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Shuang-Nan Zhang

Publications and source records attributed to Shuang-Nan Zhang.

At least 127 records · Page 7Linked to original sources

Future Perspectives for Gamma-ray Burst Detection from Space

Since their first discovery in the late 1960s, Gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early Universe by unveiling first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs have stimulated in the past $\sim$50 years the development of cutting-edge technological instruments for observations of high energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data taking mode nowadays). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and associated phenomenology.

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Evidence for a nearly orthogonal rotator in GX 301--2 with phase-resolved cyclotron resonant scattering features

Cyclotron resonant scattering features (CRSFs) are the absorption features in the X-ray spectra of strongly magnetized accretion neutron stars (NSs), which are probably the most reliable probe to the surface magnetic fields of NSs. The high mass X-ray binary GX 301--2 exhibits a very wide, variable and complicated CRSF in the average spectra, which should be two absorption lines based on NuStar and Insight-HXMT observations. With the Insight-HXMT frequent observations, we performed the phase-resolved spectroscopy and confirmed two cyclotron absorption lines in the phase-resolved spectra, with their centroid energy ratio $\sim 1.6-1.7$ in the super-critical luminosity case. A major hindrance in understanding those CRSFs is the very poorly constrained magnetic inclination angle, which is also a fundamental property of a NS and key to understanding the emission characteristics of a pulsar. Comparing the phase-resolved CRSF with simulated X-ray spectra, the magnetic inclination angle is found to be $\gtrsim 70^{\circ}$, i.e., nearly orthogonal between the NS's spin and magnetic axies. The implications of an orthogonal rotator and magnetic structure evolution in the accreting X-ray binary are also discussed.

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The influence of thermonuclear bursts on polar caps of accreting X-ray millisecond pulsar MAXI J1816-195

We report accretion-powered pulsations for the first time during thermonuclear bursts in hard X-rays, which were observed with Insight-HXMT in 2022 during the outburst of the accreting X-ray millisecond pulsar MAXI J1816-195. By stacking 73 bursts, we detected pulse profiles in 8-30 keV and 30-100 keV during bursts, which are identical to those obtained from the persistent (non-burst) emission. On average, no significant phase lag was observed between burst and persistent pulse profiles. In addition, we suggest that the interaction with burst photons can be used as a direct diagnostic to distinguish contributions from the hot plasma near polar caps and the corona around the accretion disk, which are highly degenerate in their spectral shapes.

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Timing and Spectral Analysis of HMXB 4U 1700-37 Observed with Insight-HXMT

We report timing and spectral studies of the high mass X-ray binary 4U 1700-37 using Insight-HXMT observations carried out in 2020 during its out-of-eclipse state. We found significant variations in flux on a time-scale of kilo-seconds, while the hardness (count rate ratio between 10-30 keV and 2-10 keV) remains relatively stable. No evident pulsations were found over a frequency range of $10^{-3}$-2000 Hz. During the spectral analysis, for the first time we took the configuration of different Insight-HXMT detectors' orientations into account, which allows us obtaining reliable results even if a stable contamination exists in the field-of-view. We found that the spectrum could be well described by some phenomenological models that commonly used in accreting pulsars (e.g., a power law with a high energy cutoff) in the energy range of 2-100 keV. We found hints of cyclotron absorption features around ~ 16 keV or/and ~ 50 keV.

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A Monte Carlo Simulation of the Broad Band X-ray Emission of the Accreting Millisecond X-ray pulsar MAXI J1816-195

MAXI J1816-195 is an accreting millisecond X-ray pulsar (AMXP) discovered in 2022. According to the Insight-HXMT data, the pulsations of this source extend all the way to over 100 keV, and its pulse profiles change from a single peak in low-energy range to double peaks in high-energy range. In this work, we simulate its energy spectra and pulse profiles with a Compton scattering Monte Carlo program. The simulation results suggest that the low energy X-ray source on the neutron star surface should be pencil-beamed radiations from the magnetic poles, and there should be a boundary layer in a hollow cylinder shape between the accretion disc and the neutron star surface: the up-scattering of the polar radiations in the boundary layer leads to the double peak structure of the high-energy pulse profile. Under this boundary layer geometry, we suggest that the rarity of AMXPs can be caused by the smearing of the boundary layer. To estimate the mass M and radius R of accretion-powered millisecond pulsars whose surface radiations are badly polluted by the accretion disk and boundary layer, the impact of Compton scattering in the boundary layer on the radiation should be removed before employing the X-ray pulse profile modeling method.

