arXiv subjects

Shuang-Nan Zhang

Publications and source records attributed to Shuang-Nan Zhang.

At least 199 records · Page 11Linked to original sources

arXiv2020

Discovery of oscillations above 200 keV in a black hole X-ray binary with Insight-HXMT

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.

astro-ph.HE↗

arXiv2020

The POLAR Gamma-Ray Burst Polarization Catalog

Despite over 50 years of research, many open questions remain about the origin and nature of GRBs. Polarization measurements of the prompt emission of these extreme phenomena have long been thought to be the key to answering a range of these questions. The POLAR detector was designed to produce the first set of detailed and reliable polarization measurements in an energy range of approximately 50-500 keV. During late 2016 and early 2017, POLAR detected a total of 55 GRBs. Analysis results of 5 of these GRBs have been reported in the past. The results were found to be consistent with a low or unpolarized flux. However, previous reports by other collaborations found high levels of polarization. We study the polarization for all the 14 GRBs observed by POLAR for which statistically robust inferences are possible. Additionally, time-resolved polarization studies are performed on GRBs with sufficient apparent flux. A publicly available polarization analysis tool, developed within the 3ML framework, was used to produce statistically robust results. The method allows to combine spectral and polarimetric data from POLAR with spectral data from the Fermi GBM and Swift-BAT to jointly model the spectral and polarimetric parameters. The time integrated analysis finds all results to be compatible with a low or zero polarization with the caveat that, when time-resolved analysis is possible within individual pulses, we observe moderate polarization with a rapidly changing polarization angle. Thus, time-integrated polarization results, while pointing to lower polarization are potentially an artifact of summing over the changing polarization signal and thus, washing out the true moderate polarization. Therefore, we caution against over interpretation of any time-integrated results and encourage one to wait for more detailed polarization measurements from forthcoming missions such as POLAR-2 and LEAP.

astro-ph.HE↗

arXiv2020

A peculiar cyclotron line near 16 keV detected in the 2015 outburst of 4U 0115+63?

In 2015 October, the Be/X-ray binary 4U 0115+63 underwent a type II outburst, reaching an X-ray luminosity of $\sim$ {\DP{38}\es}. During the outburst, Nuclear Spectroscopic Telescope Array ({\it NuSTAR}) performed two Target of Opportunity observations. Using the broad-band spectra from {\it NuSTAR} (3$-$79 keV), we have detected multiple cyclotron lines of the source, i.e., $\sim$ 12, 16, 22 and 33/35 keV. Obviously, the 16 keV line is not a harmonic component of the 12 keV line. As described by the phase-dependent equivalent widths of these cyclotron lines, the 16 keV and 12 keV lines are two different fundamental lines. In our work, we apply the two-poles cyclotron line model to the observation, i.e., the two line sets are formed at the same altitude ($\sim$ 0.2 km over the NS surface) of different magnetic poles, with $\sim$ {1.1\TDP{12}} and {1.4\TDP{12}} G in two poles, respectively.

astro-ph.HE↗

arXiv2020

Introducing a special collection of papers in the Journal of High Energy Astrophysics on the Early Results of China's 1st X-ray Astronomy Satellite: Insight-HXMT

Insight-HXMT is the first Chinese X-ray astronomical mission, launched successfully on June 15, 2017, from China's Jiuquan Satellite Launch Center. Insight-HXMT was designed to have a broad energy coverage in X-rays, from 1-250 keV, with excellent timing and adequate energy resolution at soft X-rays, and the largest effective area at hard X-rays. Here, we present a collection of papers of the Journal of High Energy Astrophysics on the Early Results of China's 1st X-ray Astronomy Satellite Insight-HXMT. These papers cover the in-orbit performance, the background model, and all calibration results, together with several first results on observations of binaries and details on the Galactic plane survey.

astro-ph.HE↗

arXiv2020

Background Model for the High-Energy Telescope of Insight-HXMT

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.

astro-ph.IM↗

arXiv2020

eXTP perspectives for the $ν$MSM sterile neutrino dark matter model

We discuss the potential of the eXTP X-ray telescope, in particular its Spectroscopic Focusing Array (SFA), Large Area Detector (LAD) and Wide Field Monitor (WFM) for the detection of a signal from keV-scale decaying dark matter. We show that the sensitivity of the eXTP is sufficient to improve existing constraints on the mixing angle of the neutrino Minimal extension of the Standard Model ($ν$MSM) by a factor of 5-10 within the dark matter mass range 2-50 keV, assuming 1% level of systematic uncertainty. We assert that the eXTP will be able to probe previously inaccessible range of $ν$MSM parameters and serve as a precursor for the Athena mission in decaying dark matter searches.

