arXiv subjects

L. Tao

Publications and source records attributed to L. Tao.

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.

astro-ph.HE↗

arXiv2026

Discovery of EP J175257.3-351923 as a Candidate Black Hole Low-Mass X-ray Binary

We report the discovery of a new X-ray transient, EP J175257.3-351923 (EP250916a), by the Einstein Probe (EP) near the Galactic plane. The outburst lasted for at least 250\,days, reached a peak 2--10 keV flux of $\sim 4 \times 10^{-10}$~erg~cm$^{-2}$~s$^{-1}$, and exhibited a fast-rise exponential-decay (FRED) profile typical of X-ray binary outbursts. The source remained in the hard state throughout the outburst, with only modest variations in the photon index ($\sim 1.6$--$2.2$) and no evidence for a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at $\sim 217$~keV, consistent with hard-state accretion in black-hole systems. No reliable optical or radio counterpart is detected within the Swift/XRT error circle. The inferred X-ray-to-optical and X-ray-to-radio flux ratios are consistent with a low-mass X-ray binary classification. Neither pulsations nor significant aperiodic variability are detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and multiwavelength evidence favors EP~J175257.3--351923 as a black-hole low-mass X-ray binary candidate, highlighting EP's potential to uncover a faint, previously hidden population of X-ray binaries.

astro-ph.HE↗

arXiv2026

Multi-wavelength outburst activity from EP J174942.2-384834: a very faint X-ray transient discovered by Einstein Probe

We report the discovery and multi-wavelength characterization of the Galactic transient EP J174942.2$-$384834, first detected by the Einstein Probe during a faint X-ray outburst in March 2025. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a seven-month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of $\Gamma \approx 1$-2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV - X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EP J174942.2$-$384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.

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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$\alpha$ 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.

astro-ph.SR↗

arXiv2026

SVOM discovery of a strong X-ray outburst of the blazar 1ES~1959+650 and multi-wavelength follow-up with the Neil Gehrels Swift observatory

On December 6, 2024, 1ES 1959+650, one of the X-ray brightest blazars known, underwent a high-amplitude X-ray outburst detected by SVOM, the first such discovery with this mission. The source was subsequently monitored with SVOM and Swift from December 2024 to March 2025. We report the detection and multi-wavelength follow-up of this event, and describe the temporal and spectral evolution observed during the campaign. Data from SVOM/MXT, SVOM/ECLAIRs, and Swift/XRT were analyzed with log-parabola models to track flux and spectral variability. The source was detected in a bright state over the 0.3-50 keV range. During the three months of monitoring, the X-ray flux varied significantly, showing episodes of spectral hardening at high flux levels. The spectral curvature evolved more irregularly and did not show a clear trend with flux. A shift of the Spectral Energy Distribution (SED) synchrotron peak to higher energies is seen when the flux increases. This constitutes the first blazar outburst discovered in X-rays by SVOM. The coordinated follow-up with Swift provided continuous coverage of the flare and highlights the strong complementarity of the two missions for time-domain studies of blazars. The flare shows no clear signatures of either Fermi I or Fermi II acceleration, suggesting a mixed Fermi I/II scenario.

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arXiv2026

Einstein Probe discovery of EP J171159.4-333253: an eclipsing neutron star low-mass X-ray binary with clocked bursts

EP J171159.4-333253 is a new neutron-star low-mass X-ray binary discovered in outburst by the Einstein Probe (EP) on 2025 June 23, exhibiting clocked type-I X-ray bursts, eclipses and dips. In this paper, we report on the results of the X-ray spectral and timing analyses for EP J171159.4-333253 using data collected by EP and NuSTAR during the first 21 days of the outburst. The X-ray burst recurrence time can be characterized over a subset of nine bursts spanning 1.6 days around the NuSTAR observation, and the result is $t_{\rm rec}=8196 \pm 177\,$s with indications of a possible decreasing trend. From the X-ray eclipse events, the binary orbital period and the eclipse duration are estimated to be $P_{\rm orb}=6.48301 \pm 0.00003\,$hr and $D_{\star,X} = 1245.5^{+6.9}_{-6.5}\,$s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are $M_2\approx0.6-0.8\,M_\odot$, $R_2\approx0.7-0.8\,R_\odot$ and $i\approx73-75^\circ$, respectively. We also report on joint ULTRACAM and EP observations on 2025 July 21--22, detecting the source optical counterpart and covering an eclipse in both X-ray and optical bands. The optical eclipse is wavelength-dependent and broader than in X-rays, indicating that part of the optical emission arises from an extended region in the accretion flow. Despite a moderate variation in the source flux, the properties of the persistent X-ray emission are typical of a hard spectral state. We further evaluated the ratio of the accretion energy to the thermonuclear energy to be 120--130, implying helium bursts with the accreted hydrogen being depleted in-between bursts.

