arXiv subjects

E. W. Liang

Publications and source records attributed to E. W. Liang.

At least 19 recordsLinked to original sources

arXiv2026

Early results from the SVOM Observatory Science program

We present the organisation and early results from the Observatory Science program of the Space-based multi-band astronomical Variable Objects Monitor (SVOM), based on data collected between July 2024 and December 2025. Although primarily designed for gamma-ray burst studies, SVOM's wide-field, multi-wavelength instruments enable a broad range of high-energy astrophysical investigations. We summarize the execution and performance of the General Program and Target-of-Opportunity observations, and we describe the frameworks used for serendipitous source detection and monitoring with the ECLAIRs coded-mask instrument. Over this period, SVOM carried out more than a thousand pointed observations and detected several hundred non-GRB high-energy sources, mainly X-ray binaries, as well as blazars, stellar flares, magnetars, and unidentified events. We highlight some key results, including the monitoring of the microquasar Cygnus X-1, the detection of burst oscillations from the Low-Mass X-ray Binary 4U 0614+091, the spectral-state monitoring of Aql X-1, the first SVOM detection of an X-ray blazar flare from 1ES 1959+650, and observations of a stellar flare from HD 22468. These results demonstrate SVOM's strong capabilities for time-domain astrophysics beyond its core GRB program.

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arXiv2026

The SVOM mission, its profile and its system

The SVOM (Space-based Variable Objects Monitor) mission, launched into low Earth orbit on 22 June 2024, is a French-Chinese multi-wavelength observatory dedicated to the study of the transient sky. Inspired by the Neil Gehrels Swift Observatory, it consists of an autonomous rapid-slewing satellite, linked in real time to several ground-based telescopes. The space segment comprises two X-ray/gamma-ray wide-field instruments (ECLAIRs and GRM) with real-time triggering capabilities combined with two narrow-field telescopes in X-ray (MXT) and in visible (VT). In addition, the SVOM collaboration has also developed a unique visible and NIR ground-based follow-up system to promptly respond to the gamma-ray transients detected on board. The core program of SVOM will provide new insights into the Gamma-Ray Burst physics by providing a homogeneous dataset covering both the prompt and afterglow emissions, as well as better studying the low luminosity and soft Gamma-Ray Burst populations. As a versatile satellite platform with fast slewing capabilities, SVOM also comprises a Target of Opportunity program and a General Program consisting in pointed observations scheduled over the year that will both significantly contribute to the multi-messenger and time-domain astronomy.

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arXiv2026

Multi-wavelength Study of A Superflare on RS CVn-type Star HD22468 Triggered at Hard X-ray by SVOM

Detection of stellar flares at hard X-ray is still rare at the current stage. A transient was recently detected by the hard X-ray camera, ECLAIRs onboard the SVOM mission at 11:39:01.2UT on 2025, January 09. Simultaneous monitor in the optical band on the ground by SVOM/GWAC and follow-up spectroscopy enable us to confirm that the transient is caused by a superflare on HD~22468, a RS CVn-type star. The bolometric energy released in the flare is estimated to be $\sim7.2\times10^{37}-1.7\times10^{38}\ \mathrm{erg}$. The hard X-ray spectra of the event at the peak can be reproduced by the ``apec'' model of a hot plasma with a temperature of $106^{+27}_{-22}$~MK. In the optical range, the H$α$ emission-line profile obtained at $\sim1.7$ hrs after the trigger shows a bulk blueshift of $-96\pm20\ \mathrm{km\ s^{-1}}$, which can be explained by either a chromospheric evaporation or a prominence eruption. The ejected mass is estimated to be $3.9\times10^{20}$ g for the evaporating plasma, and to be $3.2\times10^{21}\ \mathrm{g}<M_{\mathrm{p}}<8.8\times10^{21}\ \mathrm{g}$ for the erupted prominence.

