We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various ...We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various values of specific angular momentum,λ_(0).Within the range 10-50R_(g)(where R_(g)denotes the Schwarzschild radius),shocks are discernible forλ_(0)≥1.75.In the special relativistic hydrodynamic simulation whenλ_(0)=1.80,we find the merger of two shocks resulted in a dramatic increase in luminosity.We present the impact of external and internal flow collisions from the funnel region on luminosity.Notably,oscillatory behavior characterizes shocks within 1.70≤λ_(0)≤1.80.Using free–free emission as a proxy for analysis,we show that the luminosity oscillations between frequencies of 0.1-10 Hz forλ_(0)range as 1.7≤λ_(0)≤1.80.These findings offer insights into quasiperiodic oscillation emissions from certain black hole X-ray binaries,exemplified by GX 339-4.Furthermore,for the supermassive black hole at the Milky Way's center,Sgr A*,oscillation frequencies between 10^(-6)and 10^(-5)Hz were observed.This frequency range,translating to one cycle every few days,aligns with observational data from X-ray telescopes such as Chandra,Swift,and XMM-Newton.展开更多
The mass-radius relations for white dwarfs are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff （TOV） equations for hydrostatic equilibrium assuming the electron gas to be non-interacting....The mass-radius relations for white dwarfs are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff （TOV） equations for hydrostatic equilibrium assuming the electron gas to be non-interacting. We find that the Newtonian limiting mass of 1.4562 M is modified to 1.4166 M in the general relativistic case for 4He （and 12 6C） white dwarfs. Using the same general relativistic treatment, the critical mass for 56 26Fe white dwarfs is obtained as 1.2230 M. In addition, departure from the ideal degenerate equation of state （EoS） is accounted for by considering Salpeter＇s EoS along with the TOV equation, yielding slightly lower values for the critical masses, namely 1.4081 M for 4He, 1.3916M for 12C and 1.1565M for 56 26Fe white dwarfs. We also compare the critical densities for gravitational instability with the neutronization threshold densities to find that 4 2He and 12 6C white dwarfs are stable against neutronization with the critical values of 1.4081 M and 1.3916 M, respectively. However, the critical masses for 16 8O, 20 10Ne, 24 12Mg, 28 14Si,32 16S and 56 26Fewhite dwarfs are 26Fe white lower due to neutronization. Corresponding to their central densities for neutronization thresholds, we obtain their maximum stable masses due to neutronization by solving the TOV equation coupled with the Salpeter EoS.展开更多
Considered as a gedanken experiment are the conditions under which the relativistic Doppler shifting of visible electromagnetic radiation to beyond the human ocular range could reduce the incident radiance of the sour...Considered as a gedanken experiment are the conditions under which the relativistic Doppler shifting of visible electromagnetic radiation to beyond the human ocular range could reduce the incident radiance of the source, and render a luminous astrophysical body (LAB) invisible to a naked eye. This paper determines the proper distance as a function of relativistic velocity at which a luminous object attains ocular invisibility.展开更多
This study employs two-dimensional axisymmetric relativistic magnetohydrodynamic simulations to investigate the evolution of supernova remnant(SNR) and pulsar wind nebula(PWN) composite systems in two distinct interst...This study employs two-dimensional axisymmetric relativistic magnetohydrodynamic simulations to investigate the evolution of supernova remnant(SNR) and pulsar wind nebula(PWN) composite systems in two distinct interstellar medium(ISM) configurations: a uniform density distribution and a medium with a sharp density discontinuity. Compared to the uniform density distribution, the ISM with this density discontinuity better reflects the actual conditions and explains the overall morphological characteristics of specific types of SNR-PWN composite systems. These systems exhibit asymmetries, such as an SNR shell with differing radii or an inner PWN located nearer to the shell on one side. The simulation results suggest that the density discontinuity in the ISM is a contributing factor to both the shell asymmetry and the PWN displacement. Specifically, this density variation directly causes the inconsistency in the forward shock speeds of the SNR between high and low density regions, resulting in discrepancies in the shell layer radii. Furthermore, the asymmetric morphology of the PWN and its positional offset emerge through interactions with the reverse shock. The PWN tends to shift toward the SNR shell on one side. The greater the density jump in the background field, the more pronounced the shell radius differences and PWN offset become.展开更多
Energetic X-ray radiations emitted from various accretion systems are widely considered to be produced by Comptonization in the hot corona.The corona and its interaction with the disk play an essential role in the evo...Energetic X-ray radiations emitted from various accretion systems are widely considered to be produced by Comptonization in the hot corona.The corona and its interaction with the disk play an essential role in the evolution of the system and are potentially responsible for many observed features.However,many intrinsic properties of the corona are still poorly understood,especially for the geometrical configurations.The traditional spectral fitting method is not powerful enough to distinguish various configurations.In this paper,we intend to investigate the possible configurations by modeling the polarization properties of X-ray radiations.The geometries of the corona include the slab,sphere and cylinder.The simulations are implemented through the publicly available code,Lemon,which can deal with the polarized radiative transfer and different electron distributions readily.The results demonstrate clearly that the observed polarizations are dependent heavily on the geometry of the corona.The slab-like corona produces the highest polarization degrees(PDs),followed by the cylinder and sphere.One of the interesting things is that the PDs first increase gradually and then decrease with the increase of photon energy.For slab geometry,there exists a zero-point where the polarization vanishes and the polarization angle(PA)rotates by 90°.These results may potentially be verified by the upcoming missions for polarized X-ray observations,such as IXPE and e XTP.展开更多
