The occurrence of the first significant digits from real world sources is usually not equally distributed,but is consistent with a logarithmic distribution instead,known as Benford’s law.In this work,we perform a com...The occurrence of the first significant digits from real world sources is usually not equally distributed,but is consistent with a logarithmic distribution instead,known as Benford’s law.In this work,we perform a comprehensive investigation on the first digit distributions of the duration,fluence,and energy flux of gamma-ray bursts (GRBs) for the first time.For a complete GRB sample detected by the Fermi satellite,we find that the first digits of the duration and fluence adhere to Benford’s law.However,the energy flux shows a significant departure from this law,which may be due to the fact that a considerable part of the energy flux measurements is restricted by lack of spectral information.Based on the conventional duration classification scheme,we also check if the durations and fluences of long and short GRBs (with duration T_(90)>2 s and T_(90)≤2 s,respectively) obey Benford’s law.We find that the fluences of both long and short GRBs still agree with the Benford distribution,but their durations do not follow Benford’s law.Our results hint that the long–short GRB classification scheme does not directly represent the intrinsic physical classification scheme.展开更多
The prompt emission mechanism of gamma-ray bursts(GRBs)is still unclear,and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes.We performed a detailed time-...The prompt emission mechanism of gamma-ray bursts(GRBs)is still unclear,and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes.We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor.A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model.First,the parameter distributions of the time-resolved spectrum are given as follows:the low-energy spectral indexα~-0.72,high-energy spectral indexβ~2.42,the peak energy E_(p)~221.69 keV,and the energy flux F~7.49×10^(-6)erg cm^(-2)s^(-1).More than 80%of the bursts exhibit the hardest low-energy spectral indexα_(max),exceeding the synchrotron limit(-2/3).Second,the evolution patterns of a and E_(p)were statistically analyzed.The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of both E_(p)andα.The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs.Finally,we found a significant positive correlation between F and E_(p),with half of the samples exhibiting a positive correlation between F andα.We discussed the spectral evolution of different radiation models.The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process,including photo spheric radiation and synchrotron radiation.However,it may also involve only one radiation mechanism,but more complicated physical details need to be considered.展开更多
Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the rem...Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the remnants of binary strangeon star mergers could even be long-lived massive strangeon stars.We study rigidly rotating strangeon stars in the slowly rotating approximation,using the Lennard-Jones model for the equation of state.Rotation can significantly increase the maximum mass of strangeon stars with unchanged baryon numbers,enlarging the mass-range of long-lived strangeon stars.During spin-down after merger,the decrease of radius of the remnant will lead to the release of gravitational energy.Taking into account the efficiency of converting the gravitational energy luminosity to the observed X-ray luminosity,we find that the gravitational energy could provide an alternative energy source for the plateau emission of X-ray afterglow.The fitting results of X-ray plateau emission of some short gamma-ray bursts suggest that the magnetic dipole field strength of the remnants can be much smaller than that of expected when the plateau emission is powered only by spin-down luminosity of magnetars.展开更多
Gamma-ray bursts (GRBs) are extremely powerful explosions that have been traditionally classified into two categories: long bursts (LGRBs) with an observed duration T<sub>90 </sub>> 2 s, and short burst...Gamma-ray bursts (GRBs) are extremely powerful explosions that have been traditionally classified into two categories: long bursts (LGRBs) with an observed duration T<sub>90 </sub>> 2 s, and short bursts (SGRBs) with an observed duration T<sub>90</sub> T<sub>90</sub> is the time interval during which 90% of the fluence is detected. LGRBs are believed to emanate from the core-collapse of massive stars, while SGRBs are believed to result from the merging of two compact objects, like two neutron stars. Because LGRBs are produced by the violent death of massive stars, we expect that their redshift distribution should trace the star-formation rate (SFR). The purpose of our study is to investigate the extent to which the redshift distribution of LGRBs follows and reflects the SFR. We use a sample of 370 LGRBs taken from the Swift catalog, and we investigate different models for the LGRB redshift distribution. We also carry out Monte Carlo simulations to check the consistency of our results. Our results indicate that the SFR can describe the LGRB redshift distribution well for high redshift bursts, but it needs an evolution term to fit the distribution well at low redshift.展开更多