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arXiv2025

Picking-up Local GRB Candidates Based on Their Host Galaxies

Rapid identification of candidates of high-value gamma-ray bursts (GRBs), including both high-$z$ and local events, is crucial for outlining subsequent observational strategy. In this paper, we present a model that enables an on-duty astronomer to rapidly identify candidates of local GRBs prior to spectroscopy, provided that these events have been localized at an arcseconds precision. After taking into account the mass distribution of the host galaxies of GRBs, the model calculates the two-dimensional cross-match probabilities between a localized GRB and its surrounding nearby galaxies, and then returns the best match with the highest probability. The model is evaluated not only by the observed GRB sample with redshifts up to $z=4$, but also through the simulated GRB samples. By using the recently published GLADE+ galaxies catalog with a completeness of 95\% up to 500Mpc, along with the NED-LVS catalog, the Precision and Recall of the model are determined to be 0.23-0.33 and 0.75, respectively, at the best performance. A dedicated web service, which will be integrated into the SVOM Science User Support System, has been developed to deploy the model.

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arXiv2024

Potential Chromospheric Evaporation in A M-dwarf's Flare Triggered by Einstein Probe Mission

Although flares from late-type main-sequence stars have been frequently detected in multi-wavelength, the associated dynamical process has been rarely reported so far. Here, we report follow-up observations of an X-ray transient triggered by WXT onboard the Einstein Probe at UT08:45:08 in 2024, May 7. The photometry in multi-bands and time-resolved spectroscopy started at 3 and 7.5 hours after the trigger, respectively, which enables us to identify the transient as a flare of the M-dwarf 2MASS J12184187-0609123. The bolometric energy released in the flare is estimated to be $\sim10^{36}\ \mathrm{erg}$ from its X-ray light curve. The H$α$ emission-line profile obtained at about 7 hours after the trigger shows an evident blue asymmetry with a maximum velocity of $200-250\ \mathrm{km\ s^{-1}}$. The blue wing can be likely explained by the chromospheric temperature (cool) upflow associated with chromospheric evaporation, in which the mass of the evaporating plasma is estimated to be $1.2\times10^{18}$g. In addition, a prominence eruption with an estimated mass of $7\times10^{15}\mathrm{g}<M_{\mathrm{p}}<7\times10^{18}\mathrm{g}$ can not be entirely excluded.

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arXiv2023

Super-Eddington Accretion as a Possible Scenario to Form GW190425

On 2019 April 25, the LIGO/Virgo Scientific Collaboration detected a compact binary coalescence, GW190425. Under the assumption of the binary neutron star (BNS), the total mass of $3.4^{+0.3}_{-0.1}\, M_\odot$ lies five standard deviations away from the known Galactic population mean. In the standard common envelope scenario, the immediate progenitor of GW190425 is a close binary system composed of an NS and a He-rich star. With the detailed binary evolutionary modeling, we find that in order to reproduce GW190425-like events, super-Eddington accretion (e.g., $1,000\,\dot{M}_{\rm Edd}$) from a He-rich star onto the first-born NS with a typical mass of 1.33 $M_\odot$ via stable Case BB mass transfer (MT) is necessarily required. Furthermore, the immediate progenitors should potentially have an initial mass of $M_{\rm ZamsHe}$ in a range of $3.0-3.5$ $M_\odot$ and an initial orbital period of $P_{\rm init}$ from 0.08 days to 0.12 days, respectively. The corresponding mass accreted onto NSs via stable Case BB MT phase varies from $0.70\, M_\odot$ to $0.77\, M_\odot$. After the formation of the second-born NS, the BNSs are expected to be merged due to gravitational wave emission from $\sim$ 11 Myr to $\sim$ 190 Myr.

