Abstracts of Interest
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Abstract: 2111.07712
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Title:The Crab Pulsar and Nebula as seen in gamma-rays
Download PDFAbstract: Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator.
Abstract: 2111.07389
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Title:Persistent $γ$-ray emission of the blazar PKS 1510-089 in its low state: Fermi-LAT data analysis and theoretical modelling
Download PDFAbstract: Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in $\gamma$ rays via hadronuclear interactions. In the present paper we study the persistent $\gamma$-ray emission of the FSRQ PKS 1510-089 in its low state utilizing the publicly-available Fermi-LAT data, as well as using the spectrum measured with the MAGIC imaging atmospheric Cherenkov telescopes. We find an indication for an excess of $\gamma$ rays at the energy range $\gtrsim$10 GeV with respect to a simple baseline log-parabolic intrinsic spectral model. The statistical significance of the excess is $\approx$5.5$\sigma$, but it drops to $\approx$2.7$\sigma$ after the account of the systematic uncertainty of the MAGIC measurements. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the Cloudy code. To our knowledge, this is the first calculation of electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code.
Abstract: 2111.07142
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Title:Close, bright and boxy: the superluminous SN 2018hti
Download PDFAbstract: SN~2018hti was a very nearby (z=0.0614) superluminous supernova with an exceedingly bright absolute magnitude of -22.2 mag in r-band at maximum. The densely sampled pre-maximum light curves of SN~2018hti show a slow luminosity evolution and constrain the rise time to ~50 rest-frame days. We fitted synthetic light curves to the photometry to infer the physical parameters of the explosion of SN 2018hti for both the magnetar and the CSM-interaction scenarios. We conclude that one of two mechanisms could be powering the luminosity of SN 2018hti; interaction with ~10 Msun of circumstellar material or a magnetar with a magnetic field of B_p~1.3e13 G and initial period of P_spin~1.8 ms. From the nebular spectrum modelling we infer that SN 2018hti likely results from the explosion of a ~40 Msun progenitor star.
Abstract: 2111.09123
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Title:GRB 191016A: A highly collimated gamma-ray burst jet with magnetised energy injection
Download PDFAbstract: Long gamma-ray burst GRB 191016A was a bright and slow rising burst that was detected by the \textit{Swift} satellite and followed up by ground based Liverpool Telescope (LT). LT follow-up started $2411$-s after the \textit{Swift} Burst Alert Telescope (BAT) trigger using imager IO:O around the time of the late optical peak. From $3987-7687$-s, we used the LT polarimeter RINGO3 to make polarimetric and photometric observations of the GRB simultaneously in the $V,R$ and $I$ bands. The combined optical light curve shows an initial late peak followed by a decline until 6147-s, 6087-s, and 5247-s for $I,R$ and $V$ filters respectively followed by a flattening phase. There is evidence of polarization at all phases including polarization ($P = 14.6 \pm 7.2 \%$) which is coincident with the start of the flattening phase. The combination of the light curve morphology and polarization measurement favours an energy injection scenario where slower magnetised ejecta from the central engine catches up with the decelerating blast wave. We calculate the minimum energy injection to be $\Delta E / E>0.36$. At a later time combining the optical light curve from BOOTES (reported via GCN) and IO:O we see evidence of a jet break with jet opening angle 2 \degree.
Abstract: 2111.09001
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Title:A study of Intrinsic $γ$-ray Emission for {\it Fermi}/LAT-detected BL Lacs
Download PDFAbstract: BL Lacs are one subclass of blazars with highly energetic $\gamma$-ray emission, which is strongly boosted by a relativistic beaming effect. The latest catalogue of the 10 years of {\it Fermi}/LAT data Abdollahi et al. (2020) and the $\gamma$-ray Doppler factors in Pei et al. (2020) provide us with a large BL Lac sample to study their jet emission morphologies and intrinsic properties. In this paper, we collected a sample of 294 {\it Fermi} BL Lacs and probed the correlations between the $\gamma$-ray emissions and luminosity distances. Our analyses give following conclusions: (1) the observed $\gamma$-ray emissions are really boosted by the $\gamma$-ray Doppler factor, and the intrinsic $\gamma$-ray emissions are closely correlated with luminosity distances. (2) the morphology of jet emissions for HBLs may be continuous, while that for IBLs may be the case of a moving sphere in the $\gamma$-ray bands.