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Radiation Effects on Scientific CMOS Detectors for X-ray Astronomy: II. Total Ionizing Dose Irradiation

Complementary metal-oxide-semiconductor (CMOS) detectors are a competitive choice for current and upcoming astronomical missions. To understand the performance variations of CMOS detectors in space environment, we investigate the total ionizing dose effects on custom-made large-format X-ray CMOS detectors. Three CMOS detector samples were irradiated with a Co-60 source with a total dose of 70 krad and 105 krad. We test and compare the performance of these detectors before and after irradiation. After irradiation, the dark current increases by roughly 20 to 100 times, and the readout noise increases from 3 e- to 6 e-. The bias level at 50 ms integration time decreases by 13 to 18 Digital Number (DN) at -30 degree. The energy resolution increases from about 150 eV to about 170 eV at 4.5 keV at -30 degree. The conversion gain of the detectors varies for less than 2% after the irradiation. Furthermore, there are about 50 pixels whose bias at 50 ms has changed by more than 20 DN after the exposure to the radiation and about 30 to 140 pixels whose readout noise has increased by over 20 e- at -30 degree at 50 ms integration time. These results demonstrate that the performances of large-format CMOS detectors do not suffer significant degeneration in space environment.

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Timing analysis of the newly discovered black hole candidate Swift J1727.8-1613 with Insight-HXMT

We present the results obtained from an X-ray timing study of the new black hole candidate (BHC) Swift J1727.8-1613. The work is based on Hard X-ray Modulation Telescope (Insight-HXMT) observations carried out during the 2023 outburst. Prominent type-C low-frequency Quasi-periodic Oscillations (LFQPOs) are detected throughout the observations. With the substantial effective area of the Insight-HXMT at high energies, we examine the energy dependence of various parameters, including the centroid frequency, fractional rms, and phase lags of the type-C QPOs. Our findings align closely with those observed in high-inclination systems. During the initial stage of the outburst, a peaked noise component is also detected, the frequency of which is highly correlated with the LFQPO frequency, aligning with the Psaltis-Belloni-van der Klis (PBK) relation. By assuming that the peaked noise originates from the precession of the accretion disc, the spin of this source can be constrained. Our results suggest that this source may possess a high spin.

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Simulation Studies for the First Pathfinder of the CATCH Space Mission

The Chasing All Transients Constellation Hunters (CATCH) space mission is an intelligent constellation consisting of 126 micro-satellites in three types (A, B, and C), designed for X-ray observation with the objective of studying the dynamic universe. Currently, we are actively developing the first Pathfinder (CATCH-1) for the CATCH mission, specifically for type-A satellites. CATCH-1 is equipped with Micro Pore Optics (MPO) and a 4-pixel Silicon Drift Detector (SDD) array. To assess its scientific performance, including the effective area of the optical system, on-orbit background, and telescope sensitivity, we employ the Monte Carlo software Geant4 for simulation in this study. The MPO optics exhibit an effective area of $41$ cm$^2$ at the focal spot for 1 keV X-rays, while the entire telescope system achieves an effective area of $29$ cm$^2$ at 1 keV when taking into account the SDD detector's detection efficiency. The primary contribution to the background is found to be from the Cosmic X-ray Background. Assuming a 625 km orbit with an inclination of $29^\circ$, the total background for CATCH-1 is estimated to be $8.13\times10^{-2}$ counts s$^{-1}$ in the energy range of 0.5--4 keV. Based on the background within the central detector and assuming a Crab-like source spectrum, the estimated ideal sensitivity could achieve $1.9\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ for an exposure of 10$^4$ s in the energy band of 0.5--4 keV. Furthermore, after simulating the background caused by low-energy charged particles near the geomagnetic equator, we have determined that there is no need to install a magnetic deflector.