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arXiv2026

Detection of Oscillations in a Type I X-Ray Burst of 4U 0614+091 with SVOM/ECLAIRs

On 2025 January 10, a thermonuclear (Type I) X-ray burst from the neutron star low-mass X-ray binary \textit{4U~0614+091} was detected with the ECLAIRs instrument on board the \textit{SVOM} mission. We present here a time-resolved spectroscopic analysis of the burst, along with the detection of burst oscillations within a 51-second interval during the decay phase. The oscillation frequency is measured to be $\nu = 413.674 \pm 0.002\,\mathrm{Hz}$, consistent with previous reports. However, we detect a significant downward frequency drift over the burst duration, characterized by $\dot{\nu} = (-4.7 \pm 0.3) \times 10^{-3}\,\mathrm{Hz\,s^{-1}}$. This frequency evolution is atypical compared to those observed in similar burst oscillation sources. We tentatively attribute the observed drift to a Doppler shift induced by orbital motion. Under this interpretation, the inferred orbital period must be shorter than 20 minutes, placing \textit{4U~0614+091} among the most compact known low-mass X-ray binaries.

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arXiv2025

Aql X-1 from dawn 'til dusk: the early rise, fast state transition and decay of its 2024 outburst

Transient Low-Mass X-ray Binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts, typically at X-ray luminosities above $\sim$10$^{36}$ erg/s. Observations of these sources during the early rise of the outbursts have so far been very limited. However, the launch of the Einstein Probe (EP) has greatly improved our ability to detect fainter X-ray activity, unlocking access to the outburst early rise. In September 2024, EP detected the early onset of a new outburst from the neutron star LMXB Aql X-1, catching the source at a luminosity below 10$^{35}$ erg/s. In this paper we present results from a comprehensive, multi-wavelength campaign of this event, combining data from EP, NICER, NuSTAR, Swift and Las Cumbres Observatory covering the full outburst from its early rise through its decay. By comparing X-ray and optical light curves obtained with Las Cumbres Observatory during the initial rise, we show that the start of the X-ray emission lagged the optical rise by, at most, 13 days. Time-resolved X-ray spectroscopy revealed how the geometry and the physical properties of the accretion flow evolve during this early stage of the outburst, as well as at higher luminosities as the source transitioned through the canonical X-ray spectral states - hard, intermediate and soft. These data show that the source underwent a very rapid, about 12-h long, transition from the hard to the soft state about two weeks after the optical onset of the outburst. The evolution of the temperature and physical sizes of both the inner region of the disk and a black body near the NS surface suggest that at the state transition, a boundary and spreading layer likely formed. We discuss these results in the context of time-scales for outburst evolution and state transitions in accreting neutron stars and black holes.

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arXiv2025

Einstein Probe Discovery of EP J182730.0-095633: A New Black Hole X-ray Binary Candidate in Faint Outburst?