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arXiv2023

Revisiting the Properties of GW190814 and Its Formation History

GW190814 was reported during LIGO's and Virgo's third observing run with the most asymmetric component masses (a $\sim 23$ $M_{\odot}$ black hole and a $\sim2.6$ $M_{\odot}$ compact object). Under the assumption that this event is a binary black hole (BBH) merger formed through the isolated binary evolution channel, we reanalyze the publicly released data of GW190814 with the modified astrophysical priors on the effective spin $χ_{\rm eff}$, and further explore its formation history using detailed binary modeling. We show that GW190814 is likely to have been formed through the classical common envelope channel. Our findings show that the properties inferred using the modified astrophysical priors are consistent with those inferred by the uniform priors. With the newly-inferred properties of GW190814, we perform detailed binary evolution of the immediate progenitor of the BBH (namely a close binary system composed of a BH and a helium star) in a large parameter space, taking into account mass-loss, internal differential rotation, supernova kicks, and tidal interactions between the helium star and the BH companion. Our findings show that GW190814-like events could be formed in limited initial conditions just after the common envelope phase: a $\sim 23$ $M_{\odot}$ BH and a helium star of $M_{\rm ZamsHe}$ $\sim$ 8.5 $M_{\odot}$ at solar metallicity ($\sim$ 7.5 $M_{\odot}$ at 10\% solar metallicity) with an initial orbital period at around 1.0 day. Additionally, the inferred low spin of the secondary indicates that the required metallicity for reproducing GW190814-like events should not be too low (e.g., Z $\gtrsim$ 0.1 $Z_{\odot}$).

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arXiv2022

Flaring-associated Complex Dynamics in Two M-dwarfs Revealed by Fast, Time-resolved Spectroscopy

Habitability of an exoplanet is believed to be profoundly affected by activities of the host stars, although the related coronal mass ejections (CMEs) are still rarely detected in solar-like and late-type stars. We here report an observational study on flares of two M-dwarfs triggered by the high-cadence survey performed by the Ground Wide-angle Camera system. In both events, the fast, time-resolved spectroscopy enables us to identify symmetric broad H$α$ emission with not only a nearly zero bulk velocity, but also a large projected maximum velocity as high as $\sim700-800\ \mathrm{km\ s^{-1}}$. This broadening could be resulted from either Stark (pressure) effect or a flaring-associated CME at stellar limb. In the context of the CME scenario, the CME mass is estimated to be $\sim4\times10^{18}$ g and $2\times10^{19}$ g. In addition, our spectral analysis reveals a temporal variation of the line center of the narrow H$α$ emission in both events. The variation amplitudes are at tens of $\mathrm{km\ s^{-1}}$, which could be ascribed to the chromospheric evaporation in one event, and to a binary scenario in the other one. With the total flaring energy determined from our photometric monitor, we show a reinforced trend in which larger the flaring energy, higher the CME mass is.

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arXiv2021

Constrains on optical emission of FAST-detected FRB 20181130B with GWAC synchronized observations

Multi-wavelength simultaneous observations are essential to the constraints on the origin of fast radio bursts (FRBs). However, it is a significant observational challenge due to the nature of FRBs as transients with a radio millisecond duration, which occur randomly in the sky regardless of time and position. Here, we report the search for short-time fast optical bursts in the GWAC archived data associated with FRB 20181130B, which were detected by the Five Hundred Meter Spherical Radio Telescope (FAST) and recently reported. No new credible sources were detected in all single GWAC images with an exposure time of 10 s, including image with coverage of the expected arrival time in optical wavelength by taking the high dispersion measurements into account. Our results provide a limiting magnitude of 15.43$\pm0.04$ mag in R band, corresponding to a flux density of 1.66 Jy or 8.35 mag in AB system by assuming that the duration of the optical band is similar to that of the radio band of about 10 ms. This limiting magnitude makes the spectral index of $α<0.367$ from optical to radio wavelength. The possible existence of longer duration optical emission was also investigated with an upper limits of 0.33 Jy (10.10 mag), 1.74 mJy (15.80 mag) and 0.16 mJy (18.39 mag) for the duration of 50 ms, 10 s and 6060 s, respectively. This undetected scenario could be partially attributed to the shallow detection capability, as well as the high inferred distance of FRB 20181130B and the low fluence in radio wavelength. The future detectability of optical flashes associated with nearby and bright FRBs are also discussed in this paper.

