arXiv subjects

Jifeng Liu

Publications and source records attributed to Jifeng Liu.

At least 199 records · Page 11Linked to original sources

arXiv2021

Do the periodic activities of repeating fast radio bursts represent the spins of neutron stars?

Fast radio bursts (FRBs) are mysterious radio transients with millisecond durations. Recently, a periodic activity of $\sim$16 day and a possible periodicity of $\sim$159 day were detected to arise from FRB 180916.J0158+65 and FRB 121102, respectively, and the spin period of a slow-rotation magnetar was further considered to be one of possibilities to explain the periodic activities of repeating FRBs. For isolated neutron stars, the spin evolution suggests that it's difficult to reach several hours. In this work, we mainly focus on the possible maximum spin period of isolated NSs / magnetars dominated by an interaction between star's magnetic field and the disk. We find that the disk wind plays an important role in spin evolution, whose influence varies the power law index in the evolution equation of mass flow rate. For a magnetar without disk wind, the longest spin period is tens of hours. When the disk wind with a classical parameter is involved, the maximum spin period can reach hundreds of hours. But for a much extremely large index of mass flow rate due to disk wind or other angular momentum extraction processes, a spin period of $\sim$(16-160) days is still possible.

astro-ph.HE↗

arXiv2021

Insight-HXMT observations of jet-like corona in a black hole X-ray binary MAXI J1820+070

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster.

astro-ph.HE↗

arXiv2021

Maurer-Cartan characterizations and cohomologies of compatible Lie algebras

In this paper, we give Maurer-Cartan characterizations as well as a cohomology theory for compatible Lie algebras. Explicitly, we first introduce the notion of a bidifferential graded Lie algebra and thus give Maurer-Cartan characterizations of compatible Lie algebras. Then we introduce a cohomology theory of compatible Lie algebras and use it to classify infinitesimal deformations and abelian extensions of compatible Lie algebras. In particular, we introduce the reduced cohomology of a compatible Lie algebra and establish the relation between the reduced cohomology of a compatible Lie algebra and the cohomology of the corresponding compatible linear Poisson structures introduced by Dubrovin and Zhang in their study of bi-Hamiltonian structures. Finally, we use the Maurer-Cartan approach to classify nonabelian extensions of compatible Lie algebras.

math-ph↗

arXiv2021

Correction to the photometric colors of Gaia Early Data Release 3

In this work, we use the spectroscopy-based stellar color regression (SCR) method with ~ 0.7 million common stars between LAMOST DR7 and Gaia EDR3 to acquire color corrections in G - GRP and GBP - GRP. A sub-mmag precision is achieved. Our results demonstrate that improvements in the calibration process of the EDR3 have removed the color term in GBP - GRP and eliminated the discontinuity caused by the changes of instrument configurations to a great extent. However, modest systematic trends with G magnitude are still detected. The corresponding color correction terms as a function of G are provided for 9.5 < G < 17.5 mag and compared with other determinations. We conclude that the corrections given in this work are particularly suited for cases where the color-color investigations are required while for color-magnitude investigations other corrections may be better due to systematic with reddening. Possible applications of our results are discussed.

astro-ph.SR↗

arXiv2021

Correction to the photometric colors of the Gaia Data Release 2 with the stellar color regression method

The second Gaia data release (DR2) delivers accurate and homogeneous photometry data of the whole sky to an exquisite quality, reaching down to the unprecedented milli-magnitude (mmag) level for the G, GRP, and GBP passbands. However, the presence of magnitude-dependent systematic effects at the 10 mmag level limits its power in scientific exploitation. In this work, using about half-million stars in common with the LAMOST DR5, we apply the spectroscopy-based stellar color regression method to calibrate the Gaia G-GRP and GBP-GRP colors. With an unprecedented precision of about 1 mmag, systematic trends with G magnitude are revealed for both colors in great detail, reflecting changes in instrument configurations. Color dependent trends are found for the GBP-GRP color and for stars brighter than G~11.5 mag. The calibration is up to 20 mmag in general and varies a few mmag/mag. A revised color-color diagram of Gaia DR2 is given, and some applications are briefly discussed.

astro-ph.SR↗

arXiv2021

Billion-pixel X-ray camera (BiPC-X)

The continuing improvement in quantum efficiency (above 90% for single visible photons), reduction in noise (below 1 electron per pixel), and shrink in pixel pitch (less than 1 micron) motivate billion-pixel X-ray cameras (BiPC-X) based on commercial CMOS imaging sensors. We describe BiPC-X designs and prototype construction based on flexible tiling of commercial CMOS imaging sensors with millions of pixels. Device models are given for direct detection of low energy X-rays ($<$ 10 keV) and indirect detection of higher energies using scintillators. Modified Birks's law is proposed for light-yield nonproportionality in scintillators as a function of X-ray energy. Single X-ray sensitivity and spatial resolution have been validated experimentally using laboratory X-ray source and the Argonne Advanced Photon Source. Possible applications include wide field-of-view (FOV) or large X-ray aperture measurements in high-temperature plasmas, the state-of-the-art synchrotron, X-ray Free Electron Laser (XFEL), and pulsed power facilities.

