arXiv subjects

Hsiang-Kuang Chang

Publications and source records attributed to Hsiang-Kuang Chang.

At least 55 records · Page 3Linked to original sources

arXiv2023

Coronal Properties of Low-Accreting AGNs using Swift, XMM-Newton and NuSTAR Observations

We studied the broadband X-ray spectra of {\it Swift}/BAT selected low-accreting AGNs using the observations from {\it XMM-Newton}, {\it Swift}, and {\it NuSTAR} in the energy range of $0.5-150$~keV. Our sample consists of 30 AGNs with Eddington ratio, $λ_{\rm Edd}<10^{-3}$. We extracted several coronal parameters from the spectral modelling, such as the photon index, hot electron plasma temperature, cutoff energy, and optical depth. We tested whether there exists any correlation/anti-correlation among different spectral parameters. We observe that the relation of hot electron temperature with the cutoff energy in the low accretion domain is similar to what is observed in the high accretion domain. We did not observe any correlation between the Eddington ratio and the photon index. We studied the compactness-temperature diagram and found that the cooling process for extremely low-accreting AGNs is complex. The jet luminosity is calculated from the radio flux, and observed to be related to the bolometric luminosity as $L_{\rm jet} \propto L_{\rm bol}^{0.7}$, which is consistent with the standard radio-X-ray correlation.

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arXiv2023

Observational connection of non-thermal X-ray emission from pulsars with their timing properties and thermal emission

The origin and radiation mechanisms of high energy emissions from pulsars have remained mysterious since their discovery. Here we report, based on a sample of 68 pulsars, observational connection of non-thermal X-ray emissions from pulsars with their timing properties and thermal emissions, which may provide some constraints on theoretical modeling. Besides strong correlations with the spin-down power $\dot{E}$ and the magnetic field strength at the light cylinder $B_{\rm lc}$, the non-thermal X-ray luminosity in 0.5 - 8 keV, $L_{\rm p}$, represented by the power-law component in the spectral model, is found to be strongly correlated with the highest possible electric field strength in the polar gap, $E_{\rm pc}$, of the pulsar. The spectral power index $Γ_{\rm p}$ of that power-law component is also found, for the first time in the literature, to strongly correlate with $\dot{E}$, $B_{\rm lc}$ and $E_{\rm pc}$, thanks to the large sample. In addition, we found that $L_{\rm p}$ can be well described by $L_{\rm p}\propto T^{5.96\pm 0.64}R^{2.24\pm 0.18}$, where $T$ and $R$ are the surface temperature and the emitting-region radius of the surface thermal emission, represented by the black-body component in the spectral model. $Γ_{\rm p}$, on the other hand, can be well described only when timing variables are included, and the relation is $Γ_{\rm p} = \log(T^{-5.8\pm 1.93}R^{-2.29\pm 0.85}P^{-1.19\pm 0.88}\dot{P}^{0.94\pm 0.44})$ plus a constant. These relations strongly suggest the existence of connections between surface thermal emission and electron-positron pair production in pulsar magnetospheres.

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arXiv2022

Broadband X-ray Spectroscopy and Estimation of Spin of the Galactic Black Hole Candidate GRS 1758-258

We present the results of a broadband (0.5-78 keV) X-ray spectral study of the persistent Galactic black hole X-ray binary GRS 1758-258 observed simultaneously by Swift and NuSTAR. Fitting with an absorbed power-law model revealed a broad Fe line and reflection hump in the spectrum. We used different flavours of the relativistic reflection model for the spectral analysis. All models indicate the spin of the black hole in GRS 1758-258 is >0.92. The source was in the low hard state during the observation, with the hot electron temperature of the corona estimated to be kT$_e$ ~ 140 keV. The black hole is found to be accreting at ~1.5 % of the Eddington limit during the observation, assuming the black hole mass of 10 $M_{\odot}$ and distance of 8 kpc.

