arXiv subjects

C. Y. Hui

Publications and source records attributed to C. Y. Hui.

At least 37 records · Page 2Linked to original sources

arXiv2020

Searches for Pulsar-like Candidates from Unidentified Objects in the Third Catalog of Hard Fermi-LAT (3FHL) sources with Machine Learning Techniques

We report the results of searching pulsar-like candidates from the unidentified objects in the $3^{\rm rd}$ Catalog of Hard Fermi-LAT sources (3FHL). Using a machine-learning based classification scheme with a nominal accuracy of $\sim98\%$, we have selected 27 pulsar-like objects from 200 unidentified 3FHL sources for an identification campaign. Using archival data, X-ray sources are found within the $γ-$ray error ellipses of 10 3FHL pulsar-like candidates. Within the error circles of the much better constrained X-ray positions, we have also searched for the optical/infrared counterparts and examined their spectral energy distributions. Among our short-listed candidates, the most secure identification is the association of 3FHL J1823.3-1339 and its X-ray counterpart with the globular cluster Mercer 5. The $γ-$rays from the source can be contributed by a population of millisecond pulsars residing in the cluster. This makes Mercer 5 as one of the slowly growing hard $γ-$ray population of globular clusters with emission $>10$ GeV. Very recently, another candidate picked by our classification scheme, 3FHL J1405.1-6118, has been identified as a new $γ-$ray binary with an orbital period of $13.7$ days. Our X-ray analysis with a short Chandra observation has found a possible periodic signal candidate of $\sim1.4$ hrs and a putative extended X-ray tail of $\sim20$ arcsec long. Spectral energy distribution of its optical/infrared counterpart conforms with a blackbody of $T_{\rm bb}\sim40000$ K and $R_{\rm bb}\sim12R_{\odot}$ at a distance of 7.7 kpc. This is consistent with its identification as an early O star as found by infrared spectroscopy.

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arXiv2020

Repeated state change of variable gamma-ray pulsar, PSR J2021+4026

PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that shows state change of the gamma-ray emission and spin-down rate. The state change of PSR J2021+4026 was first observed at 2011 October, at which the pulsar changes the state from high gamma-ray flux/low spin-down rate state to low gamma-ray flux/high spin-down rate st\ ate. In December 2014, PSR J2021+4026 recovered the state before the 2011 state change over a timescale of a few months. We report that the long term evolution of the gamma-ray flux and timing behavior suggests that PSR J2021+4026 changed the state near 2018 February 1st and entered a new low gamma-ray flux/high spin-down rate state. At the 2018 state change, the averaged flux dropped from $(1.29\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1}}$ to $(1.12\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1\ }}$, which has the similar behavior to the case of 2011 event. The spin-down rate has increased by $\sim 3\%$ in the new state since the 2018 state change. The shapes of pulse profile and spectrum in GeV bands also changed at the 2018 event, and they are consistent with behavior at the 2011 state change. Our results probably suggest that PSR J2021+4026 is switching between different states with a timescale of several years, like some radio pulsars (e.g. PSR~B1828-11). PSR J2021+4026 will provide a unique opportunity to study the mechanism of the state switching.

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arXiv2020

An investigation on the factors affecting machine learning classifications in $γ$-ray astronomy

We have investigated a number of factors that can have significant impacts on the classification performance of $γ$-ray sources detected by Fermi Large Area Telescope (LAT) with machine learning techniques. We show that a framework of automatic feature selection can construct a simple model with a small set of features which yields better performance over previous results. Secondly, because of the small sample size of the training/test sets of certain classes in $γ$-ray, nested re-sampling and cross-validations are suggested for quantifying the statistical fluctuations of the quoted accuracy. We have also constructed a test set by cross-matching the identified active galactic nuclei (AGNs) and the pulsars (PSRs) in the Fermi LAT eight-year point source catalog (4FGL) with those unidentified sources in the previous 3$^{\rm rd}$ Fermi LAT Source Catalog (3FGL). Using this cross-matched set, we show that some features used for building classification model with the identified source can suffer from the problem of covariate shift, which can be a result of various observational effects. This can possibly hamper the actual performance when one applies such model in classifying unidentified sources. Using our framework, both AGN/PSR and young pulsar (YNG)/millisecond pulsar (MSP) classifiers are automatically updated with the new features and the enlarged training samples in 4FGL catalog incorporated. Using a two-layer model with these updated classifiers, we have selected 20 promising MSP candidates with confidence scores $>98\%$ from the unidentified sources in 4FGL catalog which can provide inputs for a multi-wavelength identification campaign.