Black hole X-ray binaries (candidates) currently identified in our galaxy are mainly transient sources, with the majority discovered through the detection of their X-ray outbursts. Among these, only four were found during faint outbursts exhibiting peak X-ray luminosities $L_{\rm X}\lesssim10^{36}~{\rm erg~s^{-1}}$, likely due to the previous lack of sensitive, wide-field monitoring instruments in the X-ray band. In this Letter, we present the discovery of an intriguing X-ray transient, EP J182730.0-095633, via the Einstein Probe (EP) and subsequent multi-wavelength follow-up studies. This transient, located on the Galactic plane, experienced a faint and brief X-ray outburst lasting about 20 days. Its X-ray spectrum is non-thermal and consistent with a power-law model with a nearly constant photon index of $\Gamma \sim2$ throughout the outburst. A long-lasting millihertz quasi-periodic oscillation (QPO) signal was detected in its X-ray light curve, centered around a frequency of $\sim0.04$ Hz. A transient near-infrared source was identified as its counterpart, although no optical emission was detectable, likely due to significant extinction. A radio counterpart was also observed, displaying an inverted radio spectrum with $\alpha\sim0.45$. The X-ray spectral and temporal characteristics, along with the multi-wavelength properties, indicate that the source is a faint low-mass X-ray binary, with the compact object likely being a black hole. This work demonstrates the potential of the EP in discovering new X-ray binaries by capturing faint-level X-ray outbursts.

astro-ph.HE↗

arXiv2025

Einstein Probe discovery of the short period intermediate polar EP J115415.8-501810

The X-ray transient source EP240309a/EP\,J115415.8$-$501810 was first detected by the Wide-Field X-ray Telescope (WXT) on board Einstein Probe (EP) during the commissioning phase. Subsequent optical observations confirmed it as a Cataclysmic Variable of the intermediate polar type with a 238.2\,s spinning white dwarf in a $\sim$3.76\,hr orbit. We report on the source discovery and follow-up studies made with the Follow-up X-ray Telescope (FXT) of EP. A periodic variation of 231\,s is detected in the 0.3$-$2\,keV band, while no obvious pulsation appears in the 2$-$10\,keV band. The spectral analysis shows that the X-ray emission could be described by an absorbed bremsstrahlung model with $kT$\textgreater\,11\,keV. The partial covering absorption, with an hydrogen column density $N_H$ = 2.0$\times 10^{22}\,\rm cm^{-2}$ and covering fraction around 0.9, is much larger than the interstellar absorption along the line of sight. According to the distance $d = 309.5$\,pc obtained from Gaia parallax, we estimate that the luminosity of this source in the 0.3$-$10\,keV range is $\sim 2\times10^{32}$\,erg\,s$^{-1}$. In addition, phase-resolved spectral analysis reveals that the detected periodic variation is mainly caused by the change in the absorption column density. In this scenario the spin modulation arises due to absorption from the pre-shock accretion flow of the X-ray emitting pole, while the optical radiation is modulated at the orbital side band ($\omega_{\rm spin} - \Omega_{\rm orbit}$) due to reprocessing in regions within the binary system. Due to its unusual transient behaviour for an intermediate polar, we have also searched for radio signals similar to those observed in the new class of long period transients. We derived upper limits with ASKAP (200--300\,$\mu$Jy\,beam$^{-1}$ between 800--1500 MHz) and MWA (40--90\,mJy\,beam$^{-1}$ between 80--300 MHz).

astro-ph.HE↗

arXiv2025

New insight into the hard X-ray emission influenced by the type-\uppercase\expandafter{\romannumeral1} bursts observed by Insight-HXMT during outburst of 4U 1636--536

By analyzing the data from Insight-HXMT and NICER, we can determine the evolution of the significance of the hard shortage in 4U 1636--536 with its spectral state, as well as the evolution of the fraction of deficit with energy. Additionally, we investigate the possible geometry and evolution of the corona in 4U 1636-536 by combining our findings with the results of spectral analysis. We find that during the soft state, the significance of possible hard X-ray shortage in bursts is almost zero. However, in the hard state, some bursts exhibit significant shortages (>3 $\sigma$), while others do not. We attempt to establish a correlation between the significance of the hard X-ray shortage and the spectral parameters, but the data quality and the limited number of bursts prevent us from finding a strong correlation. For bursts with insignificant shortages in the soft state, their fraction of the deficit remains small. However, in the hard state, the fraction of deficit for all bursts increases with energy, regardless of the significance of the shortage of individual bursts. For bursts during the hard state, we investigate the evolution of the fraction of deficit during the bursts by stacking the peaks and decays of the bursts, respectively, and find that as the flux of the bursts decreases, the energy corresponding to the maximum of the fraction of deficit becomes progressively higher. We explore the possible geometry and evolution of the corona clued by the evolution of the fraction of deficit, which is obtained from the spectral and temporal analysis.