The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are characterized by a plateau followed by a very sharp drop. The plateau could be explained within the framework of the external forward shock model but...The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are characterized by a plateau followed by a very sharp drop. The plateau could be explained within the framework of the external forward shock model but the sharp drop can not. We interpret the plateau as the afterglows of magnetized central engines, plausibly magnetars. In this model, the X-ray afterglows are powered by the internal magnetic energy dissipation and the sudden drop is caused by the collapse of the magnetar. Accordingly, the X-ray plateau photons should have a high linear polarization, which can be tested by future X-ray polarimetry.展开更多
We analyze the emission plateaus in the X-ray afterglow light curves of gamma-ray bursts (GRBs) and those in the optical light curves of type Ⅱ plateau su- pernovae (SNe Ⅱ-P) in order to study whether they have ...We analyze the emission plateaus in the X-ray afterglow light curves of gamma-ray bursts (GRBs) and those in the optical light curves of type Ⅱ plateau su- pernovae (SNe Ⅱ-P) in order to study whether they have similar late energy injection behaviors. We show that correlations of bolometric energies (or luminosities) between the prompt explosions and the plateaus for the two phenomena are similar. The energy emitted by SNe II-P are at the lower end of the range of possible energies for GRBs. The bolometric energies (or luminosities) in the prompt phase Eexpl (or Lexpl) and in the plateau phase E_plateau (or L_plateau) share relations of E_expl ∝E _0.73±0.14_plateau and L_expl ∝ L^-0.70_plateau. These results may indicate a similar late energy injection behavior that produces the observed plateaus in these two phenomena.展开更多
It seems that the wealth of information revealed by the multi-messenger observations of the binary neutron star(NS)merger event,GW170817/GRB 170817A/kilonova AT2017gfo,places irreconcilable constraints to models of th...It seems that the wealth of information revealed by the multi-messenger observations of the binary neutron star(NS)merger event,GW170817/GRB 170817A/kilonova AT2017gfo,places irreconcilable constraints to models of the prompt emission of this gamma-ray burst(GRB).The observed time delay between the merger of the two NSs and the trigger of the GRB and the thermal tail of the prompt emission can hardly be reproduced by these models simultaneously.We argue that the merger remnant should be an NS(last for,at least,a large fraction of 1 s),and that the difficulty can be alleviated by the delayed formation of the accretion disk due to the absorption of high-energy neutrinos emitted by the NS and the delayed emergence of effective viscosity in the disk.Further,we extend the consideration of the effect of the energy deposition of neutrinos emitted from the NS.If the NS is the central object of a GRB with a distance and duration similar to that of GRB 170817A,thermal emission of the thermal bubble inflated by the NS after the termination of accretion may be detectable.If our scenario is verified,it would be of interest to investigate the cooling of nascent NSs.展开更多
Gamma-ray bursts(GRBs)exhibit powerful radiation and relativistic jets similar to blazars.However,the central engine of GRBs remains unknown.In this paper,we use the fundamental plane to analyze a sample of GRBs with ...Gamma-ray bursts(GRBs)exhibit powerful radiation and relativistic jets similar to blazars.However,the central engine of GRBs remains unknown.In this paper,we use the fundamental plane to analyze a sample of GRBs with measured mass.We extend,over~12 orders of magnitude,the correlation analysis and fundamental plane with a sample of X-ray binaries,active galactic nuclei(AGNs)including blazars,and brightest cluster galaxies.The fundamental plane of our de-beamed sample,with a measured mass(logL_(R)=(0.60±0.03)logL_(X)+(0.78±0.02)logM+7.23±0.95),closely aligns with the findings of previous work on AGNs.This finding suggests that GRBs adhere to the fundamental plane of AGNs and supports the theory proposed in previous work that the central engine of GRBs may be black holes.This observation provides a plausible explanation for the striking similarities between GRBs and AGNs.展开更多
