arXiv subjects

C. Y. Hui

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

At least 73 records · Page 4Linked to original sources

arXiv2014

Identification campaign of supernova remnant candidates in the Milky Way. II. X-ray studies of G38.7-1.4

We report on XMM-Newton and Chandra observations of the Galactic supernova remnant candidate G38.7-1.4, together with complementary radio, infrared, and gamma-ray data. An approximately elliptical X-ray structure is found to be well correlated with radio shell as seen by the Very Large Array. The X-ray spectrum of G38.7-1.4 can be well-described by an absorbed collisional ionization equilibrium plasma model, which suggests the plasma is shock heated. Based on the morphology and the spectral behaviour, we suggest that G38.7-1.4 is indeed a supernova remnant belongs to a mix-morphology category.

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arXiv2014

Multi-wavelength emissions from the millisecond pulsar binary PSR J1023+0038 during an accretion active state

Recent observations strongly suggest that the millisecond pulsar binary PSR J1023+0038 has developed an accretion disk since 2013 June. We present a multi-wavelength analysis of PSR J1023+0038, which reveals that 1) its gamma-rays suddenly brightened within a few days in June/July 2013 and has remained at a high gamma-ray state for several months; 2) both UV and X-ray fluxes have increased by roughly an order of magnitude, and 3) the spectral energy distribution has changed significantly after the gamma-ray sudden flux change. Time variabilities associated with UV and X-rays are on the order of 100-500 seconds and 50-100 seconds, respectively. Our model suggests that a newly formed accretion disk due to the sudden increase of the stellar wind could explain the changes of all these observed features. The increase of UV is emitted from the disk, and a new component in gamma-rays is produced by inverse Compton scattering between the new UV component and pulsar wind. The increase of X-rays results from the enhancement of injection pulsar wind energy into the intra-binary shock due to the increase of the stellar wind. We also predict that the radio pulses may be blocked by the evaporated winds from the disk and the pulsar is still powered by rotation.

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arXiv2013

Exploring the X-ray and gamma-ray properties of the redback millisecond pulsar PSR J1723-2837

We have investigated the X-ray and $γ$-ray properties of the redback millisecond pulsar PSR J1723-2837 with XMM-Newton, Chandra and Fermi. We have discovered the X-ray orbital modulation of this binary system with the minimum that coincides with the phases of radio eclipse. The X-ray emission is clearly non-thermal in nature which can be well described by a simple power-law with a photon index of $\sim1.2$. The phase-averaged luminosity is $\sim9\times10^{31}$ erg/s in 0.3-10 keV which consumes $\sim0.2\%$ of the spin-down power. We have detected the $γ-$ray emission in $0.1-300$ GeV from this system at a significance of $\sim6σ$ for the first time. The $γ-$rays in this energy range consumes $\sim2\%$ of the spin-down power and can be modeled by a power-law with a photon index of $\sim2.6$. We discuss the high energy properties of the new redback in the context of a intrabinary shock model.

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arXiv2013

Discovery of X-ray Pulsation from the Geminga-like Pulsar PSR J2021+4026

We report the discovery of X-ray periodicity of ~265.3 ms from a deep XMM-Newton observation of the radio-quiet gamma-ray pulsar, PSR J2021+4026, located at the edge of the supernova remnant G78.2+2.1 (gamma-Cygni).The detected frequency is consistent with the gamma-ray pulsation determined by the observation of Fermi Gamma-ray Space Telescope at the same epoch. The X-ray pulse profile resembles the modulation of hot spot on the surface of the neutron star. The phase-averaged spectral analysis also suggests that the majority of the observed X-rays have a thermal origin. This is the third member in the class of radio-quiet pulsars with the significant pulsations detected from both X-rays and gamma-ray regimes.