astro-ph.HE↗

arXiv2020

Calibration of the Instrumental Response of Insight-HXMT/HE CsI Detectors for Gamma-Ray Monitoring

The CsI detectors of the High Energy X-ray Telescope of the Hard X-ray Modulation Telescope (HXMT/CsI) can be used for gamma-ray all sky monitoring and searching for the electromagnetic counterpart of gravitational wave source. The instrumental responses are mainly obtained by Monte Carlo simulation with the Geant4 tool and the mass model of both the satellite and all the payloads, which is updated and tested with the Crab pulse emission in various incident directions. Both the Energy-Channel relationship and the energy resolution are calibrated in two working modes (Normal-Gain mode & Low-Gain Mode) with the different detection energy ranges. The simulative spectral analyses show that HXMT/CsI can constrain the spectral parameters much better in the high energy band than that in the low energy band. The joint spectral analyses are performed to ten bright GRBs observed simultaneously with HXMT/CsI and other instruments (Fermi/GBM, Swift/BAT, Konus-Wind), and the results show that the GRB flux given by HXMT/CsI is systematically higher by $7.0\pm8.8\%$ than those given by the other instruments. The HXMT/CsI-Fermi/GBM joint fittings also show that the high energy spectral parameter can be constrained much better as the HXMT/CsI data are used in the joint fittings.

astro-ph.IM↗

arXiv2020

Background Model for the Low-Energy Telescope of Insight-HXMT

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.

astro-ph.IM↗

arXiv2020

Design and Calibration of the High Energy Particle Monitor onboard the Insight-HXMT

Three high energy particle monitors (HPMs) employed onboard the Hard X-ray Modulation Telescope Insight-HXMT) can detect the charged particles from South Atlantic Anomaly (SAA) and hence provide the alert trigger for switch-on/off of the main detectors. Here a typical design of HPM with high stability and reliability is adopted by taking a plastic scintillator coupled with a small photomultiplier tube (PMT). The window threshold of HPM is designed as 1 MeV and 20 MeV for the incident electron and proton, respectively. Before the launch of Insight-HXMT, we performed in details the ground calibration of HPM. The measured energy response and its dependence on temperature are taken as essential input of Geant4 simulation for estimating the HPM count rate given with an incident particle energy spectrum. This serves as a guidance for choosing a reasonable working range of the PMT high voltage once the real SAA count rate is measured by HPM in orbit. So far the three HPMs have been working in orbit for more than two years. Apart from providing reliable alert trigger, the HPMs data are used as well to map the SAA region.

physics.ins-det↗

arXiv2020

The Background Model of the Medium Energy X-ray telescope of Insight-HXMT

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.

astro-ph.IM↗

arXiv2020

In-orbit Calibration to the Point-Spread Function of Insight-HXMT

We make the in-orbit calibration to the point-spread functions (PSFs) of the collimators of the Hard X-ray Modulation Telescope with the scanning observation of the Crab. We construct the empirical adjustments to the theoretically calculated geometrical PSFs. The adjustments contain two parts: a rotating matrix to adjust the directional deviation of the collimators and a paraboloidal function to correct the inhomogeneity of the real PSFs. The parameters of the adjusting matrices and paraboloidal functions are determined by fitting the scanning data with lower scanning speed and smaller intervals during the calibration observations. After the PSF calibration, the systematic errors in source localization in the Galactic plane scanning survey are 0.010 deg, 0.015 deg, 0.113 deg for the Low-Energy Telescope (LE), the Medium-Energy telescope (ME) and the High-Energy telescope (HE), respectively; meanwhile, the systematic errors in source flux estimation are 1.8%, 1.6%, 2.7% for LE, ME and HE, respectively.

astro-ph.IM↗

arXiv2019

Studies on the time response distribution of Insigh}-HXMT/LE

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.