The broadband spectral energy distribution（SED） of blazars is generally interpreted as radiation arising from synchrotron and inverse Compton mechanisms. Traditionally,the underlying source parameters responsible fo...The broadband spectral energy distribution（SED） of blazars is generally interpreted as radiation arising from synchrotron and inverse Compton mechanisms. Traditionally,the underlying source parameters responsible for these emission processes,like particle energy density,magnetic field,etc.,are obtained through simple visual reproduction of the observed fluxes. However,this procedure is incapable of providing confidence ranges for the estimated parameters. In this work,we propose an efficient algorithm to perform a statistical fit of the observed broadband spectrum of blazars using different emission models. Moreover,we use the observable quantities as the fit parameters,rather than the direct source parameters which govern the resultant SED. This significantly improves the convergence time and eliminates the uncertainty regarding initial guess parameters. This approach also has an added advantage of identifying the degenerate parameters,which can be removed by including more observable information and/or additional constraints. A computer code developed based on this algorithm is implemented as a user-defined routine in the standard X-ray spectral fitting package,XSPEC. Further,we demonstrate the efficacy of the algorithm by fitting the well sampled SED of blazar 3 C 279 during its gamma ray flare in 2014.展开更多
Exact solutions to the Einstein field equations for class I spacetime symmetry in relativistic stars are generated.The symmetry provides a relation between the gravitational potentials that lead to generalized solutio...Exact solutions to the Einstein field equations for class I spacetime symmetry in relativistic stars are generated.The symmetry provides a relation between the gravitational potentials that lead to generalized solutions of the Einstein field equations.We choose one of the gravitational potentials on a physical basis,which allows us to obtain the other gravitational potential via an embedding approach.It is therefore possible to generate a model with astrophysical significance.The model generated satisfies physical properties like stability,causality,regularity,equilibrium and energy conditions.展开更多
We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar’s crust.We address the basics of such a model by solving the...We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar’s crust.We address the basics of such a model by solving the propagation of the perturbation approximately,and quantify the energetics and the radiation by bunches of charges in the so-called charge starved region in the magnetosphere.The(almost)simultaneous emission of newly detected X-rays from SGR 1935+2154 is tentatively associated with a reconnection behind the propagation.The strength of f-mode gravitational radiation excited by the event is quantified,and more detailed studies of the nonlinear(spiky)soliton solutions are suggested.展开更多
The afterglow of GRB 170817 A has been detected for more than three years,but the origin of the multi-band afterglow light curves remains under debate.A classical top-hat jet model is faced with difficulties in produc...The afterglow of GRB 170817 A has been detected for more than three years,but the origin of the multi-band afterglow light curves remains under debate.A classical top-hat jet model is faced with difficulties in producing a shallow rise of the afterglow light curves as observed(E_(v) ∝ T^(0.8)).Here we reconsider the model of stratified ejecta with an energy profile of E(> Γβ)=E_0(Γβ)^(-k) as the origin of the afterglow light curves of the burst,where Γ and β are the Lorentz factor and speed of the ejecta,respectively.k is the power-law slope of the energy profile.We consider that the ejecta are collimated into jets.Two kinds of jet evolutions are investigated,including a lateral-spreading jet and a non-lateral-spreading jet.We fit the multi-band afterglow light curves,including the X-ray data at one thousand days post-burst,and find that both the models of the spreading and non-spreading jets can fit the light curves well,but the observed angular size of the source and the apparent velocity of the flux centroid for the spreading jet model are beyond the observation limits,while the non-spreading jet model meets the observation limits.Some of the best-fit parameters for the non-spreading jet model,such as the number density of the circumburst medium~10^(-2) cm^(-3) and the total jet kinetic energy E ~ 4.8 × 10^(51) erg,also appear plausible.The best-fit slope of the jet energy profile is k ~7.1.Our results suggest that the afterglow of GRB 170817 A may arise from the stratified jet and that the lateral spreading of the jet is not significant.展开更多
基金supported by the National Natural Science Foundation of China(NSFC,Grant No.12073021)。
文摘We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various values of specific angular momentum,λ_(0).Within the range 10-50R_(g)(where R_(g)denotes the Schwarzschild radius),shocks are discernible forλ_(0)≥1.75.In the special relativistic hydrodynamic simulation whenλ_(0)=1.80,we find the merger of two shocks resulted in a dramatic increase in luminosity.We present the impact of external and internal flow collisions from the funnel region on luminosity.Notably,oscillatory behavior characterizes shocks within 1.70≤λ_(0)≤1.80.Using free–free emission as a proxy for analysis,we show that the luminosity oscillations between frequencies of 0.1-10 Hz forλ_(0)range as 1.7≤λ_(0)≤1.80.These findings offer insights into quasiperiodic oscillation emissions from certain black hole X-ray binaries,exemplified by GX 339-4.Furthermore,for the supermassive black hole at the Milky Way's center,Sgr A*,oscillation frequencies between 10^(-6)and 10^(-5)Hz were observed.This frequency range,translating to one cycle every few days,aligns with observational data from X-ray telescopes such as Chandra,Swift,and XMM-Newton.