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arXiv2013

Search for pulsed gamma-ray emission from globular cluster M28

Using the data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, we have searched for the gamma-ray pulsations from the direction of globular cluster M28 (NGC 6626). We report the discovery of a signal with the frequency consistent with that of the energetic millisecond pulsar (MSP) PSR B1821-24 in M28. A weighted H-test test statisic (TS) of 28.8 is attained which corresponds to a chance probability of ~1e-5 (4.3-sigma detection). With a phase-resolved analysis, the pulsed component is found to contribute ~25% of the total observed gamma-ray emission from the cluster. On the other hand, the unpulsed level provides a constraint for the underlying MSP population and the fundamental plane relations for the scenario of inverse Compton scattering. Follow-up timing observations in radio/X-ray are encouraged for further investigating this periodic signal candidate.

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arXiv2012

Orbital Phase Dependent Gamma-ray emissions from the Black Widow Pulsar

We report on evidence for orbital phase-dependence of the gamma-ray emission from PSR B1957+20 black widow system by using the data of the Fermi Large Area Telescope. We divide an orbital cycle into two regions: a region containing the inferior conjunction, and the other region containing rest of the orbital cycle. We show that the observed spectra for the different orbital regions are fitted by different functional forms. The spectrum of the orbital region containing inferior conjunction can be described by a power-law with an exponential cutoff (PLE) model, which gives the best-fit model for the orbital phase that does not contain the inferior conjunction, plus an extra component above ~2.7 GeV. The emission above 3 GeV in this region is detected with a ~7-sigma confidence level. The gamma-ray data above ~2.7 GeV are observed to be modulated at the orbital period at the ~2.3-sigma level. We anticipate that the PLE component dominating below ~2.7 GeV originates from the pulsar magnetosphere. We also show that the inverse-Compton scattering of the thermal radiation of the companion star off a "cold" ultra-relativistic pulsar wind can explain the extra component above ~2.7 GeV. The black widow pulsar PSR B1957+20 may be the member of a new class of object, in the sense that the system is showing gamma-ray emission with both magnetospheric and pulsar wind origins.

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arXiv2012

Pulsed γ-ray emission from magnetar 1E 2259+586

Anomalous x-ray pulsars (AXPs) are thought to be magnetars which are young isolated neutron stars with extremely strong magnetic fields of > 10^14Gauss. Their tremendous magnetic fields inferred from the spin parameters provide a huge energy reservoir to power the observed x-ray emission. High-energy emission above 0.3 MeV has never been detected despite intensive search. Here, we present the possible Fermi Large Area Telescope (LAT) detection of γ-ray pulsations above 200 MeV from the AXP, 1E 2259+586, which puts the current theoretical models of γ-ray emission mechanisms of magnetars into challenge. We speculate that the high-energy γ-rays originate from the outer magnetosphere of the magnetar.

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arXiv2012

X-ray studies of the Black Widow Pulsar PSR B1957+20

We report on Chandra observations of the black widow pulsar, PSR B1957+20. Evidence for a binary-phase dependence of the X-ray emission from the pulsar is found with a deep observation. The binary-phase resolved spectral analysis reveals non-thermal X-ray emission of PSR B1957+20, confirming the results of previous studies. This suggests that the X-rays are mostly due to intra-binary shock emission which is strongest when the pulsar wind interacts with the ablated material from the companion star. The geometry of the peak emission is determined in our study. The marginal softening of the spectrum of the non-thermal X-ray tail may indicate that particles injected at the termination shock is dominated by synchrotron cooling.

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arXiv2012

Detection of X-ray periodicity from a new eclipsing polar candidate XGPS-I J183251-100106

We report the results from a detailed analysis of an archival XMM-Newton observation of the X-ray source XGPS-I J183251-100106, which has been suggested as a promising magnetic cataclysmic variable candidate based on its optical properties. A single periodic signal of 1.5 hrs is detected from all EPIC cameras on board XMM-Newton. The phase-averaged X-ray spectrum can be well-modeled with a thermal bremsstrahlung of a temperature kT~50 keV. Both X-ray spectral and temporal behavior of this system suggest it as a eclipsing cataclysmic variable of AM Herculis (or polar) type.