physics.ins-det↗

arXiv2020

Non-interacting black hole binaries with Gaia and LAMOST

Until recently, black holes (BHs) could be discovered only through accretion from other stars in X-ray binaries, or in merging double compact objects. Improvements in astrometric and spectroscopic measurements have made it possible to detect BHs also in non-interacting BH binaries (nBHB) through a precise analysis of the companion's motion. In this study, using an updated version of the Startrack binary-star population modelling code and a detailed model of the Milky Way (MW) galaxy we calculate the expected number of detections for Gaia and LAMOST surveys. We develop a formalism to convolve the binary population synthesis output with a realistic stellar density distribution, star-formation history (SFH), and chemical evolution for the MW, which produces a probability distribution function of the predicted compact-binary population over the MW. This avoids the additional statistical uncertainty which is introduced by methods which Monte Carlo sample from binary population synthesis output to produce one potential specific realisation of the MW compact-binary distribution, and our method is also comparatively fast to such Monte Carlo realisations. Specifically, we predict $\sim41$-$340$ nBHBs to be observed by Gaia, although the numbers may drop to $\sim10$-$70$ if the recent ($\lesssim100\;$ Myr) star formation is low ($\sim1\;M_\odot$/yr ). For LAMOST we predict $\lesssim14$ detectable nBHBs, which is lower partially because its field-of-view covers just $\sim6\%$ of the Galaxy.

astro-ph.HE↗

arXiv2020

Short-range order in GeSn alloy

Group IV alloys have been long viewed as homogeneous random solid solutions since they were first perceived as Si-compatible, direct-band-gap semiconductors 30 years ago. Such a perception underlies the understanding, interpretation and prediction of alloys' properties. However, as the race to create scalable and tunable device materials enters a composition domain far beyond alloys' equilibrium solubility, a fundamental question emerges as to how random these alloys truly are. Here we show, by combining statistical sampling and large-scale ab initio calculations, that GeSn alloy, a promising group IV alloy for mid-infrared technology, exhibits a clear, short-range order for solute atoms within its entire composition range. Such short-range order is further found to substantially affect the electronic properties of GeSn. We demonstrate the proper inclusion of this short-range order through canonical sampling can lead to a significant improvement over previous predictions on alloy's band gaps, by showing an excellent agreement with experiments within the entire studied composition range. Our finding thus not only calls for an important revision of current structural model for group IV alloy, but also suggests short-range order may generically exist in different types of alloys.

cond-mat.mtrl-sci↗

arXiv2020

Electrically injected GeSn lasers with peak wavelength up to 2.7 micrometer at 90 K

GeSn lasers enable monolithic integration of lasers on the Si platform using all-group-IV direct-bandgap materials. Although optically pumped GeSn lasers have made significant progress, the study of the electrically injected lasers has just begun only recently. In this work, we present explorative investigations of electrically injected GeSn heterostructure lasers with various layer thicknesses and material compositions. The cap layer total thickness was varied between 240 and 100 nm. At 10 K, a 240-nm-SiGeSn capped device had a threshold current density Jth = 0.6 kA/cm2 compared to Jth = 1.4 kA/cm2 of a device with 100-nm-SiGeSn cap due to an improved modal overlap with the GeSn gain region. Both devices had a maximum operating temperature Tmax = 100 K. Device with cap layers of Si0.03Ge0.89Sn0.08 and Ge0.95Sn0.05, respectively, were also compared. Due to less effective carrier (electron) confinement, the device with a 240-nm-GeSn cap had a higher threshold Jth = 2.4 kA/cm2 and lower maximum operating temperature Tmax = 90 K, compared to those of the 240-nm-SiGeSn capped device with Jth = 0.6 kA/cm2 and Tmax = 100 K. In the study of the active region material, the device with Ge0.85Sn0.15 active region had a 2.3 times higher Jth and 10 K lower Tmax, compared to the device with Ge0.89Sn0.11 in its active region. This is likely due to higher defect density in Ge0.85Sn0.15 rather than an intrinsic issue. The longest lasing wavelength was measured as 2682 nm at 90 K. The investigations provide guidance to the future structure design of GeSn laser diodes to further improve the performance.