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arXiv2022

Absorption Variability of the Highly Obscured Active Galactic Nucleus NGC 4507

We present a detailed study of the highly obscured active galaxy NGC 4507, performed using four Nuclear Spectroscopic Telescope Array (NuSTAR) observations carried out between May and August in 2015 (~ 130 ks in total). Using various phenomenological and physically motivated torus models, we explore the properties of the X-ray source and those of the obscuring material. The primary X-ray emission is found to be non-variable, indicating a stable accretion during the period of the observations. We find the equatorial column density of the obscuring materials to be ~ 2 x 10^24 cm^-2 while the line of sight column density to be ~ 7 - 8 x 10^23 cm^-22. The source is found to be deeply buried with the torus covering factor ~ 0.85. We observe variability in the line-of-sight column density on a timescale of < 35 days. The covering factor of the Compton-Thick material is found to be ~ 0.35, in agreement with the results of recent X-ray surveys. From the variability of the line-of-sight column density, we estimate that the variable absorbing material is likely located either in the BLR or in the torus.

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arXiv2022

Properties of 2017-18 'failed' Outburst of GX 339-4

The Galactic transient black hole candidate GX 339-4 is a very interesting object to study as it showed both complete and failed types of outbursts. We studied both spectral and temporal properties of the 2017-18 outburst of the source using archival data of NICER and AstroSat instruments. This 2017-18 outburst is found to be failed in nature, as during the entire period of the outburst, the source was only in the hard spectral state. Source spectra were highly dominated with the non-thermal fluxes. When we tried to fit spectra with phenomenological models, most of the spectra were fitted with only the powerlaw model, and only six spectra required disk black body plus powerlaw models. While fitting spectra with the physical two-component advective flow (TCAF) model, we observed that the flow was highly dominated by the sub-Keplerian halo rate. The presence of stronger shock at a larger radius from the black hole was also observed during the rising and declining phases of the outburst. A prominent signature of $0.31$~Hz QPO is observed with its four harmonics. Mass of the black hole was also estimated from our spectral analysis with the TCAF model as $10.76^{+0.77}_{-1.07}~M_\odot$.

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arXiv2021

Statistical tests of young radio pulsars with/without supernova remnants: implying two origins of neutron stars

The properties of the young pulsars and their relations to the supernova remnants (SNRs) have been the interesting topics. At present, 383 SNRs in the Milky Way galaxy have been published, which are associated with 64 radio pulsars and 46 pulsars with high energy emissions. However, we noticed that 630 young radio pulsars with spin periods of less than half a second have been not yet observed the SNRs surrounding or nearby them, which arises a question of that could the two types of young radio pulsars with/without SNRs hold distinctive characteristics? Here, we employ the statistical tests on the two groups of young radio pulsars with (52) and without (630) SNRs to reveal if they share different origins. Kolmogorov-Smirnov (K-S) and Mann-Whitney-Wilcoxon (M-W-W) tests indicate that the two samples have the different distributions with parameters of spin period ($P$), derivative of spin period ($\dot P$), surface magnetic field strength ($B$), and energy loss rate ($\dot E$). Meanwhile, the cumulative number ratio between the pulsars with and without SNRs at the different spindown ages decreases significantly after $\rm10-20\,Kyr$. So we propose that the existence of the two types of supernovae (SNe), corresponding to their SNR lifetimes, which can be roughly ascribed to the low-energy and high-energy SNe. Furthermore, the low-energy SNe may be formed from the $\rm8-12\,M_{\odot}$ progenitor, e.g., possibly experiencing the electron capture, while the main sequence stars of $\rm12-25\,M_{\odot}$ may produce the high-energy SNe probably by the iron core collapse.

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arXiv2021

The Compton Spectrometer and Imager Project for MeV Astronomy

The Compton Spectrometer and Imager (COSI) is a 0.2-5 MeV Compton telescope capable of imaging, spectroscopy, and polarimetry of astrophysical sources. Such capabilities are made possible by COSI's germanium cross-strip detectors, which provide high efficiency, high resolution spectroscopy and precise 3D positioning of photon interactions. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization, and detecting and localizing multimessenger sources. The instantaneous field of view (FOV) for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts or other gamma-ray flares over >50% of the sky. We have completed a Phase A concept study to consider COSI as a Small Explorer (SMEX) satellite mission, and here we discuss the advances COSI-SMEX provides for astrophysics in the MeV bandpass.

