Dong Lai, opening remark

• Dong Lai

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room

Signatures of high-energy neutrinos from sources for r-process nuclei

• Yongzhong Qian (Univ. of Minnesota)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Collapsars and neutron star mergers are similar in that both host accretion disks and jets. Both are also possible sites for production of heavy nuclei by rapid neutron capture (the r-process). High-energy neutrinos produced during jet propagation can interact with low-energy neutrinos from decay of r-process nuclei or thermal emission of the accretion disk. The potential signatures of such interaction will be discussed.

Model of the very high-energy Galactic gamma-ray emission from discrete distributions of supernova remnants

• Samy Kaci

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Recently, the new generation of gamma-ray telescopes (HAWC, AS-gamma, LHAASO...) opened a new observational window of our Galaxy. It is expected that the ultra-high-energy gamma-ray data provided by these experiments will help answering some of the biggest open questions in cosmic-ray physics. During my talk, I will report my latest results and show how observations of the Galactic diffuse gamma-ray background can help to determine the nature of the so-called Galactic PeVatrons and to constrain their number and the propagation mechanism of cosmic rays in the interstellar medium.

Evidence of hybrid gamma-ray emission from SNR G150.3+4.5

• Yuan Li

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room SNR G150.3+4.5 was first identified in radio and has a hard GeV spectrum with a $\sim 1.5^\circ$ radius. Radio observations revealed a bright arc with an index of $\sim -0.40$ in contrast to the index of $\sim -0.69$ for the other part. This arc is coincident with the point-like Fermi source 4FGL J0426.5+5434 and ultra-high-energy (UHE) source 1LHAASO J0428+5531. The rest of the SNR however has a hard GeV spectrum and a soft TeV spectrum, implying a spectral cutoff or break near 1 TeV. Since there is no X-ray counterpart and no pulse signal detected, the nature of $\gamma$-ray emission from the SNR and the point-like source are puzzling. We reanalyze the $\gamma$-ray emission using 14 yr data recorded by Fermi Large Area Telescope and find that the spectrum of the northern half-sphere is compatible with a broken power-law with a break at 146 $\pm$ 11 GeV and photon indices of $\Gamma_{\rm{NorthLobe}}$ =$1.54\pm0.04_{\rm{stat}}\pm0.07_{\rm{syst}}$ ($2.28\pm0.08_{\rm{stat}}\pm0.12_{\rm{syst}}$) below (above) the break, the southern half-sphere can be described well with a single power-law with $\Gamma_{\rm{SouthLobe}}$ =$1.95\pm0.07_{\rm{stat}}\pm0.09_{\rm{syst}}$. Since the southern half-sphere is well correlated with CO emission, we propose that the $\gamma$-ray emission of the northern half-sphere is dominated by relativistic electrons via the inverse-Compton processes, while the southern half-sphere is dominated by cosmic rays via the hadronic processes. 4FGL J0426.5+5434 can result from illumination of a cloud by escaping cosmic rays or recent shock-cloud interaction. Observations from LHAASO-KM2A then favor the possibility of a cosmic-ray PeVatron candidate, while leptonic scenarios can not be ruled out. Further multi-wavelength observations are warranted to confirm the hadronic nature of 1LHAASO J4028+5531.

Asymmetric cosmic-ray diffusion around sources: "Mirage" sources and large offsets

• Yiwei Bao

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room We show that a large asymmetric halo may be misidentified as multiple "mirage" sources, and that asymmetric diffusion could lead to a very large offset between the injection site and the identified halo. We add background noise into the region and try to identify the sources. We utilize the concept of asymmetric diffusion to elucidate several observed sources that were previously challenging to interpret. Our model offers intuitive explanations for these observations and has the potential to help identify a broad range of sources in the future.