文摘The mass-radius relations for white dwarfs are investigated by solving the Newtonian as well as Tolman-Oppenheimer-Volkoff （TOV） equations for hydrostatic equilibrium assuming the electron gas to be non-interacting. We find that the Newtonian limiting mass of 1.4562 M is modified to 1.4166 M in the general relativistic case for 4He （and 12 6C） white dwarfs. Using the same general relativistic treatment, the critical mass for 56 26Fe white dwarfs is obtained as 1.2230 M. In addition, departure from the ideal degenerate equation of state （EoS） is accounted for by considering Salpeter＇s EoS along with the TOV equation, yielding slightly lower values for the critical masses, namely 1.4081 M for 4He, 1.3916M for 12C and 1.1565M for 56 26Fe white dwarfs. We also compare the critical densities for gravitational instability with the neutronization threshold densities to find that 4 2He and 12 6C white dwarfs are stable against neutronization with the critical values of 1.4081 M and 1.3916 M, respectively. However, the critical masses for 16 8O, 20 10Ne, 24 12Mg, 28 14Si,32 16S and 56 26Fewhite dwarfs are 26Fe white lower due to neutronization. Corresponding to their central densities for neutronization thresholds, we obtain their maximum stable masses due to neutronization by solving the TOV equation coupled with the Salpeter EoS.
文摘Considered as a gedanken experiment are the conditions under which the relativistic Doppler shifting of visible electromagnetic radiation to beyond the human ocular range could reduce the incident radiance of the source, and render a luminous astrophysical body (LAB) invisible to a naked eye. This paper determines the proper distance as a function of relativistic velocity at which a luminous object attains ocular invisibility.
基金supported by the National Natural Science Foundation of China(NSFC,grants No.12393852)the Yunnan Fundamental Research Projects(grant No.202501AS070068)the Program of Graduate Research and Innovation Fund Project of Yunnan University(KC-24249493).
文摘This study employs two-dimensional axisymmetric relativistic magnetohydrodynamic simulations to investigate the evolution of supernova remnant(SNR) and pulsar wind nebula(PWN) composite systems in two distinct interstellar medium(ISM) configurations: a uniform density distribution and a medium with a sharp density discontinuity. Compared to the uniform density distribution, the ISM with this density discontinuity better reflects the actual conditions and explains the overall morphological characteristics of specific types of SNR-PWN composite systems. These systems exhibit asymmetries, such as an SNR shell with differing radii or an inner PWN located nearer to the shell on one side. The simulation results suggest that the density discontinuity in the ISM is a contributing factor to both the shell asymmetry and the PWN displacement. Specifically, this density variation directly causes the inconsistency in the forward shock speeds of the SNR between high and low density regions, resulting in discrepancies in the shell layer radii. Furthermore, the asymmetric morphology of the PWN and its positional offset emerge through interactions with the reverse shock. The PWN tends to shift toward the SNR shell on one side. The greater the density jump in the background field, the more pronounced the shell radius differences and PWN offset become.
基金the National Natural Science Foundation of China(NSFC,Grant Nos.U2031111,11573060,12073069,11661161010 and 11673060)。
文摘Energetic X-ray radiations emitted from various accretion systems are widely considered to be produced by Comptonization in the hot corona.The corona and its interaction with the disk play an essential role in the evolution of the system and are potentially responsible for many observed features.However,many intrinsic properties of the corona are still poorly understood,especially for the geometrical configurations.The traditional spectral fitting method is not powerful enough to distinguish various configurations.In this paper,we intend to investigate the possible configurations by modeling the polarization properties of X-ray radiations.The geometries of the corona include the slab,sphere and cylinder.The simulations are implemented through the publicly available code,Lemon,which can deal with the polarized radiative transfer and different electron distributions readily.The results demonstrate clearly that the observed polarizations are dependent heavily on the geometry of the corona.The slab-like corona produces the highest polarization degrees(PDs),followed by the cylinder and sphere.One of the interesting things is that the PDs first increase gradually and then decrease with the increase of photon energy.For slab geometry,there exists a zero-point where the polarization vanishes and the polarization angle(PA)rotates by 90°.These results may potentially be verified by the upcoming missions for polarized X-ray observations,such as IXPE and e XTP.