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arXiv2012

Investigation of CTA 1 with Suzaku Observation

We report on an 105 ks Suzaku observation of the supernova remnant CTA 1 (G119.5+10.2). The Suzaku soft X-ray observation was carried out with both timing mode and imaging mode. A ~ 10' extended feature, which is interpreted as a bow-shock component of the pulsar wind nebula (PWN), is revealed in this deep observation for the first time. The nebular spectrum can be modelled by a power-law with a photon index of ~ 1.8 which suggests a slow synchrotron cooling scenario. The photon index is approximately constant across this extended feature. We compare and discuss our observations of this complex nebula with previous X-ray investigations. We do not obtain any significant pulsation from the central pulsar in the soft (0.2-12 keV) and hard (10-60 keV) X-ray data. The non-detection is mainly due to the loss of the precise imaging ability to accurately determine the source contribution. The spectra of XIS and HXD can be directly connected without a significant spectral break according to our analysis. Future observations of NuSTAR and Astro-H would be able to resolve the contamination and provide an accurate hard X-ray measurement of CTA 1.

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arXiv2012

Identification campaign of supernova remnant candidates in the Milky Way - I: Chandra observation of G308.3-1.4

ROSAT all-sky survey (RASS) data have provided another window to search for supernova remnants (SNRs). In reexamining this data archive, a list of unidentified extended X-ray objects have been suggested as promising SNR candidate. However, most of these targets have not yet been fully explored by the state-of-art X-ray observatories. For selecting a pilot target for a long-term identification campaign, we have observed the brightest candidate, G308.3-1.4, with Chandra X-ray observatory. An incomplete shell-like X-ray structure which well-correlated with the radio shell emission at 843 MHz has been revealed. The X-ray spectrum suggests the presence of a shock-heated plasma. All these evidences confirm G308.3-1.4 as a SNR. The brightest X-ray point source detected in this field-of-view is also the one locates closest to the geometrical center of G308.3-1.4, which has a soft spectrum. The intriguing temporal variability and the identification of optical/infrared counterpart rule out the possibility of an isolated neutron star. On the other hand, the spectral energy distribution from Ks band to R band suggests a late-type star. Together with a putative periodicity of \sim1.4 hrs, the interesting excesses in V, B bands and H-alpha suggest this source as a promising candidate of a compact binary survived in a supernova explosion (SN).

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arXiv2012

Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

The Fermi Gamma-ray Space Telescope has revolutionized our knowledge of the gamma-ray pulsar population, leading to the discovery of almost 100 gamma-ray pulsars and dozens of gamma-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and gamma-ray pulsars, until now all of the known gamma-ray MSPs have been visible in the radio. Here we report the discovery of a "radio-quiet" gamma-ray emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and gamma-ray properties of the source are consistent with known gamma-ray pulsars. We also found a 4.63-hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 solar-mass late-type companion star. Based on the profile of the optical and X-ray light-curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intra-binary shock. No radio detection of the source has been reported yet and although no gamma-ray/radio pulsation has been found, we estimated that the spin period of the MSP is ~3-5 ms based on the inferred gamma-ray luminosity.

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arXiv2011

XMM-Newton observation of PSR B2224+65 and its jet

We have investigated the pulsar PSR B2224+65 and its X-ray jet with XMM-Newton. Apart from the long X-ray jet which is almost perpendicular to the direction of proper motion, a putative extended feature at the pulsar position, which oriented in the opposite direction of the proper motion, is also suggested by this deep X-ray imaging. Non-detection of any coherent X-ray pulsation disfavors the magnetospheric origin of the X-rays observed from the position of PSR B2224+65 and hence suggest that the interpretation of pulsar wind nebula is more viable. We have also probed the origin of PSR B2224+65 and identified a runaway star, which possibly originated from the Cygnus OB9 association, as a candidate for the former binary companion of the neutron star's progenitor.