Gamma-ray Astronomy with the Large High Altitude Air Shower Observatory (LHAASO)

• Hao Zhou

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room The Large High Altitude Air Shower Observatory (LHAASO), located in China at 4410m above sea level, is a hybrid extensive air shower detector array. Utilizing different detection techniques, LHAASO monitors the northern gamma-ray sky from a few hundreds of GeV to beyond PeV energies. LHAASO has been fully operational since July 2021. Recently, the first LHAASO catalog published 90 gamma-ray sources. 43 of them are PeVatron candidates with gamma-ray emission above 100 TeV. Among these sources, a giant gamma-ray bubble with energy up to 2 PeV is detected in the Cygnus region, indicating a super-PeVatron cosmic-ray accelerator that is likely associated with young massive star cluster Cygnus OB2. These findings confirmed the existence of PeV cosmic-ray accelerators in our Galaxy and opened up an era of ultra-high-energy gamma-ray astronomy. On Oct 9, 2022, the brightest gamma-ray burst ever seen in history happened in the field-of-view of LHAASO. Tens of thousands of very-high-energy gamma-ray photons were recored. I will briefly summarize recent highlights from LHAASO and relevent analysis topics being done at TDLI.

Progress in kilonova study and discussion on binary black holes and mass transfer

• Kenta Hotokezaka (Univ. of Tokyo)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room In the first part of the talk, I will review progress in kilonova studies, including theoretical modelings, elemental identification, and JWST observation of a kilonova candidate associated with a long GRB 230307A. In the second part of the talk, I will discuss the origin of binary black holes and the potential roles of mass transfer from a massive star to a black hole based on our new radiation hydrodynamic simulation.

Particle acceleration to ultra-high energies at extragalactic jet termination shocks

• Gwenael Giacinti

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Extragalactic plasma jets are some of the few astrophysical environments able to confine ultra-high-energy cosmic rays, but whether they are capable of accelerating these particles is unknown. In this work, we revisit particle acceleration at relativistic magnetized shocks beyond the local uniform field approximation, by considering the global transverse structure of the jet. Using large two-dimensional particle-in-cell simulations of a relativistic electron-ion plasma jet, we show that the termination shock forming at the interface with the ambient medium accelerates particles up to the confinement limit. The radial structure of the jet magnetic field leads to a relativistic velocity shear that excites a von Kármán vortex street in the downstream medium trailing behind an over-pressured bubble filled with cosmic rays. Particles are efficiently accelerated at each crossing of the shear flow boundary layers. These findings support the idea that extragalactic plasma jets may be capable of producing ultra-high-energy cosmic rays. This extreme particle acceleration mechanism may also apply to microquasar jets.

Quasi-periodic eruptions as EM counterparts of extreme mass ratio inpirals

• Zhen Pan

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts with recurrence times about a few hours to a few weeks from galactic nuclei. Though the debates on the origin of QPEs have not completely settled down, more and more analyses favor the interpretation that QPEs are the result of collisions between a stellar mass object (a stellar mass black hole or a main sequence star) and an accretion disk around a supermassive black hole (SMBH) in galactic nuclei. If this interpretation is correct, QPEs will be invaluable in probing the orbits of stellar mass objects in the vicinity of SMBHs, and further inferring the formation of extreme mass ratio inspirals (EMRIs). Among all the analyzed 5 QPE sources where a resonable number of flares are available, two distinct EMRI populations are identified: 4 EMRIs are of low orbital eccentricity (consistent with 0) which are expected to be born in the wet EMRI formation channel, and 1 mildly eccentric EMRI (with $e\approx 0.25$) is consistent with the predictions of both the dry loss-cone formation channel and the Hills mechanism.

Large-scale dynamos and variabilities in thin accretion discs

• Hongzhe Zhou

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room The dynamics of accreting and outgoing flows around compact objects depends crucially on the strengths and configurations of the magnetic fields therein, especially of the large-scale fields that remain coherent beyond turbulence scales. Possible origins of these large-scale magnetic fields include flux advection and disc dynamo actions. However, most numerical simulations have to adopt an initially strong large-scale field rather than allow them to be self-consistently advected or amplified, due to limited computational resources. The situation can be partially cured by using sub-grid models where dynamo actions only reachable at high resolutions are mimicked by artificial terms in low-resolution simulations. In this work, thin-disc models are coupled with local shearing-box simulation results to facilitate more realistic sub-grid dynamo implementations. I will also briefly talk about the possibility of optical-UV variabilities from AGNs driven by such disk dynamos.