文摘The broadband spectral energy distribution（SED） of blazars is generally interpreted as radiation arising from synchrotron and inverse Compton mechanisms. Traditionally,the underlying source parameters responsible for these emission processes,like particle energy density,magnetic field,etc.,are obtained through simple visual reproduction of the observed fluxes. However,this procedure is incapable of providing confidence ranges for the estimated parameters. In this work,we propose an efficient algorithm to perform a statistical fit of the observed broadband spectrum of blazars using different emission models. Moreover,we use the observable quantities as the fit parameters,rather than the direct source parameters which govern the resultant SED. This significantly improves the convergence time and eliminates the uncertainty regarding initial guess parameters. This approach also has an added advantage of identifying the degenerate parameters,which can be removed by including more observable information and/or additional constraints. A computer code developed based on this algorithm is implemented as a user-defined routine in the standard X-ray spectral fitting package,XSPEC. Further,we demonstrate the efficacy of the algorithm by fitting the well sampled SED of blazar 3 C 279 during its gamma ray flare in 2014.
基金the University of Dodoma in Tanzania for the continuous support and encouragement to complete this research workthe Ministry of Education,Science and Technology of Tanzania for sponsorshipthe South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation that facilitate the research funding。
文摘Exact solutions to the Einstein field equations for class I spacetime symmetry in relativistic stars are generated.The symmetry provides a relation between the gravitational potentials that lead to generalized solutions of the Einstein field equations.We choose one of the gravitational potentials on a physical basis,which allows us to obtain the other gravitational potential via an embedding approach.It is therefore possible to generate a model with astrophysical significance.The model generated satisfies physical properties like stability,causality,regularity,equilibrium and energy conditions.
基金financial support.J.E.H.has been supported by the CNPq Agency(Brazil)the FAPESP foundation(S?o Paulo,Brazil)。
文摘We revisit in this work a model for repeating Fast Radio Bursts based of the release of energy provoked by the magnetic field dynamics affecting a magnetar’s crust.We address the basics of such a model by solving the propagation of the perturbation approximately,and quantify the energetics and the radiation by bunches of charges in the so-called charge starved region in the magnetosphere.The(almost)simultaneous emission of newly detected X-rays from SGR 1935+2154 is tentatively associated with a reconnection behind the propagation.The strength of f-mode gravitational radiation excited by the event is quantified,and more detailed studies of the nonlinear(spiky)soliton solutions are suggested.
基金supported by the National Natural Science Foundation of China(Nos.U1831135 and 11833003)Yunnan Natural Science Foundation(2014FB188)+2 种基金support by the National Key Research and Development Programs of China(2018YFA0404204)the National Natural Science Foundation of China(grant Nos.11833003 and U2038105)the Program for Innovative Talents,Entrepreneur in Jiangsu。
文摘The afterglow of GRB 170817 A has been detected for more than three years,but the origin of the multi-band afterglow light curves remains under debate.A classical top-hat jet model is faced with difficulties in producing a shallow rise of the afterglow light curves as observed(E_(v) ∝ T^(0.8)).Here we reconsider the model of stratified ejecta with an energy profile of E(> Γβ)=E_0(Γβ)^(-k) as the origin of the afterglow light curves of the burst,where Γ and β are the Lorentz factor and speed of the ejecta,respectively.k is the power-law slope of the energy profile.We consider that the ejecta are collimated into jets.Two kinds of jet evolutions are investigated,including a lateral-spreading jet and a non-lateral-spreading jet.We fit the multi-band afterglow light curves,including the X-ray data at one thousand days post-burst,and find that both the models of the spreading and non-spreading jets can fit the light curves well,but the observed angular size of the source and the apparent velocity of the flux centroid for the spreading jet model are beyond the observation limits,while the non-spreading jet model meets the observation limits.Some of the best-fit parameters for the non-spreading jet model,such as the number density of the circumburst medium~10^(-2) cm^(-3) and the total jet kinetic energy E ~ 4.8 × 10^(51) erg,also appear plausible.The best-fit slope of the jet energy profile is k ~7.1.Our results suggest that the afterglow of GRB 170817 A may arise from the stratified jet and that the lateral spreading of the jet is not significant.