Abstract: 2111.06946
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Title:Leptonic non-thermal emission from supernova remnants evolving in the circumstellar magnetic field
Download PDFAbstract: The very-high-energy (VHE; E > 100 GeV) gamma-ray emission observed from a number of Supernova remnants (SNRs) indicates particle acceleration to high energies at the shock of the remnants and a potentially significant contribution to Galactic cosmic rays. It is extremely difficult to determine whether protons (through hadronic interactions and subsequent pion decay) or electrons (through inverse Compton scattering on ambient photon fields) are responsible for this emission. For a successful diagnostic, a good understanding of the spatial and energy distribution of the underlying particle population is crucial. Most SNRs are created in core-collapse explosions and expand into the wind bubble of their progenitor stars. This circumstellar medium features a complex spatial distribution of gas and magnetic field which naturally strongly affects the resulting particle population. In this work, we conduct a detailed study of the spectro-spatial evolution of the electrons accelerated at the forward shock of core-collapse SNRs and their non-thermal radiation, using the RATPaC code that is designed for the time- and spatially dependent treatment of particle acceleration at SNR shocks. We focus on the impact of the spatially inhomogeneous magnetic field through the efficiency of diffusion and synchrotron cooling. It is demonstrated that the structure of the circumstellar magnetic field can leave strong signatures in the spectrum and morphology of the resulting non-thermal emission.
Abstract: 2111.09546
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Title:Probing the magnitude of asymmetries in the lateral density distribution of electrons in EAS
Download PDFAbstract: The lateral density distributions (LDD) of inclined cosmic ray air shower are asymmetric and the corresponding iso-density contours are of increasing eccentric ellipses with zenith angles of different showers. The polar asymmetry of the iso-density contours introduces a significant shift of the EAS core, which is quantitatively expressed as a gap length (GL) parameter between the EAS core and the center of the modified density pattern consisting of several equi-density ellipses. The LDD of EAS particles is usually approximated by a particular type of lateral density function (LDF) which is generally assumed to be polar symmetric about the EAS axis, and cannot describe the asymmetric LDDs accurately. A polar angle-dependent modified lateral density function of EASs has been derived analytically by considering the effect of attenuation of EAS particles in the atmosphere. From the simulation studies, it has been found that the GL manifests sensitivity to the cosmic ray mass composition. The cosmic ray mass sensitivity of the lateral shower age is also re-examined by applying the modified LDF to the simulated data.
Abstract: 2111.09534
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Title:Resurrection of Non-thermal Emissions from Type Ib/c Supernova Remnants
Download PDFAbstract: Supernova remnants (SNRs) are important objects in investigating the links among supernova (SN) explosion mechanism(s), progenitor stars, and cosmic-ray acceleration. Non-thermal emission from SNRs is an effective and promising tool for probing their surrounding circumstellar media (CSM) and, in turn, the stellar evolution and mass-loss mechanism(s) of massive stars. In this work, we calculate the time evolution of broadband non-thermal emissions from Type Ib/c SNRs whose CSM structures are derived from the mass-loss history of their progenitors. Our results predict that Type Ib/c SNRs make a transition of brightness in radio and $\gamma$-ray bands from an undetectable dark for a certain period to a re-brightening phase. This transition originates from their inhomogeneous CSM structures in which the SNRs are embedded within a low-density wind cavity surrounded by a high-density wind shell and the ambient interstellar medium (ISM). The "resurrection" in non-thermal luminosity happens at an age of ~1,000 yrs old for a Wolf-Rayet star progenitor evolved within a typical ISM density. Combining with the results of Type II SNR evolution recently reported by Yasuda et al. (2021), this result sheds light on a comprehensive understanding of non-thermal emissions from SNRs with different SN progenitor types and ages, which is made possible for the first time by the incorporation of realistic mass-loss histories of the progenitors.
Abstract: 2111.09391
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Title:Establishing accretion flares from massive black holes as a major source of high-energy neutrinos
Download PDFAbstract: High-energy neutrinos have thus far been observed in coincidence with time-variable emission from three different accreting black holes: a gamma-ray flare from a blazar (TXS 0506+056), an optical transient following a stellar tidal disruption (AT2019dsg), and an optical outburst from an active galactic nucleus (AT2019fdr). Here we present a unified explanation for the latter two of these sources: accretion flares that reach the Eddington limit. A signature of these events is a luminous infrared reverberation signal from circumnuclear dust that is heated by the flare. Using this property we construct a sample of similar sources, revealing a third event coincident with a PeV-scale neutrino. This sample of three accretion flares is correlated with high-energy neutrinos at a significance of 3.7 sigma. Super-Eddington accretion could explain the high particle acceleration efficiency of this new population.
