arXiv subjects

B. Zhang

Publications and source records attributed to B. Zhang.

At least 127 records · Page 7Linked to original sources

arXiv2017

Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B

The ejecta composition is an open question in gamma-ray bursts (GRB) physics. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function, consistent with a synchrotron radiation origin and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component suggesting a likely hybrid jet composition. Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being $\sim$ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L$_{\rm p, iso}\sim 4\times 10^{53}$ erg/s) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.

astro-ph.HE↗

arXiv2017

Theoretical Description Of GRB 160625B with Wind-to-ISM Transition and Implications for a Magnetized Outflow

GRB 160625B, one of the brightest bursts in recent years, was simultaneously observed by Fermi and Swift satellites, and ground-based optical telescopes in three different events separated by long periods of time. In this paper the non-thermal multiwavelength observations of GRB 160625B are described and a transition phase from wind-type-like medium to interstellar medium between the early (event II) and the late (event III) afterglow is found. The multiwavelength observations of the early afterglow are consistent with the afterglow evolution starting at $\sim$ 150 s in a stellar wind medium whereas the observations of the late afterglow are consistent with the afterglow evolution in interstellar medium (ISM). The wind-to-ISM transition is calculated to be at $\sim 8\times 10^3$ s when the jet has decelerated, at a distance of $\sim$ 1 pc from the progenitor. Using the standard external shock model, the synchrotron and synchrotron self-Compton emission from reverse shock is required to model the GeV $γ$-ray and optical observations in the early afterglow, and synchrotron radiation from the adiabatic forward shock to describe the X-ray and optical observations in the late afterglow. The derived values of the magnetization parameter, the slope of the fast decay of the optical flash and the inferred magnetic fields suggest that Poynting flux-dominated jet models with arbitrary magnetization could account for the spectral properties exhibited by GRB 160625B.

astro-ph.HE↗

arXiv2017

A New Stellar Outburst Associated with the Magnetic Activities of the K-type Dwarf in a White-dwarf Binary

1SWASP\,J162117.36$+$441254.2 was originally classified as an EW-type binary with a period of 0.20785\,days. However, it was detected to have undergone a stellar outburst on June 3, 2016. Although the system was latter classified as a cataclysmic variable (CV) and the event was attributed as a dwarf-nova outburst, the physical reason is still unknown. This binary has been monitored photometrically since April 19, 2016 and many light curves were obtained before, during and after the outburst. Those light and color curves observed before the outburst indicate that the system is a special CV. The white dwarf is not accreting material from the secondary and there are no accretion disks surrounding the white dwarf. By comparing the light curves obtained from April 19 to September 14, 2016, it was found that magnetic activity of the secondary is associated with the outburst. We show strong evidence that the $L_1$ region on the secondary was heavily spotted before and after the outburst and thus quench the mass transfer, while the outburst is produced by a sudden mass accretion of the white dwarf. These results suggest that J162117 is a good astrophysical laboratory to study stellar magnetic activity and its influences on CV mass transfer and mass accretion.

astro-ph.SR↗

arXiv2017

Dark Energy Survey Year 1 Results: Cross-Correlation Redshifts - Methods and Systematics Characterization

We use numerical simulations to characterize the performance of a clustering-based method to calibrate photometric redshift biases. In particular, we cross-correlate the weak lensing (WL) source galaxies from the Dark Energy Survey Year 1 (DES Y1) sample with redMaGiC galaxies (luminous red galaxies with secure photometric redshifts) to estimate the redshift distribution of the former sample. The recovered redshift distributions are used to calibrate the photometric redshift bias of standard photo-$z$ methods applied to the same source galaxy sample. We apply the method to three photo-$z$ codes run in our simulated data: Bayesian Photometric Redshift (BPZ), Directional Neighborhood Fitting (DNF), and Random Forest-based photo-$z$ (RF). We characterize the systematic uncertainties of our calibration procedure, and find that these systematic uncertainties dominate our error budget. The dominant systematics are due to our assumption of unevolving bias and clustering across each redshift bin, and to differences between the shapes of the redshift distributions derived by clustering vs photo-$z$'s. The systematic uncertainty in the mean redshift bias of the source galaxy sample is $Δz \lesssim 0.02$, though the precise value depends on the redshift bin under consideration. We discuss possible ways to mitigate the impact of our dominant systematics in future analyses.

