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We report analysis of $\textit{Insight}$-HXMT observations of the newly discovered accreting millisecond pulsar SRGA J144459.2$-$604207. During the outburst, detected in 2024 February by $\textit{eROSITA}$, the broadband persistent spectrum was well fitted by an absorbed Comptonization model. We detected 60 type I X-ray bursts in the $\textit{Insight}$-HXMT medium energy (ME) data, and 37 were also detected with the low-energy (LE) telescope. By superimposing the $\textit{Insight}$-HXMT/LE/ME/HE light curves of 37 bursts with similar profiles and intensities, we measured a deficit of X-rays in the 40$-$70 keV energy band. By analyzing the time-resolved X-ray burst spectra, we determine the mean ratio of persistent to burst flux of $\alpha=71\pm7$. We estimate the average hydrogen mass fraction in the fuel at ignition, as $\bar{X} = 0.342 \pm 0.033$, and constrain the burst fuel composition as $X_0\lesssim0.4$. We found that 14 out of 60 X-ray bursts exhibited photospheric expansion, and thus we estimated the distance to the source as $10.03\pm 0.71$ kpc. Combined with $\textit{IXPE}$ observations, the burst recurrence time were increasing from 1.55 to 8 hr as the local mass accretion rate decreasing, which can be described as $\Delta T_{\rm rec}\sim \dot{m}^{-0.91\pm0.02}$., Comment: 13 pages, 9 figures, submitted to ApJ, comments are welcome

The solar composition problem has puzzled astrophysicists for more than 20 years. Recent measurements of carbon-nitrogen-oxygen (CNO) neutrinos by the Borexino experiment show a $\sim2\sigma$ tension with the "low-metallicity" determinations. $^{14}$N$(p,\gamma)^{15}$O, the slowest reaction in the CNO cycle, plays a crucial role in the standard solar model (SSM) calculations of CNO neutrino fluxes. Here we report a direct measurement of the $^{14}$N$(p,\gamma)^{15}$O reaction, in which $S$-factors for all transitions were simultaneously determined in the energy range of $E_p=110-260$ keV for the first time. Our results resolve previous discrepancies in the ground-state transition, yielding a zero-energy $S$-factor $S_{114}(0) = 1.92\pm0.08$ keV b which is 14% higher than the $1.68\pm0.14$ keV b recommended in Solar Fusion III (SF-III). With our $S_{114}$ values, the SSM B23-GS98, and the latest global analysis of solar neutrino measurements, the C and N photospheric abundance determined by the Borexino experiment is updated to $N_{\mathrm{CN}}=({4.45}^{+0.69}_{-0.61})\times10^{-4}$. This new $N_{\mathrm{CN}}$ value agrees well with latest "high-metallicity" composition, however, is also consistent with the "low-metallicity" determination within $\sim 1\sigma$ C.L., indicating that the solar metallicity problem remains an open question. In addition, the significant reduction in the uncertainty of $S_{114}$ paves the way for the precise determination of the CN abundance in future large-volume solar neutrino measurements.

Massive stars end their life as core-collapse supernovae, amongst which some extremes are Type Ic broad-lined supernovae associated with long-duration gamma-ray bursts (LGRBs) having powerful relativistic jets. Their less-extreme brethren make unsuccessful jets that are choked inside the stars, appearing as X-ray flashes or low-luminosity GRBs. On the other hand, there exists a population of extragalactic fast X-ray transients (EFXTs) with timescales ranging from seconds to thousands of seconds, whose origins remain obscure. Known sources that contribute to the observed EFXT population include the softer analogs of LGRBs, shock breakouts of supernovae, or unsuccessful jets. Here, we report the discovery of the bright X-ray transient EP240414a detected by the Einstein Probe (EP), which is associated with the Type Ic supernova SN 2024gsa at a redshift of 0.401. The X-ray emission evolution is characterised by a very soft energy spectrum peaking at < 1.3 keV, which makes it distinct from known LGRBs, X-ray flashes, or low-luminosity GRBs. Follow-up observations at optical and radio bands revealed the existence of a weak relativistic jet that interacts with an extended shell surrounding the progenitor star. Located on the outskirts of a massive galaxy, this event reveals a new population of explosions of Wolf-Rayet stars characterised by a less powerful engine that drives a successful but weak jet, possibly owing to a progenitor star with a smaller core angular momentum than in traditional LGRB progenitors., Comment: 43 pages, 9 figures, 4 tables, submitted. Comments are welcome