physics.app-ph↗

arXiv2020

Stellar X-ray activity across the Hertzsprung-Russell diagram. I. Catalogs

Stellar magnetic activity provides substantial information on the magnetic dynamo and the coronal heating process. We present a catalog of X-ray activity for about 6000 stars, based on the $Chandra$ and $Gaia$ DR2 data. We also classified more than 3000 stars as young stellar objects, dwarf stars, or giant stars. By using the stars with valid stellar parameters and classifications, we studied the distribution of X-ray luminosity ($L_X$) and the ratio of X-ray-to-bolometric luminosities ($R_X$), the positive relation between $L_X$, $R_X$, and hardness ratio, and the long-term X-ray variation. This catalog can be used to investigate some important scientific topics, including the activity-rotation relation, the comparison between different activity indicators, and the activities of interesting objects (e.g., A-type stars and giants). As an example, we use the catalog to study the activity-rotation relation, and find that the young stellar objects, dwarfs, and giants fall on a single sequence in the relation $R_X$ versus Rossby number, while the giants do not follow the relation $R_X$ versus $P_{\rm rot}^{-2}R^{-4}$ valid for dwarfs.

astro-ph.SR↗

arXiv2020

Monte Carlo Modeling and Design of Photon Energy Attenuation Layers (PALs) for 10-30x Quantum Yield Enhancement in Si-based Hard X-ray Detectors

High-energy (>20keV) X-ray photon detection at high quantum yield, high spatial resolution and short response time has long been an important area of study in physics. Scintillation is a prevalent method but limited in various ways. Directly detecting high-energy X-ray photons has been a challenge to this day, mainly due to low photon-to-photoelectron conversion efficiencies. Commercially available state-of-the-art Si direct detection products such as the Si charge-coupled device (CCD) are inefficient for >10keV photons. Here, we present Monte Carlo simulation results and analyses to introduce a highly effective yet simple high-energy X-ray detection concept with significantly enhanced photon-to-electron conversion efficiencies composed of two layers: a top high-Z photon energy attenuator layer (PAL) and a bottom Si detector. We use the principle of photon energy down conversion, where high-energy X-ray photon energies are attenuated down to and below 10keV via inelastic scattering suitable for efficient photoelectric absorption by Si. Our Monte Carlo simulation results demonstrate that 10-30x increase in quantum yield can be achieved using PbTe PAL on Si, potentially advancing high-resolution, high-efficiency X-ray detection using PAL-enhanced Si CMOS image sensors.

physics.ins-det↗

arXiv2020

KIC 12268220: A $δ$ Scuti Pulsating Star and an Active Protohelium White Dwarf in an Eclipsing Binary System

We present a photometric, spectroscopic, asteroseismic, and evolutionary analysis of the Algol-type eclipsing binary KIC 12268220. We find the O'Connell effect and anticorrelated eclipse timing variations in the Kepler light curve, revealing the presence of large starspots. Radial velocities and atmospheric parameters are obtained from ground-based spectroscopic observations. Combined with the radial velocity measurements and Gaia-derived total luminosity, our light-curve modeling yields the solution of the physical parameters for both the primary and secondary components. We find 14 independent frequencies arising from the $δ$ Scuti primary, and the observed frequencies agree with the frequency range of unstable modes from nonadiabatic calculations. Based on the conclusion from previous literature, we run a grid of models to study the evolution process of our system. The evolutionary tracks of our model suggest that the low-mass ($\sim 0.23\,M_\odot$) evolved secondary shows a similar evolutionary state to the R CMa-type system, which might evolve to an EL CVn system.

astro-ph.SR↗

arXiv2020

Phase-dependent study of near-infrared disk emission lines in LB-1

The mass, origin and evolutionary stage of the binary system LB-1 has been the subject of intense debate, following the claim that it hosts an $\sim$70$M_{\odot}$ black hole, in stark contrast with the expectations for stellar remnants in the Milky Way. We conducted a high-resolution, phase-resolved spectroscopic study of the near-infrared Paschen lines in this system, using the 3.5-m telescope at Calar Alto Observatory. We find that Pa$β$ and Pa$γ$ (after proper subtraction of the stellar absorption component) are well fitted with a standard double-peaked model, typical of disk emission. We measured the velocity shifts of the red and blue peaks at 28 orbital phases: the line center has an orbital motion in perfect antiphase with the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s for different choices of lines and profile modelling. We interpret this curve as proof that the disk is tracing the orbital motion of the primary, ruling out the circumbinary disk and the hierarchical triple scenarios. The phase-averaged peak-to-peak half-separation (proxy for the projected rotational velocity of the outer disk) is $\sim$70 km s$^{-1}$, larger than the stellar orbital velocity and also inconsistent with a circumbinary disk. From those results, we infer a primary mass 4--8 times higher than the secondary mass. Moreover, we show that the ratio of the blue and red peaks (V/R intensity ratio) has a sinusoidal behaviour in phase with the secondary star, which can be interpreted as the effect of external irradiation by the secondary star on the outer disk. Finally, we briefly discuss our findings in the context of alternative scenarios recently proposed for LB-1. Definitive tests between alternative solutions will require further astrometric data from $Gaia$.