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arXiv2019

High Energy Radiation from Spider Pulsars

The population of millisecond pulsars (MSPs) has been expanded considerably in the last decade. Not only is their number increasing, but also various classes of them have been revealed. Among different classes of MSPs, the behaviour of black widows and redbacks are particularly interesting. These systems consist of an MSP and a low-mass companion star in compact binaries with an orbital period of less than a day. In this article, we give an overview of the high energy nature of these two classes of MSPs. Updated catalogues of black widows and redbacks are presented and their X ray/$γ$-ray properties are reviewed. Besides the overview, using the most updated eight-year Fermi Large Area Telescope point source catalog, we have compared the $γ$-ray properties of these two MSP classes. The results suggest that the X-rays and $γ$-rays observed from these MSPs originate from different mechanisms. Lastly, we will also mention the future prospects of studying these spider pulsars with the novel methodologies as well as upcoming observing facilities.

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arXiv2019

Consequences of Energetic Magnetar-Like Outbursts of Nearby Neutron Stars: $^{14}$C Events and the Cosmic Electron Spectrum

Four significant events of rapid $^{14}$C increase have taken place within the past several thousand years. The physical origin of these rapid increases is still a mystery but must be associated with extremely energetic cosmic processes. Pulsars are highly magnetized neutron stars that emit a beam of electromagnetic radiations. Any sudden release of the energy stored in the magnetic multipole field will trigger outbursts similar to the giant flares of magnetars. Here we show that the relativistic outflow from the outbursts of a nearby pulsar interacting with the interstellar medium generates a shock, which accelerates electrons to trillions of electron volts. The high-energy photons from synchrotron emission of the shock interact with Earth's atmosphere, producing the cosmogenic nuclide $^{14}$C, which can cause the rapid $^{14}$C increases discovered in tree rings. These same relativistic electrons can account for a significant fraction of the cosmic electron spectrum in the trillion electron volts energy range, as observed by space-borne satellites. Since these outburst events can significantly affect our environment, monitoring nearby pulsars for such outbursts may be important in the future.

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arXiv2019

Face changing companion of the redback millisecond pulsar PSR J1048+2339

We present optical observations of the redback millisecond pulsar PSR J1048+2339, which is a 4.66 ms radio pulsar in a compact binary with an orbital period of six hours. We obtained high-quality light curves of PSR J1048+2339 with the Lulin 1 m Telescope. The system shows two distinct six-hour orbital modulations, in which an ellipsoidal modulation changes into a sinusoidal-like profile in less than 14 days. In addition to the change, the brightness of the companion increased by one magnitude, suggesting that the latter type of modulation is caused by the pulsar wind heating of the companion and that the heating became dominant in the system. While the changes are not unexpected, such a timescale is the shortest among similar systems. We performed modeling analysis to extract the properties of the system. We obtained a derived pulsar mass of 2.1 M$_{\odot }$ and a companion star mass of 0.4 M$_{\odot }$ for the system. The irradiation power increased by a factor of 6 during which the pulsar wind heating dominates. We also report on the two archival Chandra X-ray observations and discuss several possibilities that might cause the varying heating on the companion.

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arXiv2018

Discovery of an X-ray nebula in the field of millisecond pulsar PSR J1911-1114

We have discovered an extended X-ray feature which is apparently associated with millisecond pulsar (MSP) PSR J1911-1114 from a XMM-Newton observation, which extends for ~1' and the radio timing position of PSR J1911-1114 is in the mid point of the feature. The orientation of the feature is similar to the proper motion direction of PSR J1911-1114. Its X-ray spectrum can be well-modeled by an absorbed power-law with a photon index of $Γ=1.8^{+0.3}_{-0.2}$. If this feature is confirmed to be a pulsar wind nebula (PWN), this will be the third case that an X-ray PWN found to be powered by a MSP.

