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IGR J17091-3624, in addition to GRS 1915+105, is the only black hole X-ray binary that displays ``heartbeat"-like variability characterized by structured flares with high amplitudes. In this study, we conduct a detailed phase-resolved analysis of the recently identified ``heartbeat"-like Class X variability in IGR J17091-3624 during its 2022 outburst, utilizing data from NICER and NuSTAR observations. A shortage in the high-energy (>20 keV) X-ray flux is detected at peak phases of the soft X-ray flare at a ~15 sigma confidence level from the phase-folded light curves. Furthermore, our phase-resolved spectral analysis reveals variations in the spectral shape, particularly showing significant synchronous variations in the disk temperature and flux with the count rate. These findings imply that the flare is primarily driven by instabilities within the accretion disk, consistent with previous studies on the well-known Class rho variability in GRS 1915+105. However, we also observe a positive correlation between the disk temperature and flux over the flare cycle, which differs from a loop relation between the two parameters found in the Class rho variability. This could suggest differences in underlying physical processes between the two variability classes. Variations in the Componization component during flares are also observed: the electron temperature and covering fraction show anti-correlations with the disk flux, revealing potential interactions between the accretion disk and the corona during these flares., Comment: Accepted for publication in The Astrophysical Journal

Low-frequency quasi-periodic oscillations (LFQPOs) are commonly observed in X-ray light curves of black hole X-ray binaries (BHXRBs); however, their origin remains a topic of debate. In order to thoroughly investigate variations in spectral properties on the QPO timescale, we utilized the Hilbert-Huang transform technique to conduct phase-resolved spectroscopy across a broad energy band for LFQPOs in the newly discovered BHXRB Swift J1727.8-1613. This is achieved through quasi-simultaneous observations from Neutron star Interior Composition ExploreR (NICER), Nuclear Spectroscopic Telescope ARray (NuSTAR), and Hard X-ray Modulation Telescope (Insight-HXMT). Our analysis reveals that both the non-thermal and disk-blackbody components exhibit variations on the QPO timescale, with the former dominating the QPO variability. For the spectral parameters, we observe modulation of the disk temperature, spectral indices, and reflection fraction with the QPO phase with high statistical significance (>5\sigma). Notably, the variation in the disk temperature is found to precede the variations in the non-thermal and disk fluxes by ~0.4-0.5 QPO cycles. We suggest that these findings offer further evidence that the type-C QPO variability is a result of geometric effects of the accretion flow., Comment: Accepted for pulication in The Astrophysical Journal

We conducted an analysis of 45 bursts observed from 4U 1636$-$53. To investigate the mechanism behind the light curve profiles and the impact of thermonuclear X-ray bursts on the accretion environment in accreting neutron star low-mass X-ray binaries. This analysis employed both light curve and time-resolved spectroscopy methodologies, with data collected by the \textit{Insight}-HXMT instrument. We found that 30 bursts exhibited similar light curve profiles and were predominantly in the hard state, and two photospheric radius expansion (PRE) bursts were in the soft state. The light curves of most bursts did not follow a single exponential decay but displayed a dual-exponential behavior. The initial exponent had a duration of approximately 6 s. We utilized both the standard method and the `$f_{\rm a}$' method to fit the burst spectra. The majority of the `$f_{\rm a}$' values exceeded 1, indicating an enhancement of the persistent emission during the burst. Under the two comptonization components assumption, we suggest that the scattering of burst photons by the inner corona may mainly contribute to the persistent emission enhancement. We also observed an inverse correlation between the maximum $f_{\rm a}$ and the persistent emission flux in the non-PRE burst. This anti-correlation suggests that when the accretion rate is lower, there is a greater enhancement of persistent emission during the burst peak. The prediction based on Poynting-Robertson drag (P-R drag) aligns with this observed anti-correlation., Comment: accepted by MNRAS

We have performed a spectral-timing analysis on the black hole X-ray binary MAXI J1535--571 during its 2017 outburst, with the aim of exploring the evolution of the inner accretion flow geometry. X-ray reverberation lags are observed in the hard-intermediate state (HIMS) and soft-intermediate state (SIMS) of the outburst. During the HIMS, the characteristic frequency of the reverberation lags $\nu_0$ (the frequency at which the soft lag turns to zero in the lag-frequency spectra) increases when the spectrum softens. This reflects a reduction of the spatial distance between the corona and accretion disc, when assuming the measured time lags are associated with the light travel time. We also find a strong correlation between $\nu_0$ and type-C Quasi Periodic Oscillation (QPO) centroid frequency $\nu_{QPO}$, which can be well explained by the Lense-Thirring (L-T) precession model under a truncated disk geometry. Despite the degeneracy in the spectral modellings, our results suggest that the accretion disc is largely truncated in the low hard state (LHS), and moves inward as the spectrum softens. Combine the spectral modelling results with the $\nu_0$ - $\nu_{QPO}$ evolution, we are inclined to believe that this source probably have a truncated disk geometry in the hard state.