We report on the broadband spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498$-$2921 during its April 2023 outburst using data from NICER (1$-$10 keV), NuSTAR (3$-$79 keV), Insight-HXMT (2$-$150 keV), and INTEGRAL (30$-$150 keV). We detect significant 401 Hz pulsations across the 0.5$-$150 keV band. The pulse fraction increases from $\sim$2% at 1 keV to $\sim$13% at 66 keV. Five type-I X-ray bursts have been detected, including three photospheric radius expansion bursts, with a rise time of $\sim$2 s and an exponential decay time of $\sim$5 s. The recurrence time is $\sim$9.1 h, which can be explained by unstable thermonuclear burning of hydrogen-deficient material on the neutron star surface. The quasi-simultaneous 1$-$150 keV broadband spectra from NICER, NuSTAR, and INTEGRAL can be well fitted by an absorbed reflection model, relxillCp, and a Gaussian line of instrumental origin. The Comptonized emission from the hot corona is characterized by a photon index $\Gamma$ of $\sim$1.8 and an electron temperature $kT_{\rm e}$ of $\sim$40 keV. We obtain a low inclination angle $i\sim34^{\circ}$. The accretion disk shows properties of strong ionization, $\log(\xi/{\rm erg~cm~s^{-1}})\sim4.5$, over-solar abundance, $A_{\rm Fe}\sim 7.7$, and high density, $\log(n_{\rm e}/{\rm cm^{-3}})\sim 19.5$. However, a lower disk density with normal abundance and ionization could also be possible. From the inner disk radius $R_{\rm in}=1.67R_{\rm ISCO}$ and the long-term spin-down rate of $-3.1(2)\times10^{-15}~{\rm Hz~s^{-1}}$, we constrain the magnetic field of IGR J17498$-$2921 in the range of $(0.9-2.4)\times10^8$ G., Comment: Published in A&A

Long gamma-ray bursts (GRBs) are believed to originate from core collapse of massive stars. High-redshift GRBs can probe the star formation and reionization history of the early universe, but their detection remains rare. Here we report the detection of a GRB triggered in the 0.5--4 keV band by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated as EP240315a, whose bright peak was also detected by the Swift Burst Alert Telescope and Konus-Wind through off-line analyses. At a redshift of $z=4.859$, EP240315a showed a much longer and more complicated light curve in the soft X-ray band than in gamma-rays. Benefiting from a large field-of-view ($\sim$3600 deg$^2$) and a high sensitivity, EP-WXT captured the earlier engine activation and extended late engine activity through a continuous detection. With a peak X-ray flux at the faint end of previously known high-$z$ GRBs, the detection of EP240315a demonstrates the great potential for EP to study the early universe via GRBs., Comment: 41 pages, 8 figures, 7 tables

The second brightest GRB in history, GRB230307A provides an ideal laboratory to study the details of GRB prompt emission thanks to its extraordinarily high photon statistics and its single broad pulse overall shape characterized by an energy-dependent fast-rise-exponential-decay (FRED) profile. Here we demonstrate that its broad pulse is composed of many rapidly variable short pulses, rather than being the superposition of many short pulses on top of a slow component. Such a feature is consistent with the picture of many mini-jets due to local magnetic reconnection events in a large emission zone far from the GRB central engine, as envisaged in the internal-collision-induced magnetic reconnection and turbulence (ICMART) model, but raises a great challenge to the internal shock models that attribute all variability components to collisions among different shells. Since relativistic mini-jets demand strong magnetization in the outflow, this work provides strong evidence for a Poynting-flux-dominated jet composition of this bright GRB., Comment: 7 pages and 2 figures in the main text. 27 pages and 9 figures in total

The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of a peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties are consistent with a compact star merger origin, as suggested by its association with a kilonova. Intriguingly, an extended X-ray emission component shows up as the $\gamma$-ray emission dies out, signifying the likely emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with half opening angle of approximately $\sim 3.4^\circ (R_{GRB}/10^{15}~{cm})^{-1/2}$, where $R_{GRB}$ is the GRB prompt emission radius., Comment: 54 pages, 11 figures, 5 tables, accepted for publication in National Science Review

Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a space-borne instrument dedicated to monitoring high-energy transients, including Terrestrial Gamma-ray Flashes (TGFs) and Terrestrial Electron Beams (TEBs). We implemented a TGF/TEB search algorithm for GECAM, with which 147 bright TGFs, 2 typical TEBs and 2 special TEB-like events are identified during an effective observation time of $\sim$9 months. We show that, with gamma-ray and charged particle detectors, GECAM can effectively identify and distinguish TGFs and TEBs, and measure their temporal and spectral properties in detail. A very high TGF-lightning association rate of $\sim$80\% is obtained between GECAM and GLD360 in east Asia region., Comment: The paper was accepted by Geophysical Research Letters on June 16th, 2023

Fast transitions between different types of power density spectra (PDS) happening over timescales of several tens of seconds are rare phenomena in black hole X-ray binaries. In this paper, we report a broadband spectral-timing analysis of the fast transitions observed in the 2021 outburst of GX 339-4 using NICER and HXMT observations. We observe transitions between band-limited noise-dominated PDS and type-B quasi-periodic oscillations (QPOs), and their rapid appearance or disappearance. We also make a detailed comparison between the fast transitions in GX 339-4 with those seen in MAXI J1820+070 and MAXI J1348--630. By comparing the spectra of the periods with and without type-B QPOs, we find that the spectral ratios above 10 keV are nearly constant or slightly decreasing, and the values are different between sources. Below 10 keV, the flux change of the Comptonization component is inversely proportional to the flux change of the thermal component, suggesting that the appearance of type-B QPOs is associated with a redistribution of the accretion power between the disc and the Comptonizing emission region. The spectral ratios between the periods with type-B QPO and those with broadband noise are significantly different from that with type-B QPO and without type-B QPO, where the ratios (type-B QPO/broadband noise) show a maximum at around 4 keV and then decrease gradually towards high energies. Finally, we discuss the possible change of the geometry of the inner accretion flow and/or jet during the transitions.

We report the X-ray timing results of the black hole candidate MAXI J1820+070 during its 2018 outburst using the Hard X-ray Modulation Telescope (Insight-HXMT) and Neutron Star Interior Composition Explorer Mission (NICER) observations. Low frequency quasi-periodic oscillations (LFQPOs) are detected in the low/hard state and the hard intermediate state, which lasted for about 90 days. Thanks to the large effective area of Insight-HXMT at high energies and NICER at low energies, we are able to present the energy dependence of the LFQPO characteristics and phase lags from 0.2 keV to 200 keV, which has never been explored by previous missions. We find that the centroid frequency of the LFQPOs do not change significantly with energy, while the full width at half maximum (FWHM) and fractional rms show a complex evolution with energy. The LFQPO phase lags at high energies and low energies show consistent energy-dependence relations taking the ~2 keV as reference. Our results suggest that the LFQPOs from high energy come from the LT precession of the relativistic jet, while the low-energy radiation is mainly from the perpendicular innermost regions of the accretion disk.

GRB 221009A is the brightest Gamma-Ray Burst (GRB) detected in more than 50 years of study. In this paper, we present observations in the X-ray and optical domains after the GRB obtained by the GRANDMA Collaboration (which includes observations from more than 30 professional and amateur telescopes) and the Insight-HXMT Collaboration. We study the optical afterglow with empirical fitting from GRANDMA+HXMT data, augmented with data from the literature up to 60 days. We then model numerically, using a Bayesian approach, the GRANDMA and HXMT-LE afterglow observations, that we augment with Swift-XRT and additional optical/NIR observations reported in the literature. We find that the GRB afterglow, extinguished by a large dust column, is most likely behind a combination of a large Milky-Way dust column combined with moderate low-metallicity dust in the host galaxy. Using the GRANDMA+HXMT-LE+XRT dataset, we find that the simplest model, where the observed afterglow is produced by synchrotron radiation at the forward external shock during the deceleration of a top-hat relativistic jet by a uniform medium, fits the multi-wavelength observations only moderately well, with a tension between the observed temporal and spectral evolution. This tension is confirmed when using the extended dataset. We find that the consideration of a jet structure (Gaussian or power-law), the inclusion of synchrotron self-Compton emission, or the presence of an underlying supernova do not improve the predictions, showing that the modelling of GRB22109A will require going beyond the most standard GRB afterglow model. Placed in the global context of GRB optical afterglows, we find the afterglow of GRB 221009A is luminous but not extraordinarily so, highlighting that some aspects of this GRB do not deviate from the global known sample despite its extreme energetics and the peculiar afterglow evolution., Comment: Accepted to ApJL for the special issue, 37 pages, 23 pages main text, 6 tables, 13 figures

