arXiv subjects

Jian-He Zheng

Publications and source records attributed to Jian-He Zheng.

13 recordsLinked to original sources

arXiv2026

First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets

Neutron star mergers (NSMs) are confirmed gravitational wave sources. Identifying an early electromagnetic counterpart for these events is therefore crucial for rapid localization and multimessenger follow-up. However, the associated Gamma-Ray Bursts (GRBs) are highly collimated and are therefore easily missed by off-axis observers. An early, less beamed counterpart is essential for identifying the majority of mergers. In this Letter, we investigate the cooling emission from jet-driven cocoons produced by short gamma-ray burst jets propagating through merger ejecta. We perform hydrodynamic simulations and radiative post-processing to calculate the early X-ray emission over a wide range of viewing angles. We find that the mildly relativistic cocoon produces bright soft X-ray transients for off-axis observers, with luminosities of $10^{46-48}{\rm erg\,s^{-1}}$ and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of $0.1$--$1\,{\rm keV}$. For observers at $θ_{\rm v}=10^{\circ}$, the cocoon emission is detectable by Einstein Probe (EP) out to $z\simeq 0.3$. For nearby events like GW170817, it remains detectable up to $θ_{\rm v}\simeq 45^{\circ}$. The predicted detection rate for EP is $0.5\,{\rm yr^{-1}}$ in the canonical model. In future multimessenger campaigns, rapid UV/optical/IR follow-up of such X-ray triggers can subsequently identify the associated kilonova and jet afterglow emission, which will confirm the origin.

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arXiv2026

Fast X-ray Transients produced by Off-axis Jet-Cocoons from Long Gamma-Ray Bursts

Fast X-ray transients (FXTs) have been detected for over a decade, yet their origins are still enigmatic. The observed association between FXTs and broad-lined Type Ic supernovae (SNe Ic-BL) suggests that some may share the same progenitor with Long Gamma-Ray Bursts. In this work, we numerically simulate the long-term evolution of a relativistic jet propagating from inside the progenitor star up to the photon diffusion radius of the cocoon. Then we post-process the hydrodynamic results and calculate the cocoon cooling emission for various viewing angles from the jet axis. We find that, for viewing angles $θ_{\rm v}=10^{\circ}$-$20^{\circ}$, the off-axis cocoon emission can produce FXTs with luminosity $L_{\rm X}\simeq 10^{47-48} {\rm\, erg\,s^{-1}}$ and duration $t_{\rm X}\simeq 10$-$100\,$s. The observed spectra are quasi-thermal with the peak energy $E_{\rm peak}\simeq0.8$ keV. These properties naturally explain observational features of { a fraction of FXTs}, including their high luminosity, soft spectra, and lack of gamma-ray counterparts. The Rayleigh-Jeans tail of the FXT spectra extends to the UV, producing an early UV flash simultaneously. As the cocoon expands and cools, the emission peak shifts to UV and optical bands, resulting in a bright optical plateau lasting for $\sim1$ day with color temperature $T_{\rm UV/opt} \simeq (1{-}3)\times10^{4}\,$K and bolometric luminosity $L_{\rm bol}\simeq10^{41-42} {\rm\, erg\,s^{-1}}$, before the emergence of supernova emission. Although our model underpredicts the UV/optical luminosity at $\sim1$ day for some events (e.g. EP 240414a), it still provides useful diagnostics for identifying the origins of FXTs.

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arXiv2025

EP250108a/SN2025kg: A Magnetar-powered Gamma-Ray Burst Supernova Originating from a Close Helium-star Binary via Isolated Binary Evolution

SN\,2025kg, linked to EP250108a, is among the brightest broad-lined Type Ic supernova (SN Ic-BL) known, showing unique helium absorptions, a late-time broad H$α$, and an early bump. In this {\em{Letter}}, we propose a jet-cocoon origin to explain EP250108a as off-axis cooling emission from a mildly relativistic inner cocoon viewed at $\sim45^\circ$ and the early bump of SN\,2025kg as the outer cocoon cooling emission, both constraining an energy of $\sim(1-2)\times10^{52}{\rm{erg}}$ and a progenitor radius of $\sim5\,R_\odot$. To explain SN\,2025kg's exceptionally luminous peak, potential energy injection into the $\sim2.5\,M_\odot$ ejecta from a magnetar with initial period $\sim1.7\,{\rm{ms}}$ and magnetic field $\sim2\times10^{15}{\rm{G}}$ may be required, implying a rapidly rotating $\sim4\,M_\odot$ progenitor. Thus, the progenitor may be a low-mass helium star with an extended helium envelope, supported by helium absorption lines and an inferred weak pre-SN wind. Hydrogen-rich material may reside in the inner ejecta layers, as suggested by the late-time broad H$α$, possibly originating from main-sequence companion material evaporated by the magnetar wind. Since the observed near-solar metallicity challenges the popular quasi-chemically homogeneous evolution channel, the rapidly rotating helium-star progenitor of EP250108a/SN\,2025kg might attain angular momentum by being tidally spun up by a main-sequence companion in a close binary formed through isolated binary evolution.