For the practical use of femtosecond laser ablation, inputs of higher laser intensity are preferred to attain high-throughput material removal. However, the use of higher laser intensities for increasing ablation rate...For the practical use of femtosecond laser ablation, inputs of higher laser intensity are preferred to attain high-throughput material removal. However, the use of higher laser intensities for increasing ablation rates can have detrimental effects on ablation quality due to excess heat generation and air ionization. This paper employs ablation using BiBurst femtosecond laser pulses, which consist of multiple bursts(2 and 5 bursts) at a repetition rate of 64 MHz, each containing multiple intra-pulses(2–20 pulses) at an ultrafast repetition rate of 4.88 GHz, to overcome these conflicting conditions. Ablation of silicon substrates using the BiBurst mode with 5 burst pulses and 20 intra-pulses successfully prevents air breakdown at packet energies higher than the pulse energy inducing the air ionization by the conventional femtosecond laser pulse irradiation(single-pulse mode). As a result, ablation speed can be enhanced by a factor of23 without deteriorating the ablation quality compared to that by the single-pulse mode ablation under the conditions where the air ionization is avoided.展开更多
A negative correlation was found to exist between the low-energy spectral index and the redshift of gamma-ray bursts(GRBs)by Amati et al.It was later confirmed by Geng&Huang and Gruber et al.,but the correlation w...A negative correlation was found to exist between the low-energy spectral index and the redshift of gamma-ray bursts(GRBs)by Amati et al.It was later confirmed by Geng&Huang and Gruber et al.,but the correlation was also found to be quite dispersive when the sample size was significantly expanded.In this study,we have established two even larger samples of GRBs to further examine the correlation.One of our samples consists of316 GRBs detected by the Swift satellite,and the other one consists of 80 GRBs detected by the Fermi satellite.It is found that there is no correlation between the two parameters for the Swift sample,but there does exist a weak negative correlation for the Fermi sample.The correlation becomes even more significant when the spectral index at the peak flux is considered.It is argued that the absence of the correlation in the Swift sample may be due to the fact that Swift has a very narrow energy response so that it could not measure the low-energy spectral index accurately enough.Further studies based on even larger GRB samples are solicited.展开更多
Bursts of GHz repetition rate pulses involve more ablation mechanisms than single femtosecond pulses.Efficient ablation by GHz laser pulses is a multi-step process,consisting of a first thermal incubation phase,follow...Bursts of GHz repetition rate pulses involve more ablation mechanisms than single femtosecond pulses.Efficient ablation by GHz laser pulses is a multi-step process,consisting of a first thermal incubation phase,followed by a highly efficient ablation phase.GHz ablation therefore combines thermal and non-thermal ablation mechanisms.With an optimal choice of the burst duration,the ablation efficiency can be highly enhanced.Long bursts,comprising tens of pulses to hundreds of pulses,are needed to take full advantage of the increase in ablation efficiency.展开更多
We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and LAXPC instruments on the AstroSat satellite.Unlike the 1998 ...We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and LAXPC instruments on the AstroSat satellite.Unlike the 1998 and 2010 outbursts,where a giant outburst was followed by several low intensity periodic outbursts,the 2018 and 2021 outbursts were single outbursts.The X-ray pulsations are detected over a broad energy band covering 0.5–80 keV from the compact object.We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts.Energy resolved pulse profiles generated in several bands in 0.5–80 keV show that the pulse shape varies with the energy.The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts.The best fit spectral models require presence of cyclotron resonant scattering feature at about 43 keV in the energy spectra of both the outbursts.We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations.Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be 2.7×10^(37) erg s^(-1) for the 2018 observations and 2.3×10^(37) erg s^(-1) for the 2021 observations.We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.展开更多
As one class of the most important objects in the universe,magnetars can produce a lot of different frequency bursts including X-ray bursts.In Cai et al.,75 X-ray bursts produced by magnetar SGR J1935+2154 during an a...As one class of the most important objects in the universe,magnetars can produce a lot of different frequency bursts including X-ray bursts.In Cai et al.,75 X-ray bursts produced by magnetar SGR J1935+2154 during an active period in 2020 are published,including the duration and net photon counts of each burst,and waiting time based on the trigger time difference.In this paper,we utilize the power-law model,dN(x)/dx∝(x+x_0)~((-α)_x),to fit the cumulative distributions of these parameters.It can be found that all the cumulative distributions can be well fitted,which can be interpreted by a self-organizing criticality theory.Furthermore,we check whether this phenomenon still exists in different energy bands and find that there is no obvious evolution.These findings further confirm that the X-ray bursts from magnetars are likely to be generated by some self-organizing critical process,which can be explained by a possible magnetic reconnection scenario in magnetars.展开更多