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Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

The Imaging X-ray Polarimetry Explorer (IXPE) measured with high significance the X-ray polarization of the brightest Z-source Scorpius X-1, resulting in the nominal 2-8 keV energy band in a polarization degree of 1.0(0.2)% and a polarization angle of 8(6)° at 90% of confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broad-band spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasi-periodic oscillations. From a spectro-polarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% of confidence level, compatible with expectations for an electron-scattering dominated optically thick atmosphere at the Sco X-1 inclination of 44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3(0.4)%, in agreement with expectations for a slab of Thomson optical depth of ~7 and an electron temperature of ~3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or to a change in the corona geometry with the accretion flow.

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Studies on the soft intermediate state X-ray flare of MAXI J1535-571 during its 2017 outburst

We analyzed an observation with the Nuclear Spectroscopic Telescope Array of the black-hole X-ray binary MAXI J1535-571 in the soft intermediate state, in which we detected a 2.5-ks long flare. Our spectral fitting results suggest that MAXI J1535-571 possesses a high spin of 0.97 (-0.10/+0.02) and a low inclination of approximately 24 deg. We observed a gradual increase in the inner disc radius, as determined from fits to the continuum spectrum. This trend is inconsistent with an increased flux ratio of the thermal component, as well as the source evolving towards the soft state. This inconsistency may be attributed to a gradual decrease of the color correction factor. Additionally, with a flare velocity of approximately 0.5 c and a higher hardness ratio during the flare period, the quasi-simultaneous detection of a type-B QPO in the Neutron Star Interior Composition Explorer data, and quasi-simultaneous ejecta launch through radio observations collectively provide strong evidence supporting the possibility that the flare originated from a discrete jet ejection.

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Insight-HXMT observations of thermonuclear X-ray bursts in 4U 1636-53

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.

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Observation of GRB 221009A early afterglow in X/$γ$-ray energy band

The early afterglow of a Gamma-ray burst (GRB) can provide critical information on the jet and progenitor of the GRB. The extreme brightness of GRB 221009A allows us to probe its early afterglow in unprecedented detail. In this letter, we report comprehensive observation results of the early afterglow of GRB 221009A (from $T_0$+660 s to $T_0$+1860 s, where $T_0$ is the \textit{Insight}-HXMT/HE trigger time) in X/$γ$-ray energy band (from 20 keV to 20 MeV) by \textit{Insight}-HXMT/HE, GECAM-C and \textit{Fermi}/GBM. We find that the spectrum of the early afterglow in 20 keV-20 MeV could be well described by a cutoff power-law with an extra power-law which dominates the low and high energy bands respectively. The cutoff power-law $E_{\rm peak}$ is $\sim$ 30 keV and the power-law photon index is $\sim$ 1.8 throughout the early afterglow phase. By fitting the light curves in different energy bands, we find that a significant achromatic break (from keV to TeV) is required at $T_0$ + 1246$^{+27}_{-26}$ s (i.e. 1021 s since the afterglow starting time $T_{\rm AG}$=$T_0$+225 s), providing compelling evidence of a jet break. Interestingly, both the pre-break and post-break decay slopes vary with energy, and these two slopes become closer in the lower energy band, making the break less identifiable. Intriguingly, the spectrum of the early afterglow experienced a slight hardening before the break and a softening after the break. These results provide new insights into the understanding of this remarkable GRB.