We report the evolution of the X-ray pulsations of EXO 2030+375 during its 2021 outburst using the observations from \textit{Insight}-HXMT. Based on the accretion torque model, we study the correlation between the spin frequency derivatives and the luminosity. Pulsations can be detected in the energy band of 1--160 keV. The pulse profile evolves significantly with luminosity during the outburst, leading to that the whole outburst can be divided into several parts with different characteristics. The evolution of the pulse profile reveals the transition between the super-critical (fan-beam dominated) and the sub-critical accretion (pencil-beam dominated) mode. From the accretion torque model and the critical luminosity model, based on a distance of 7.1 kpc, the inferred magnetic fields are $(0.41-0.74) \times 10^{12}$ G and $(3.48-3.96) \times 10^{12}$ G, respectively, or based on a distance of 3.6 kpc, the estimated magnetic fields are $(2.4-4.3) \times 10^{13}$ G and $(0.98-1.11)\times 10^{12}$ G, respectively. Two different sets of magnetic fields both support the presence of multipole magnetic fields of the NS.

A double-peak X-ray burst from the Galactic magnetar SGR J1935+2154 was discovered as associated with the two radio pulses of FRB 200428 separated by 28.97+-0.02 ms. Precise measurements of the timing and spectral properties of the X-ray bursts are helpful for understanding the physical origin of fast radio bursts (FRBs). In this paper, we have reconstructed some information about the hard X-ray events, which were lost because the High Energy X-ray Telescope (HE) onboard the Insight-HXMT mission was saturated by this extremely bright burst, and used the information to improve the temporal and spectral analyses of the X-ray burst. The arrival times of the two X-ray peaks by fitting the new Insight-HXMT/HE lightcurve with multi-Gaussian profiles are 2.77+-0.45 ms and 34.30+-0.56 ms after the first peak of FRB 200428, respectively, while these two parameters are 2.57+-0.52 ms and 32.5+-1.4 ms if the fitting profile is a fast rise and exponential decay function. The spectrum of the two X-ray peaks could be described by a cutoff power-law with cutoff energy ~60 keV and photon index ~1.4, the latter is softer than that of the underlying bright and broader X-ray burst when the two X-ray peaks appeared., Comment: 11 pages, 7 figures

Pulsating ultraluminous X-ray sources (PULXs) are accreting pulsars with apparent X-ray luminosity exceeding $10^{39}\, \rm erg\ s^{-1}$. We perform Monte-Carlo simulations to investigate whether high collimation effect (or strong beaming effect) is dominant in the presence of accretion outflows, for the fan beam emission of the accretion column of the neutron stars in PULXs. We show that the three nearby PULXs (RX J0209.6$-$7427, Swift J0243.6+6124 and SMC X-3), namely the three musketeers here, have their main pulsed emission not strongly collimated even if strong outflows exist. This conclusion can be extended to the current sample of extragalactic PULXs, if accretion outflows are commonly produced from them. This means that the observed high luminosity of PULXs is indeed intrinsic, which can be used to infer the existence of very strong surface magnetic fields of $\sim10^{13-14}$ G, possibly multipole fields. However, if strong outflows are launched from the accretion disks in PULXs as a consequence of disk spherization by radiation pressure, regular dipole magnetic fields of $\sim10^{12}$ G may be required, comparable to that of the three musketeers, which have experienced large luminosity changes from well below their Eddington limit ($2\times10^{38}\, \rm erg\ s^{-1}$ for a NS) to super-Eddington and their maximum luminosity fills the luminosity gap between Galactic pulsars and extragalactic PULXs., Comment: 26 pages, 8 figures, 1 table. Accepted by ApJ

Using wavelet analysis and power density spectrum, we investigate two transient quasi-periodic oscillations (QPOs) observed in MAXI J1535$-$571 observed with Insight-HXMT. The transient QPOs have a centroid frequency of $\sim 10$ Hz with a FWHM $\sim 0.6$ Hz and an rms amplitude $\sim 14\%$. Energy spectra of QPO and non-QPO regimes are also separated and analyzed, and the spectra become softer with higher $E_{cut}$ in the non-QPO regime compared to the QPO regime. Our results suggest that the transient QPOs detected in MJD 58016 and 58017 are still the type-C QPO, and the source remains in its HIMS. The duration of all type-C QPO signals based on wavelet is positively correlated with the mean count rate above $\sim 10$ keV, implying appearance of QPOs in different time scales should be coupled with the corona. The transient QPO properties could be related to the jet or flares, perhaps the partial ejection of the corona is responsible for the disappearance of the type-C QPO., Comment: 10 pages, MNRAS in press

