超高能伽马射线的天体物理起源

中国大科学装置之一的高海拔宇宙线观测站开创了超高能伽马射线天文学的新时代,至今已作出多项重要的科学发现,为人们破解银河系宇宙线起源之谜带来了关键线索,并为研究极端天体中的极端物理过程提供了独特的探针。文章主要介绍其探测到的超高能伽马射线源,以及相关天体的物理图像。

Observation and Analysis of VLF Nocturnal Multimode Interference Phenomenon based on Waveguide Mode Theory

Very low frequency(VLF)signals are propagated between the ground-ionosphere.Multimode interference will cause the phase to show oscillatory changes with distance while propagating at night,leading to abnormalities in the received VLF signal.This study uses the VLF signal received in Qingdao City,Shandong Province,from the Russian Alpha navigation system to explore the multimode interference problem of VLF signal propagation.The characteristics of the effect of multimode interference phenomena on the phase are analyzed according to the variation of the phase of the VLF signal.However,the phase of VLF signals will also be affected by the X-ray and energetic particles that are released during the eruption of solar flares,therefore the two phenomena are studied in this work.It is concluded that the X-ray will not affect the phase of VLF signals at night,but the energetic particles will affect the phase change,and the influence of energetic particles should be excluded in the study of multimode interference phenomena.Using VLF signals for navigation positioning in degraded or unavailable GPS conditions is of great practical significance for VLF navigation systems as it can avoid the influence of multimode interference and improve positioning accuracy.

Lobe-dominatedγ-Ray Emission of Compact Symmetric Objects

Theγ-ray emitting compact symmetric objects(CSOs)PKS 1718-649,NGC 3894,and TXS 0128+554 are lobedominated in the radio emission.In order to investigate theirγ-ray radiation properties,we analyze the~14yr Fermi/LAT observation data of the three CSOs.They all show the low luminosity(1041-1043 erg s-1)and no significant variability in theγ-ray band.Theirγ-ray average spectra can be well fitted by a power-law function.These properties ofγ-rays are clearly different from theγ-ray emitting CSOs CTD 135 and PKS 1413+135,for which theγ-rays are produced by a restarted aligned jet.In the L_(γ)-Γ_(γ)plane,the three CSOs are also located at the region occupied by radio galaxies(RGs)while CTD 135 and PKS 1413+135 display a similar feature to blazars.Together with a similar radio emission property toγ-ray emitting RGs Cen A and Fornax A,we speculate that theγ-rays of the three CSOs stem from their extended mini-lobes.The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model,where theirγ-rays are produced by the inverse Compton process of the relativistic electrons in extended regions.By extrapolating the observed Fermi/LAT spectra to the very high energy band,we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in the future.

First Digit Distributions of Gamma-Ray Bursts

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.

Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts

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.

A Comparative Study of the Power-law Relationship between the Pulse width and Energy of Precursor and Main Burst

In gamma-ray burst prompt emission,there is still no consistent conclusion if the precursor and main burst share the same origin.In this paper,we try to study this issue based on the relationship between pulse width and energy of the precursor and main burst.We systematically search the light curve data observed by Swift/BAT and Fermi/GBM,and find 13 long bursts with well-structured precursors and main bursts.After fitting the precursor light curve of each different energy channel with the Norris function,we find that there is not only a power-law relationship between precursor width and energy,but also a power-law relationship between the ratio of the rising width to the decaying width and energy.By comparing the relationship between the precursors and the main burst pulses,we find that the distribution of the precursors and the relationship between the power-law indices are roughly the same as those of the main burst.In addition,it is found that the precursor width distribution as well as the upper limit of the pulse width ratio does not exceed 1 and both are asymmetric,which are also consistent with the main burst.These indicate that the precursor and the main burst are indistinguishable,and the precursor and the main burst may have the same physical origin.

