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As the brightest gamma-ray burst ever observed, GRB 221009A provided a precious opportunity to explore spectral line features. In this article, we performed a comprehensive spectroscopy analysis of GRB 221009A jointly with GECAM-C and Fermi /GBM data to search for emission and absorption lines. For the first time we investigated the line feature throughout this GRB including the most bright part where many instruments suffered problems, and identified prominent emission lines in multiple time intervals. The central energy of the Gaussian emission line evolves from about 37 to 6 MeV, with a nearly constant ratio (about 10%) between the line width and central energy. Particularly, we find that both the central energy and the energy flux of the emission line evolve with time as a power law decay with power law index of −1 and −2, respectively. We suggest that the observed emission lines most likely origin from the blue-shifted electron positron pair annihilation 511 keV line. We find that a standard high latitude emission scenario cannot fully interpret the observation, thus we propose that the emission line comes from some dense clumps with electron positron pairs traveling together with the jet. In this scenario, we can use the emission line to directly, for the first time, measure the bulk Lorentz factor of the jet (Γ) and reveal its time evolution (i.e., Γ ∼ t −1 ) during the prompt emission. Interestingly, we find that the flux of the annihilation line in the co-moving frame keeps constant. These discoveries of the spectral line features shed new and important lights on the physics of GRB and relativistic jet.

LS I +61° 303 is one of the rare gamma-ray binaries 1 that emit most of their luminosity in photons with energies beyond 100 MeV (ref.  2 ). It is well characterized—the ~26.5 day orbital period is clearly detected at many wavelengths 2 – 4 —and other aspects of its multifrequency behaviour make it the most interesting example of its class. The morphology of high-resolution radio images changes with orbital phase, displaying a cometary tail pointing away from the high-mass star component 5 and LS I +61° 303 also shows superorbital variability 3 , 6 – 9 . A couple of energetic (~10 37  erg s −1 ), short, magnetar-like bursts have been plausibly ascribed to it 10 – 13 . Although the phenomenology of LS I +61° 303 has been the subject of theoretical scrutiny for decades, there has been a lack of certainty regarding the nature of the compact object in the binary that has hampered our understanding of the source. Here, using observations with the Five-hundred-meter Aperture Spherical radio Telescope, we report the existence of transient radio pulsations from the direction of LS I +61° 303 with a period P  = 269.15508 ± 0.00016 ms at a significance of >20 σ . These pulsations strongly argue for the existence of a rotating neutron star within LS I +61° 303. Well-observed gamma-ray binary system LS I +61° 303 consists of a high-mass star and a compact object whose nature is unknown. Here, transient radio pulsations detected with the sensitive FAST telescope suggest that the compact object is a rotating neutron star.

The Be X-ray pulsar SMC X-3 underwent an extra long and ultraluminous giant outburst from 2016 August to 2017 March. The peak X-ray luminosity is up to ∼1039erg/s, suggesting a mildly super-Eddington accretion onto the strongly magnetized neutron star. It therefore bridges the gap between the Galactic Be/X-ray binaries (LXpeak≤1038erg/s) and the ultraluminous X-ray pulsars (LXpeak≥1040erg/s) found in nearby galaxies. A number of observations were carried out to observe the outburst. In this paper, we perform a comprehensive phase-resolved analysis on the high quality data obtained with the Nustar and XMM-Newton, which were observed at a high and intermediate luminosity levels. In order to get a better understanding on the evolution of the whole extreme burst, we take the Swift results at the low luminosity state into account as well. At the early stage of outburst, the source shows a double-peak pulse profile, the second main peak approaches the first one and merges into the single peak at the low luminosity. The second main peak vanishes beyond 20 keV, and its radiation becomes much softer than that of the first main peak. The line widths of fluorescent iron line vary dramatically with phases, indicating a complicated geometry of accretion flows. In contrast to the case at low luminosity, the pulse fraction increases with the photon energy. The significant small pulse fraction detected below 1 keV can be interpreted as the existence of an additional thermal component located at far away from the central neutron star.