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arXiv2021

Detection of Flare-associated CME Candidates on Two M-dwarfs by GWAC and Fast, Time-resolved Spectroscopic Follow-ups

The flare-associated stellar coronal mass ejection (CME) in solar-like and late type stars is quite essential for the habitability of an exoplanet. In this paper, we report detection of flare-associated CMEs in two M-dwarfs, thanks to the high cadence survey carried out by the Ground Wide-angle Camera system and the fast photometric and spectroscopic follow-ups. The flare energy in $R-$band is determined to be $1.6\times10^{35}\ \mathrm{erg}$ and $8.1\times10^{33}\ \mathrm{erg}$ based on the modeling of their light curves. The time-resolved spectroscopyic observations start at about 20 and 40 minutes after the trigger in both cases. The large projected maximum velocity of $\sim500-700\ \mathrm{km\ s^{-1}}$ suggests that the high velocity wing of their H$α$ emission lines are most likely resulted from a CME event in both stars, after excluding the possibility of chromospheric evaporation and coronal rain. The masses of the CMEs are estimated to be $1.5-4.5\times10^{19}\ \mathrm{g}$ and $7.1\times10^{18}\ \mathrm{g}$.

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arXiv2020

A $ΔR\sim 9.5$ mag Super Flare of An Ultracool Star Detected by $\text{SVOM/GWAC}$ System

In this paper, we report the detection and follow-ups of a super stellar flare GWAC\,181229A with an amplitude of $ΔR\sim$9.5 mag on a M9 type star by $\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The estimated bolometric energy $E_{bol}$ is $(5.56-9.25)\times10^{34}$ ergs, which places the event to be one of the most powerful flares on ultracool stars. The magnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a cadence of 15 seconds, a new component near the peak time with a very steep decay is detected in the $R$-band light curve, followed by the two-component flare template given by Davenport et al. (2014). An effective temperature of $5340\pm40$ K is measured by a blackbody shape fitting to the spectrum at the shallower phase during the flare. The filling factors of the flare are estimated to be $\sim$30\% and 19\% at the peak time and at 54 min after the first detection. The detection of the particular event with large amplitude, huge-emitted energy and a new component demonstrates that a high cadence sky monitoring cooperating with fast follow-up observations is very essential for understanding the violent magnetic activity.

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arXiv2020

No pulsed radio emission during a bursting phase of a Galactic magnetar

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of unknown origin observed at extragalactic distances. It has been long speculated that magnetars are the engine powering repeating bursts from FRB sources, but no convincing evidence has been collected so far\cite{sun19}. Recently, the Galactic magnetar SGR J1935+2154 entered an active phase by emitting intense soft Gamma-ray bursts. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft Gamma-ray / hard X-ray flare. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard X-rays) data. During the third session, 29 soft Gamma-ray repeater (SGR) bursts were detected in Gamma-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB -- SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.

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arXiv2019

Photometric and spectroscopic Studies of Superoutbursts of Three Dwarf Novae Independently Identified by The SVOM/GWAC System in 2018

We report our photometric and spectroscopic follow-up observations of the superoutbursts of three dwarf novae (GWAC\,180415A, GWAC\,181017A and GWAC\,181211A) identified independently by the Ground Wide-angle Cameras system, one of the ground-based instruments of the China-France SVOM mission. Based on a combination of our photometry and that taken from the AAVSO, our period analysis of the superhumps enables us to determine the mass ratios to be 0.0967-0.1163, 0.1879-0.1883 and 0.0981-0.1173 for GWAC\,180415A, GWAC\,181017A and GWAC\,181211A, respectively. GWAC\,180415A can be firmly identified as a WZ sge-type dwarf novae due to its long duration ($\sim2$ weeks) multiple rebrightenings with amplitudes of 3-4 magnitudes, the early superhump associated with a double-wave modulation and the low mass ratio. The inferred low mass ratio and location in the $\varepsilon-P_{\mathrm{orb}}$ diagram suggest that GWAC\,181211A is a WZ sge-type dwarf novae candidate. The measured Balmer decrements suggest the Balmer line emission is produced from an optical thick region in GWAC\,180415A and GWAC\,181017A, and from an optical thin region in GWAC\,181211A.