astro-ph.CO↗

arXiv2017

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in alfa-FeRh-based junctions driven by the magnetic phase transition of alfa-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one alfa-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the alfa-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Techniques for Accurate Parallax Measurements for 6.7-GHz Methanol Masers

The BeSSeL Survey is mapping the spiral structure of the Milky Way by measuring trigonometric parallaxes of hundreds of maser sources associated with high-mass star formation. While parallax techniques for water masers at high frequency (22 GHz) have been well documented, recent observations of methanol masers at lower frequency (6.7 GHz) have revealed astrometric issues associated with signal propagation through the ionosphere that could significantly limit parallax accuracy. These problems displayed as a "parallax gradient" on the sky when measured against different background quasars. We present an analysis method in which we generate position data relative to an "artificial quasar" at the target maser position at each epoch. Fitting parallax to these data can significantly mitigate the problems and improve parallax accuracy.

astro-ph.IM↗

arXiv2016

The Deep and Transient Universe in the SVOM Era: New Challenges and Opportunities - Scientific prospects of the SVOM mission

To take advantage of the astrophysical potential of Gamma-Ray Bursts (GRBs), Chinese and French astrophysicists have engaged the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor). Major advances in GRB studies resulting from the synergy between space and ground observations, the SVOM mission implements space and ground instrumentation. The scientific objectives of the mission put a special emphasis on two categories of GRBs: very distant GRBs at z$>$5 which constitute exceptional cosmological probes, and faint/soft nearby GRBs which allow probing the nature of the progenitors and the physics at work in the explosion. These goals have a major impact on the design of the mission: the on-board hard X-ray imager is sensitive down to 4 keV and computes on line image and rate triggers, and the follow-up telescopes on the ground are sensitive in the NIR. At the beginning of the next decade, SVOM will be the main provider of GRB positions and spectral parameters on very short time scale. The SVOM instruments will operate simultaneously with a wide range of powerful astronomical devices. This rare instrumental conjunction, combined with the relevance of the scientific topics connected with GRB studies, warrants a remarkable scientific return for SVOM. In addition, the SVOM instrumentation, primarily designed for GRB studies, composes a unique multi-wavelength observatory with rapid slew capability that will find multiple applications for the whole astronomy community beyond the specific objectives linked to GRBs. This report lists the scientific themes that will benefit from observations made with SVOM, whether they are specific GRB topics, or more generally all the issues that can take advantage of the multi-wavelength capabilities of SVOM.

astro-ph.IM↗

arXiv2016

The first photometric analysis of the W-subtype contact binary UCAC4 436-062932 with O'Connell effect

Two sets of light curves in $V$ $R_c$ $I_c$ bands for a newly discovered binary system UCAC4 436-062932 are obtained and analyzed using the Wilson-Devinney (W-D) code. The two sets of light curves get almost consistent results. The determined mass ratio is about $q = 2.7$ and the less massive component is nearly $250K$ hotter than the more massive one. The solutions conclude that UCAC4 436-062932 is a W-subtype shallow contact (with a contact degree of $f = 20\,\%$) binary system. Since the O'Connell effect appears on one set of the light curves, theories proposed to explain the effect are discussed. We assume that spot model may be the more plausible one to the O'Connell effect appeared on the asymmetric light curves of the binary system UCAC4 436-062932. Therefore, we add a cool spot on the surface of the more massive star (component with lower effective temperature) and get a quite approving results for the light curve fitting. It will provide evidence to support the spot model in the explanatory mechanism of O'Connell effect.

astro-ph.SR↗

arXiv2016

The photometric investigation of the newly discovered W UMa type binary system GSC 03122-02426