Abstract: 2111.09320
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Title:High-energy neutrinos from X-rays flares of blazars frequently observed by the Swift X-Ray Telescope
Download PDFAbstract: Blazar flares have been suggested as ideal candidates for enhanced neutrino production. While the neutrino signal of $\gamma$-ray flares has been widely discussed, the neutrino yield of X-ray flares has received less attention. Here, we compute the predicted neutrino signal from X-ray flares detected in 66 blazars observed more than 50 times with the X-ray Telescope (XRT) on board the Neil Gehrels Swift Observatory. We consider a scenario where X-ray flares are powered by synchrotron radiation of relativistic protons, and neutrinos are produced through photomeson interactions between protons with their own synchrotron X-ray photons. Using the 1 keV X-ray light curves for flare identification, the 0.5-10 keV fluence of each flare as a proxy for the all-flavour neutrino fluence, and the IceCube point-source effective area for different detector configurations, we calculate the number of muon and antimuon neutrinos above 100 TeV expected for IceCube from each flaring source. The bulk of the neutrino events from the sample originates from flares with durations $\sim 1-10$ d. Accounting for the X-ray flare duty cycle of the sources in the sample, which ranges between $\sim2$ and 24 per cent, we compute an average yearly neutrino rate for each source. The median of the distribution (in logarithm) is $\sim0.03$ yr$^{-1}$, with Mkn 421 having the highest predicted rate $1.2\pm 0.3$ yr$^{-1}$, followed by 3C 273 $(0.33\pm0.03$ yr$^{-1})$ and PG 1553+113 ($0.25\pm0.02$ yr$^{-1}$). Next-generation neutrino detectors together with regular X-ray monitoring of blazars could constrain the duty cycle of hadronic X-ray flares.
Abstract: 2111.09334
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Title:Dynamics of relativistic electrons in non-uniform magnetic fields and its applications in quantum computing and astrophysics
Download PDFAbstract: We explore the two-dimensional motion of relativistic electrons when they are trapped in magnetic fields having spatial power-law variation. Its impacts include lifting of degeneracy that emerged in the case of the constant magnetic field, special alignment of Landau levels of spin-up and spin-down electrons depending on whether the magnetic field is increasing or decreasing from the centre, splitting of Landau levels of electrons with zero angular momentum from that of positive one and the change in the equation of state of matter. Landau quantization (LQ) in variable magnetic fields has interdisciplinary applications in a variety of disciplines ranging from condensed matter to quantum information. As examples, we discuss the increase in quantum speed of the electron in presence of spatially increasing magnetic field; and the attainment of super Chandrasekhar mass of white dwarfs by taking into account LQ and Lorentz force simultaneously.
Abstract: 2111.09390
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Title:The candidate tidal disruption event AT2019fdr coincident with a high-energy neutrino
Download PDFAbstract: The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time X-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.
Abstract: 2111.10299
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Title:Improved Characterization of the Astrophysical Muon-Neutrino Flux with 9.5 Years of IceCube Data
Download PDFAbstract: We present a measurement of the high-energy astrophysical muon-neutrino flux with the IceCube Neutrino Observatory. The measurement uses a high-purity selection of ~650k neutrino-induced muon tracks from the Northern celestial hemisphere, corresponding to 9.5 years of experimental data. With respect to previous publications, the measurement is improved by the increased size of the event sample and the extended model testing beyond simple power-law hypotheses. An updated treatment of systematic uncertainties and atmospheric background fluxes has been implemented based on recent models. The best-fit single power-law parameterization for the astrophysical energy spectrum results in a normalization of $\phi_{\mathrm{@100TeV}}^{\nu_\mu+\bar{\nu}_\mu} = 1.44_{-0.26}^{+0.25} \times 10^{-18}\,\mathrm{GeV}^{-1}\mathrm{cm}^{-2}\mathrm{s}^{-1}\mathrm{sr}^{-1}$ and a spectral index $\gamma_{\mathrm{SPL}} = 2.37_{-0.09}^{+0.09}$, constrained in the energy range from $15\,\mathrm{TeV}$ to $5\,\mathrm{PeV}$. The model tests include a single power law with a spectral cutoff at high energies, a log-parabola model, several source-class specific flux predictions from the literature and a model-independent spectral unfolding. The data is well consistent with a single power law hypothesis, however, spectra with softening above one PeV are statistically more favorable at a two sigma level.