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arXiv2018

Investigation of the High-Energy Emission from the Magnetar-like Pulsar PSR J1119-6127 after the 2016 Outburst

PSR J1119-6127 is a radio pulsar that behaved magnetar like bursts, and we performed a comprehensive investigation of this pulsar using the archival high-energy observations obtained after its outburst in 2016 July. After the 2016 outburst, specific regions on the neutron star surface were heated up to > 0.3 and > 1 keV from about 0.2 keV. A hard non-thermal spectral component with a photon index < 0.5 related to the magnetospheric emission can be resolved from the NuSTAR spectra above 10 keV. We find that the thermal emitting regions did not cool down and gradually shrank by about (20-35)% four months after the outburst. Hard X-ray pulsations were detected with NuSTAR immediately after the outburst, at 5 sigma confidence level and with a background-subtracted pulsed fraction of (40+-10)%. However, the signal became undetectable after a few days. Using Fermi data, we found that the gamma-ray emission in 0.5-300 GeV was suppressed along with the disappearance of the radio pulsations. This is likely caused by a reconfiguration of the magnetic field. We also discovered that the timing noise evolved dramatically, and the spin-down rate significantly increased after the 2016 glitch. We proposed that post-outburst temporal and spectral behaviors from radio to gamma-ray bands were caused by changes of the magnetosphere structure, pair plasma injection and the shrinking emission sites on the neutron star.

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arXiv2018

On the orbital properties of millisecond pulsar binaries

We report a detailed analysis of the orbital properties of binary millisecond pulsar (MSP) with a white dwarf (WD) companion. Positive correlations between the orbital period $P_{\rm b}$ and eccentricity $ε$ are found in two classes of MSP binaries with a He WD and with a CO/ONeMg WD, though their trends are different. The distribution of $P_{\rm b}$ is not uniform. Deficiency of sources at $P_{\rm b}\sim35-50$~days (Gap 1) have been mentioned in previous studies. On the other hand, another gap at $P_{\rm b}\sim2.5-4.5$~days (Gap 2) is identified for the first time. Inspection of the relation between $P_{\rm b}$ and the companion masses $M_{\rm c}$ revealed the subpopulations of MSP binaries with a He WD separated by Gap 1, above which $P_{\rm b}$ is independent of $M_{\rm c}$ (horizontal branch) but below which $P_{\rm b}$ correlates strongly with $M_{\rm c}$ (lower branch). Distinctive horizontal branch and lower branch separated by Gap 2 were identified for the MSP binaries with a CO/ONeMg WD at shorter $P_{\rm b}$ and higher $M_{\rm c}$. Generally, $M_{\rm c}$ are higher in the horizontal branch than in the lower branch for the MSP binaries with a He WD. These properties can be explained in terms of a binary orbital evolution scenario in which the WD companion was ablated by a pulsar wind in the post mass-transfer phase.

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arXiv2018

X-ray Census of Millisecond Pulsars in the Galactic field

We have conducted a systematic survey for the X-ray properties of millisecond pulsars (MSPs). Currently, there are 47 MSPs with confirmed X-ray detections. We have also placed the upper limits for the X-ray emission from the other 36 MSPs by using the archival data. We have normalized their X-ray luminosities $L_{x}$ and their effective photon indices $Γ$ into a homogeneous data set, which enable us to carry out a detailed statistical analysis. Based on our censored sample, we report a relation of $L_{x}\simeq10^{31.05}\left(\dot{E}/10^{35}\right)^{1.31}$ erg/s (2-10 keV) for the MSPs. The inferred X-ray conversion efficiency is found to be lower than previously reported estimate that could be affected by selection bias. $L_{x}$ also correlates/anti-correlates with the magnetic field strength at the light cylinder $B_{LC}$/characteristic age $τ$. On the other hand, there is no correlation between $L_{x}$ and their surface magnetic field strength $B_{s}$. We have further divided the sample into four classes: (i) black-widows, (ii) redbacks, (iii) isolated MSPs and (iv) other MSP binaries, and compare the properties among them. We noted that while the rotational parameters and the orbital periods of redbacks and black-widow are similar, $L_{x}$ of redbacks are significantly higher than those of black-widows in the 2-10 keV band. Also the $Γ$ of redbacks are apparently smaller than those of black-widows, which indicates the X-ray emission of redbacks are harder than that of black-widows. This can be explained by the different contribution of intrabinary shocks in the X-ray emission of these two classes.