The fate of 7 - 70 solar mass stars: from electron capture supernovae to asymmetric mergers and black hole binaries

• John Antoniadis (Institute of Astrophysics - FORTH)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room I will briefly review the late evolution of massive stars that evolve towards electron-capture and core-collapse supernovae, in the context of recent multi-messenger observations. I will then introduce some novel formation scenarios that could help understand the mass spectrum of compact objects and its evolution across Cosmic time.

Temperature properties in two-temperature accretion flows onto a black hole

• Yosuke Mizuno

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. Simplified assumptions about electron thermodynamics are normally employed in GRMHD simulations of accretion onto black holes. To counter this, we developed a self-consistent approach to study two-temperature accretion flows around a Kerr black hole. In this work, we have investigated magnetized and radiatively cooled two-temperature accretion flows around a Kerr black hole. We found that the inclusion of radiative cooling impacts the thermodynamical properties of both the ions and electrons which are important for radiative images.

Flares from Sagittarius A∗: Nonthermal Emission from Magnetic Flux Rope

• Hongxuan Jiang

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Sagittarius A∗, the supermassive black hole at the center of our galaxy, exhibits flares across various wavelengths. However, the origin of these flares remains elusive. Among the potential explanations, magnetic flux ropes emerging from the black hole are an intriguing candidate. These flux ropes are thought to form through reconnection processes within the jet sheath. In this study, we employ nonthermal polarized general relativistic radiation transfer (GRRT) calculation to investigate the emission properties of these flux ropes and the background accretion flows. Our calculations incorporate a subgrid electron heating model by turbulence or magnetic reconnection to determine the electron temperature and the electron distribution function. Across the radio-to-infrared spectrum, both models align well with the observed spectrum. However, the reconnection model falls short in generating sufficiently strong X-ray emission compared to observations (a magnitude lower), while the turbulence model exhibits excellent agreement across all frequencies. The flares produced by our turbulence model present comparable durations of flaring and quiescent intensities to those observed. Notably, the polarization signature in our simulation forms a circular feature in the Stokes Q−U map, coinciding with the flares. The polarization rate from the turbulence model is within the 25−30% range, consistent with the 20−40% range observed value. GRRT results suggest that this polarized emission originates from the flaring region, primarily contributed by the flux ropes. The presence of ordered magnetic fields within the flux ropes likely accounts for the high polarization rate observed during flares.

X-ray reflection spectra from GRMHD simulated thin disks

• Swarnim Shashank (Fudan Univ.)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room X-ray reflection spectroscopy is a prominent method employed to investigate the inner regions of accretion disks surrounding black holes. This technique provides opportunities to measure black hole spins and also test fundamental physics in the presence of strong gravitational fields. The reliability of these measurements heavily relies on the accuracy of the reflection models utilized for spectral analysis, which can be subject to scrutiny. We use GRMHD simulations of thin accretion disks and model their X-ray reflection spectra. The GRMHD disks provide a more realistic scenario compared to the analytical disk profiles typically used in data analysis models. By simulating observations assuming the response of the current X-ray instruments, we assess the ability of current reflection models to accurately recover the input parameters.

Axion star condensation around primordial black holes and microlensing limits

• Ziwen Yin

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Dark matter is one of the biggest mysteries in physics and astronomy today. While multiple dark matter candidates have been proposed, such as the axion and primordial black holes (PBHs), and their interplay deserves further study. In this talk, I will focus on the formation of axion minihalos around PBHs and the condensation of axion stars inside them, and reveal distinct morphological characteristics of these structures compared to isolated axion star scenarios. Furthermore, I will explore the implications of these results when applied to gravitational microlensing from extended objects, providing constraints on the fraction of these objects contributing to microlensing events from the EROS-2 survey.

Non-thermal emissions from accretion flows: Neutrinos from AGN and multi-wavelength signatures from isolated black holes

• Shigeo Kimura (Tohoku Univ.)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room Magnetic activities in black hole accretion flows can produce non-thermal particles, which leads to non-thermal emissions. In the first half of my talk, I will talk about high-energy neutrino emission from accretion flows of supermassive black holes. We construct a high-energy neutrino emission model where hot plasma at the vicinity of the supermassive black holes accelerate protons by stochastic acceleration. In the second half, I will talk about multi-wavelength emission from isolated black holes wandering in interstellar medium. Recently, an isolated black hole is discovered by a microlensing event. Motivated by this, I will introduce how to find isolated black holes using multi-wavelength data and their potential to be PeVatrons and LHAASO sources.