Lorentz Invariance is a foundational principle in modern physics, but some recent quantum gravity theories have hinted that it may be violated at extremely high energies. Gamma-ray bursts (GRBs) provide a promising to...Lorentz Invariance is a foundational principle in modern physics, but some recent quantum gravity theories have hinted that it may be violated at extremely high energies. Gamma-ray bursts (GRBs) provide a promising tool for checking and constraining any deviations from Lorentz Invariance due to their huge energies and cosmological distances. Gamma-ray bursts, which are the most intense and powerful explosions in the universe, are traditionally divided into long bursts whose observed duration exceeds 2 s, and short bursts whose observed duration is less than 2 s. In this study, we employ a recent sample of 46 short GRBs to check for any deviation from Lorentz Invariance. We analyze the spectral lag of the bursts in our data sample and check for any redshift dependence in the GRB rest frame, which would indicate a violation of Lorentz Invariance. Our results are consistent, to within 1σ, with no deviation from Lorentz Invariance.展开更多
According to a recent calculation, 10<sup>58</sup> erg of radiant energy was released by Sgr A*, when it formed the Fermi bubbles. Here, it is argued that this explosion constituted a long gamma-ray burst. .
基金supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(grant No.XDB0550400)the Key Research Program of Frontier Sciences(grant No.ZDBS-LY-7014)of Chinese Academy of Sciences+1 种基金the National Natural Science Foundation of China(NSFC,Grant Nos.12373053 and 12321003)the Natural Science Foundation of Jiangsu Province(grant No.BK20221562)。
文摘The occurrence of the first significant digits from real world sources is usually not equally distributed,but is consistent with a logarithmic distribution instead,known as Benford’s law.In this work,we perform a comprehensive investigation on the first digit distributions of the duration,fluence,and energy flux of gamma-ray bursts (GRBs) for the first time.For a complete GRB sample detected by the Fermi satellite,we find that the first digits of the duration and fluence adhere to Benford’s law.However,the energy flux shows a significant departure from this law,which may be due to the fact that a considerable part of the energy flux measurements is restricted by lack of spectral information.Based on the conventional duration classification scheme,we also check if the durations and fluences of long and short GRBs (with duration T_(90)>2 s and T_(90)≤2 s,respectively) obey Benford’s law.We find that the fluences of both long and short GRBs still agree with the Benford distribution,but their durations do not follow Benford’s law.Our results hint that the long–short GRB classification scheme does not directly represent the intrinsic physical classification scheme.
基金performed under the auspices of the Science and Technology Foundation of Guizhou Province(grant No.Qian Ke He Ji Chu ZK[2021]027)Major Science and Technology Program of Xinjiang Uygur Autonomous Region through No.2022A03013-1+1 种基金the National Key Research and Development Program of China(No.2022YFC2205202)the National Natural Science Foundation of China grants 12288102,12041304 and 11847102。
文摘The prompt emission mechanism of gamma-ray bursts(GRBs)is still unclear,and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes.We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor.A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model.First,the parameter distributions of the time-resolved spectrum are given as follows:the low-energy spectral indexα~-0.72,high-energy spectral indexβ~2.42,the peak energy E_(p)~221.69 keV,and the energy flux F~7.49×10^(-6)erg cm^(-2)s^(-1).More than 80%of the bursts exhibit the hardest low-energy spectral indexα_(max),exceeding the synchrotron limit(-2/3).Second,the evolution patterns of a and E_(p)were statistically analyzed.The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of both E_(p)andα.The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs.Finally,we found a significant positive correlation between F and E_(p),with half of the samples exhibiting a positive correlation between F andα.We discussed the spectral evolution of different radiation models.The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process,including photo spheric radiation and synchrotron radiation.However,it may also involve only one radiation mechanism,but more complicated physical details need to be considered.