Abstract: 2111.09970
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Title:Search for GeV-scale Dark Matter Annihilation in the Sun with IceCube DeepCore
Download PDFAbstract: The Sun provides an excellent target for studying spin-dependent dark matter-proton scattering due to its high matter density and abundant hydrogen content. Dark matter particles from the Galactic halo can elastically interact with Solar nuclei, resulting in their capture and thermalization in the Sun. The captured dark matter can annihilate into Standard Model particles including an observable flux of neutrinos. We present the results of a search for low-energy ($<$ 500 GeV) neutrinos correlated with the direction of the Sun using 7 years of IceCube data. This work utilizes, for the first time, new optimized cuts to extend IceCube's sensitivity to dark matter mass down to 5 GeV. We find no significant detection of neutrinos from the Sun. Our observations exclude capture by spin-dependent dark matter-proton scattering with cross-section down to a few times $10^{-41}$ cm$^2$, assuming there is equilibrium with annihilation into neutrinos/anti-neutrinos for dark matter masses between 5 GeV and 100 GeV. These are the strongest constraints at GeV energies for dark matter annihilation directly to neutrinos.
Abstract: 2111.09962
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Title:Observation of Variations in Cosmic Ray Single Count Rates During Thunderstorms and Implications for Large-Scale Electric Field Changes
Download PDFAbstract: We present the first observation by the Telescope Array Surface Detector (TASD) of the effect of thunderstorms on the development of cosmic ray single count rate intensity over a 700 km$^{2}$ area. Observations of variations in the secondary low-energy cosmic ray counting rate, using the TASD, allow us to study the electric field inside thunderstorms, on a large scale, as it progresses on top of the 700 km$^{2}$ detector, without dealing with the limitation of narrow exposure in time and space using balloons and aircraft detectors. In this work, variations in the cosmic ray intensity (single count rate) using the TASD, were studied and found to be on average at the $\sim(0.5-1)\%$ and up to 2\% level. These observations were found to be both in excess and in deficit. They were also found to be correlated with lightning in addition to thunderstorms. These variations lasted for tens of minutes; their footprint on the ground ranged from 6 to 24 km in diameter and moved in the same direction as the thunderstorm. With the use of simple electric field models inside the cloud and between cloud to ground, the observed variations in the cosmic ray single count rate were recreated using CORSIKA simulations. Depending on the electric field model used and the direction of the electric field in that model, the electric field magnitude that reproduces the observed low-energy cosmic ray single count rate variations was found to be approximately between 0.2-0.4 GV. This in turn allows us to get a reasonable insight on the electric field and its effect on cosmic ray air showers inside thunderstorms.
Abstract: 2111.09929
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Title:Scaling violation in interaction of cosmic ray hadrons and the nature of the 3 PeV knee in the spectrum of primary cosmic rays
Download PDFAbstract: Energy spectra of the most energetic hadrons in the core region of extensive air showers (EAS) were studied in dependence on the EAS energy $E_0$ in the hybrid experiment HADRON at the Tien Shan mountain cosmic ray station. For the first time by direct measurement it was found in this experiment that the slope of the power energy spectrum of EAS hadrons diminishes itself locally, and the average hadron energy, correspondingly, increases between the $E_0$ borders 3PeV and 20PeV. Such behavior agrees with threshold appearance in EAS, in the same energy range, of a long-flying penetrative component which was earlier revealed at the Tien Shan station. Now we reconsider this Tien Shan result in comparison with the new data of modern collider experiments. The analysis permits to state unambiguously an astrophysical nature of the penetrative EAS component, and to discuss its probable connection with the origin of the famous knee in the energy spectrum of cosmic rays at $E_0\simeq 3$ PeV.