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The Self-organized Criticality Behaviors of Two New Parameters in SGR J1935+2154

The minimum variation timescale (MVT) and spectral lag of hundreds of X-ray bursts (XRBs) from soft gamma-ray repeater (SGR) J1935+2154 were analyzed in detail for the first time in our recent work, which are important probes for studying the physical mechanism and radiation region. In this work, we investigate their differential and cumulative distributions carefully and find that they follow power-law models. Besides, the distributions of fluctuations in both parameters follow the Tsallis $q$-Gaussian distributions and the $q$ values are consistent for different scale intervals. Therefore, these results indicate that both parameters are scale-invariant, which provides new parameters for the study of self-organized criticality systems. Interestingly, we find that the $q$ values for MVT and spectral lag are similar with duration and fluence, respectively.

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RMS-flux slope in MAXI J1820+070: a measure of the disk-corona coupling

Linear RMS-flux relation has been well established in different spectral states of all accreting systems. In this work, we study the evolution of the frequency-dependent RMS-flux relation of MAXI J1820+070 during the initial decaying phase of the 2018 outburst with Insight-HXMT over a broad energy range 1-150 keV. As the flux decreases, we first observe a linear RMS-flux relation at frequencies from 2 mHz to 10 Hz, while such a relation breaks at varying times for different energies, leading to a substantial reduction in the slope. Moreover, we find that the low-frequency variability exhibits the highest sensitivity to the break, which occurs prior to the hard-to-hard state transition time determined through time-averaged spectroscopy, and the time deviation increases with energy. The overall evolution of the RMS-flux slope and intercept suggests the presence of a two-component Comptonization system. One component is radially extended, explaining the strong disk-corona coupling before the break, while the other component extends vertically, contributing to the reduction of the disk-corona coupling after the break. A further vertical expansion of the latter component is required to accommodate the dynamic evolution observed in the RMS-flux slope. In conclusion, we suggest that the RMS-flux slope in 1-150 keV band can be employed as an indicator of the disk-corona coupling and the hard-to-hard state transition in MAXI J1820+070 could be partially driven by the changes in the corona geometry.

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Broadband X-ray timing and spectral characteristics of the accretion-powered millisecond X-ray pulsar MAXI J1816$-$195

We studied the broadband X-ray timing and spectral behaviors of the newly confirmed accreting millisecond X-ray pulsar MAXI J1816$-$195 during its 2022 outburst. We used the data from Insight-HXMT ME/HE, NICER and NuSTAR which cover the energy range between 0.8$-$210 keV. A coherent timing analysis of solely Insight-HXMT HE data across the full outburst revealed a complex behavior of the timing residuals, also prominently visible in independent Insight-HXMT ME and NICER data, particularly at rising part of the outburst and at the very end in NICER data. Therefore, we broke down the full outburst into a (noisy) rising part, covering only about five days from MJD 59737.0 to 59741.9, and a decaying part lasting for 19 days across MJD 59741.9$-$59760.6. Fitting for the decaying part a timing model including a frequency $ν$ and frequency time derivative $\dotν$ component yielded a value of $(+9.0\pm2.1)\times10^{-14}~{\rm Hz~s^{-1}}$ for $\dotν$, which could be interpreted as a spin-up under our model assumptions. We detected the X-ray pulsations up to $\sim$95 keV in a combination of Insight-HXMT HE observations. The pulse profiles were quite stable over the whole outburst and could be well described by a truncated Fourier series using two harmonics, the fundamental and the first overtone. Both components kept alignment in the range 0.8$-$64 keV. The joint and time-averaged NICER and Insight-HXMT spectra in the energy range 1$-$150 keV were well fitted by the absorbed Comptonization model compps plus disk blackbody with two additional Gaussian components. Using the bolometric flux and spin-up values both evaluated during the decay phase, we determined a magnetic field strength of $(0.2-2)\times10^8$ G for MAXI J1816$-$195.