基金supported by the National SKA Program of China(Nos.2020SKA0120300,2020SKA0120100)the Outstanding Young and Middle-aged Science and Technology Innovation Teams from Hubei colleges and universities(No.T2021026)the Young Top-notch Talent Cultivation Program of Hubei Province,and the Key Laboratory Opening Fund(MOE)of China(grant No.QLPL2021P01)。
文摘Strangeon stars,which are proposed to describe the nature of pulsar-like compact stars,have passed various observational tests.The maximum mass of a non-rotating strangeon star could be high,which implies that the remnants of binary strangeon star mergers could even be long-lived massive strangeon stars.We study rigidly rotating strangeon stars in the slowly rotating approximation,using the Lennard-Jones model for the equation of state.Rotation can significantly increase the maximum mass of strangeon stars with unchanged baryon numbers,enlarging the mass-range of long-lived strangeon stars.During spin-down after merger,the decrease of radius of the remnant will lead to the release of gravitational energy.Taking into account the efficiency of converting the gravitational energy luminosity to the observed X-ray luminosity,we find that the gravitational energy could provide an alternative energy source for the plateau emission of X-ray afterglow.The fitting results of X-ray plateau emission of some short gamma-ray bursts suggest that the magnetic dipole field strength of the remnants can be much smaller than that of expected when the plateau emission is powered only by spin-down luminosity of magnetars.
文摘Gamma-ray bursts (GRBs) are extremely powerful explosions that have been traditionally classified into two categories: long bursts (LGRBs) with an observed duration T<sub>90 </sub>> 2 s, and short bursts (SGRBs) with an observed duration T<sub>90</sub> T<sub>90</sub> is the time interval during which 90% of the fluence is detected. LGRBs are believed to emanate from the core-collapse of massive stars, while SGRBs are believed to result from the merging of two compact objects, like two neutron stars. Because LGRBs are produced by the violent death of massive stars, we expect that their redshift distribution should trace the star-formation rate (SFR). The purpose of our study is to investigate the extent to which the redshift distribution of LGRBs follows and reflects the SFR. We use a sample of 370 LGRBs taken from the Swift catalog, and we investigate different models for the LGRB redshift distribution. We also carry out Monte Carlo simulations to check the consistency of our results. Our results indicate that the SFR can describe the LGRB redshift distribution well for high redshift bursts, but it needs an evolution term to fit the distribution well at low redshift.
文摘The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are characterized by a plateau followed by a very sharp drop. The plateau could be explained within the framework of the external forward shock model but the sharp drop can not. We interpret the plateau as the afterglows of magnetized central engines, plausibly magnetars. In this model, the X-ray afterglows are powered by the internal magnetic energy dissipation and the sudden drop is caused by the collapse of the magnetar. Accordingly, the X-ray plateau photons should have a high linear polarization, which can be tested by future X-ray polarimetry.
基金supported by the National Basic Research Program of China (973 program, Grant Nos. 2012CB821800 and 2009CB824800)the National Natural Science Foundation of China(Grant Nos. 11225314, 11103026 and 10935001)
文摘We analyze the emission plateaus in the X-ray afterglow light curves of gamma-ray bursts (GRBs) and those in the optical light curves of type Ⅱ plateau su- pernovae (SNe Ⅱ-P) in order to study whether they have similar late energy injection behaviors. We show that correlations of bolometric energies (or luminosities) between the prompt explosions and the plateaus for the two phenomena are similar. The energy emitted by SNe II-P are at the lower end of the range of possible energies for GRBs. The bolometric energies (or luminosities) in the prompt phase Eexpl (or Lexpl) and in the plateau phase E_plateau (or L_plateau) share relations of E_expl ∝E _0.73±0.14_plateau and L_expl ∝ L^-0.70_plateau. These results may indicate a similar late energy injection behavior that produces the observed plateaus in these two phenomena.