Abstract: 2111.09889
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Title:Multiwavelength View of the M87 Black Hole Captured by the Event Horizon Telescope
Download PDFAbstract: In 2017, the first image of the center of the M87 galaxy was captured by the Event Horizon Telescope (EHT). It revealed a ring morphology and a size consistent with theoretical expectations for the light pattern around a weakly accreting supermassive black hole of ~6.5 billion solar masses. In parallel to the EHT measurements, an extensive multi-wavelength (MWL) campaign with ground- and space-based facilities from radio all the way up to the TeV energy range was organized. In this note we will give an overview of the results from this campaign. M87 was found to be in a historically low state. At X-ray energies the emission from the core dominates over HST-1. We present the most complete simultaneous, MWL spectrum of the active nucleus to date.
Abstract: 2111.10169
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Title:A search for neutrino emission from cores of Active Galactic Nuclei
Download PDFAbstract: The sources of the majority of the high-energy astrophysical neutrinos observed with the IceCube neutrino telescope at the South Pole are unknown. So far, only a gamma-ray blazar was compellingly associated with the emission of high-energy neutrinos. In addition, several studies suggest that the neutrino emission from the gamma-ray blazar population only accounts for a small fraction of the total astrophysical neutrino flux. In this work we probe the production of high-energy neutrinos in the cores of Active Galactic Nuclei (AGN), induced by accelerated cosmic rays in the accretion disk region. We present a likelihood analysis based on eight years of IceCube data, searching for a cumulative neutrino signal from three AGN samples created for this work. The neutrino emission is assumed to be proportional to the accretion disk luminosity estimated from the soft X-ray flux. Next to the observed soft X-ray flux, the objects for the three samples have been selected based on their radio emission and infrared color properties. For the largest sample in this search, an excess of high-energy neutrino events with respect to an isotropic background of atmospheric and astrophysical neutrinos is found, corresponding to a post-trial significance of 2.60$\sigma$. Assuming a power-law spectrum, the best-fit spectral index is $2.03^{+0.14}_{-0.11}$, consistent with expectations from particle acceleration in astrophysical sources. If interpreted as a genuine signal with the assumptions of a proportionality of X-ray and neutrino fluxes and a model for the sub-threshold flux distribution, this observation implies that at 100 TeV, 27$\%$ - 100$\%$ of the observed neutrinos arise from particle acceleration in the core of AGN.
Abstract: 2111.08153
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Full Text: [ PostScript, PDF]
Title:Gamma-ray spectral properties of the Galactic globular clusters: constraint on the numbers of millisecond pulsars
Download PDFAbstract: We study the gamma-ray spectra of 30 globular clusters (GCs) thus far detected with the Fermi Gamma-ray Space Telescope. Presuming that gamma-ray emission of a GC comes from millisecond pulsars (MSPs) contained in, a model that generates spectra for the GCs is built based on the gamma-ray properties of the detected MSP sample. We fit the GCs' spectra with the model, and for 27 of them, their emission can be explained with arising from MSPs. The spectra of the other three, NGC 7078, 2MS-GC01, and Terzan 1, can not be fit with our model, indicating that MSPs' emission should not be the dominant one in the first two and the third one has a unique hard spectrum. We also investigate six nearby GCs that have relatively high encounter rates as the comparison cases. The candidate spectrum of NGC 6656 can be fit with that of one MSP, supporting its possible association with the gamma-ray source at its position. The five others do not have detectable gamma-ray emission. Their spectral upper limits set limits of mostly $\leq 1$ MSPs in them, consistent with the numbers of radio MSPs found in them. The estimated numbers of MSPs in the gamma-ray GCs are generally larger than those reported for radio pulsars, suggesting more MSPs in them than those currently revealed. Our studies of the gamma-ray GCs and the comparison nearby GCs indicate that the encounter rate should not be the only factor determining the number of MSPs a GC contains.
Abstract: 2111.09470
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Full Text: [ PostScript, PDF]
Title:Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment
Download PDFAbstract: In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hours of the supernova explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen Camera. The fast evolution of the early flash was captured by intensive intranight observations via the Tomo-e Gozen high-cadence survey. Numerical simulations show that such a prominent and fast early emission is most likely generated from an interaction between $0.01~M_{\odot}$ circumstellar material (CSM) extending to a distance of $\sim$$10^{13}~\text{cm}$ and supernova ejecta soon after the explosion, indicating a confined dense CSM formation at the final evolution stage of the progenitor of SN 2020hvf. Based on the CSM-ejecta interaction-induced early flash, the overluminous light curve, and the high ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf ("super-$M\rm_{Ch}$ WD"). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-$M\rm_{Ch}$ WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova.
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