astro-ph.IM↗

arXiv2019

Observation data pre-processing and scientific data products generation of POLAR

POLAR is a compact space-borne detector initially designed to measure the polarization of hard X-rays emitted from Gamma-Ray Bursts in the energy range 50-500keV. This instrument was launched successfully onboard the Chinese space laboratory Tiangong-2 (TG-2) on 2016 September 15. After being switched on a few days later, tens of gigabytes of raw detection data were produced in-orbit by POLAR and transferred to the ground every day. Before the launch date, a full pipeline and related software were designed and developed for the purpose of quickly pre-processing all the raw data from POLAR, which include both science data and engineering data, then to generate the high level scientific data products that are suitable for later science analysis. This pipeline has been successfully applied for use by the POLAR Science Data Center in the Institute of High Energy Physics (IHEP) after POLAR was launched and switched on. A detailed introduction to the pipeline and some of the core relevant algorithms are presented in this paper.

astro-ph.IM↗

arXiv2019

The effective magnetic field decay of radio pulsars: insights from the statistical properties of their spin frequency's second derivatives

We present a new method to investigate the effective magnetic field decay of isolated neutron stars, from the analysis of the long-term timing data of a large sample of radio pulsars \citep{2010MNRAS.402.1027H}. There are some differences between the distributions of frequency's second derivatives of the pulsar spins with different effective field decay timescales. Kolmogorov-Smirnov tests are performed to reexamine the consistency of distributions of the simulated and reported data for a series values of decay timescales. \textbf{We show that the timescale of the effective field decay exceeds $\sim 5~\rm {Myr}$ for pulsars with spin-down age $τ_{\rm C}< 10^{7}~{\rm yr}$ or $\sim 100~\rm {Myr}$ for pulsars with $10^{7}<τ_{\rm C}< 10^{9}~{\rm yr}$ in the sample. The result does not depend on any specific theories of the field evolution, the inclination decay or the variation in the moment of inertia. It is also found that the extent of the closed line region of the magnetic field is close to the light cylinder $r_{\rm lc}$, i.e., the corotating radius $r_{\rm c}\approx r_{\rm lc}$ is a good approximation for the observed pulsar population.

astro-ph.HE↗

arXiv2019

What can PSR J1640-4631 tell us about the internal physics of this neutron star?

Gravitational wave emissions (GWEs) of pulsars could not only make them promising targets for continuous gravitational wave searches but also leave imprints in their timing data. We interpret the measured braking index of PSR J1640-4631 with a model involving both the GWE and dipole magnetic field decay. Combining the timing data of PSR J1640-4631 and the theory of magnetic field decay, we propose a new approach of constraining the number of precession cycles, $ξ$, which is highly uncertain currently but can be tightly related to the interior physics of a neutron star and its GWE. We suggest that future observation of the tilt angle $χ$ of PSR J1640-4631 would not merely help to constrain $ξ$ but also possibly provide information about the internal magnetic field configuration of this pulsar. We find that $ξ$ would be larger than previous estimates unless a tiny angle $χ\lesssim5^\circ$ is observed. Furthermore, a measured angle $χ\gtrsim12^\circ$ would indicate $ξ\gtrsim 10^6$, which is at least ten times larger than that suggested previously.

astro-ph.HE↗

arXiv2019

Detailed polarization measurements of the prompt emission of five Gamma-Ray Bursts

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.

astro-ph.HE↗

arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

astro-ph.HE↗

arXiv2018

The Large Area Detector onboard the eXTP mission

The eXTP (enhanced X-ray Timing and Polarimetry) mission is a major project of the Chinese Academy of Sciences (CAS) and China National Space Administration (CNSA) currently performing an extended phase A study and proposed for a launch by 2025 in a low-earth orbit. The eXTP scientific payload envisages a suite of instruments (Spectroscopy Focusing Array, Polarimetry Focusing Array, Large Area Detector and Wide Field Monitor) offering unprecedented simultaneous wide-band X-ray spectral, timing and polarimetry sensitivity. A large European consortium is contributing to the eXTP study and it is expected to provide key hardware elements, including a Large Area Detector (LAD). The LAD instrument for eXTP is based on the design originally proposed for the LOFT mission within the ESA context. The eXTP/LAD envisages a deployed 3.4 m2 effective area in the 2-30 keV energy range, achieved through the technology of the large-area Silicon Drift Detectors - offering a spectral resolution of up to 200 eV FWHM at 6 keV - and of capillary plate collimators - limiting the field of view to about 1 degree. In this paper we provide an overview of the LAD instrument design, including new elements with respect to the earlier LOFT configuration.

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