Investigating the Evolution of Amati Parameters with Redshift

Gamma-ray bursts(GRBs) are among the brightest objects in the Universe and, hence, can be observed up to a very high redshift. Properly calibrated empirical correlations between intensity and spectral correlations of GRBs can be used to estimate the cosmological parameters. However, the possibility of the evolution of GRBs with redshift is a long-standing puzzle. In this work, we used 162 long-duration GRBs to determine whether GRBs below and above a certain redshift have different properties. The GRBs are split into two groups, and we fit the Amati relation for each group separately. Our findings demonstrate that estimations of the Amati parameters for the two groups are substantially dissimilar. We perform simulations to investigate whether the selection effects could cause the difference. Our analysis shows that the differences may be either intrinsic or due to systematic errors in the data, and the selection effects are not their true origin. However, in-depth analysis with a new data set comprised of 119 long GRBs shows that intrinsic scatter may partly be responsible for such effects.

GRB 210610B:The Internal and External Plateau as Evidence for the Delayed Outflow of Magnetar

After launching a jet,outflows of magnetar were used to account for the achromatic plateau of afterglow and the early X-ray flux plateau known as“internal plateau”.The lack of detecting magnetic dipole emission together with the energy injection feature in a single observation poses confusion until the long gamma-ray burst(GRB)210610B is detected.GRB 210610B is presented with an optical bump following an early X-ray plateau during the afterglow phase.The plateau followed by a steep decline flux overlays in the steadily decaying X-ray flux with indexα_(X,1)~2.06,indicating an internal origin and that can be fitted by the spin-down luminosity law with the initial plateau luminosity log_(10)L_(X)~48.29 erg s~(-1)and the characteristic spin-down timescale T~2818 s.A subsequent bump begins at~4000 s in the R band with a rising indexα_(R,1)~-0.30 and peaks at~14125 s,after which a decay indexα_(R,2)~0.87 and finally transiting to a steep decay withα_(R,3)~1.77 achieve the closure relation of the external shock for the normal decay phase as well as the magnetar spin-down energy injection phase,provided that the average value of the photon indexΓ_γ=1.80 derived from the spectral energy distributions(SEDs)between the X-ray and optical afterglow.The closure relation also works for the late X-ray flux.Akin to the traditional picture of GRB,the outflow powers the early X-ray plateau by dissipating energy internally and collides with the leading decelerating blast burst as time goes on,which could interpret the exotic feature of GRB 210610B.We carry out a Markov Chain Monte Carlo simulation and obtain a set of best parameters:■.The artificial light curve can fit the afterglow data well.After that,we estimated the average Lorentz factor and the X-ray radiation efficiency of the later ejecta are 35%and 0.13%,respectively.

GRB 200612A:An Ultralong Gamma-Ray Burst Powered by Magnetar Spinning Down

GRB 200612A could be classified as an ultralong gamma-ray burst due to its prompt emission lasting up to~1020 s and the true timescale of the central engine activity t_(burst)≥4×10^(4) s.The late X-ray light curve with a decay index ofα=7.53 is steeper than the steepest possible decay from an external shock model.We propose that this X-ray afterglow can be driven by dipolar radiation from the magnetar spindown during its early stage,while the magnetar collapsed into the black hole before its spindown,resulting in a very steep decay of the late X-ray light curve.The optical data show that the light curve is still rising after 1.1 ks,suggesting a late onset.We show that GRB 200612A’s optical afterglow light curve is fitted with the forward shock model by Gaussian structured off-axis jet.This is a special case among GRBs,as it may be an ultralong gamma-ray burst powered by a magnetar in an off-axis observation scenario.

New γ-Ray Sources Identified in All-sky Surveys Based on Fermipy's Advanced Algorithm

We employ an efficient method for identifying γ-ray sources across the entire sky,leveraging advanced algorithms from Fermipy,and cleverly utilizing the Galactic diffuse background emission model to partition the entire sky into72 regions,thereby greatly enhancing the efficiency of discovering new sources throughout the sky through multithreaded parallel computing.After confirming the reliability of the new method,we applied it for the first time to analyze data from the Fermi Large Area Telescope(Fermi-LAT)encompassing approximately 15.41 yr of all-sky surveys.Through this analysis,we successfully identified 1379 new sources with significance levels exceeding 4σ,of which 497 sources exhibited higher significance levels exceeding 5σ.Subsequently,we performed a systematic analysis of the spatial extension,spectra,and light variation characteristics of these newly identified sources.We identified 21 extended sources and 23 sources exhibiting spectral curvature above 10 GeV.Additionally,we identified 44 variable sources above 1 GeV.