X-ray polarization observations of pulsar wind nebulae (PWNe) provide crucial insights into magnetic field structures and particle acceleration mechanisms. While the Imaging X-ray Polarimetry Explorer (IXPE) has made significant contributions to PWN studies, its limited effective area restricts observations to only the brightest sources, leaving many fainter nebulae unexplored. We evaluate the polarization capabilities of the enhanced X-ray Timing and Polarimetry mission (eXTP) for studying PWNe and establish a methodology for simulating eXTP Polarimetry Focusing Array (PFA) observations using modified IXPEOBSSIM. We develop and validate a simulation framework with appropriate response functions and instrumental background models, conducting comprehensive simulations of twelve PWNe selected from the SNRcat catalogue across various evolutionary stages and brightness levels. Our simulations demonstrate that eXTP provides approximately a factor of 2 improvement in minimum detectable polarization at the 99\\% confidence level (MDP\\(_{99}\\)) compared to IXPE. For the brightest targets (N157B, G54.1+0.3, and Mouse), 1 Ms observations achieve MDP\\(_{99}\\) values of 4-5\\%. The area with significant polarization detection for extended sources like Vela PWN is nearly twice as large as achievable with IXPE. These enhanced capabilities will significantly expand the sample of PWNe with robust X-ray polarization measurements, enabling systematic studies of magnetic field structures, particle acceleration mechanisms, and PWN-environment interactions across different evolutionary phases.

We conduct a detailed spectral analysis of the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124 in its sub-Eddington regime, using Insight-HXMT and NICER observations during multiple outbursts including the 2018 giant outburst. We discover a new transition at \\(L_{\\rm t} \\approx 4.5 \\times 10^{37}\\ {\\rm erg\\ s^{-1}}\\), accompanied by systematic evolution of spectral parameters, in particular a significant turnover in the blackbody normalization. This transition luminosity in the sub-Eddington regime represents the fifth transition identified so far in Swift J0243.6+6124, further highlighting the complexity of its accretion-powered emission. We interpret the transition in terms of a multipolar magnetic-field configuration, where weak (\\(\\sim 2.8 \\times 10^{12}\\ {\\rm G}\\)) and strong (\\(\\sim 1.6 \\times 10^{13}\\ {\\rm G}\\)) magnetic poles dominate the emission at different accretion rates. On the magnetospheric scale, this configuration is equivalent to an effective dipole field of \\(\\sim 6.6 \\times 10^{12}\\ {\\rm G}\\), while allowing the local surface field to exceed \\(10^{13}\\ {\\rm G}\\).

GRB 220426A is a bright gamma-ray burst (GRB) dominated by the photospheric emission. We perform several tests to speculate the origin of this photospheric emission. The dimensionless entropy \\(\\eta\\) is large, which is not usual if we assume that it is a pure hot fireball launched by neutrino-antineutrino annihilation mechanism only. Moreover, the outflow has larger \\(\\eta\\) with lower luminosity \\(L\\) in the first few seconds, so that the trend of time-resolved \\(\\eta-L\\) can not be described as a monotonically positive correlation between \\(\\eta\\) and \\(L\\). A hybrid outflow with almost completely thermalized Poynting flux could account for the quasi-thermal spectrum as well as large \\(\\eta\\). More importantly, the existence of magnetic field could affect the proton density and neutron-proton coupling effect, so that it could account for the observed trend of time-resolved \\(\\eta-L\\). The other origins for the photospheric emission, such as non-dissipative hybrid outflow or magnetic reconnection, are not supported because their radiation efficiencies are low, which is not consistent with non-detection of the afterglow for GRB 220426A. Therefore, we think the hybrid outflow may be the most likely origin.