As users shift from interacting actively with devices with screens to interacting seamlessly with smart environments, novel models of user authentication will be needed to maintain the security and privacy of user data. To understand users' attitudes toward new models of authentication (e.g., voice recognition), we surveyed 117 Amazon Turk workers and 43 computer science students about their authentication preferences, in contexts when others are present and different usability metrics. Our users placed less trust in natural authentication modalities (e.g., body gestures) than traditional modalities (e.g., passwords) due to concerns about accuracy or security. Users were also not as willing to use natural authentication modalities except in the presence of people they trust due to risk of exposure and feelings of awkwardness. We discuss the implications for designing natural multimodal authentication and explore the design space around users' current mental models for the future of secure and usable smart technology., Comment: 23 pages, 4 figures

GRB 221009A is the brightest gamma-ray burst ever detected since the discovery of this kind of energetic explosions. However, an accurate measurement of the prompt emission properties of this burst is very challenging due to its exceptional brightness. With joint observations of \textit{Insight}-HXMT and GECAM-C, we made an unprecedentedly accurate measurement of the emission during the first $\sim$1800 s of GRB 221009A, including its precursor, main emission (ME, which dominates the burst in flux), flaring emission and early afterglow, in the hard X-ray to soft gamma-ray band from $\sim$ 10 keV to $\sim$ 6 MeV. Based on the GECAM-C unsaturated data of the ME, we measure a record-breaking isotropic equivalent energy ($E_{\rm iso}$) of $\bf \sim 1.5 \times 10^{55}$ erg, which is about eight times the total rest-mass energy of the Sun. The early afterglow data require a significant jet break between 650 s and 1100 s, most likely at $\sim950$ s from the afterglow starting time $T_{AG}$, which corresponds to a jet opening angle of $\sim {0.7^\circ} \ (\eta_\gamma n)^{1/8}$, where $n$ is the ambient medium density in units of $\rm cm^{-3}$ and $\eta_\gamma$ is the ratio between $\gamma$-ray energy and afterglow kinetic energy. The beaming-corrected total $\gamma$-ray energy $E_{\gamma}$ is $\sim 1.15 \times10^{51} \ (\eta_\gamma n)^{1/4}$ erg, which is typical for long GRBs. These results suggest that this GRB may have a special central engine, which could launch and collimate a very narrowly beamed jet with an ordinary energy budget, leading to exceptionally luminous gamma-ray radiation per unit solid angle. Alternatively, more GRBs might have such a narrow and bright beam, which are missed by an unfavorable viewing angle or have been detected without distance measurement., Comment: Submitted to National Science Review. This paper is under press embargo, contact the corresponding author for details

Purpose: We present the five-year in-orbit background evolution of Insight-HXMT since the launch, as well as the effects of the background model in data analysis. Methods: The backgrounds of the three main payloads, i.e., Low-Energy Telescope, Medium-Energy Telescope and High-Energy Telescope, are described, respectively. The evolution of the background over time is obtained by simply comparing the background in every year during the in-orbit operation of Insight-HXMT. Results: The major observational characteristics of the Insight-HXMT in-orbit background are presented, including the light curve, spectrum, geographical distribution, and long-term evolution. The systematic error in background estimation is investigated for every year. Conclusion: The observational characteristics of the five-year in-orbit background are consistent with our knowledge of the satellite design and the space environment, and the background model is still valid for the latest observations of Insight-HXMT., Comment: 22 pages, 25 figures, accepted for publication in RDTM