astro-ph.HE↗

arXiv2025

Insight-HXMT observations on thermonuclear X-ray bursts from 4U~1608--52 in the low/hard state: the energy-dependant hard X-ray deficit and cooling saturation of the corona

During thermonuclear bursts, it is suspected that {\bf the cooling of the corona by the burst emission} may be the cause of hard X-ray {\bf deficits}. Although this {\bf deficit} has been observed in nine sources, it has not been observed {\bf from} 4U~1608--52, a nearby prolific burster. Therefore, the authenticity and universality of the hard X-ray {\bf deficit} may be in question. To investigate this suspicion, Insight-HXMT performed cadence observations during the low/hard state of 4U~1608--52 in September 2022 and detected 10 thermonuclear X-ray bursts. Two of these bursts show a double-peaked structure in the soft X-ray band, which could be caused by the high temperature of the burst emission and a marginal photospheric radius expansion (PRE) around the burst peak time. This is indicated by their peak fluxes being up to the Eddington limit and having a large color factor at the peak of the bursts. The hard X-ray deficit is significantly observed during bursts at $>$ 30 keV. Furthermore, the fraction of this deficit shows saturation at 50\% for the first 8 bursts. This saturation may indicate that the corona is layered and only a part of the corona is cooled by the bursts. For example, the part close to the NS surface is cooled while the rest remains intact during bursts. This result provides a clue to the geometry of the corona, e.g., a possible scenario is that the corona has two forms: a quasi-spheric corona between the NS and the disk, and a disk-corona on both surfaces of the disk.

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arXiv2025

New insight into the Rapid Burster by Insight-HXMT

We report the timing and spectral analyses upon of the type II X-ray bursts from the Rapid Burster (MXB 1730--335) observed by Insight-HXMT and Swift/XRT. By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging than the soft X-rays by 3 seconds. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be non-thermal, thanks to the broad band coverage of Insight-HXMT. These findings put new insights into the type-II bursts and require a temporally showing-up corona for possible interpretation.

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arXiv2025

A possible overall scenario for the outburst evolution of MAXI J1820+070 revealed by Insight-HXMT

We study the spectral and temporal properties of the black hole X-ray transient binary MAXI J1820+070 during the 2018 outburst with Insight-HXMT observations. The outburst of MAXI J1820+070 can be divided into three intervals. For the two intervals of the outburst, we find that low-energy (below 140 keV) photos lag high-energy (140-170 keV) ones, while in the decay of the outburst, high-energy photons lag low-energy photons, both with a time scale of the order of days. Based on these results, the canonical hysteresis effect of the 'q' shape in the hardness-intensity diagram can be reformed into a roughly linear shape by taking into account the lag corrections between different energy bands. Time analysis shows that the high-frequency break of hard X-rays, derived from the power density spectrum of the first interval of the outburst is, in general, larger and more variable than that of soft X-rays. The spectral fitting shows that the coverage fraction of the hard X-rays drops sharply at the beginning of the outburst to around 0.5, then increases slightly. The coverage fraction drops to roughly zero once the source steps into a soft state and increases gradually to unity when the source returns to a low hard state. We discuss the possible overall evolution scenario of corona hinted from these discoveries.

astro-ph.HE↗

arXiv2024

Continuous evolution of the polarization properties in the transient X-ray pulsar RX J0440.9+4431/LS V +44 17

We present a detailed time-resolved and phase-resolved polarimetric analysis of the transient X-ray pulsar RX J0440.9+4431/LS V +44 17, using data from the Imaging X-ray Polarimetry Explorer (IXPE) during the 2023 giant outburst. We conducted a time-resolved analysis by dividing the data into several intervals for each observation. This analysis reveals a continuous rotation of the phase-averaged polarization angle (PA) across the observations performed during the super-critical and sub-critical regimes. To investigate the origin of the PA rotation, we performed a pulse phase-resolved polarimetric analysis over four time intervals, each spanning approximately three days. Applying the rotating vector model (RVM), the geometric parameters of the system were determined for each interval. Despite the short time gap of just $\sim$ 20 days, we observed significant variation in the RVM parameters between the first interval and the subsequent three, indicating the presence of an additional polarized component alongside the RVM component. Using a two-polarized component model with the assumption that this additional component remains constant across pulse phases, we calculated the phase-averaged PA and polarized flux of both the variable and constant components. The phase-averaged PA of each component remained relatively stable over time, but the polarized flux of the constant component decreased, while that of the variable component increased. The observed rotation of the PA is attributed to the gradual shift in the polarized flux ratio between the two components and is not directly related to the different accretion regimes.