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arXiv2025

EP240414a: Off-axis View of a Jet-Cocoon System from an Expanded Progenitor Star

When a relativistic jet is launched following the core-collapse of a star, its interaction with the stellar envelope leads to the formation of a hot cocoon, which produces various viewing-angle-dependent observational phenomena following the breakout from the surface. We study the observational signatures of fast X-ray transient (FXT) EP240414a, which may originate from a jet-cocoon system viewed slightly off-axis. In our model, (1) the prompt X-ray emission lasting $\sim\! 100\,{\rm{s}}$ is attributed to the cooling emission from the inner cocoon (shocked jet material); (2) the $\sim\! 0.1\,{\rm{d}}$ X-ray emission comes from the inner cocoon's afterglow; (3) the $\sim\! 0.4\,{\rm{d}}$ thermal-dominated optical emission arises from the cooling of the outer cocoon (shocked stellar material); (4) the $\sim\! 3\,{\rm{d}}$ non-thermal optical component and subsequent radio emission can be explained by the afterglow from a jet with a viewing angle of $10^{\circ}\lesssim θ_{\rm{v}}\lesssim15^\circ$; and (5) the associated broad-lined Type Ic supernova only dominates the optical emission after $\sim\! 7\rm\, d$. Both the jet inferred from the off-axis afterglow and the inner cocoon constrained by the cooling emission are found to have similar kinetic energies, on the order of $10^{51}\,{\rm{erg}}$. We find that the progenitor's radius is $\sim3\,R_\odot$ as constrained by the { inner cocoon's} cooling emissions, indicating that the pre-explosion star may be a massive helium star that is slightly inflated. More FXTs associated with off-axis jets and supernovae will be further examined by the Einstein Probe, leading to a deeper understanding of jet-cocoon systems.

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arXiv2024

On the External Inverse Compton Scattering off the Prompt Emission in GRB 221009A

The light curve of the TeV emission in GRB 221009A displays a smooth transition from an initial rapid rise to a slower rise and eventually a decay phase. The smooth temporal profile of the TeV emission suggests that it mainly results from an external shock. The temporal overlap between the prompt KeV-MeV emission and the early TeV afterglow indicates that external inverse Compton scattering (EIC) between the prompt KeV-MeV photons and the afterglow electrons is inevitable. Since the energy density of the prompt emission is much higher than that of the afterglow during the early phase, the EIC process dominates the cooling of afterglow electrons. The EIC scattering rate is influenced by the anisotropy of the seed photon field, which depends on the radii of the internal dissipation ($R_{\rm dis}$), where the prompt emission is produced, and that of the external shock ($R_{\rm ext}$), where the afterglow emission is produced. We investigate the EIC process for different values of $R_{\rm dis}/R_{\rm ext}$. We find that, for varying $ R_{\rm dis}/R_{\rm ext} $, the EIC scattering rate can differ by a factor of $\sim 2$. For GRB 221009A, the EIC emission is dominated during the early rising phase of the TeV afterglow. It then transitions to a phase dominated by the synchrotron self-Compton (SSC) emission as the intensity of the prompt emission decreases. Additionally, we investigate the effect of $γγ$ absorption in the TeV afterglow caused by prompt MeV photons and find that it is insufficient to explain the early rapid rise in the TeV afterglow, even in the case of $R_{\rm dis}/R_{\rm ext} \sim 1$.

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arXiv2024

Probing the nova shock physics with future gamma-ray observations of the upcoming outburst from T Coronae Borealis