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arXiv2019

The mini-GWAC optical follow-up of the gravitational wave alerts: results from the O2 campaign and prospects for the upcoming O3 run

The second observational campaign of gravitational waves organized by the LIGO/Virgo Collaborations has led to several breakthroughs such as the detection of gravitational wave signals from merger systems involving black holes or neutrons stars. During O2,14 gravitational wave alerts were sent to the astronomical community with sky regions covering mostly over hundreds of square degrees. Among them, 6 have been finally confirmed as real astrophysical events. Since 2013, a new set of ground-based robotic telescopes called GWAC and its pathfinder mini-GWAC have been developed to contribute to the various challenges of themulti-messenger and time domain astronomy. The GWAC system is built up in the framework of the ground-segment system of the SVOM mission that will be devoted to the study of the multi-wavelength transient sky in the next decade. During O2, only the mini-GWAC telescopenetwork was fully operational. Due to the wide field of view and fast automatic follow-up capabilities of the mini-GWAC telescopes, they were well adapted to efficiently cover the sky localization areas of the gravitational wave event candidates. In this paper, we present the mini-GWAC pipeline we have set up to respond to the GW alerts and we report our optical follow-up observations of 8 GW alerts detected during the O2 run. Our observations provided the largest coverage of the GW localization areas in a short latency made by any optical facility. We found tens of optical transient candidates in our images, but none of those could be securely associated with any confirmed black hole-black hole merger event. Based on this first experience and the near future technical improvements of our network system, we will be more competitive to detect the optical counterparts from some gravitational wave events that will be detected during the upcoming O3 run, especially those emerging from binary neutron star mergers.

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arXiv2019

Learning Transient Universe in Near-Ultraviolet By Wide-angle Cameras

We perform a detailed analysis and simulations on the transient detection capability in the near-ultraviolet (NUV) band by focusing on some major local transient events. These events include the tidal disruption event due to a supermassive blackhole, the shock breakout of a core-collapse supernova and the flare of a late-type star. Our simulations show that a set of small wide-angle NUV cameras can allow us to detect and study numerous galactic and extra-galactic transient events. Based on the analysis and simulations, here we propose a space-based NUV sky patrol mission by updating the proposal that was originally submitted to the Chinese Space Station mission in 2011. The mission proposed here is composed of a set of eight small wide-field NUV cameras each with a diameter of 20cm. The total sky area simultaneously covered by the NUV cameras is as large as 3000$\mathrm{deg^2}$. The survey cadence ranges from 30 to 300s. The transient events are required to be detected by a dedicated on-board software in real time.

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arXiv2016

eXTP -- enhanced X-ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of about 3.4 m^2, between 6 and 10 keV, and spectral resolution <250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm^2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees FoV. The eXTP international consortium includes mostly major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

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arXiv2015

Gamma-ray Burst Cosmology

Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the Universe, which emit up to $8.8\times10^{54}$ erg isotropic equivalent energy in the hard X-ray band. The high luminosity makes them detectable out to the largest distances yet explored in the Universe. GRBs, as bright beacons in the deep Universe, would be the ideal tool to probe the properties of high-redshift universe: including the cosmic expansion and dark energy, star formation rate, the reionization epoch and the metal enrichment history of the Universe. In this article, we review the luminosity correlations of GRBs, and implications for constraining the cosmological parameters and dark energy. Observations show that the progenitors of long GRBs are massive stars. So it is expected that long GRBs are tracers of star formation rate. We also review the high-redshift star formation rate derived from GRBs, and implications for the cosmic reionization history. The afterglows of GRBs generally have broken power-law spectra, so it is possible to extract intergalactic medium (IGM) absorption features. We also present the capability of high-redshift GRBs to probe the pre-galactic metal enrichment and the first stars.

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arXiv2014

The Optical Luminosity Function of Gamma-ray Bursts deduced from ROTSE-III Observations

We present the optical luminosity function (LF) of gamma-ray bursts (GRBs) estimated from a uniform sample of 58 GRBs from observations with the Robotic Optical Transient Search Experiment III (ROTSE-III). Our GRB sample is divided into two sub-samples: detected afterglows (18 GRBs), and those with upper limits (40 GRBs). The $R$ band fluxes 100s after the onset of the burst for these two sub-samples are derived. The optical LFs at 100s are fitted by assuming that the co-moving GRB rate traces the star-formation rate. The detection function of ROTSE-III is taken into account during the fitting of the optical LFs by using Monte Carlo simulations. We find that the cumulative distribution of optical emission at 100s is well-described with an exponential rise and power-law decay (ERPLD), broken power-law (BPL), and Schechter LFs. A single power-law (SPL) LF, on the other hand, is ruled out with high confidence.

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