基金the National SKA Program of China(2020SKA0120100)research projects of Henan Science and Technology Committee(212300410378)the National NaturalScience Foundationof China(NSFC)grant(U1938116).
文摘It seems that the wealth of information revealed by the multi-messenger observations of the binary neutron star(NS)merger event,GW170817/GRB 170817A/kilonova AT2017gfo,places irreconcilable constraints to models of the prompt emission of this gamma-ray burst(GRB).The observed time delay between the merger of the two NSs and the trigger of the GRB and the thermal tail of the prompt emission can hardly be reproduced by these models simultaneously.We argue that the merger remnant should be an NS(last for,at least,a large fraction of 1 s),and that the difficulty can be alleviated by the delayed formation of the accretion disk due to the absorption of high-energy neutrinos emitted by the NS and the delayed emergence of effective viscosity in the disk.Further,we extend the consideration of the effect of the energy deposition of neutrinos emitted from the NS.If the NS is the central object of a GRB with a distance and duration similar to that of GRB 170817A,thermal emission of the thermal bubble inflated by the NS after the termination of accretion may be detectable.If our scenario is verified,it would be of interest to investigate the cooling of nascent NSs.
基金partially supported by the Natural Science Foundation of Yunnan Province(grant No.202101AU070006)the Young and Middle-Aged Academic and Technical Reserve Talents of Yunnan Province(202205AC160087)。
文摘Gamma-ray bursts(GRBs)exhibit powerful radiation and relativistic jets similar to blazars.However,the central engine of GRBs remains unknown.In this paper,we use the fundamental plane to analyze a sample of GRBs with measured mass.We extend,over~12 orders of magnitude,the correlation analysis and fundamental plane with a sample of X-ray binaries,active galactic nuclei(AGNs)including blazars,and brightest cluster galaxies.The fundamental plane of our de-beamed sample,with a measured mass(logL_(R)=(0.60±0.03)logL_(X)+(0.78±0.02)logM+7.23±0.95),closely aligns with the findings of previous work on AGNs.This finding suggests that GRBs adhere to the fundamental plane of AGNs and supports the theory proposed in previous work that the central engine of GRBs may be black holes.This observation provides a plausible explanation for the striking similarities between GRBs and AGNs.
基金supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067246。
文摘For the practical use of femtosecond laser ablation, inputs of higher laser intensity are preferred to attain high-throughput material removal. However, the use of higher laser intensities for increasing ablation rates can have detrimental effects on ablation quality due to excess heat generation and air ionization. This paper employs ablation using BiBurst femtosecond laser pulses, which consist of multiple bursts(2 and 5 bursts) at a repetition rate of 64 MHz, each containing multiple intra-pulses(2–20 pulses) at an ultrafast repetition rate of 4.88 GHz, to overcome these conflicting conditions. Ablation of silicon substrates using the BiBurst mode with 5 burst pulses and 20 intra-pulses successfully prevents air breakdown at packet energies higher than the pulse energy inducing the air ionization by the conventional femtosecond laser pulse irradiation(single-pulse mode). As a result, ablation speed can be enhanced by a factor of23 without deteriorating the ablation quality compared to that by the single-pulse mode ablation under the conditions where the air ionization is avoided.