We investigated the quasi-periodic oscillation (QPO) features in the accretion-powered X-ray pulsar Cen X-3 observed by Insight-HXMT. For two observations in 2020 when Cen X-3 was in an extremely soft state, the power density spectrum revealed the presence of obvious QPO features at $\sim$40 mHz with an averaged fractional rms amplitude of $\sim9\%$. We study the mHz QPO frequency and rms amplitude over orbital phases, and find that the QPO frequency is $\sim$33-39 mHz at the orbital phase of 0.1-0.4, increasing to $\sim$37-43 mHz in the orbital phase of 0.4-0.8, but has no strong dependence on X-ray intensity. We also carried out an energy-dependent QPO analysis, the rms amplitude of the mHz QPOs have a decreasing trend as the energy increases from 2 to 20 keV. In addition, the QPO time-lag analysis shows that the time delay is $\sim 20$ ms (a hard lag) in the range of $\sim$5-10 keV, and becomes negative (time lag of $-(20-70)$ ms) above $\sim 10$ keV. The different QPO theoretical models are summarized and discussed. In the end, we suggest that these energy-dependent timing features as well as the origin of mHz QPOs in Cen X-3 may be ascribed to an instability when the accretion disk is truncated near the corotation radius., Comment: 9 pages, MNRAS in press

The uniqueness of the positive ground state solutions of fractional Shrodinger equations with a harmonic potential has not been covered by the breakthrough method developed in [1, 2]. It has remained an open question for years. [3] and [5] were quite recently able to prove the existence of the ground state solutions and to derive important properties but they failed to address the uniqueness. [3] left it as an open question, and [5] run numerical simulations that were in favor of the uniqueness. Very recently, the authors of this note developed a general and unified method to prove the uniqueness of the ground state solutions of a large class of variational problems, they also exhibited many examples to which their approach applies. After the publication of [4], some colleagues reached out to us to know whether our method applies to the fractional Shrodinger equation with a harmonic potential. In this short note, we will explain how it applies, and we will also state some existence/ non-existence and multiplicity solutions of the normalized, Comment: All comments are welcome

We present detailed timing and spectral analyses of the transient X-ray pulsar RX J0209.6$-$7427 in the Small Magellanic Cloud during its 2019 giant outburst. With a better known distance than most galactic X-ray pulsars, its peak luminosity is determined to be $(1.11\pm0.06)\times 10^{39}\, \rm erg\ s^{-1}$; it is thus a {\it bonda fide} pulsating ultraluminous X-ray source (PULX). Owing to the broad energy band of \textit{Insight}-HXMT, its pulsed X-ray emission was detected from 1 keV up to the 130$-$180 keV band, which is the highest energy emission detected from any PULXs outside the Milky Way. This allows us to conclude that its main pulsed X-ray emission is from the "fan beam" of the accretion column, and its luminosity is thus intrinsic. We also estimate its magnetic field of (4.8$-$8.6)$\times10^{12}$ G or (1.7$-$2.2)$\times10^{13}$ G, from its spin evolution or transition in the accretion column structure during the outburst; we suggest that the two values of the magnetic field strength correspond to the dipole and multipole magnetic fields of the neutron star, similar to the recent discovery in the Galactic PULX Swift J0243.6+6124. Therefore, the nature of the neutron star and its ULX emission can be understood within the current theoretical frame of accreting neutron stars. This may have implications for understanding the nature of those farther away extragalactic PULXs., Comment: 31 pages, 14 figures, 5 tables. Accepted by ApJ

