分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The parameters of cosmic ray electrons (CRe) and the strength of magnetic field are crucial for studying the particle acceleration of shocks. The recent confirmation of the bipolarity of the eROSITA bubbles suggests that the well-known NPS/Loop I should be a 10--kpc sized relic in the Galactic halo instead of a small local structure near the Sun. By deriving the energy density of CRe and magnetic field strength accounting for the NPS, unprecedentedly precise parameter constraints on particle acceleration for weak shocks in the halo medium can be provided. The parameters of CRe and magnetic field can be derived independently by modeling the gamma-ray and the radio data of the NPS via inverse Compton scattering and synchrotron emission, respectively. Our main results are: (1), the energy density of CRe is (3-6)$\times 10^{-14}$ erg cm$^{-3}$, and the spectral index is $p\simeq 2.0\pm 0.1$ below the cooling break energy of about 5 GeV; (2), the magnetic field strength is 3 $\mu$G; (3), the shock acceleration efficiency of CRe is close to 1%. Given the Mach number of 1.5, the high acceleration efficiency and flat spectrum of CRe suggest that re-acceleration of preexisting relativistic electrons could exist in the NPS. Alternatively, they could be accelerated by a faster shock in the early epoch, and transport to the current position with an unusually large perpendicular diffusion coefficient. In addition, the cooling break energy suggests that the cooling timescale is $10^7$ yr, in agreement with the age of the eROSITA bubbles.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Guobin Mou
Dongze Sun
Taotao Fang
Wei Wang
Ruiyu Zhang
Feng Yuan
Yoshiaki Sofue
Tinggui Wang
Zhicheng He
摘要： The eROSITA bubbles are detected via the instrument with the same name. The northern bubble shows noticeable asymmetric features, including distortion to the west and enhancement in the eastern edge, while the southern counterpart is significantly dimmer. Their origins are debated. Here, we performed hydrodynamic simulations showing that asymmetric eROSITA bubbles favor a dynamic, circumgalactic medium wind model, but disfavor other mechanisms such as a non-axisymmetric halo gas or a tilted nuclear outflow. The wind from the east by north direction in Galactic coordinates blows across the northern halo with a velocity of about 200 km s$^{-1}$, and part of it enters the southern halo. This creates a dynamic halo medium and redistributes both density and metallicity within. This naturally explains the asymmetric bubbles in both the morphology and surface brightness. Our results suggest that our Galaxy is accreting low-abundance circumgalactic medium from one side while providing outflow feedback.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The parameters of cosmic ray electrons (CRe) and the strength of magnetic field are crucial for studying the particle acceleration of shocks. The recent confirmation of the bipolarity of the eROSITA bubbles suggests that the well-known NPS/Loop I should be a 10--kpc sized relic in the Galactic halo instead of a small local structure near the Sun. By deriving the energy density of CRe and magnetic field strength accounting for the NPS, unprecedentedly precise parameter constraints on particle acceleration for weak shocks in the halo medium can be provided. The parameters of CRe and magnetic field can be derived independently by modeling the gamma-ray and the radio data of the NPS via inverse Compton scattering and synchrotron emission, respectively. Our main results are: (1), the energy density of CRe is (3-6)$\times 10^{-14}$ erg cm$^{-3}$, and the spectral index is $p\simeq 2.0\pm 0.1$ below the cooling break energy of about 5 GeV; (2), the magnetic field strength is 3 $\mu$G; (3), the shock acceleration efficiency of CRe is close to 1%. Given the Mach number of 1.5, the high acceleration efficiency and flat spectrum of CRe suggest that re-acceleration of preexisting relativistic electrons could exist in the NPS. Alternatively, they could be accelerated by a faster shock in the early epoch, and transport to the current position with an unusually large perpendicular diffusion coefficient. In addition, the cooling break energy suggests that the cooling timescale is $10^7$ yr, in agreement with the age of the eROSITA bubbles.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Tidal disruption events (TDEs) that occur in active galactic nuclei (AGN) with dusty tori are a special class of sources. TDEs can generate ultrafast and large opening-angle wind, which will almost inevitably collide with the preexisting AGN dusty tori a few years later after the TDE outburst. The wind-torus interactions drive two kinds of shocks: the bow shocks at the windward side of the torus clouds, and the cloud shocks inside the torus clouds. In a previous work, we proved that the shocked clouds will give rise to considerable X-ray emissions which can reach $10^{41-42}$ erg s$^{-1}$ (so called \emph{years delayed X-ray afterglows}). In this work, we focus on the radiations of high energy particles accelerated at both shocks. Benefitting from the