基金supported by the National Natural Science Foundation of China(grant Nos.12233002,12041306,12147103,U1938201)by the National SKA Program of China(No.2020SKA0120300)+1 种基金by the National Key R&D Program of China(2021YFA0718500)by the Youth Innovations and Talents Project of Shandong Provincial Colleges and Universities(grant No.201909118)。
文摘A negative correlation was found to exist between the low-energy spectral index and the redshift of gamma-ray bursts(GRBs)by Amati et al.It was later confirmed by Geng&Huang and Gruber et al.,but the correlation was also found to be quite dispersive when the sample size was significantly expanded.In this study,we have established two even larger samples of GRBs to further examine the correlation.One of our samples consists of316 GRBs detected by the Swift satellite,and the other one consists of 80 GRBs detected by the Fermi satellite.It is found that there is no correlation between the two parameters for the Swift sample,but there does exist a weak negative correlation for the Fermi sample.The correlation becomes even more significant when the spectral index at the peak flux is considered.It is argued that the absence of the correlation in the Swift sample may be due to the fact that Swift has a very narrow energy response so that it could not measure the low-energy spectral index accurately enough.Further studies based on even larger GRB samples are solicited.
文摘Bursts of GHz repetition rate pulses involve more ablation mechanisms than single femtosecond pulses.Efficient ablation by GHz laser pulses is a multi-step process,consisting of a first thermal incubation phase,followed by a highly efficient ablation phase.GHz ablation therefore combines thermal and non-thermal ablation mechanisms.With an optimal choice of the burst duration,the ablation efficiency can be highly enhanced.Long bursts,comprising tens of pulses to hundreds of pulses,are needed to take full advantage of the increase in ablation efficiency.
文摘We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and LAXPC instruments on the AstroSat satellite.Unlike the 1998 and 2010 outbursts,where a giant outburst was followed by several low intensity periodic outbursts,the 2018 and 2021 outbursts were single outbursts.The X-ray pulsations are detected over a broad energy band covering 0.5–80 keV from the compact object.We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts.Energy resolved pulse profiles generated in several bands in 0.5–80 keV show that the pulse shape varies with the energy.The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts.The best fit spectral models require presence of cyclotron resonant scattering feature at about 43 keV in the energy spectra of both the outbursts.We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations.Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be 2.7×10^(37) erg s^(-1) for the 2018 observations and 2.3×10^(37) erg s^(-1) for the 2021 observations.We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.
基金supported by the National Key R&D Program of China(2021YFA0718500)the National Natural Science Foundation of China under grants U2038106 and 12065017partially by the Jiangxi Provincial Natural Science Foundation under grant 20224ACB211001。
文摘As one class of the most important objects in the universe,magnetars can produce a lot of different frequency bursts including X-ray bursts.In Cai et al.,75 X-ray bursts produced by magnetar SGR J1935+2154 during an active period in 2020 are published,including the duration and net photon counts of each burst,and waiting time based on the trigger time difference.In this paper,we utilize the power-law model,dN(x)/dx∝(x+x_0)~((-α)_x),to fit the cumulative distributions of these parameters.It can be found that all the cumulative distributions can be well fitted,which can be interpreted by a self-organizing criticality theory.Furthermore,we check whether this phenomenon still exists in different energy bands and find that there is no obvious evolution.These findings further confirm that the X-ray bursts from magnetars are likely to be generated by some self-organizing critical process,which can be explained by a possible magnetic reconnection scenario in magnetars.
文摘Lorentz Invariance is a foundational principle in modern physics, but some recent quantum gravity theories have hinted that it may be violated at extremely high energies. Gamma-ray bursts (GRBs) provide a promising tool for checking and constraining any deviations from Lorentz Invariance due to their huge energies and cosmological distances. Gamma-ray bursts, which are the most intense and powerful explosions in the universe, are traditionally divided into long bursts whose observed duration exceeds 2 s, and short bursts whose observed duration is less than 2 s. In this study, we employ a recent sample of 46 short GRBs to check for any deviation from Lorentz Invariance. We analyze the spectral lag of the bursts in our data sample and check for any redshift dependence in the GRB rest frame, which would indicate a violation of Lorentz Invariance. Our results are consistent, to within 1σ, with no deviation from Lorentz Invariance.
文摘According to a recent calculation, 10<sup>58</sup> erg of radiant energy was released by Sgr A*, when it formed the Fermi bubbles. Here, it is argued that this explosion constituted a long gamma-ray burst. .