We present a systematic analysis of type C quasi-periodic oscillation (QPO) observations of H 1743--322 throughout the Rossi X-ray Timing Explorer (RXTE) era. We find that, while different outbursts have significant flux differences, they show consistent positive correlations between the QPO fractional root-mean-square (rms) amplitude and non-thermal fraction of the emission, which indicate an independence of the intrinsic QPO rms on individual outburst brightness in H 1743--322. However, the dependence of the QPO rms on frequency is different between the outburst rise and decay phases, where QPO fractional rms of the decay phase is significantly lower than that of the rise phase at low frequencies. The spectral analysis also reveals different ranges of coronal temperature between the two outburst stages. A semi-quantitative analysis shows that the Lense-Thirring precession model could be responsible for the QPO rms differences, requiring a variable coronal geometric shape. However, the variable-Comptonization model could also account for the findings. The fact that the rms differences and the hysteresis traces in the hardness-intensity diagram (HID) accompany each other indicates a connection between the two phenomena. By correlating the findings with QPO phase lags and the quasi-simultaneous radio flux previously published, we propose there could be corona-jet transitions in H 1743--322 similar to those that have been recently reported in GRS 1915+105., Comment: 21 pages, 12 figures

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., Comment: Accepted by ApJL

After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \& 2022; ten thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing the source as the 5th accreting neutron star (NS) exhibiting double/triple-peaked type-I X-ray bursts; the other bursts showed photospheric radius expansion (PRE). The properties of double-peaked non-PRE burst indicate that it could be related to a stalled burning front. For the five bright PRE bursts, apart from the emission from the neutron star (NS) surface, we find the residuals both in the soft ($<$3 keV) and hard ($>$10 keV) X-ray band. Time-resolved spectroscopy reveals that the excess can be attributed to an enhanced pre-burst/persistent emission or the Comptonization of the burst emission by the corona/boundary-layer. We find, the burst emission shows a rise until the photosphere touches down to the NS surface rather than the theoretical predicted constant Eddington luminosity. The shortage of the burst emission in the early rising phase is beyond the occlusion by the disk. We speculate that the findings above correspond to that the obscured part (not only the lower part) of the NS surface is exposed to the line of sight due to the evaporation of the obscured material by the burst emission, or the burst emission is anisotropic ($\xi>1$) in the burst early phase. In addition, based on the average flux of PRE bursts at their touch-down time, we derive a distance estimation as 10.4 kpc., Comment: arXiv admin note: substantial text overlap with arXiv:2208.13556; text overlap with arXiv:2208.12124

A type-I burst could influence the accretion process through radiation pressure and Comptonization both for the accretion disk and the corona/boundary layer of an X-ray binary, and vice versa. We investigate the temporal evolution of a bright photospheric radius expansion (PRE) burst of 4U 1608-52 detected by Insight-HXMT in 1-50 keV, with the aim of studying the interplay between the burst and persistent emission. Apart from the emission from the neutron star (NS) surface, we find the residuals both in the soft (<3 keV) and hard (>10 keV) X-ray band. Time-resolved spectroscopy reveals that the excess can be attributed to an enhanced pre-burst/persistent emission or the Comptonization of the burst emission by the corona/boundary layer. The Comptonization model is a convolution thermal-Comptonization model (thcomp in XSPEC) and the Comptonization parameters are fixed at the values derived from the persistent emission. We find, during the PRE phase, after the enhanced pre-burst/persistent emission or the Comptonization of the burst emission is removed, the NS surface emission shows a plateau, and then a rise until the photosphere touches down to the NS surface, resulting in a flux peak at that moment. We speculate that the findings above correspond to that the obscured lower part of the NS surface by the disk is exposed to the line of sight due to the inner disk evaporation by the burst emission. The consistency between the fa model and convolution thermal-Comptonization model indicates the interplay between thermonuclear bursts and accretion environments. These phenomena did not usually show up by the conventional blackbody model fitting, which may be due to low count rate and narrow energy coverage in previous observations., Comment: ApJ in press. arXiv admin note: text overlap with arXiv:2208.12124, arXiv:1910.08220