The $B$ $V$ $R_c$ $I_c$ bands light curves of the newly discovered binary system \astrobj{GSC 03122-02426} are obtained and analyzed using the Wilson-Devinney (W-D) code. The solutions suggest that the mass ratio of the binary system is $q = 2.70$ and the less massive component is $422K$ hotter than the more massive one. We conclude that \astrobj{GSC 03122-02426} is a W-subtype shallow contact (with a contact degree of $f = 15.3\,\%$) binary system. It may be a newly formed contact binary system which is just under geometrical contact and will evolve to be a thermal contact binary system. The high orbital inclination ($i = 81.6^{\circ}$) implies that \astrobj{GSC 03122-02426} is a total eclipsing binary system and the photometric parameters obtained by us are quite reliable. We also estimate the absolute physical parameters of the two components in \astrobj{GSC 03122-02426}, which will provide fundamental information for the research of contact binary systems. The formation and evolutionary scenario of \astrobj{GSC 03122-02426} is discussed.

astro-ph.SR↗

arXiv2016

Photometric and period investigation of the late F-type overcontact binary II UMa

II UMa is a late F-type (F5) contact binary with a close-in tertiary and a distant visual companion. According to the four-color ($B$ $V$ $R_c$ $I_c$) light curves' solutions of II UMa, it is a high fill-out (f=$86.6\,\%$) and low mass ratio ($q = 0.172$) contact binary system, which indicate that it is at the late evolutionary stage of late-type tidal-locked binary stars. The mass of the primary star and secondary one are calculated to be $M_1 = 1.99M_\odot$, $M_2 = 0.34M_\odot$. The primary star has evolved from ZAMS, but it is still before TAMS, and the secondary star is even more evolved. Considering the mass ratio ($M_3/M_1 = 0.67$) obtained by spectroscopic observations, the mass of the close-in tertiary is estimated to be $M_3 = 1.34M_\odot$. The period variations of the binary system is investigated for the first time. According to the observed-calculated ($O$-$C$) curve analysis, a continuous period increase at a rate of $dP/dt=4.88\times{10^{-7}}day\cdot year^{-1}$ is determined. It may be just a part of a cyclic period change, or the combinational period change of a parabolic variation and a cyclic one. More times of minimum light are needed to confirm this. The presence of the tertiary component may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system during the pre-contact stage.

astro-ph.SR↗

arXiv2016

A Solar-type Stellar Companion to a Deep Contact Binary in a Quadruple System

The four-color ($B$ $V$ $R_c$ $I_c$) light curves of V776 Cas are presented and analyzed using the Wilson-Devinney (W-D) method. It is discovered that V776 Cas is an early F-type (F2V) overcontact binary with a very high contact degree ($ f=64.6\,\%$) and an extremely low mass ratio ($q=0.130$), which indicate that it is at the final evolutionary stage of cool short-period binaries. The mass of the primary and secondary stars are calculated to be $M_1 = 1.55(\pm0.04)M_\odot$, $M_2 = 0.20(\pm0.01)M_\odot$. V776 Cas is supposed to be formed from an initially detached binary system via the loss of angular momentum due to the magnetic wind. The initial mass of the present primary and secondary components are calculated to be $M_{1i} = 0.86(\pm0.10)M_\odot$ and $M_{2i} = 2.13(\pm0.04)M_\odot$. The observed-calculated ($O$-$C$) curve exhibits a cyclic period variation, which is due to the light-travel time effect (LTTE) caused by the presence of a third component with a period of 23.7 years. The mass of the third component is estimated to be $M_3 = 1.04(\pm0.03)M_\odot$ and the orbital inclination of the third component is calculated to be $i' = 33.1^{\circ}$. The distance of the binary system to the mass center of the triple system is calculated to be $a'_{12} = 3.45AU$. The presence of the close-in tertiary component may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system.

astro-ph.SR↗

arXiv2016

The 80 Ms follow-up of the X-ray afterglow of GRB 130427A challenges the standard forward shock model