In theory, burst activity of the magnetar can lead to the formation of fireballs trapped by the magnetic field and corotating with the star. However, the smoking-gun observational evidence of the fireball is elusive. We envisage that the fireball emission should occasionally be eclipsed by the magnetar, especially when the burst duration is comparable to the magnetar's spin period. In this work, we first discover a peculiar type of burst whose light curve has a plateau-like feature among the long bursts of the magnetar SGR J1935+2154 detected by GECAM and Fermi/GBM. Then, based on these bursts, we identified four burst candidates with eclipse-like characteristics. By fitting their light curves with the eclipse fireball model, the viewing angle of the magnetar relative to its spin axis is estimated to be \\(17^\\circ \\pm 10^\\circ\\), and the distances from the fireballs to the magnetar are found to be more than 5 times the magnetar's radius, indicating that the fireballs are suspended in the magnetosphere rather than adhering to the magnetar surface. Furthermore, we find that this configuration is well consistent with the implication of the cyclotron resonance scattering feature we found in their spectra. Our results suggest that some intermediate X-ray bursts may originate from magnetic reconnection within the magnetosphere rather than the starquake.

Thermonuclear X-ray bursts occur on the surface of an accreting neutron star (NS), and their characteristics and interplay with the surrounding circumstance could be a clue to understand the nature of the NS and accretion process. For this purpose, Insight-HXMT has performed high cadence observations on the bright thermonuclear X-ray burster--4U~1608--52 during its outburst in July and August 2022; nine bursts were detected, including seven bursts with the photospheric radius expansion (PRE). Time-resolved spectroscopy of the bright PRE bursts reveals that an enhancement of accretion rate or the Comptonization of the burst emission by the corona could reduce the residuals when fitting their spectra with the conventional model--blackbody. The inferred energy increment rate of the burst photon gained from the corona is up to \\(\\sim\\)40\\%, even though the bursts have different peak fluxes and locate at different accretion rates. Moreover, the flux shortage of the rising PRE is observed in the bursts at a high mass accretion rate, but not for the burst with a faint persistent emission, which has been predicted theoretically but first observed in this work. If the flux shortage is due to the disk obscuration, i.e., the burst emission is anisotropic, the phenomenon above could indicate that the anisotropy of the burst emission is accretion rate dependent, which could also be evidence of the truncated disk in the low/hard state.

After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \\& 2022, the onset and tail of the outbursts were observed by NICER, which give us a peerless opportunity to study the state transition and its underlying mechanism. In this work, we take both the NS surface and accretion disk emission as the seed photons of the Comptonization and derive their spectral evolution in a bolometric luminosity range of 1\\%--15\\%\\(L_{\\rm Edd}\\). In the high/soft state, the inferred inner disk radius and the NS radius are consistent well, which implies that the accretion disk is close to the NS surface. For the decay stage, we report a steep change of the accretion disk emission within one day, i.e., the soft-to-hard transition, which could be due to the propeller effect and the corresponding neutron star surface magnetic field is 1.8--2.2\\(\\times10^{8}\\) G. Moreover, the inner disk radius is truncated at the corotation radius, which is similar to the propeller effect detected from 4U~1608--52. The absence of the propeller effect in the hard-to-soft state transition implies that the transition between the magnetospheric accretion and the disk accretion is not the sole cause of the state transitions.

Gamma-ray bursts (GRBs) are believed to launch relativistic jets, which generate prompt emission by internal processes, and produce long-lasting afterglows by driving external shocks into surrounding medium. However, how the jet powers the external shock is poorly known. The unprecedented observations of the keV-MeV emission with GECAM and the TeV emission with LHAASO of the brightest-of-all-time GRB 221009A offer a great opportunity to study the prompt-to-afterglow transition and the impact of jet on the early dynamics of external shock. In this letter, we find that the cumulative light curve of keV-MeV emission could well fit the rising stage of the TeV light curve of GRB 221009A, with a time delay, \\(4.45^{+0.26}_{-0.26}\\)\\,s, of TeV emission. Moreover, both the rapid increase in the initial stage and the excess from about \\T+260\\,s to 270\\,s in the TeV light curve are tracking the light-curve bumps in the prompt keV-MeV emission. The close relation between the keV-MeV and TeV emission reveals the continuous energy-injection into the external shock. Assuming an energy-injection rate exactly following the keV-MeV flux of GRB 221009A, including the very early precursor, we build a continuous energy-injection model where the jet Lorentz factor is derived from the TeV time delay, and the TeV data is well fitted, with the TeV excesses interpreted by inverse Compton (IC) scatterings of the inner-coming prompt emission by the energetic electrons in external shock.