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arXiv2021

Accretion Properties of MAXI J1813-095 during its Failed Outburst in 2018

We present the results obtained from detailed timing and spectral studies of a black hole candidate MAXI~J1813-095 using {\it Swift}, {\it NICER}, and {\it NuSTAR} observations during its 2018 outburst. The timing behaviour of the source is mainly studied by using {\it NICER} light curves in a $0.5-10$ keV range. We did not find any signature of quasi-periodic oscillations in the power density spectra of the source. We carry out spectral analysis with a combined disk blackbody \& power-law model, and a physical two-component advective flow (TCAF) model. From the combined {\tt disk blackbody} \& {\tt power-law} model, we extracted thermal and non-thermal fluxes, photon index, and inner disk temperature. We also find evidence for weak reflection in the spectra. We have tested the physical TCAF model on a broadband spectrum from {\it NuSTAR} and {\it Swift}/XRT. The parameters like mass accretion rates, the size of the Compton clouds and the shock strength are extracted. Our result shows that the source remained in the hard state during the entire outburst which indicates a `failed' outburst. We estimate the mass of the black hole as $7.4 \pm 1.5$ $M_{\odot}$ from the spectral study with the TCAF model. We use {\tt LAOR} model for the Fe K$α$ line emission. From this, the spin parameter of the black hole is estimated as $a^* > 0.76$. The inclination angle of the system is estimated to be in the range of $28^{\circ} - 45^{\circ}$ from the reflection model. We estimate the source distance to be $\sim 6$ kpc.

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arXiv2021

High Frequency Radio Observations of Two Magnetars, PSR J1622$-$4950 and 1E 1547.0$-$5408

We investigated the radio spectra of two magnetars, PSR J1622$-$4950 and 1E 1547.0$-$5408, using observations from the Australia Telescope Compact Array and the Atacama Large Millimeter/submillimeter Array taken in 2017. Our observations of PSR J1622$-$4950 show a steep spectrum with a spectral index of $-$1.3 $\pm$ 0.2 in the range of 5.5-45 GHz during its re-activating X-ray outburst in 2017. By comparing the data taken at different epochs, we found significant enhancement in the radio flux density. The spectrum of 1E 1547.0$-$5408 was inverted in the range of 43-95 GHz, suggesting a spectral peak at a few hundred gigahertz. Moreover, we obtained the X-ray and radio data of radio magnetars, PSR J1622$-$4950 and SGR J1745$-$2900, from literature and found two interesting properties. First, radio emission is known to be associated with X-ray outburst but has different evolution. We further found that the rising time of the radio emission is much longer than that of the X-ray during the outburst. Second, the radio magnetars may have double peak spectra at a few GHz and a few hundred GHz. This could indicate that the emission mechanism is different in the cm and the sub-mm bands. These two phenomenons could provide a hint to understand the origin of radio emission and its connection with the X-ray properties.

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arXiv2021

The power-law component of the X-ray emissions from pulsar wind nebulae and their pulsars

To look for possible phenomenological connections between pulsar's timing properties and emissions from pulsar wind nebulae and their pulsars, we studied the power-law component of the X-ray emissions from 35 pulsar wind nebulae which have a detected pulsar in X-rays. Our major results are in the following: (1) The power-law component of the X-ray luminosities, in the energy range from 0.5 keV to 8 keV, of the nebulae and of the pulsar both show a strong correlation with the pulsar spin-down power ($\dot{E}$), consistent with earlier studies. However, equally significant correlations with the magnetic field strength at the light cylinder ($B_{\rm lc}$) are also found. The similar significance level of the correlations with $\dot{E}$ and with $B_{\rm lc}$ suggests that not only $\dot{E}$ but also $B_{\rm lc}$ plays an important role in understanding these power-law emissions. (2) Thermal X-ray emissions are detected in 12 pulsars among the 35 samples. With derived temperature as one additional variable, we found that the photon indices of pulsar's non-thermal X-ray power-law spectra can be well described by a linear function of $\log P$, $\log\dot{P}$ and temperature logarithm $\log T$. It indicates that the surface temperature of neutron stars plays an important role in determining the energy distribution of the radiating pair plasma in pulsar's magnetospheres.