GRB 130427A was the brightest gamma-ray burst detected in the last 30 years. With an equivalent isotropic energy output of $8.5\times10^{53}$ erg and redshift $z=0.34$, it uniquely combined very high energetics with a relative proximity to Earth. As a consequence, its X-ray afterglow has been detected by sensitive X-ray observatories such as XMM-Newton and Chandra for a record-breaking baseline longer than 80 million seconds. We present the X-ray light-curve of this event over such an interval. The light-curve shows a simple power-law decay with a slope $α= 1.309 \pm 0.007$ over more than three decades in time (47 ks - 83 Ms). We discuss the consequences of this result for a few models proposed so far to interpret GRB 130427A, and more in general the significance of this outcome in the context of the standard forward shock model. We find that this model has difficulty in explaining our data, in both cases of constant density and stellar wind circumburst media, and requires far-fetched values for the physical parameters involved.

astro-ph.HE↗

arXiv2016

Fast response electromagnetic follow-ups from low latency GW triggers

We investigate joint low-latency gravitational wave (GW) detection and prompt electromagnetic (EM) follow-up observations of coalescing binary neutron stars (BNSs). Assuming that BNS mergers are associated with short duration gamma ray bursts (SGRBs), we evaluate if rapid EM follow-ups can capture the prompt emission, early engine activity or reveal any potential by-products such as magnetars or fast radio bursts. To examine the expected performance of extreme low-latency search pipelines, we simulate a population of coalescing BNSs and use these to estimate the detectability and localisation efficiency at different times before merger. Using observational SGRB flux data corrected to the range of the advanced GW interferometric detectors, we determine what EM observations could be achieved from low-frequency radio up to high energy $γ$-ray. We show that while challenging, breakthrough multi-messenger science is possible through low latency pipelines.

astro-ph.HE↗

arXiv2016

Capturing the electromagnetic counterparts of binary neutron star mergers through low latency gravitational wave triggers

We investigate the prospects for joint low-latency gravitational wave (GW) detection and prompt electromagnetic (EM) follow-up observations of coalescing binary neutron stars (BNSs). For BNS mergers associated with short duration gamma-ray bursts (SGRBs), we for the first time evaluate the feasibility of rapid EM follow-ups to capture the prompt emission, early engine activity or reveal any potential by-products such as magnetars or fast radio bursts. To achieve our goal, we first simulate a population of coalescing BNSs using realistic distributions of source parameters and estimate the detectability and localisation efficiency at different times before merger. We then use a selection of facilities with GW follow-up agreements in place, from low-frequency radio to high energy $γ$-ray to assess the prospects of prompt follow-up. We quantify our assessment using observational SGRB flux data extrapolated to be within the horizon distances of the advanced GW interferometric detectors LIGO and Virgo and to the prompt phase immediately following the binary merger. Our results illustrate that while challenging, breakthrough multi-messenger science is possible with EM follow-up facilities with fast responses and wide fields-of-view. We demonstrate that the opportunity to catch the prompt stage ($<$ 5s) of SGRBs, can be enhanced by speeding up the detection pipelines of both GW observatories and EM follow-up facilities. We further show that the addition of an Australian instrument to the optimal detector network could possibly improve the angular resolution by a factor of two and thereby contribute significantly to GW-EM multi-messenger astronomy.