The detection of cyclotron resonance scattering features (CRSFs) is the only way to directly and reliably measure the magnetic field near the surface of a neutron star (NS). The broad energy coverage and large collection area of \emph{Insight}-HXMT in the hard X-ray band allowed us to detect the CRSF with the highest energy known to date, reaching about 146 keV during the 2017 outburst of the first galactic pulsing ultraluminous X-ray source (pULX) Swift J0243.6+6124. During this outburst, the CRSF was only prominent close to the peak luminosity $\sim 2\times10^{39}$ erg s$^{-1}$, the highest to date in any of the Galactic pulsars. The CRSF is most significant in the spin phase region corresponding to the main pulse of the pulse profile, and its centroid energy evolves with phase from 120 to 146 keV. We identify this feature as the fundamental CRSF, since no spectral feature exists at $60-70$ keV. This is the first unambiguous detection of an electron CRSF from an ULX. We also estimate a surface magnetic field $\sim1.6\times10^{13}$ G for Swift J0243.6+6124. Considering that the dipole magnetic field strengths, inferred from several independent estimates of magnetosphere radius, are at least an order of magnitude lower than our measurement, we argue that the detection of the highest energy CRSF reported here unambiguously proves the presence of multipole field components close to the surface of the neutron star. Such a scenario has previously been suggested for several pulsating ULXs, including Swift J0243.6+6124, and our result represents the first direct confirmation of this scenario., Comment: Accepted for publication in ApJL

We have performed phase-resolved spectral analysis of the accreting pulsar 1A~0535+262 based on observations of Insight-HXMT during the 2020 Type-II outburst of the source. We focus on the two-dimensional dependence of the cyclotron resonance scattering features (CRSFs) along the outburst time and at different phases. The fundamental CRSF line (f-CRSF) shows different time- and phase-dependent behaviors. At higher luminosity, the phase profile of the f-CRSF energy changes from a single peak to double peaks, with the transition occurring at MJD 59185. On the contrary, the first harmonic CRSF (1-st CRSF) at $\sim$ 100 keV is only detected within a narrow phase range (0.8$-$1.0) accompanied by a shallow f-CRSF line. Based on these results, we speculate that when the source enters the supercritical regime, the higher accretion column can significantly enhance the harmonic line at a narrow phase through an "anti-pencil" beam at a higher energy band. At the same time, it will also affect the behavior of the fundamental line.

PSR B1259-63 is a $\gamma$-ray binary system, where the compact object is a pulsar. The system has an orbital period of 1236.7 days and shows peculiar $\gamma$-ray flares (in 100\,MeV--300\,GeV) after its periastron time. We analyzed the \textit{Fermi}-LAT observation of PSR B1259-63 during its latest periastron passage, as well as its previous three periastrons. The bright GeV flares started about 60 days after the periastron epoch in 2021. This delay is larger than that around the 2017 periastron and much larger than earlier periastrons. The delay of the GeV flux peak time in each periastron passage is apparent in our results. We discussed the possible origin of this delay and made a prediction of the GeV flux peak time in next periastron passage, based on observation of the previous delays., Comment: accepted for publication in Universe

Accretion is an essential physical process in black-hole X-ray binaries (BHXRBs) and active galactic nuclei. The properties of accretion flows and their radiation were originally considered to be uniquely determined by the mass accretion rate of the disk; however, the ``hysteresis effect'' observed during outbursts of nearly all BHXRBs seriously challenges this paradigm. The hysteresis effect is referred to that the hard-to-soft state transition in the fast-rise stage occurs at much higher luminosity than the soft-to-hard state transition in the slow-decay stage. That is, the same source can show different spectral/temporal properties at the same luminosity. Phenomenologically, this effect is also represented as the so-called ``q''-shaped hardness-intensity diagram, which has been proposed as a unified scene for BHXRBs. However, there is still a lack of quantitative theoretical interpretation and observational understanding on the ``q''-diagram. Here, we present a detailed time-lag analysis of a recently found BHXRB, MAXI J1348-630, intensively monitored by Insight-HXMT over a broad energy band (1-150 keV). We find the first observational evidence that the observed time-lag between radiations of the accretion disk and the corona leads naturally to the hysteresis effect and the ``q''-diagram. Moreover, complemented by the quasi-simultaneous Swift data, we achieve a panorama of the accretion flow: the hard X-ray outburst from the corona heats and subsequently induces the optical brightening in the outer disk with nearly no lag; thereafter, the enhanced accretion in the outer disk propagates inward, generating the delayed soft X-ray outburst at the viscous timescale of ~ 8-12 days., Comment: 8 pages, 7 figures, published in ApJL

We present the observational results from a detailed timing analysis of the black hole candidate EXO 1846-031 during its outburst in 2019 with the observations of Insight-HXMT, NICER and MAXI. This outburst can be classfied roughly into four different states. Type-C quasi-periodic oscillations (QPOs) observed by NICER (about 0.1-6Hz) and Insight-HXMT (about 0.7-8Hz) are also reported in this work. Meanwhile, we study various physical quantities related to QPO frequency.The QPO rms-frequency relationship in three energy band 1-10 keV indicates that there is a turning pointing in frequency around 2 Hz,which is similar to that of GRS 1915+105. A possible hypothesis for the relationship above may be related to the inclination of the source, which may require a high inclination to explain it. The relationships between QPO frequency and QPO rms,hardness,total fractional rms and count rate have also been found in other transient sources, which can indicate that the origin of type-C QPOs is non-thermal.