Nova shocks behave like scaled-down supernova remnant shocks with a lifetime of only a few weeks or months, thereby providing a unique opportunity to study the dynamics of non-relativistic shocks as well as the shock acceleration physics. Recently, GeV and TeV gamma-ray emissions from an outburst of the recurrent nova RS Ophiuchi have been observed. The light curves of the gamma-ray emissions suggest that they arise from an external shock, which is formed as the nova ejecta interacts with the ambient medium. The shock is thought to transition from an adiabatic shock to a radiative one at later times, but no such later observations are available for RS Ophiuchi. The spectral evolution of the gamma-ray outburst of RS Ophiuchi was not well measured, and hence the related particle acceleration mechanisms are not well understood. T Coronae Borealis (T CrB) is another recurrent nova in Milky Way and its last outburst was nearly ten times optically brighter than RS Ophiuchi. Recently the optical light curve of T CrB displayed a state transition behavior before the eruption, and it has been predicted that T CrB will undergo an outburst in the near future. By performing a theoretical investigation, we find that Fermi-LAT could probably capture the transition of the shock from the adiabatic phase to the radiative phase at the GeV band if the ambient wind medium is dense with $A_{\star}\geq1$. Due to its higher brightness, we also find that imaging atmospheric Cherenkov telescopes such as MAGIC and VERITAS, and extensive air shower experiments such as LHAASO could detect the nova outburst and measure the gamma-ray spectrum in the very-high-energy ($>0.1\,{\rm TeV}$) band more precisely. This can be used to constrain the high-energy cutoff index in the accelerated proton spectrum and the acceleration efficiency, which will shed light on the particle acceleration physics in nova shocks.

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arXiv2024

Choked precessing jets in tidal disruption events and high-energy neutrinos

It has been suggested that relativistic jets might have been commonly formed in tidal disruption events (TDEs), but those with relatively weak power could be choked by the surrounding envelope. The discovery of high-energy neutrinos possibly associated with some normal TDEs may support this picture in the hypothesis that the neutrinos are produced by choked jets. Recently, it was noted that disrupted stars generally have misaligned orbits with respect to the supermassive black hole spin axis and highly misaligned precessing jets are more likely to be choked. Here we revisit the jet break-out condition for misaligned precessing jets by considering the jet could be collimated by the cocoon pressure while propagating in the disk wind envelope. The jet head opening angle decreases as the jet propagates in the envelope, but the minimum power of a successful jet remains unchanged in terms of the physical jet power. We further calculate the neutrino flux from choked precessing jets, assuming that the cocoon energy does not exceed the kinetic energy of the disk wind. We find that neutrino flux from highly misaligned choked jets is sufficient to explain the neutrinos from AT2019aalc, while it is marginal to explain the neutrinos from AT2019dsg and AT2019fdr. The latter could be produced by weakly misaligned choked jets, since the duty cycle that the jet sweeps across increases as the misaligned angle decreases. We also show that the population of choked TDE jets could contribute to ~10% of the observed diffuse neutrino flux measured by IceCube.

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arXiv2024

A Narrow Uniform Core with a Wide Structured Wing: Modeling the TeV and Multi-wavelength Afterglows of GRB 221009A

The TeV afterglow of the BOAT GRB 221009A was interpreted as arising from a narrow jet while the radio to X-ray afterglows were interpreted as arising from a wide structured jet. However, there is no model explaining the TeV and lower-energy multi-wavelength afterglows simultaneously. We here investigate a two-component jet model, including a narrow uniform core with a wide structured wing, to explain both the multi-wavelength afterglows that last up to 100 days. We find that to explain the early TeV afterglow with the inverse-Compton process, we need a circum-burst density higher than $\gtrsim 0.1{\rm cm^{-3}}$, while the radio afterglow and the H.E.S.S. upper limit combine to constrain the density to be lower at larger radii. Thus, a decreasing density profile with radius is favored. Considering that the rising TeV light curve during the afterglow onset favors a constant-density medium, we invoke a stratified density profile, including a constant-density profile at small radii and a wind density profile at large radii. We find that the two-component jet model with such a stratified density profile can explain the TeV, X-ray and optical afterglows of GRB 221009A, although the radio fluxes exceed the observed ones by a factor of two at later epochs. The discrepancy in the radio afterglow could be resolved by invoking some non-standard assumption about the microphysics of afterglow shocks. The total kinetic energy of the two components in our model is $\lesssim 10^{52}{\rm erg}$, significantly smaller than that in the single structured jet models.

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arXiv2024

The BOAT GRB 221009A: a Poynting-Flux-Dominated Narrow Jet Surrounded by a Matter-Dominated Structured Jet Wing

We argue that the broad-band observations of the brightest-of-all-time (BOAT) GRB 221009A reveal a physical picture involving two jet components: a narrow ($\sim 0.6$ degree half opening angle) pencil-beam jet that has a Poynting-flux-dominated jet composition, and a broader matter-dominated jet with an angular structure. We discuss various observational evidence that supports such a picture. To treat the problem, we develop an analytical structured jet model for both forward and reverse shock emission from the matter dominated structured jet wing during the deceleration phase. We discuss the physical implications of such a two-component jet configuration for this particular burst and for GRBs in general. We argue that some bright X-ray flares could be similar narrow jets viewed slightly outside the narrow jet cone and that narrow jets may exist in many more GRBs without being detected.