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arXiv2011

Gamma-ray properties of globular clusters and the "fundamental planes"

We report on the discovery of gamma-ray emission from several globular clusters (GCs), including Terzan 5, the second known gamma-ray GCs. By now, more than a dozen GCs are known to emit gamma-rays of energies above 100 MeV, thus enabling us to carry out the first detailed correlation study with several cluster properties. We found strong correlations between the observed gamma-ray luminosities and four cluster parameters: stellar encounter rate, metallicity [Fe/H], and energy densities of the soft photons at the cluster locations. These "fundamental planes" of gamma-ray GCs put an intimate relation of the observed gamma-rays to the underlying millisecond pulsar population and have important implications on the origin of the gamma-ray emission of GCs.

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arXiv2011

XMM-Newton confirmation of a new intermediate polar: XMMU J185330.7-012815

We report the results from a detailed spectro-imaging and temporal analysis of an archival XMM-Newton observation of a new intermediate polar XMMU J185330.7-012815. Its X-ray spectrum can be well-described by a multi-temperature thermal plasma model with the K-lines of heavy elements clearly detected. Possible counterparts of XMMU J185330.7-012815 have been identified in optical and UV bands. The low value of the inferred X-ray-to-UV and X-ray-to-optical flux ratios help to safely rule out the possibility as an isolated neutron star. We confirm the X-ray periodicity of ~238 s, but different from the previous preliminary result, we do not find any convincing evidence of phase-shift in this observation. We further investigate its properties through an energy-resolved temporal analysis and find the pulsed fraction monotonically increases with energy.

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arXiv2011

Discovery of gamma-ray emission from the supernova remnant Kes 17 with Fermi Large Area Telescope

We report the discovery of GeV emission at the position of supernova remnant Kes 17 by using the data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Kes 17 can be clearly detected with a significance of ~12 sigma in the 1 - 20 GeV range. Moreover, a number of gamma-ray sources were detected in its vicinity. The gamma-ray spectrum of Kes 17 can be well described by a simple power-law with a photon index of ~ 2.4. Together with the multi-wavelength evidence for its interactions with the nearby molecular cloud, the gamma-ray detection suggests that Kes 17 is a candidate acceleration site for cosmic-rays.

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arXiv2011

Discovery of GeV gamma-ray emission from PSR B1259-63/LS 2883

The binary system PSR B1259-63/LS 2883 consists of a 47.8 ms radio pulsar that orbits the companion Be star with a period of 3.4 years in a highly eccentric orbit. The system is well sampled in radio, X-rays, and TeV gamma-rays, and shows orbital-phase-dependent variability in all observed frequencies. Here we report on the discovery of >100 MeV gamma-rays from PSR B1259-63/LS 2883 through the 2010 pariastron passage. Using data collected with the Large Area Telescope aboard Fermi from 33 days before pariastron to 75 days after pariastron, PSR B1259-63/LS 2883 is detected at a significance of 13.6 standard deviations. The gamma-ray light curve is highly variable over the above period, with changing photon index that correlates with gamma-ray flux. In particular, two major flares that occur after the pariastron passage were observed. The onset of gamma-ray emission occurs close to, but not at the same orbital phases as, the two disk passages that occur ~1 month before and ~1 month after the pariastron passage. The fact that the GeV orbital light curve is different from that of the X-ray and TeV light curves strongly suggests that GeV gamma-ray emission originates from a different component. We speculate that the observed GeV flares may be resulting from Doppler boosting effects.

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arXiv2011

Exploring the dark accelerator HESS J1745-303 with Fermi Large Area Telescope

We present a detailed analysis of the gamma-ray emission from HESS J1745-303 with the data obtained by the Fermi Gamma-ray Space Telescope in the first ~29 months observation.The source can be clearly detected at the level of ~18-sigma and ~6-sigma in 1-20 GeV and 10-20 GeV respectively. Different from the results obtained by the Compton Gamma-ray Observatory, we do not find any evidence of variability. Most of emission in 10-20 GeV is found to coincide with the region C of HESS J1745-303. A simple power-law is sufficient to describe the GeV spectrum with a photon index of ~2.6. The power-law spectrum inferred in the GeV regime can be connected to that of a particular spatial component of HESS J1745-303 in 1-10 TeV without any spectral break. These properties impose independent constraints for understanding the nature of this "dark particle accelerator".

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