction. Furthermore, we suggest that iPTF13ehe is a genuine core-collapse supernova rather than a pulsational pair-instability supernova candidate. Further studies on similar SLSNe in the future would eventually shed light on their explosion and energy-source mechanisms.

astro-ph.HE↗

arXiv2016

Reverse Shock Emission Driven By Post-Merger Millisecond Magnetar Winds: Effects of the Magnetization Parameter

The study of short-duration gamma-ray bursts provides growing evidence that a good fraction of double neutron star mergers lead to the formation of stable millisecond magnetars. The launch of Poynting flux by the millisecond magnetars could leave distinct electromagnetic signatures that reveal the energy dissipation processes in the magnetar wind. In previous studies (Wang & Dai 2013b; Wang et al. 2015), we assume that the magnetar wind becomes completely lepton-dominated so that electrons/positrons in the magnetar wind are accelerated by a diffusive shock. However, theoretical modeling of pulsar wind nebulae shows that in many cases the magnetic field energy in the pulsar wind may be strong enough to suppress diffusive shock acceleration. In this paper, we investigate the reverse shock emission as well as the forward shock emission with an arbitrary magnetization parameter $σ$ of a magnetar wind. We find that the reverse shock emission strongly depends on $σ$, and in particular, $σ\sim 0.3$ leads to the strongest reverse shock emission. Future observations would be helpful to diagnose the composition of the magnetar wind.

astro-ph.HE↗

arXiv2016

Optical Transients Powered by Magnetars: Dynamics, Light Curves, and Transition to the Nebular Phase

Millisecond magnetars can be formed via several channels: core-collapse of massive stars, accretion-induced collapse of white dwarfs (WDs), double WD mergers, double neutron star (NS) mergers, and WD-NS mergers. Because the mass of ejecta from these channels could be quite different, their light curves are also expected to be diverse. We evaluate the dynamic evolution of optical transients powered by millisecond magnetars. We find that the magnetar with short spin-down timescale converts its rotational energy mostly into the kinetic energy of the transient, while the energy of a magnetar with long spin-down timescale goes into radiation of the transient. This leads us to speculate that hypernovae could be powered by magnetars with short spin-down timescales. At late times the optical transients will gradually evolve into a nebular phase because of the photospheric recession. We treat the photosphere and nebula separately because their radiation mechanisms are different. In some cases the ejecta could be light enough that the magnetar can accelerate it to a relativistic speed. It is well known that the peak luminosity of a supernova (SN) occurs when the luminosity is equal to the instantaneous energy input rate, as shown by Arnett (1979). We show that photospheric recession and relativistic motion can modify this law. The photospheric recession always leads to a delay of the peak time $t_{\mathrm{pk}}$ relative to the time $t_{\times }$ at which the SN luminosity equals the instantaneous energy input rate. Relativistic motion, however, may change this result significantly.

astro-ph.HE↗

arXiv2016

Accurate and fast fiber transfer delay measurement based on phase discrimination and frequency measurement

An accurate and fast fiber transfer delay measurement method is demonstrated. As a key technique, a simple ambiguity resolving process based on phase discrimination and frequency measurement is used to overcome the contradiction between measurement accuracy and system complexity. The optimized system achieves a high accuracy of 0.3 ps with a 0.1 ps resolution, and a large dynamic range up to 50 km as well as no dead zone.

physics.ins-det↗

arXiv2016

Probing the Birth of Post-merger Millisecond Magnetars by X-ray and Gamma-ray Emission

There is growing evidence that a stable magnetar could be formed from the coalescence of double neutron stars. In previous papers, we investigated the signature of formation of stable millisecond magnetars in radio and optical/ultraviolet bands by assuming that the central rapidly rotating magnetar deposits its rotational energy in the form of a relativistic leptonized wind. We found that the optical transient PTF11agg could be the first evidence for the formation of post-merger millisecond magnetars. To enhance the probability of finding more evidence for the post-merger magnetar formation, it is better to extend the observational channel to other photon energy bands. In this paper we propose to search the signature of post-merger magnetar formation in X-ray and especially gamma-ray bands. We calculate the SSC emission of the reverse shock powered by post-merger millisecond magnetars. We find that the SSC component peaks at $1\,{\rm GeV}$ in the spectral energy distribution and extends to $\gtrsim 10\,{\rm TeV}$ for typical parameters. These energy bands are quite suitable for Fermi/LAT and CTA, which, with their current observational sensitivities, can detect the SSC emission powered by post-merger magnetars up to $1\,{\rm Gpc}$. NuSTAR, sensible in X-ray bands, can detect the formation of post-merger millisecond magnetars at redshift $z\sim 1$. Future improvement in sensitivity of CTA can also probe the birth of post-merger millisecond magnetars at redshift $z\sim 1$. However, because of the $γ$-$γ$ collisions, strong high-energy emission is clearly predicted only for ejecta masses lower than $10^{-3}M_\odot$.