strong radiation field at the inner edge of the torus, the inverse Compton scatterings of AGN photons by relativistic electrons at bow shocks dominate the overall gamma-ray radiation. The gamma-ray luminosity can reach $10^{41}~{\rm erg s^{-1}} (L_{\rm kin}/10^{45}{\rm erg s^{-1}})$, where $L_{\rm kin}$ is the kinetic luminosity of TDE wind. Synchrotron radiation at bow shocks contributes to the radio afterglow with a luminosity of 10$^{38-39} ~{\rm erg s^{-1}} (L_{\rm kin}/10^{45}{\rm erg s^{-1}})$ at 1-10 GHz if the magnetic field is 100 mGauss, and extends to infrared with a luminosity of $\sim 10^{39-40}~{\rm erg s^{-1}} (L_{\rm kin}/10^{45}{\rm erg s^{-1}})$. Our scenario provides a prediction of the years delayed afterglows in multiple wavebands for TDEs and reveals their connections.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Cassiopeia A (Cas A) supernova remnant shows strong radiation from radio to gamma-ray bands. The mechanism of gamma-ray radiation in Cas A and its possible contribution to PeV cosmic rays are still under debate. The X-ray imaging reveals an asymmetric profile of Cas A, suggesting the existence of a jet-like structure. In this work, we propose an asymmetrical model for Cas A, consisting of a fast moving jet-like structure and a slowly expanding isotropic shell. This model can account for the multi-wavelength spectra of Cas A, especially for the power-law hard X-ray spectrum from $\sim$ 60 to 220 keV. The GeV to TeV emission from Cas A should be contributed by both hadronic and leptonic processes. Moreover, the jet-like structure may produce a gamma-ray flux of $\sim 10^{-13}\rm erg\ cm^{-2}\ s^{-1}$ at $\sim 100$ TeV, to be examined by LHAASO and CTA.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zhicheng He
Guilin Liu
Tinggui Wang
Guobin Mou
Richard Green
Weihao Bian
Huiyuan Wang
Luis C. Ho
Mouyuan Sun
Lu Shen
Nahum Arav
Chen Chen
Qingwen Wu
Hengxiao Guo
Zesen Lin
Junyao Li
Weimin Yi
摘要： Quasar outflows may play a crucial role in regulating the host galaxy, although the spatial scale of quasar outflows remain a major enigma, with their acceleration mechanism poorly understood. The kinematic information of outflow is the key to understanding its origin and acceleration mechanism. Here, we report the galactocentric distances of different outflow components for both a sample and an individual quasar. We find that the outflow distance increases with velocity, with a typical value from several parsecs to more than one hundred parsecs, providing direct evidence for an acceleration happening at a scale of the order of 10 parsecs. These outflows carry ~1% of the total quasar energy, while their kinematics are consistent with a dust driven model with a launching radius comparable to the scale of a dusty torus, indicating that the coupling between dust and quasar radiation may produce powerful feedback that is crucial to galaxy evolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Tidal disruption event (TDE) can launch an ultrafast outflow. If the black hole is surrounded by large amounts of clouds, outflow-cloud interaction will generate bow shocks, accelerate electrons and produce radio emission. Here we investigate the interaction between a non-relativistic outflow and clouds in active galaxies, which is manifested as outflow-BLR (broad line region) interaction, and can be extended to outflow-torus interaction. This process can generate considerable radio emission, which may account for the radio flares appearing a few months later after TDE outbursts. Benefitting from efficient energy conversion from outflow to shocks and the strong magnetic field, outflow-cloud interaction may play a non-negligible, or even dominating role in generating radio flares in a cloudy circumnuclear environment if the CNM density is no more than 100 times the Sgr A*-like one. In this case, the evolution of radio spectra can be used to directly constrain the properties of outflows.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Guobin Mou
Dongze Sun
Taotao Fang
Wei Wang
Ruiyu Zhang
Feng Yuan
Yoshiaki Sofue
Tinggui Wang
Zhicheng He
摘要： The eROSITA bubbles are detected via the instrument with the same name. The northern bubble shows noticeable asymmetric features, including distortion to the west and enhancement in the eastern edge, while the southern counterpart is significantly dimmer. Their origins are debated. Here, we performed hydrodynamic simulations showing that asymmetric eROSITA bubbles favor a dynamic, circumgalactic medium wind model, but disfavor other mechanisms such as a non-axisymmetric halo gas or a tilted nuclear outflow. The wind from the east by north direction in Galactic coordinates blows across the northern halo with a velocity of about 200 km s$^{-1}$, and part of it enters the southern halo. This creates a dynamic halo medium and redistributes both density and metallicity within. This naturally explains the asymmetric bubbles in both the morphology and surface brightness. Our results suggest that our Galaxy is accreting low-abundance circumgalactic medium from one side while providing outflow feedback.
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