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Rapid dimming followed by a state transition: a study of the highly variable nuclear transient AT 2019avd over 1000+ days

The tidal disruption of a star around a supermassive black hole (SMBH) offers a unique opportunity to study accretion onto a SMBH on a human-timescale. We present results from our 1000+ days NICER, Swift and Chandra monitoring campaign of AT 2019avd, a nuclear transient with TDE-like properties. Our primary finding is that approximately 225 days following the peak of X-ray emission, there is a rapid drop in luminosity exceeding two orders of magnitude. This X-ray drop-off is accompanied by X-ray spectral hardening, followed by a 740-day plateau phase. During this phase, the spectral index decreases from 6.2+-1.1 to 2.3+-0.4, while the disk temperature remains constant. Additionally, we detect pronounced X-ray variability, with an average fractional root mean squared amplitude of 47%, manifesting over timescales of a few dozen minutes. We propose that this phenomenon may be attributed to intervening clumpy outflows. The overall properties of AT 2019avd suggest that the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity state, possibly associated with a compact jet. This evolution follows a pattern in the hardness-intensity diagram similar to that observed in stellar-mass black holes, supporting the mass invariance of accretion-ejection processes around black holes.

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Radiation effects on scientific CMOS sensors for X-ray astronomy: I. proton irradiation

Complementary metal-oxide-semiconductor (CMOS) sensors are a competitive choice for future X-ray astronomy missions. Typically, CMOS sensors on space astronomical telescopes are exposed to a high dose of irradiation. We investigate the impact of irradiation on the performance of two scientific CMOS (sCMOS) sensors between -30 to 20 degree at high gain mode (7.5 times), including the bias map, readout noise, dark current, conversion gain, and energy resolution. The two sensors are irradiated with 50 MeV protons with a total dose of 5.3*10^10 p/cm^2. After the exposure, the bias map, readout noise and conversion gain at various temperatures are not significantly degraded, nor is the energy resolution at -30 degree. However, after the exposure the dark current has increased by hundreds of times, and for every 20 degree increase in temperature, the dark current also increases by an order of magnitude. Therefore, at room temperature, the fluctuations of the dark currents dominate the noise and lead to a serious degradation of the energy resolution. Moreover, among the 4k * 4k pixels, there are about 100 pixels whose bias at 50 ms has changed by more than 10 DN (~18 e-), and about 10 pixels whose readout noise has increased by over 15 e- at -30 degree. Fortunately, the influence of the dark current can be reduced by decreasing the integration time, and the degraded pixels can be masked by regular analysis of the dark images. Some future X-ray missions will likely operate at -30 degree, under which the dark current is too small to significantly affect the X-ray performance. Our investigations show the high tolerance of the sCMOS sensors for proton radiation and prove their suitability for X-ray astronomy applications.

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An Aluminum-coated sCMOS sensor for X-Ray Astronomy

In recent years, tremendous progress has been made on scientific Complementary Metal Oxide Semiconductor (sCMOS) sensors, making them a promising device for future space X-ray missions. We have customized a large-format sCMOS sensor, G1516BI, dedicated for X-ray applications. In this work, a 200 nm thick aluminum layer is successfully sputtered on the surface of this sensor. This Al-coated sensor, named EP4K, shows consistent performance with the uncoated version. The readout noise of the EP4K sensor is around 2.5 e- and the dark current is less than 0.01 e-/pixel/s at -30 degree. The maximum frame rate is 20 Hz in the current design. The ratio of single pixel events of the sensor is 45.0%. The energy resolution can reach 153.2 eV at 4.51 keV and 174.2 eV at 5.90 keV at -30 degree. The optical transmittance of the aluminum layer is approximately 1e-8 to 1e-10 for optical lights from 365 to 880 nm, corresponding to an effective aluminum thickness of around 140 to 160 nm. The good X-ray performance and low optical transmittance of this Al-coated sCMOS sensor make it a good choice for space X-ray missions. The Lobster Eye Imager for Astronomy (LEIA), which has been working in orbit for about one year, is equipped with four pieces of EP4K sensors. Furthermore, 48 pieces of EP4K sensors are used on the Wide-field X-ray Telescope (WXT) on the Einstein Probe (EP) satellite, which will be launched at the end of 2023.

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