The fast transitions between different types of quasi-periodic oscillations (QPOs) are generally observed in black hole transient sources (BHTs). We present a detailed study on the timing and spectral properties of the transitions of type-B QPOs in MAXI~J1348--630, observed by \emph{Insight}-HXMT. The fractional rms variability--energy relationship and energy spectra reveal that type-B QPOs probably originate from jet precession. Compared to weak power-law dominated power spectrum, when type-B QPO is present, the corresponding energy spectrum shows an increase in Comptonization component and the need for {\tt\string xillverCp} component, and a slight increase of height of the corona when using {\tt\string relxilllp} model. Therefore, we suggest that a coupled inner disk-jet region is responsible for the observed type-B QPOs transitions. The time scale for the appearance/disappearance of type-B QPOs is either long or short (seconds), which may indicate an instability of disk-jet structure. For these phenomena, we give the hypothesis that the Bardeen-Petterson effect causes disk-jet structure to align with BH spin axis, or that the disappearance of small-scale jets bound by the magnetic flux tubes lead to the disappearance of type-B QPOs. We observed three events regarding the B/C transitions, one of which occurred in a short time from $\sim 9.2$ Hz (C) to $\sim 4.8$ Hz (B). The energy spectral analysis for the other two transitions shows that when type-C QPO is present, the Comptonization flux is higher, the spectrum is harder and the inner radius of disk changes insignificantly. We suggest that type-C QPOs probably originate from relatively stronger jets or corona., Comment: 26 pages, 9 figures

We studied the transient Galactic black hole candidate MAXI J0637-430 with data from Insight-HXMT, Swift and XMM-Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637-430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disk, we suggest a most likely distance <7 kpc. Compared with other black hole transients, MAXI J0637-430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at ~ 0.7 keV and <0.1 times Eddington, respectively); a short decline timescale; a low soft-to-hard transition luminosity (<0.01 times Eddington). We argue that such properties are consistent with a small binary separation, short binary period (P ~ 2 hr), and low-mass donor star (M2 ~ 0.2 M_sun). Moreover, spectral modelling shows that a single disk-blackbody component is not a good fit to the thermal emission. Soft spectral residuals, and deviations from the standard L_disk ~ T^4 in relation, suggest the need for a second thermal component. We propose and discuss various scenarios for such component, in addition to those presented in previous studies of this source. For example, a gap in the accretion disk between a hotter inner ring near the innermost stable orbit, and a cooler outer disk. Another possibility is that the second thermal component is the thermal plasma emission from an ionized outflow.

We present a detailed study of the high mass X-ray binary Vela X-1, using observations performed by Insight-HXMT in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. The cyclotron line energy is found to be at ~21-27 keV and 43-50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that two line centroid energy ratio E2/E1 evolved from ~2 before MJD 58900 to ~1.7 after that. The harmonic cyclotron line energy has no relation to the luminosity but the fundamental line energy shows a positive correlation with X-ray luminosity, suggesting that Vela X-1 is located in the sub-critical accreting regime. In addition, the pulse phase-resolved spectroscopy in Vela X-1 is performed. Both the CRSF and continuum parameters show strong variability over the pulse phase with the ratio of two line energies about 2 near the peak phases, and down to ~1.6 around off-peak phases. Long-term significant variations of the absorption column density and its evolution over the pulse phase may imply the existence of the clumpy wind structure near the neutron star., Comment: 10 pages, MNRAS in press

In this paper, wavelet analysis is used to study spectral-timing properties of MAXI J1535-571 observed by Insight-HXMT. The low-frequency quasi-periodic oscillations (QPOs) are detected in nine observations. Based on wavelet analysis, the time intervals with QPO and non-QPO are isolated separately, and the corresponding spectra with QPO and non-QPO are analyzed. We find that the spectra with QPO (hereafter QPO spectra) are softer than those without QPO (hereafter non-QPO spectra) in the hard intermediate state (HIMS). While in the soft intermediate state (SIMS), the QPO spectra are slightly harder. The disk temperature of QPO regime is slightly lower during HIMS, but becomes higher during SIMS. The cutoff energies of QPO spectra and non-QPO spectra do not show significant differences. The flux ratio of the disk to total flux is higher for the time intervals with non-QPO than that of QPO regime. We propose that these differences in the spectral properties between QPO and non-QPO regimes could be explained in the scenario of Lense-Thirring precession, and the reversal of the QPO/non-QPO behavior between HIMS and SIMS may be associated with appearance/disappearance of a type-B QPO which might origin from the precession of the jet., Comment: 13 pages, 9 figures in main part, and 3 figures in the appendix; MNRAS in press, the first submission in 2021-06