Low-frequency quasi-periodic oscillations (LFQPOs) are commonly found in black hole X-ray binaries, and their origin is still under debate. The properties of LFQPOs at high energies (above 30 keV) are closely related to the nature of the accretion flow in the innermost regions, and thus play a crucial role in critically testing various theoretical models. The Hard X-ray Modulation Telescope (Insight-HXMT) is capable of detecting emissions above 30 keV, and is therefore an ideal instrument to do so. Here we report the discovery of LFQPOs above 200 keV in the new black hole MAXI J1820+070 in the X-ray hard state, which allows us to understand the behaviours of LFQPOs at hundreds of kiloelectronvolts. The phase lag of the LFQPO is constant around zero below 30 keV, and becomes a soft lag (that is, the high-energy photons arrive first) above 30 keV. The soft lag gradually increases with energy and reaches ~0.9s in the 150-200 keV band. The detection at energies above 200 keV, the large soft lag and the energy-related behaviors of the LFQPO pose a great challenge for most currently existing models, but suggest that the LFQPO probably originates from the precession of a small-scale jet., Comment: 36 pages, 12 figures, 3 tables; published in Nature Astronomy

Accurate background estimation is essential for spectral and temporal analysis in astrophysics. In this work, we construct the in-orbit background model for the High-Energy Telescope (HE) of the Hard X-ray Modulation Telescope (dubbed as Insight-HXMT). Based on the two-year blank sky observations of Insight-HXMT/HE, we first investigate the basic properties of the background and find that both the background spectral shape and intensity have long-term evolution at different geographical sites. The entire earth globe is then divided into small grids, each with a typical area of 5x5 square degrees in geographical coordinate system. For each grid, an empirical function is used to describe the long-term evolution of each channel of the background spectrum; the intensity of the background can be variable and a modification factor is introduced to account for this variability by measuring the contemporary flux of the blind detector. For a given pointing observation, the background model is accomplished by integrating over the grids that are passed by the track of the satellite in each orbit. Such a background model is tested with both the blank sky observations and campaigns for observations of a series of celestial sources. The results show an average systematic error of 1.5% for the background energy spectrum (26-100 keV) under a typical exposure of 8 ks, and <3% for background light curve estimation (30-150 keV). Therefore, the background model introduced in this paper is included in the Insight-HXMT software as a standard part specialized for both spectral and temporal analyses., Comment: 27 pages, 16 figures, accepted by Journal of High Energy Astrophysics

With more than 150 blank sky observations at high Galactic latitude, we make a systematic study to the background of the Low Energy Telescope (LE) of the Hard X-ray Modulation Telescope (dubbed as Insight-HXMT). Both the on-ground simulation and the in-orbit observation indicate that the background spectrum mainly has two components. One is the particle background that dominates above 7 keV and its spectral shape is consistent in every geographical locations. Another is the diffuse X-ray background that dominates below 7 keV and has a stable spectrum less dependent of the sky region. The particle background spectral shape can be obtained from the blind detector data of all the blank sky observations, and the particle background intensity can be measured by the blind detector at 10-12.5 keV. The diffuse X-ray background in the high Galactic latitude can also be obtained from the blank sky spectra after subtracting the particle background. Based on these characteristics, we develop the background model for both the spectrum and the light curve. The systematic error for the background spectrum is investigated with different exposures (T_exp). For the spectrum with T_exp=1 ks, the average systematic errors in 1-7 keV and 1-10 keV are 4.2% and 3.7%, respectively. We also perform the systematic error analyses of the background light curves with different energy bands and time bins. The results show that the systematic errors for the light curves with different time bins are <8% in 1-10 keV., Comment: 24 pages, 17 figures, accepted by Journal of High Energy Astrophysics