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arXiv2018

Broadband High-Energy Emissions of the Redback Millisecond Pulsar PSR J2129-0429

We present the first results from a joint XMM-Newton and NuSTAR observation of the gamma-ray emitting millisecond pulsar compact binary PSR J2129-0429. X-ray emission up to about 40 keV is detected and the joint spectrum can be modeled with a power-law plus a neutron star atmosphere model. At a distance of 1.4 kpc, the 0.3-79 keV luminosity is $3.56\times10^{32}$ erg s$^{-1}$. We also detected the 0.64-day binary orbital period with a double-peaked structure across the wavebands. By combining the updated Fermi GeV data, we modeled the broadband spectral energy distribution as well as the X-ray modulation with an intrabinary model involving shock interaction between pulsar wind and outflow from the companion star. Lastly, we reported a high-resolution X-ray image provided by Chandra to rule out the proposed pulsar wind nebula associated with PSR J2129-0429.

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arXiv2018

The X-ray modulation of PSR J2032+4127/MT91 213 during the Periastron Passage in 2017

We present the Neil Gehrels Swift Observatory (Swift), Fermi Large Area Telescope (Fermi-LAT), and Karl G. Jansky Very Large Array (VLA) observations of the gamma-ray binary PSR J2032+4127/MT91 213, of which the periastron passage has just occurred in November 2017. In the Swift X-ray light curve, the flux was steadily increasing before mid-October 2017, however, a sharp X-ray dip on a weekly time-scale is seen during the periastron passage, followed by a post-periastron X-ray flare lasting for ~20 days. We suggest that the X-ray dip is caused by (i) an increase of the magnetization parameter at the shock, and (ii) the suppression due to the Doppler boosting effect. The 20-day post-periastron flare could be a consequence of the Be stellar disk passage by the pulsar. An orbital GeV modulation is also expected in our model, however, no significant variability is seen in the Fermi-LAT light curve. We suspect that the GeV emission resulted from the interaction between the binary's members is hidden behind the bright magnetospheric emission of the pulsar. Pulsar gating technique would be useful to remove the magnetospheric emission and recover the predicted GeV modulation, if an accurate radio timing solution over the periastron passage is provided in the future.

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arXiv2017

A non-thermal pulsed X-ray emission of AR~Scorpii

We report the analysis result of UV/X-ray emission from AR~Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M-type star surface rather than from the accretion column on the WD's star similar to the usual IPs. Beside, the observed X-ray emission also modulates with WD's spin with a pulse fraction of $\sim 14\%$. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR~Scorpii are accelerated to a relativistic speed, and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, the evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M-type star surface heats up the plasma to a temperature of several keV, and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WD's closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.

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arXiv2017

Rapid X-ray variations of the Geminga pulsar wind nebula

A recent study by Posselt et al. (2017) reported the deepest X-ray investigation of the Geminga pulsar wind nebula (PWN) by using \emph{Chandra X-ray Observatory}. In comparison with previous studies of this system, a number of new findings have been reported and we found these suggest the possible variabilities in various components of this PWN. This motivates us to carry out a dedicated search for the morphological and spectral variations of this complex nebula. We have discovered variabilities on timescales from a few days to a few months from different components of the nebula. The fastest change occurred in the circumstellar environment at a rate of 80 per cent of the speed of light. One of the most spectacular results is the wiggling of a half light-year long tail as an extension of the jet, which is significantly bent by the ram pressure. The jet wiggling occurred at a rate of about 20 per cent of the speed of light. This twisted structure can possibly be a result of a propagating torsional Alfven wave. We have also found evidence of spectral hardening along this tail for a period of about nine months.