astro-ph.HE↗

arXiv2015

The Most Luminous Supernova ASASSN-15lh: Signature of a Newborn Rapidly-Rotating Strange Quark Star

In this paper we show that the most luminous supernova discovered very recently, ASASSN-15lh, could have been powered by a newborn ultra-strongly-magnetized pulsar, which initially rotates near the Kepler limit. We find that if this pulsar is a neutron star, its rotational energy could be quickly lost as a result of gravitational-radiation-driven r-mode instability; if it is a strange quark star, however, this instability is highly suppressed due to a large bulk viscosity associated with the nonleptonic weak interaction among quarks and thus most of its rotational energy could be extracted to drive ASASSN-15lh. Therefore, we conclude that such an ultra-energetic supernova provides a possible signature for the birth of a strange quark star.

astro-ph.HE↗

arXiv2015

Highly accurate fiber transfer delay measurement with large dynamic range

A novel and efficient method for fiber transfer delay measurement is demonstrated. Fiber transfer delay measurement in time domain is converted into the frequency measurement of the modulation signal in frequency domain, accompany with a coarse and easy ambiguity resolving process. This method achieves a sub-picosecond resolution, with an accuracy of 1 picosecond, and a large dynamic range up to 50 km as well as no measurement dead zone.

physics.ins-det↗

arXiv2015

Intense source of cold cesium atoms based on a two-dimensional magneto-optical trap with independent axial cooling and pushing

We report our studies on an intense source of cold cesium atoms based on a two-dimensional magneto-optical trap with independent axial cooling and pushing. The new-designed source, proposed as 2D-HP MOT, uses hollow laser beams for axial cooling and a thin pushing laser beam for cold atomic beam extraction. Regulated independently by the pushing beam, the atomic flux can be substantially optimized. The atomic flux maximum obtained in the 2D-HP MOT is $4.02\times 10^{10}$ atoms/s, increased by 60 percent compared to the traditional 2D$^+$ MOT in our experiment. Moreover, with the pushing power 10 $μ$W and detuning $0Γ$, the 2D-HP MOT can generate a rather intense cold cesium atomic beam with the concomitant light shift suppressed by 20 times in magnitude. The axial velocity distribution of the cold cesium beams centers at 6.8 m/s with a FMHW of about 2.8 m/s. The dependences of the atomic flux on the pushing power and detuning are studied. The experimental results are in good agreement with the theoretical model.

physics.atom-ph↗

arXiv2015

Temperature and number evolution of cold cesium atoms inside a wall-coated glass cell

We report an experimental study on the temperature and number evolution of cold cesium atoms diffusively cooled inside a wall-coated glass cell by measuring the absorption profile of the 62S1/2 (F=4)-62P3/2 (F'=5) transition line with a weak probe laser in the evolution process. We found that the temperature of the cold atoms first gradually decreases from 16 mK to 9 mK, and then rapidly increases. The number of cold atoms first declines slowly from 2.1*10^9 to 3.7*10^8 and then falls drastically. A theoretical model for the number evolution is built and includes the instantaneous temperature of the cold atoms and a fraction p, which represents the part of cold cesium atoms elastically reflected by the coated cell wall. The theory is overall in good agreement with the experimental result and a nonzero value is obtained for the fraction p, which indicates that the cold cesium atoms are not all heated to the ambient temperature by a single collision with the coated cell wall. These results can provide helpful insight for precision measurements based on diffuse laser cooling.

physics.atom-ph↗

arXiv2015

Square Kilometer Array Telescope - Precision Reference Frequency Synchronisation via 1f-2f Dissemination

The Square Kilometer Array (SKA) is an international effort to build the world's largest radio telescope, with one square kilometer collecting area. Besides its ambitious scientific objectives, such as probing the cosmic dawn and cradle of life, SKA also demands several revolutionary technological breakthroughs, with ultra-high precision synchronisation of the frequency references for thousands of antennas being one of them. In this report, aimed at applications to SKA, we demonstrate a frequency reference synchronization and dissemination scheme with the phase noise compensation function placed at the client site. Hence, one central hub can be linked to a large number of client sites, forming a star-shaped topology. As a performance test, the 100 MHz reference signal from a Hydrogen maser clock is disseminated and recovered at two remote sites. Phase noise characteristics of the recovered reference frequency signal coincides with that of the hydrogen-maser source and satisfies SKA requirement.

physics.ins-det↗

arXiv2015

Dissemination stability and phase noise specification of fiber-cascaded RF frequency dissemination

Over the past decade, fiber based frequency dissemination has achieved significant progresses in following aspects. To study the dissemination stability and phase noise specifications of the fiber-cascaded RF frequency dissemination system, we perform a lab-top experiment using three sets of RF modulated frequency dissemination systems. They are linked by 50 km + 50 km +45 km fiber spools. The dissemination stabilities of each segments and whole system are measured simultaneously. After that, the phase noise of each segment is also measured and the result shows that the phase noise spectrums of recovered frequency signal can be optimized via narrow band phase lock loop.

physics.ins-det↗