The Medium Energy X-ray Telescope (ME) is one of the main payloads of the Hard X-ray Modulation Telescope (dubbed as Insight-HXMT). The background of Insight-HXMT/ME is mainly caused by the environmental charged particles and the background intensity is modulated remarkably by the geomagnetic field, as well as the geographical location. At the same geographical location, the background spectral shape is stable but the intensity varies with the level of the environmental charged particles. In this paper, we develop a model to estimate the ME background based on the ME database that is established with the two-year blank sky observations of the high Galactic latitude. In this model, the entire geographical area covered by Insight-HXMT is divided into grids of $5^{\circ}\times5^{\circ}$ in geographical coordinate system. For each grid, the background spectral shape can be obtained from the background database and the intensity can be corrected by the contemporary count rate of the blind FOV detectors. Thus the background spectrum can be obtained by accumulating the background of all the grids passed by Insight-HXMT during the effective observational time. The model test with the blank sky observations shows that the systematic error of the background estimation in $8.9-44.0$ keV is $\sim1.3\%$ for a pointing observation with an average exposure $\sim5.5$ ks. We also find that the systematic error is anti-correlated with the exposure, which indicates the systematic error is partly contributed by the statistical error of count rate measured by the blind FOV detectors., Comment: to be published in Journal of High Energy Astrophysics

The Hard X-ray Modulation Telescope (HXMT) named Insight is China's first X-ray astronomical satellite. The Low Energy X-ray Telescope (LE) is one of its main payloads onboard. The detectors of LE adopt swept charge device CCD236 with L-shaped transfer electrodes. Charges in detection area are read out continuously along specific paths, which leads to a time response distribution of photons readout time. We designed a long exposure readout mode to measure the time response distribution. In this mode, CCD236 firstly performs exposure without readout, then all charges generated in preceding exposure phase are read out completely. Through analysis of the photons readout time in this mode, we obtained the probability distribution of photons readout time.

LS 5039 is a well-known $\gamma$-ray binary system which consists of an unknown compact object and a massive companion O star. It shows rather stable emissions at high energies over years and hence serves as an ideal laboratory to investigate the emission mechanism for such peculiar systems which emit prominent $\gamma$-rays. To this end, we take the orbital phase resolved energy spectrum as observed by \fermi over 10 years. We divide the orbit into four orbital phases, each with an orbital phase range of 0.25, centered at 0.00, 0.25, 0.50 and 0.75 respectively, where the phase 0.0 is the periastron and phase 0.5 is the apastron. The phases around 0.25 and 0.75 are symmetric and hence are supposed to have identical local acceleration environment. The spectral analysis shows that, the \fermi spectra are largely different from these two symmetric orbital phases: the emission from orbital phase 0.25 turns out to be significantly stronger than that from 0.75. This result does not fit a scenario that $\gamma$-rays are Doppler boosted emission from bow shock tails if LS 5039 has a shock configuration similar to PSR B1259-63, and indicates that the inverse Compton scatterings between the shock accelerated plasma and the stellar particle environment is the underline procedure. We also find that the previous report for a disappearance of the orbital modulation at 3--20 GeV is due to the similar spectral turn-over energies of the different orbital phases. The spectral properties of periastron and apastron regions are addressed in the context of the measurements in phase regions around 0.25 and 0.75., Comment: 7 pages, 3 figures. Published in RAA

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

PSR B1259-63 is a $\gamma$-ray emitting high mass X-ray binary system, in which the compact object is a millisecond pulsar. The system has an orbital period of 1236.7 d and shows peculiar $\gamma$-ray flares when the neutron star moves out of the stellar disk of the companion star. The $\gamma$-ray flare events were firstly discovered by using Fermi-LAT around the 2010 periastron passage, which was repeated for the 2014 and 2017 periastron passages. We analyze the Fermi-LAT data for all the three periastron passages and found that in each flare the energy spectrum can be represented well by a simple power law. The $\gamma$-ray light curves show that in 2010 and 2014 after each periastron there are two main flares, but in 2017 there are four flares including one precursor about 10 d after the periastron passage. The first main flares in 2010 and 2014 are located at around 35 d after the periastron passage, and the main flare in 2014 is delayed by roughly 1.7 d with respect to that in 2010. In the 2017 flare, the source shows a precursor about 10 d after the periastron passage, but the following two flares become weaker and lag behind those in 2010 by roughly 5 d. The strongest flares in 2017 occurred 58 d and 70 d after the periastron passage. These results challenge the previous models., Comment: 8 pages, 3 figures, accepted for publication in RAA