Multiple isotopes of samarium element are the isotopes produced by the s process, and 154Sm is produced by the r process. In addition, 144Sm is p nuclei in nuclear astrophysics. The measurement of these can help us to better understand the results of relevant photonuclear reaction experiments. On the other hand, 149Sm is a 235U fission product with a 1% yield, its cross sections are important to reactor neutronics. In this work, the neutron capture yield of the natural samarium target was measured at the back-streaming white neutron beamline (Back-n) of the China Spallation Neutron Source (CSNS), and the resonance parameters were analyzed by SAMMY code. The resonance peaks and the neutron separation energies contributed by the different isotopes are considered individually. The results of the capture yield found signs of the possibility of two resonance peaks at 8 eV, which awaits further experimental examination. Cross-section was calculated according to resonance parameters and was compared with other experimental results and evaluation databases of ENDF/B-VIII.0 and CENDL-3.2. A clear difference between ENDF/B VIII.0 and CENDL-3.2 database appears at 23.2 eV, the experimental result at this energy is smaller than data of ENDF/B VIII.0 database but CENDL-3.2 database. Most of the controversial experimental results invariably come from the samarium 149 isotope., Comment: 10 pages,7 figures

Here we present a detailed study of the broadband noise in the power density spectra of the black hole X-ray binary MAXI J1820+070 during the hard state of its 2018 outburst, using the Hard X-ray Modulation Telescope (Insight-HXMT) observations. The broadband noise shows two main humps, which might separately correspond to variability from a variable disk and two Comptonization regions. We fitted the two humps with multiple Lorentzian functions and studied the energy-dependent properties of each component up to 100--150 keV and their evolution with spectral changes. The lowest frequency component is considered as the sub-harmonic of QPO component and shows different energy dependence compared with other broadband noise components. We found that although the fractional rms of all the broadband noise components mainly decrease with energy, their rms spectra are different in shape. Above $\sim$ 20--30 keV, the characteristic frequencies of these components increase sharply with energy, meaning that the high-energy component is more variable on short timescales. Our results suggest that the hot inner flow in MAXI J1820+070 is likely to be inhomogeneous. We propose a geometry with a truncated accretion disk, two Comptonization regions.

We present the catalog of Interplanetary Network (IPN) localizations for 199 short-duration gamma-ray bursts (sGRBs) detected by the Konus-Wind (KW) experiment between 2011 January 1 and 2021 August 31, which extends the initial sample of IPN localized KW sGRBs (arXiv:1301.3740) to 495 events. We present the most comprehensive IPN localization data on these events, including probability sky maps in HEALPix format., Comment: Published in ApJS

Using the observations of the high-energy (HE) detector of the Hard X-ray Modulation Telescope (Insight-HXMT) for Scorpius X-1 in 2018, we search for hard X-ray tails in the hard X-ray spectra in ~30-200 keV. The hard X-ray tails are found throughout the Z-track on the hardness-intensity diagram and they harden and fade away from the horizontal branch (HB), through the normal branch (NB), to the flaring branch (FB). Comparing the hard X-ray spectra between Cyg X-1 and Sco X-1, it is concluded that the hard X-ray spectrum of Cyg X-1 shows high-energy cutoff, implying a hot corona in it, but the high-energy cutoff does not reveal in the hard X-ray spectrum of Sco X-1. Jointly fitting the HE spectrum with the medium-energy and low-energy spectra of Sco X-1 in ~2-200 keV, it is suggested that the upscattering Comptonization of the neutron star (NS) emission photons by the energetic free-falling electrons onto the NS or by the hybrid electrons in the boundary layer between the NS and the accretion disk could be responsible for the hard X-ray tails of Sco X-1 on the HB and NB, but neither of the two mechanisms can be responsible for the hard X-ray tail on the FB. Some possible origins for the peculiar hard X-ray tail of FB are argued., Comment: 26 pages, 7 figures