Recent insights on the maximum mass of Neutron Stars

• Lívia S. Rocha (Univ. of Sao Paulo)

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room The maximum mass reached by a neutron star is a long standing problem. Although a general consensus has been built on the impossibility of forming neutron stars with masses far beyond 2 solar masses, the last decade raised mounting evidence on the existence of very massive neutron stars, besides of an absence of a mass gap between neutron stars and black holes. The only two sources of gravitational wave signal from the merger of two neutron stars detected so far raised a tension on whether the limit should be below or above 2.3 solar masses. In this talk we will present an overview of recent results favoring very massive neutron stars, and explore the effect of the orbital inclination angle on the constraint of a maximum value from the mass distribution of galactic systems. Finally, we systematically quantify the evidence against the existence of the mass gap. Through this exploration, we aim to illuminate the current state of understanding while highlighting avenues for future investigation in this captivating field.

Interesting aspects of low angular momentum accretion flow around a Kerr black hole

• Indu K. Dihingia

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room In this talk, we focus on the role of angular momentum in determining the properties of accretion flows around Kerr black holes. Utilizing numerical simulations employing general relativistic magnetohydrodynamics (GRMHD), we explore how different angular momentum profiles influence the flow dynamics. We find distinct characteristics of intermediate angular momentum flows, which exhibit higher density, pressure, and temperature near the black hole, which may have observational consequences. Our findings reveal power-law scalings for density and pressure, emphasizing deviations for non-axisymmetric flows. Furthermore, we observe the shifting of the sonic surface with varying angular momentum. Finally, we propose that intermediate angular momentum flows offer some insights into the complexities observed in the supermassive black hole Sgr A*, which requires more study.

Modeling the inner part of the jet in M87: Confronting jet morphology with theory

• Hai Yang

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room The formation of jets in black hole accretion systems is a long-standing problem. It has been proposed that a jet can be formed by extracting the rotation energy of the black hole (“BZ-jet”) or the accretion flow (“disk-jet”). While both models can produce collimated relativistic outflows, neither has successfully explained the observed jet morphology. By using general relativistic magnetohydrodynamic simulations and considering nonthermal electrons accelerated by magnetic reconnection that is likely driven by magnetic eruption in the underlying accretion flow, we obtain images by radiative transfer calculations and compared them to millimeter observations of the jet in M87. We find that the BZ-jet originating from a magnetically arrested disk around a high-spin black hole can well reproduce the jet morphology, including its width and limb-brightening feature.

Jiao-Tong University Spectroscopic Telescope (JUST)

• Fabo Feng

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room The Jiao Tong University Spectroscopic Telescope (JUST) is a 4.4-meter f/6.0 segmented-mirror telescope in Lenghu, Qinghai Province, China, dedicated to spectroscopic observations. It features an 18-segment primary mirror (1.1 m each) and two Nasmyth platforms—one with a 10′ field of view and the other extending to 1.2° with correction optics. A tertiary mirror facilitates switching between the two foci. The telescope houses three instruments, including a multi-fiber medium-resolution spectrometer for mapping galaxies, an IFU array of 500 fibers or a long-slit spectrograph for fast transient source follow-ups, and a high-resolution spectrometer for identifying exoplanets and characterizing hot exoplanet atmospheres. Scheduled for its first light in 2026, JUST is poised to establish itself as China's premier spectroscopic telescope, promising substantial contributions to the realms of cosmology, time-domain astronomy, and exoplanet research. I will also introduce Tianyu, or JUST-pilot, a 1-meter telescope with a field of view of 10 square degrees. Tianyu is dedicated to searching for long period transiting planets and time-domain events. Tianyu will be located in Lenghu as well and will start scientific operation in 2026.

End remark

• Dong Lai

Room: Tsung-Dao Lee Institute/N4F-N400 - meeting room