astro-ph.HE↗

arXiv2016

Chemical abundance analysis of 19 barium stars

We aim at deriving accurate atmospheric parameters and chemical abundances of 19 barium (Ba) stars, including both strong and mild Ba stars, based on the high signal-to-noise ratio and high resolution Echelle spectra obtained from the 2.16 m telescope at Xinglong station of National Astronomical Observatories, Chinese Academy of Sciences. The chemical abundances of the sample stars were obtained from an LTE, plane-parallel and line-blanketed atmospheric model by inputting the atmospheric parameters (effective temperatures, surface gravities, metallicity and microturbulent velocity) and equivalent widths of stellar absorption lines. These samples of Ba stars are giants indicated by atmospheric parameters, metallicities and kinematic analysis about UVW velocity. Chemical abundances of 17 elements were obtained for these Ba stars. Their light elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn and Ni) are similar to the solar abundances. Our samples of Ba stars show obvious overabundances of neutron-capture (n-capture) process elements relative to the Sun. Their median abundances of [Ba/Fe], [La/Fe] and [Eu/Fe] are 0.54, 0.65 and 0.40, respectively. The YI and ZrI abundances are lower than Ba, La and Eu, but higher than the light elements for the strong Ba stars and similar to the iron-peak elements for the mild stars. There exists a positive correlation between Ba intensity and [Ba/Fe]. For the n-capture elements (Y, Zr, Ba, La), there is an anti-correlation between their [X/Fe] and [Fe/H]. We identify nine of our sample stars as strong Ba stars with [Ba/Fe]>0.6 where seven of them have Ba intensity Ba=2-5, one has Ba=1.5 and another one has Ba=1.0. The remaining ten stars are classified as mild Ba stars with 0.17<[Ba/Fe]<0.54.

astro-ph.SR↗

arXiv2015

Superconductivity enhancement in the S-doped Weyl semimetal candidate MoTe2

Two-dimensional (2D) transition-metal dichalcogenide (TMDs) MoTe2 has attracted much attention due to its predicted Weyl semimetal (WSM) state and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that the superconductivity in MoTe2 single crystal can be much enhanced by the partial substitution of the Te ions by the S ones. The maximum of the superconducting temperature TC of MoTe1.8S0.2 single crystal is about 1.3 K. Compared with the parent MoTe2 single crystal (TC=0.1 K), nearly 13-fold in TC is improved in MoTe1.8S0.2 one. The superconductivity has been investigated by the resistivity and magnetization measurements. MoTe2-xSx single crystals belong to weak coupling superconductors and the improvement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S-ion substitution. A dome-shape superconducting phase diagram is obtained in the S-doped MoTe2 single crystals. MoTe2-xSx materials may provide a new platform for our understanding of superconductivity phenomena and topological physics in TMDs.

cond-mat.supr-con↗

arXiv2015

The SVOM gamma-ray burst mission

We briefly present the science capabilities, the instruments, the operations, and the expected performance of the SVOM mission. SVOM (Space-based multiband astronomical Variable Objects Monitor) is a Chinese-French space mission dedicated to the study of Gamma-Ray Bursts (GRBs) in the next decade. The SVOM mission encompasses a satellite carrying four instruments to detect and localize the prompt GRB emission and measure the evolution of the afterglow in the visible band and in X-rays, a VHF communication system enabling the fast transmission of SVOM alerts to the ground, and a ground segment including a wide angle camera and two follow-up telescopes. The pointing strategy of the satellite has been optimized to favor the detection of GRBs located in the night hemisphere. This strategy enables the study of the optical emission in the first minutes after the GRB with robotic observatories and the early spectroscopy of the optical afterglow with large telescopes to measure the redshifts. The study of GRBs in the next decade will benefit from a number of large facilities in all wavelengths that will contribute to increase the scientific return of the mission. Finally, SVOM will operate in the era of the next generation of gravitational wave detectors, greatly contributing to searches for the electromagnetic counterparts of gravitational wave triggers at Xray and gamma-ray energies.

astro-ph.IM↗

arXiv2015

Longitudinal Spin Seebeck Effect in Silver Strip on CoFe Film

We report the experimental observation of the spin Seebeck effect (SSE) in Ag/CoFe noble metal/magnetic metal bilayers with a longitudinal structure. Thermal voltages jointly generated by the anomalous Nernst effect (ANE) and the SSE were detected across the Ag/CoFe/Cu strip with a perpendicular thermal gradient. To effectively separate the SSE and the ANE part of the thermal voltages, we compared the experimental results between the Ag/CoFe/Cu strip and Cu/CoFe/Cu strip, where two samples processed with the heating power instead of the temperature difference through the thin CoFe film. The respective contributions of the ANE and SSE to thermal voltage were determined, and they have the ratio of 4:1. The spin current injected through CoFe/Ag interface is calculated to be 1.76 mA/W.

cond-mat.mtrl-sci↗