astro-ph.SR↗

arXiv2020

Distances to the Supernova Remnants in the Inner Disk

Distance measurements of supernova remnants (SNRs) are essential and important. Accurate estimates of physical size, dust masses, and some other properties of SNRs depend critically on accurate distance measurements. However, the determination of SNR distances is still a tough task. Red clump stars (RCs) have a long history been used as standard candles. In this work, we take RCs as tracers to determine the distances to a large group of SNRs in the inner disk. We first select RC stars based on the near-infrared (IR) color-magnitude diagram (CMD). Then, the distance to and extinction of RC stars are calculated. To extend the measurable range of distance, we combine near-IR photometric data from the 2MASS survey with the deeper UKIDSS and VVV surveys. With the help of the Gaia parallaxes, we also remove contaminants including dwarfs and giants. Because an SN explosion compresses the surrounding interstellar medium, the SNR region would become denser and exhibit higher extinction than the surroundings. The distance of a SNR is then recognized by the position where the extinction and its gradient is higher than that of the ambient medium. A total of 63 SNRs' distances in the Galactic inner disk are determined and divided into three Levels A, B, and C with decreasing reliability. The distances to 43 SNRs are well determined with reliability A or B. The diameters and dust masses of SNRs are estimated with the obtained distance and extinction.

astro-ph.GA↗

arXiv2020

LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS): Scientific goals and survey plan

Since September 2018, LAMOST starts a new 5-year medium-resolution spectroscopic survey (MRS) using bright/gray nights. We present the scientific goals of LAMOST-MRS and propose a near optimistic strategy of the survey. A complete footprint is also provided. Not only the regular medium-resolution survey, but also a time-domain spectroscopic survey is being conducted since 2018 and will be end in 2023. According to the detailed survey plan, we expect that LAMOST-MRS can observe about 2 million stellar spectra with ~7500 and limiting magnitude of around G=15 mag. Moreover, it will also provide about 200 thousand stars with averagely 60-epoch observations and limiting magnitude of G~14 mag. These high quality spectra will give around 20 elemental abundances, rotational velocities, emission line profiles as well as precise radial velocity with uncertainty less than 1 km/s. With these data, we expect that LAMOST can effectively leverage sciences on stellar physics, e.g. exotic binary stars, detailed observation of many types of variable stars etc., planet host stars, emission nebulae, open clusters, young pre-main-sequence stars etc.

astro-ph.SR↗

arXiv2020

Electrically injected GeSn lasers on Si operating up to 100 K

The significant progress of GeSn material development has enabled a feasible solution to the long-desired monolithically integrated lasers on the Si platform. While there are many reports focused on optically pumped lasers, GeSn lasers through electrical injection have not been experimentally achieved yet. In this work, we report the first demonstration of electrically injected GeSn lasers on Si. A GeSn/SiGeSn heterostructure diode grown on a Si substrate was fabricated into ridge waveguide laser devices and tested under pulsed conditions. Special considerations were given for the structure design to ensure effective carrier confinement and optical confinement that lead to lasing. Lasing was observed at temperatures from 10 to 100 K with emission peaks at around 2300 nm. The minimum threshold of 598 A/cm2 was recorded at 10 K and the threshold increased to 842 A/cm2 at 77 K. The spectral linewidth of a single peak was measured as small as 0.13 nm (0.06 meV). The maximum characteristic temperature was extracted as 99 K over the temperature range of 10-77 K.

physics.app-ph↗

arXiv2020

The Formation of a 70 Msun Black Hole at High Metallicity

A 70Msun BH was discovered in Milky Way disk in a long period and almost circular detached binary system (LB-1) with a high metallicity 8Msun B star companion. Current consensus on the formation of BHs from high metallicity stars limits the black hole mass to be below 20Msun. Using simple evolutionary model, we show that the formation of a 70Msun BH in high metallicity environment is possible if stellar wind mass loss rates are reduced by factor of 5. As observations indicate, a fraction of massive stars have surface magnetic fields which may quench the wind mass-loss, independently of stellar mass and metallicity. We also computed detailed stellar evolution models and we confirm such a scenario. A non-rotating 85Msun model at Z=0.014 with decreased winds ends up as a 71Msun star prior core-collapse with a 32Msun helium core and a 28Msun CO core. Such star avoids pair-instability pulsation supernova mass loss and may form a 70Msun BH in the direct collapse. Stars that can form such BHs expand to significant size with radius of R>600Rsun, exceeding the size of LB-1 orbit. Therefore, we can explain the formation of BHs upto 70Msun at high metallicity and this result is independent from LB-1. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.

astro-ph.HE↗

arXiv2019

A wide star-black-hole binary system from radial-velocity measurements

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$α$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.

astro-ph.SR↗