The Influence of the Sun and Moon on the Observation of Very High Energy Gamma-ray Sources Using EAS Arrays

With great advance of ground-based extensive air shower arrays,such as LHAASO and HAWC,many very high energy(VHE)gamma-ray sources have been discovered and are being monitored regardless of the day and the night.Hence,the Sun and Moon would have some impacts on the observation of gamma-ray sources,which have not been taken into account in previous analysis.In this paper,the influence of the Sun and Moon on the observation of very high energy gamma-ray sources when they are near the line of sight of the Sun or Moon is estimated.The tracks of all the known VHE sources are scanned and several VHE sources are found to be very close to the line of sight of the Sun or Moon during some period.The absorption of very high energy gamma rays by sunlight is estimated with detailed method and some useful conclusions are achieved.The main influence is the block of the Sun and Moon on gamma rays and the shadow on the cosmic ray background.The influence is investigated considering the detector angular resolution and some strategies on data analysis are proposed to avoid the underestimation of the gamma-ray emission.

Gas microchannel plate-pixel detector for X-ray polarimetry

POLAR-2 is a gamma-ray burst(GRB)polarimeter that is designed to study the polarization in GRB radiation emissions,aiming to improve our knowledge of related mechanisms.POLAR-2 is expected to utilize an on-board polarimeter that is sensitive to soft X-rays(2-10 keV),called low-energy polarization detector.We have developed a new soft X-ray polari-zation detector prototype based on gas microchannel plates(GMCPs)and pixel chips(Topmetal).The GMCPs have bulk resistance,which prevents charging-up effects and ensures gain stability during operation.The detector is composed of low outgassing materials and is gas-sealed using a laser welding technique,ensuring long-term stability.A modulation factor of 41.28%±0.64% is obtained for a 4.5 keV polarized X-ray beam.A residual modulation of 1.96%±0.58% at 5.9 keV is observed for the entire sensitive area.

A Possible γ-Ray Pulsation from PSR J1740-5340B in the Globular Cluster NGC 6397

Recently, a new radio millisecond pulsar(MSP) J1740-5340B, hosted in the globular cluster(GC) NGC 6397,was reported with a 5.78 ms spin period in an eclipsing binary system with a 1.97 days orbital period. Based on a modified radio ephemeris updated by tool tempo2, we analyze the ~15 yr γ-ray data obtained from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detect PSR J1740-5340B's γ-ray pulsation at a confidence level of ~4σ with a weighted H-test value of ~26. By performing a phase-resolved analysis, the γ-ray luminosity in on-pulse interval of PSR J1740-5340B is L_(γ)~ 3.8 × 10^(33) erg s^(-1) using NGC 6397's distance of 2.48 kpc. And γ-rays from the on-pulse part of PSR J1740-5340B contribute ~90% of the total observed γ-ray emissions from NGC 6397. No significant γ-ray pulsation of another MSP J1740-5340A in the GC is detected.Considering that the previous four cases of MSPs in GCs, more data in γ-ray, X-ray, and radio are encouraged to finally confirm the γ-ray emissions from MSP J1740-5340B, especially starving for a precise ephemeris.

A Localization Method of High Energy Transients for All-sky Gamma-ray Monitor

Fast and reliable localization of high-energy transients is crucial for characterizing the burst properties and guiding the follow-up observations.Localization based on the relative counts of different detectors has been widely used for all-sky gamma-ray monitors.There are two major methods for this count distribution localization:χ^(2)minimization method and the Bayesian method.Here we propose a modified Bayesian method that could take advantage of both the accuracy of the Bayesian method and the simplicity of the χ^(2)method.With comprehensive simulations,we find that our Bayesian method with Poisson likelihood is generally more applicable for various bursts than the χ^(2)method,especially for weak bursts.We further proposed a location-spectrum iteration approach based on the Bayesian inference,which could alleviate the problems caused by the spectral difference between the burst and location templates.Our method is very suitable for scenarios with limited computation resources or timesensitive applications,such as in-flight localization software,and low-latency localization for rapidly follow-up observations.