astro-ph.HE↗

arXiv2024

estimating the spin of MAXI J1348--630 from intermediate and soft states using \textit{Insight}-HXMT data

We present a broadband spectral analysis of the soft-intermediate and soft states using \textit{Insight}-HXMT observations of the black hole binary MAXI J1348--630 during its outburst in 2019. We employ a combination of reflection and continuum fitting methods to measure the spin of the black hole and determine a spin of $a=0.79\pm0.13$, which is consistent with most of the previous results. Additionally, we investigate some sources of systematic uncertainty that could bias the measurement. The valid spectral hardening factor can partially account for the spin evolution observed during the outburst. Other potential factors, such as assumptions about the corona's structure and their interaction with reflected photons, can also affect the accuracy of spin measurements. Furthermore, our analysis reveals that MAXI J1348--630 exhibits a significantly high disc density ($\sim10^{22}\,\rm{cm}^{-3}$), with a moderate iron abundance of approximately 2.5 times solar.

astro-ph.HE↗

arXiv2024

A Revised Spin of the Black Hole in GRS 1716-249 with a New Distance

GRS 1716-249 is a stellar-mass black hole in a low-mass X-ray binary that underwent a gaint outburst in 2016/17. In this paper we use simultaneous observations of Insight-HXMT and NuSTAR to determine its basic parameters. The observations were performed during the softest part of the outburst, and the spectra show clear thermal disk emission and reflection features. We have fitted the X-ray energy spectra using the joint fitting method of the continuum and reflection components with the kerrbb2+ relxill model. Since there is a possibility that the distance to this source was previously underestimated, we use the latest distance parameter of 6.9 kpc in our study, in contrast to previous work in which the distance was set at 2.4 kpc. Through spectral fitting of fixing black hole mass at 6.4 $M_{\rm \odot}$, we observe a strong dependence of the derived spin on the distance: $a_{*}=0.972_{-0.005}^{+0.004}$ at an assumed distance of 2.4 kpc and $a_{*}=0.464_{-0.007}^{+0.016}$ at an assumed distance of 6.9 kpc, at a confidence level of 90%. If considering the uncertainties in the distance and black hole mass, there will be a wider range of spin with $a_{*}$ < 0.78. The fitting results with the new distance indicate that GRS 1716-249 harbors a moderate spin black hole with an inclined ($i\sim 40-50^{\circ}$) accretion disk around it. Additionally, we have also found that solely using the method of the reflection component fitting but ignoring the constraints on the spin from the accretion disk component will result in an extremely high spin.

astro-ph.HE↗

arXiv2024

Einstein Probe discovery of EP J005245.1-722843: a rare BeWD binary in the Small Magellanic Cloud?

On May 27 2024, the Wide-field X-ray Telescope onboard the Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud (SMC) during its commissioning phase. Prompt follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope and NICER have revealed a very soft, thermally emitting source (kT$\sim$0.1 keV at the outburst peak) with an X-ray luminosity of $L\sim4\times10^{38}$ erg s$^{-1}$, labelled EP J005245.1-722843. This super-soft outburst faded very quickly in a week time. Several emission lines and absorption edges were present in the X-ray spectrum, including deep Nitrogen (0.67 keV) and Oxygen (0.87 keV) absorption edges. The X-ray emission resembles the SSS phase of typical nova outbursts from an accreting white dwarf (WD) in a binary system, despite the X-ray source being historically associated with an O9-B0e massive star exhibiting a 17.55 days periodicity in the optical band. The discovery of this super-soft outburst suggests that EP J005245.1-722843 is a BeWD X-ray binary: an elusive evolutionary stage where two main-sequence massive stars have undergone a common envelope phase and experienced at least two episodes of mass transfer. In addition, the very short duration of the outburst and the presence of Ne features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne-O WD in the system.

astro-ph.HE↗