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arXiv2020

Feasibility of Observing Gamma-ray Polarization from Cygnus X-1 Using a CubeSat

Instruments flown on CubeSats are small. Meaningful applications of CubeSats in astronomical observations rely on the choice of a particular subject that is feasible for CubeSats. Here we report the result of a feasibility study for observing gamma-ray polarization from Cygnus X-1 using a small Compton polarimeter on board a 3U CubeSat. Silicon detectors and cerium bromide scintillators were employed in the instrument models that we discussed in this study. Through Monte Carlo simulations with Geant4-based MEGAlib package, we found that, with a 10-Ms on-axis, zenith-direction observation in a low inclination, low altitude earth orbit radiation background environment, the minimum detectable polarization degree can be down to about 10\% in 160 - 250 keV, 20\% in 250 - 400 keV, and 65\% in 400 - 2000 keV. A 3U CubeSat dedicated to observing Cygnus X-1 can therefore yield useful information on the polarization state of gamma-ray emissions from the brightest persistent X-ray black-hole binary in the sky.

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arXiv2020

Study of Accretion Flow Dynamics of V404 Cygni during its 2015 Outburst

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after $26$ years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using {\it Swift}/XRT and {\it Swift}/BAT data in the energy range of $0.5-150$ keV. We have done spectral analysis with the two-component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field $B$ for the outburst and found that the highest $B \sim 900$~Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the mismatch of the cooling timescale and infall timescale of the post-shock matter.

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arXiv2019

Concept study of a small Compton polarimeter to fly on a CubeSat

Application of cubesats in astronomical observations has been getting more and more mature in recent years. Here we report a concept study of a small Compton polarimeter to fly on a cubesat for observing polarization of soft gamma-rays from a black-hole X-ray binary, Cygnus X-1. Polarization states provide very useful diagnostics on the emission mechanism and the origin of those gamma rays. In our study, we conducted Monte Carlo simulations to decide the basic design of this small polarimeter. Silicon detectors and cerium bromide scintillators were employed in this study. We estimated its on-axis Compton efficiency at different energies and its data telemetry requirement when flying in a low earth orbit. Our results indicate that it is feasible to achieve high signal-to-noise ratio for observing Cyg X-1 with such a small instrument. Based on this study, we will proceed to have a more realistic design and look for opportunities of a cubesat space mission.

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arXiv2019

The Compton Spectrometer and Imager

In this Astro2020 APC White Paper, we describe a Small Explorer (SMEX) mission concept called the Compton Spectrometer and Imager. COSI is a Compton telescope that covers the bandpass often referred to as the "MeV Gap" because it is the least explored region of the whole electromagnetic spectrum. COSI provides a significant improvement in sensitivity along with high-resolution spectroscopy, enabling studies of 511 keV electron-positron annihilation emission and measurements of several radioactive elements that trace the Galactic history of supernovae. COSI also measures polarization of gamma-ray bursts (GRBs), accreting black holes, and pulsars as well as detecting and localizing multimessenger sources. In the following, we describe the COSI science, the instrument, and its capabilities. We highlight many Astro2020 science WPs that describe the COSI science in depth.