Detection of Increasingγ-Ray Emissions from 4FGL J1718.5+4237 with Fermi-LAT

We report a gradual brighteningγ-ray source,4FGL J1718.5+4237,in 0.1-500.0 GeV,which may be associated with a blazar NVSS J171822+423948 with a redshift of~2.7.We analyzed 15.25 yr ofγ-ray data recorded by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detected significantγ-ray emissions in the direction of the blazar with a test statistic(TS)of~135.Based on timing analysis using a 1 yr time bin,we have observed a gradual brightening inγ-ray emissions from the target.In our analysis,we categorize them into two states:Quiet(TS~0)and Loud(TS~226)states,with the distinction occurring in 2016 August(MJD57602.69).From the Quiet state to the brightest period(the last data point),theγ-ray flux in 0.1-500.0 GeV increased by more than 12-fold from<0.2×10^(-8)photons cm^(-1)s^(-1)to 2.6×10^(-8)photons cm^(-1)s^(-1).Additionally,we studied the spectral properties in detail for the Loud state and the overall data.While no significant variation was detected,both exhibited a spectral indexΓof~2.6.The origin of the brighteningγ-ray emissions from the target is not yet clear.Future long-term multi-wavelength observations and studies will provide insight into the astrophysical mechanisms of the target.

我科学家观测到宇宙最高能量伽马谱线

近日,中国科学院高能物理研究所牵头的科研团队通过分析极目空间望远镜和费米卫星的联合观测数据,在伽马暴中发现能量高达37 MeV的伽马射线谱线。这是迄今观测到的宇宙天体产生的能量最高、证据最确凿的谱线。相关研究成果2024年7月25日以封面论文形式发表在《中国科学:物理学力学天文学》(英文版)上。

伽玛射线暴:各向同性火球还是柱形喷流?

多数伽玛射线暴的光学余辉衰减较平缓且光变曲线未见明显拐折,通常认为它们产生于各向同性火球, GRBs 970228、970508、971214、980329及980703等就是典型的例子.但柱型喷流模型其实也能对这类伽玛射线暴的余辉给出极好的拟合,因此它们完全有可能产生于柱形喷流,而并非一定是来自各向同性火球.

SVOM Kicks Off to Explore Gamma-ray Bursts

At sharp 15:00 GMT+8 on June 22,the Space Variable Objects Monitor(SVOM),a satellite designed to observe and explore gamma-ray bursts(GRBs),was sent into preset orbit from a satellite launching center in Xichang city of northwestern China.Jointly developed by the Chinese Academy of Sciences(CAS)and the French Space Agency(FSA),this satellite is the most powerful so far in terms of multi-waveband GRB observations,and is expected to play an important role in GRB research and related space science and astronomy.

IN THIS ISSUE

IN FOCUS|SVOM Kicks Off for GRB Observations Jointly developed by the Chinese Academy of Sciences(CAS)and the French Space Agency(FSA),a satellite for observations on gamma-ray bursts(GRBs)was sent into preset orbit on June 22.Named the Space Variable Objects Monitor(SVOM),it is the most powerful so far in terms of multi-waveband GRB observations,and is expected to play an important role in GRB research and related space science and astronomy.For more,please turn to page 72.

γ射线暴的研究进展(Ⅰ):γ暴的观测特征及能源机制

文中评述了过去20年中对γ暴的观测结果，包括它的时间特性、能谱特征和空间分布。γ暴的时间结构非常复杂，持续时间相差很大，从小于1s到1000s，平均大约为15s。它的能谱表现为从10keV到10MeV间的连续谱，有的能谱中还含有吸收线和发射线。空间分布表明γ暴源是各向同性的，这说明γ暴要么位于银盘里，要么位于银晕里，或者位于宇宙学距离上。我们还简要讨论了中子星作为γ暴能源的几种机制。