We report the discovery of an anti-correlation between the soft and the hard X-ray lightcurves of the X-ray binary Aql X-1 when bursting. This behavior may indicate that the corona is cooled by the soft X-ray shower fed by the type-I X-ray bursts, and that this process happens within a few seconds. Stacking the Aql X-1 lightcurves of type-I bursts, we find a shortage in the 40--50 keV band, delayed by 4.5$\pm$1.4 s with respect to the soft X-rays. The photospheric radius expansion (PRE) bursts are different in that neither a shortage nor an excess shows up in the hard X-ray lightcurve., Comment: accepted by ApJL

The coronae in X-ray binaries (XRBs) still remain poorly understood, although they have been believed for a long time to play a key role in modeling the characteristic outbursts of XRBs. Type-I X-ray bursts, the thermonuclear flashes happening on the surface of a neutron star (NS), can be used as a probe to the innermost region of a NS XRB, where the corona is believed to be located very close to the NS. We report the discovery of a tiny life cycle of the corona that is promptly co-evolved with the type-I bursts superimposed on the outburst of the NS XRB IGR J17473$-$2721. This finding may serve as the first evidence of directly seeing the rapid disappearance and formation of a corona in an XRB with a cooling/heating timescale of less than a second, which can strongly constrain the accretion models in XRBs at work., Comment: The Astrophysical Journal, 2012, 752, L34

The 2008 outburst of the atoll source IGR J17473--2721 was observed by INTEGRAL, RXTE and Swift. Tens of type-I X-ray bursts were found in this outburst. Joint observations provide sufficient data to look into the behavior of IGR J17473--2721 at the rising part of the outburst. We find that the joint energy spectra can be well fitted with a model composed of a blackbody and a cutoff power-law, with a cutoff energy decreasing from $ \sim$ 150 keV to $\sim$ 40 keV as the source leaves the quiescent state toward the low hard state. This fits into a scenario in which the corona is cooled by the soft X-rays along the outburst evolution, as observed in several other atoll sources. By using the flux measured in the 1.5--30 keV band of the type-I bursts during the outburst, we find that the linear relationship between the burst duration and the flux still holds for those bursts that occur at the decaying part of the low hard state, but with a different slope than the overall one that was estimated with the bursts happening in the whole extent of, and for the rest of the low hard state. The significance of such a dichotomy in the type-I X-ray bursts is $\sim$ 3 $\sigma$ under an F-test. Similar results are hinted at as well with the broader energy-band that was adopted recently. This dichotomy may be understood in a scenario where part of the accreting material forms a corona on the way of falling onto the surface of the neutron star during the decaying part of the low hard state. Based on the accretion rates of the preceding LHS, estimated from type-I X-ray bursts and from persistent emission, at least for IGR J17473-2721, most of the accretion material may fall on the neutron star (NS) surface in the LHS. Considering the burst behavior in the context of the outburst indicates a corona formed on top of the disk rather than on the NS surface., Comment: accepted by A&A

Two outbursts were observed by RXTE in the history of the atoll source IGR J17473-2721. During the most recent outburst in 2008, the source showed a complete series of spectral states/transitions. The neutron star system was prolific in type-I X-ray bursts, and we investigate them in the context of complete outbursts evolution. A total exposure of ~ 309 ks was collected by RXTE during the two outbursts of IGR J17473-2721. We carried out a systematic search for type-I bursts in this data set. For each burst found, we investigated the burst profile, the peak flux, and their dependence on the accretion rate along the evolution of the outbursts. Eighteen type-I X-ray bursts were found from IGR J17473-2721: two from the outburst in 2005 and the other 16 from the recent outburst in 2008. Among them, 3 bursts show photospheric radius expansion (PRE). The distance to the source is estimated as 6.4 kpc with a 15% uncertainty based on the three bursts that show PRE. In the recent outburst, there are 6 bursts showing up in the low/hard state prior to the state transition to a high/soft state, 3 bursts at the end phase of the high/soft state, and 7 in the following low/hard state. The blackbody radius of these bursts presents a variety of interesting features. We find that at the end of the recent outburst, the profile of the blackbody radius is anti-correlated with the blackbody temperature and the burst flux. The durations of the type-I burst are found to correlate with the Eddington ratio and to have two parallel evolution groups. Along the decreasing Eddington ratio, the burst duration decreases and ends in each group the PRE bursts occurred. This provides new clues to the type-I bursts in the context of outbursts for atoll XRBs., Comment: in press at A & A