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arXiv2017

Polarimetric Analysis of the Long Duration Gamma Ray Burst GRB 160530A With the Balloon Borne Compton Spectrometer and Imager

A long duration gamma-ray burst, GRB 160530A, was detected by the Compton Spectrometer and Imager (COSI) during the 2016 COSI Super Pressure Balloon campaign. As a Compton telescope, COSI is inherently sensitive to the polarization of gamma-ray sources in the energy range 0.2-5.0 MeV. We measured the polarization of GRB 160530A using 1) a standard method (SM) based on fitting the distribution of azimuthal scattering angles with a modulation curve, and 2) an unbinned, maximum likelihood method (MLM). In both cases, the measured polarization level was below the $99\%$ confidence minimum detectable polarization levels of $72.3 \pm 0.8\%$ (SM) and $57.5 \pm 0.8\%$ (MLM). Therefore, COSI did not detect polarized gamma-ray emission from this burst. Our most constraining $90\%$ confidence upper limit on the polarization level was $46\%$ (MLM).

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arXiv2017

Maximum Likelihood Compton Polarimetry with the Compton Spectrometer and Imager

Astrophysical polarization measurements in the soft gamma-ray band are becoming more feasible as detectors with high position and energy resolution are deployed. Previous work has shown that the minimum detectable polarization (MDP) of an ideal Compton polarimeter can be improved by $\sim 21\%$ when an unbinned, maximum likelihood method is used instead of the standard approach of fitting a sinusoid to a histogram of azimuthal scattering angles. Here we outline a procedure for implementing this maximum likelihood approach for real, non-ideal polarimeters. As an example, we use the recent observation of GRB 160530A with the Compton Spectrometer and Imager. We find that the MDP for this observation is reduced by $20\%$ when the maximum likelihood method is used instead of the standard method.

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arXiv2017

Searching for High Energy, Horizon-scale Emissions from Galactic Black Hole Transients during Quiescence

We search for the gamma-ray counterparts of stellar-mass black holes using long-term Fermi archive to investigate the electrostatic acceleration of electrons and positrons in the vicinity of the event horizon, by applying the pulsar outer-gap model to their magnetosphere. When a black hole transient (BHT) is in a low-hard or quiescent state, the radiatively inefficient accretion flow cannot emit enough MeV photons that are required to sustain the force-free magnetosphere in the polar funnel via two-photon collisions. In this charge-starved gap region, an electric field arises along the magnetic field lines to accelerate electrons and positrons into ultra-relativistic energies. These relativistic leptons emit copious gamma-rays via the curvature and inverse-Compton (IC) processes. It is found that these gamma-ray emissions exhibit a flaring activity when the plasma accretion rate stays typically between 0.01 and 0.005 percent of the Eddington value for rapidly rotating, stellar-mass black holes. By analyzing the detection limit determined from archival Fermi/LAT data, we find that the 7-year averaged duty cycle of such flaring activities should be less than 5% and 10% for XTE J1118+480 and 1A 0620-00, respectively, and that the detection limit is comparable to the theoretical prediction for V404 Cyg. It is predicted that the gap emission can be discriminated from the jet emission, if we investigate the high-energy spectral behaviour or observe nearby BHTs during deep quiescence simultaneously in infrared wavelength and very-high energies.

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arXiv2017

Enhanced gamma radiation toward the rotation axis from the immediate vicinity of extremely rotating black holes

We investigate the acceleration of electrons and positrons by magnetic-field-aligned electric fields in the polar funnel of an accreting black hole (BH). Applying the pulsar outer-gap theory to BH magnetospheres, we find that such a lepton accelerator arises in the immediate vicinity of the event horizon due to frame-dragging, and that their gamma-ray luminosity increases with decreasing accretion rate. Furthermore, we demonstrate that the gamma-ray flux is enhanced along the rotation axis by more than an order of magnitude if the BH spin increases from $a=0.90M$ to $a=0.9999M$. As a result, if a ten-solar-mass, almost-maximally rotating BH is located within 3 kpc, when its accretion rate is between 0.005% and 0.01% of the Eddington rate, its high-energy flare becomes detectable with the Fermi/Large Area Telescope, provided that the flare lasts longer than 1.2 months and that we view the source nearly along the rotation axis. In addition, its very-high-energy flux is marginally detectable with the Cherenkov Telescope Array, provided that the flare lasts longer than a night and that our viewing angle is about 45 degrees with respect to the rotation axis.

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