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arXiv2023

Fermi-LAT Detection of a GeV Afterglow from a Compact Stellar Merger

It is usually thought that long-duration gamma-ray bursts (GRBs) are associated with massive star core collapse, whereas short-duration GRBs are associated with mergers of compact stellar binaries. The discovery of a kilonova associated with a nearby (350 Mpc) long-duration GRB-GRB 211211A, however, indicates that the progenitor of this long-duration GRB is a compact object merger. Here we report the Fermi-LAT detection of gamma-ray ($>100 {\rm \ MeV}$) afterglow emission from GRB 211211A, which lasts $\sim$20,000 s after the burst, the longest event for conventional short-duration GRBs ever detected. We suggest that this gamma-ray emission results from afterglow synchrotron emission. The soft spectrum of GeV emission may arise from a limited maximum synchrotron energy of only a few hundreds of MeV at $\sim$20,000 s. The unusually long duration of the GeV emission could be due to the proximity of this GRB and the long deceleration time of the GRB jet that is expanding in a low-density circumburst medium, consistent with the compact stellar merger scenario.

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arXiv2023

Choked jets in expanding envelope as the origin of the neutrino emission associated with Tidal Disruption Events

Three tidal disruption event (TDE) candidates (AT2019dsg, AT2019fdr, AT2019aalc) have been found to be coincident with high-energy astrophysical neutrinos in multi-messenger follow-ups. Recent studies suggest the presence of a quasi-spherical, optically thick envelope around the supermassive black holes in TDEs, resulted from stellar debris after the disruption. The envelope may expand outwardly with a velocity of $\sim 10^4\, {\rm km/s}$, as indicated by the emission line widths. We study whether the neutrino signal can be explained by choked relativistic jets inside the expanding envelope. While powerful jets, such as that in Swift J1644+57, can successfully break out from the envelope, those with relatively weak power could be choked by the envelope. Choked jets can still accelerate cosmic rays and produce high-energy neutrinos via interaction with the thermal photons in the envelope. We explore the parameter space of the jets that can produce detectable neutrino flux while being choked in the expanding envelope. We find that the cumulative neutrino numbers of AT2019fdr and AT2019aalc are consistent with the expected range imposed by observations, while the allowed parameter space for AT2019dsg is small.The neutrino time delay relative to the optical peak time of TDEs can be explained as the jet propagation time in the envelope before being choked. The discovery of TDE-associated neutrino events may suggest that jets might have been commonly formed in TDEs, as expected from super-Eddington accretion, but most of them are too weak to break out from the expanding envelopes.

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arXiv2022

Interpretation of the light curve of GeV-TeV emission from the 2021 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf (WD) and either a low-mass Sun-like star (classical novae) or, a red giant. GeV gamma-ray emission has been detected from a dozen of classical novae and from one novae in symbiotic system (V407 Cyg) by Fermi-LAT. For classical novae, gamma-ray emission is generally thought to be related to internal shocks formed as fast outflow collides with the slow outflow. However, for V407 Cyg, the origin of the gamma-ray emission has been debated, as both internal shock and external shock, resulted from the collision between the nova ejecta and the ambient wind of the giant companion, were suggested to explain the gamma-ray data. Recently, bright GeV and TeV gamma-ray emission has been detected from a nova in symbiotic system, RS Ophiuchi, during its 2021 outburst, which shows a remarkably smooth power-law decay in time up to about one month after the outburst. We show that this temporal decay behavior can be interpreted as arising from an adiabatic external shock expanding in the red giant wind. In this interpretation, the gamma-rays are produced by shock-accelerated protons interacting with the dense wind through the hadronic process. We also derive the scaling relations for the decay slopes for both adiabatic and radiative nova shocks in the self-similar deceleration phase.

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arXiv2021

Modeling the fast optical transient SN 2019bkc/ATLAS19dqr with a central engine and implication for its origin

Modern wide-field high-cadence surveys have revealed the significant diversity of optical transient phenomena in their luminosity and timescale distributions, which led to the discovery of some mysterious fast optical transients (FOTs). These FOTs can usually rise and decline remarkably in a timescale of a few days to weeks, which are obviously much rapider than ordinary supernovae. SN 2019bkc/ATLAS19dqr is one of the fastest detected FOTs so far and, meanwhile, it was found to be un-associated with a host galaxy. These discoveries provide a good chance to explore the possible origins of FOTs. So, we model the light curves of SN 2019bkc in details. It is found that SN 2019bkc can be well explained by the thermal emission of an explosion ejecta that is powered by a long-lasting central engine. The engine could be a spinning-down millisecond magnetar or a fallback accretion onto a compact object. Combining the engine property, the mass of the ejecta, and the hostlessness of SN 2019bkc, we suggest that this FOT is likely to originate from a merger of a white dwarf and a neutron star.

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