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arXiv2017

Mode change of a gamma-ray pulsar, PSR J2021+4026

A glitch of a pulsar is known as a sudden increase in the spin frequency and spin-down rate (frequency time derivative), and it can be caused by a sudden rel\ ease of the stress built up in the solid crust of the star or pinned vortices in the superfluid interior. PSR J2021+4026 is the first pulsar that shows a significant change in the gamma-ray flux and pulse profile at the glitch that occurred around 2011 October 16. We report the results of timing and spectral analysis of PSR~J2021+4026 using $\sim$ 8~yr Fermi-LAT data. We find that the pulsar stayed at a high spin-down rate ($\sim 4\%$ higher than the pre-glitch value) and a low gamma-ray state ($\sim 18\%$ lower) for about 3~yr after the glitch. Around 2014 December, the spin-down rate and gamma-ray flux gradually returned to pre-glitch values within a time scale of a few months. The phase-resolved spectra and pulse profiles after the relaxation are also consistent with those before the glitch. The observed long-term evolution of the spin-down rate and the gamma-ray flux indicates that the glitch triggered a mode change in the global magnetosphere. We speculate that the glitch changed the local magnetic field structure around the polar cap and/or the inclination angle of the dipole axis, leading to a change in the electric current circulating in the magnetosphere.

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arXiv2017

Swift, XMM-Newton, and NuSTAR observations of PSR J2032+4127/MT91 213

We report our recent Swift, NuSTAR, and XMM-Newton X-ray and Lijiang optical observations on PSR J2032+4127/MT91 213, the gamma-ray binary candidate with a period of 45-50 years. The coming periastron of the system was predicted to be in November 2017, around which high-energy flares from keV to TeV are expected. Recent studies with Chandra and Swift X-ray observations taken in 2015/16 showed that its X-ray emission has been brighter by a factors of ~10 than that before 2013, probably revealing some on-going activities between the pulsar wind and the stellar wind. Our new Swift/XRT lightcurve shows no strong evidence of a single vigorous brightening trend, but rather several strong X-ray flares on weekly to monthly timescales with a slowly brightening baseline, namely the low state. The NuSTAR and XMM-Newton observations taken during the flaring and the low states, respectively, show a denser environment and a softer power-law index during the flaring state, implying that the pulsar wind interacted with stronger stellar winds of the companion to produce the flares. These precursors would be crucial in studying the predicted giant outburst from this extreme gamma-ray binary during the periastron passage in late 2017.

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arXiv2017

A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837

We report on the first NuSTAR observation of the gamma-ray emitting millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is clearly detected and the simultaneous NuSTAR and Swift spectrum is well described by an absorbed power-law with a photon index of ~1.3. We also find X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at the 14.8-hr binary orbital period. All these are entirely consistent with previous X-ray observations below 10 keV. This new hard X-ray observation of PSR J1723-2837 provides strong evidence that the X-rays are from the intrabinary shock via an interaction between the pulsar wind and the outflow from the companion star. We discuss how the NuSTAR observation constrains the physical parameters of the intrabinary shock model.

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arXiv2017

Gamma-ray Emission of the Kes 73/1E 1841-045 Region Observed with the Fermi Large Area Telescope

The supernova remnant Kes 73 and/or the magnetar 1E 1841-045 at its center can deposit a large amount of energy to the surroundings and is potentially responsible for particle acceleration. Using the data taken with the Fermi Large Area Telescope (LAT), we confirmed the presence of an extended source whose centroid position is highly consistent with this magnetar/supernova-remnant pair. Its emission is intense from 100 MeV to $>$100 GeV. Its LAT spectrum can be decoupled into two components which are respectively governed by two different mechanisms. According to the young age of this system, the magnetar is seemingly a necessary and sufficient source for the downward-curved spectrum below 10 GeV, as the observed $<$10 GeV flux is too high for the supernova remnant to account for. On the other hand, the supernova remnant is reasonably responsible for the hard spectrum above 10 GeV. Further studies of this region in the TeV regime is required, so that we can perform physically meaningful comparisons of the $>$10 GeV spectrum and the TeV spectrum.

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