We present a spectral study of the black hole candidate MAXI J1348-630 during its 2019 outburst, based on monitoring observations with Insight-HXMT and Swift. Throughout the outburst, the spectra are well fitted with power-law plus disk-blackbody components. In the soft-intermediate and soft states, we observed the canonical relation L ~ T_in^4 between disk luminosity L and peak colour temperature T_in, with a constant inner radius R_in (traditionally identified with the innermost stable circular orbit). At other stages of the outburst cycle, the behaviour is more unusual, inconsistent with the canonical outburst evolution of black hole transients. In particular, during the hard rise, the apparent inner radius is smaller than in the soft state (and increasing), and the peak colour temperature is higher (and decreasing). This anomalous behaviour is found even when we model the spectra with self-consistent Comptonization models, which take into account the up-scattering of photons from the disk component into the power-law component. To explain both those anomalous trends at the same time, we suggest that the hardening factor for the inner disk emission was larger than the canonical value of ~1.7 at the beginning of the outburst. A more physical trend of radii and temperature evolution requires a hardening factor evolving from ~3.5 at the beginning of the hard state to ~1.7 in the hard intermediate state. This could be evidence that the inner disk was in the process of condensing from the hot, optically thin medium and had not yet reached a sufficiently high optical depth for its emission spectrum to be described by the standard optically-thick disk solution., Comment: 20 pages, 10 figures, 4 tables, accepted by ApJ

One month after launching Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM), a bright thermonuclear X-ray burst from 4U~0614+09, was observed on January 24, 2021. We report the time-resolved spectroscopy of the burst and a burst oscillation detection at 413 Hz with a fractional amplitude 3.4\% (rms). This coincides with the burst oscillation previously discovered with \textit{Swift}/BAT \citep{Strohmayer2008}, and therefore confirms the spin frequency of this source. This burst is the brightest one in the normal bursts (except the superburst) ever detected from 4U~0614+09, which leads to an upper limit of distance estimation as 3.1 kpc. The folded light curve during the burst oscillation shows a multi-peak structure, which is the first case observed during a single burst oscillation in nonpulsating sources. The multi-peak profile could be due to additional harmonics of the burst oscillation, which is corresponding to several brighter/fainter spots at the stellar surface.

As the main detector of the GECAM satellite, the calibration of the energy response and detection efficiency of the GRD detector is the main content of the ground-based calibration. The calibration goal requires the calibrated energy points to sample the full energy range (8 keV-2 MeV) as much as possible. The low energy band (8-160 keV) is calibrated with the X-ray beam, while the high energy band (>160 keV) with radioactive sources. This article mainly focuses on the calibration of the energy response and detection efficiency in the 8-160 keV with a refined measurement around the absorption edges of the lanthanum bromide crystal. The GRD performances for different crystal types, data acquisition modes, working modes, and incident positions are also analyzed in detail. We show that the calibration campaign is comprehensive, and the calibration results are generally consistent with simulations as expected., Comment: 16 pages,46 figures

The GECAM mission consists of two identical microsatellites (GECAM-A and GECAM-B). Each satellite is equipped with 25 gamma-ray detectors (GRD) and 8 charged particle detectors (CPD). The main scientific objective of the GECAM mission is to detect gamma-ray bursts (GRBs) associated with the gravitational wave events produced by the merging of binary compact stars. After the launch on Dec. 10, 2020 , we carried out a series of on orbit tests. This paper introduces the test results of the GECAM-B satellite. According to the in-flight performance, the energy band for gamma-ray detection of GECAM-B is from about 7 keV to 3.5 MeV. GECAM-B can achieve prompt localization of GRBs. For the first time, GECAM-B realized a quasi-real-time transmission of trigger information using Beidou-3 RDSS. Keywords GECAM, gamma-ray burst, gravitational wave, GRD, CPD, Comment: 16 pages, 8 figures

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio bursts. Recent observations of a Galactic FRB suggest that at least some FRBs originate from magnetars, but the origin of cosmological FRBs is still not settled. Here we report the detection of 1863 bursts in 82 hr over 54 days from the repeating source FRB~20201124A. These observations show irregular short-time variation of the Faraday rotation measure (RM), which probes the density-weighted line-of-sight magnetic field strength, of individual bursts during the first 36 days, followed by a constant RM. We detected circular polarisation in more than half of the burst sample, including one burst reaching a high fractional circular polarisation of 75%. Oscillations in fractional linear and circular polarisations as well as polarisation angle as a function of wavelength were detected. All of these features provide evidence for a complicated, dynamically evolving, magnetised immediate environment within about an astronomical unit (au; Earth-Sun distance) of the source. Our optical observations of its Milky-Way-sized, metal-rich host galaxy reveal a barred spiral, with the FRB source residing in a low stellar density, interarm region at an intermediate galactocentric distance. This environment is inconsistent with a young magnetar engine formed during an extreme explosion of a massive star that resulted in a long gamma-ray burst or superluminous supernova., Comment: 28 pages, 10 figures, updated to match the published version