Searcharxiv⌕ Search

arXiv subjects

C. Y. Hui

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

At least 91 records · Page 5Linked to original sources
arXiv2011

The Fundamental Plane of Gamma-ray Globular Clusters

We have investigated the properties of a group of $γ$-ray emitting globular clusters (GCs) which have recently been uncovered in our Galaxy. By correlating the observed $γ$-ray luminosities $L_γ$ with various cluster properties, we probe the origin of the high energy photons from these GCs. We report $L_γ$ is positively correlated with the encounter rate $Γ_{c}$ and the metalicity $\left[{\rm Fe/H}\right]$ which place an intimate link between the gamma-ray emission and the millisecond pulsar population. We also find a tendency that $L_γ$ increase with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that $L_γ$, soft photon densities, and $Γ_{c}$/$\left[{\rm Fe/H}\right]$ possibly span fundamental planes which potentially provide better predictions for the $γ$-ray properties of GCs.

astro-ph.HE↗
arXiv2011

Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752

Globular clusters (GCs) are emerging as a new class of gamma-ray emitters, thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now, eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs out of all known GCs that have not been studied in details before. In this paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC 6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and NGC 6541, in which gamma-rays were found within the GC tidal radius. With one of the highest metallicity among all GCs in the Milky Way, the gamma-ray luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs. In addition, we confirm a previous report of significant gamma-ray emitting region next to NGC 6441. We briefly discuss the observed offset of gamma-rays from some GC cores. The increasing number of known gamma-ray GCs at distances out to ~10 kpc is important for us to understand the gamma-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the GCs.

astro-ph.HE↗
arXiv2010

The Origin of Gamma-Rays from Globular Clusters

Fermi has detected gamma-ray emission from eight globular clusters. We suggest that the gamma-ray emission from globular clusters may result from the inverse Compton scattering between relativistic electrons/positrons in the pulsar wind of MSPs in the globular clusters and background soft photons including cosmic microwave/relic photons, background star lights in the clusters, the galactic infrared photons and the galactic star lights. We show that the gamma-ray spectrum from 47 Tuc can be explained equally well by upward scattering of either the relic photons, the galactic infrared photons or the galactic star lights whereas the gamma-ray spectra from other seven globular clusters are best fitted by the upward scattering of either the galactic infrared photons or the galactic star lights. We also find that the observed gamma-ray luminosity is correlated better with the combined factor of the encounter rate and the background soft photon energy density. Therefore the inverse Compton scattering may also contribute to the observed gamma-ray emission from globular clusters detected by Fermi in addition to the standard curvature radiation process. Furthermore, we find that the emission region of high energy photons from globular cluster produced by inverse Compton scattering is substantially larger than the core of globular cluster with a radius >10pc. The diffuse radio and X-rays emitted from globular clusters can also be produced by synchrotron radiation and inverse Compton scattering respectively. We suggest that future observations including radio, X-rays, and gamma-rays with energy higher than 10 GeV and better angular resolution can provide better constraints for the models.

astro-ph.HE↗
arXiv2010

Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026

In this paper, we report a detailed investigation of the multiwavelength properties of a newly detected gamma-ray pulsar, PSR J2021+4026, in both observational and theoretical aspects. We firstly identify an X-ray source in the XMM-Newton serendipitous source catalogue, 2XMM J202131.0+402645, located within the 95% confidence circle of PSR J2021+4026. With an archival Chandra observation, this identification provides an X-ray position with arcsecond accuracy which is helpful in facilitating further investigations. Searching for the pulsed radio emission at the position of 2XMM J202131.0+402645 with a 25-m telescope at Urumqi Astronomical Observatory resulted in null detection and places an upper-limit of 0.1~mJy for any pulsed signal at 18~cm. Together with the emission properties in X-ray and gamma-ray, the radio quietness suggests PSR J2021+4026 to be another member of Geminga-like pulsars. In the radio sky survey data, extended emission features have been identified in the gamma-ray error circle of PSR J2021+4026. We have also re-analyzed the gamma-ray data collected by FERMI's Large Area Telescope. We found that the X-ray position of 2XMM J202131.0+402645 is consistent with that of the optimal gamma-ray timing solution. We have further modeled the results in the context of outer gap model which provides us with constraints for the pulsar emission geometry such as magnetic inclination angle and the viewing angle. We have also discussed the possibility of whether PSR J2021+4026 has any physical association with the supernova remnant G78.2+2.1 (gamma-Cygni).

astro-ph.HE↗
arXiv2010

Dynamical Formation of Millisecond Pulsars in Globular Clusters

The cumulative luminosity distribution functions (CLFs) of radio millisecond pulsars (MSPs) in globular clusters (GCs) and in the Galactic field at a frequency of 1.4 GHz have been examined. Assuming a functional form, $N \propto L^q$ where $N$ is the number of MSPs and $L$ is the luminosity at 1.4 GHz, it is found that the CLFs significantly differ with a steeper slope, $q=-0.83 \pm 0.05$, in GCs than in the Galactic field ($q=-0.48 \pm 0.04$), suggesting a different formation or evolutionary history of MSPs in these two regions of the Galaxy. To probe the production mechanism of MSPs in clusters, a search of the possible relationships between the MSP population and cluster properties was carried out. The results of an investigation of 9 GCs indicate positive correlations between the MSP population and the stellar encounter rate and metallicity. This provides additional evidence suggesting that stellar dynamical interactions are important in the formation of the MSP population in GCs.

astro-ph.HE↗
arXiv2010

Fermi Discovery of Gamma-ray Emission from the Globular Cluster Terzan 5

We report the discovery of gamma-ray emission from the Galactic globular cluster Terzan 5 using data taken with the Fermi Gamma-ray Space Telescope, from 2008 August 8 to 2010 January 1. Terzan 5 is clearly detected in the 0.5-20 GeV band by Fermi at ~27σlevel. This makes Terzan 5 as the second gamma-ray emitting globular cluster seen by Fermi after 47 Tuc. The energy spectrum of Terzan 5 is best represented by an exponential cutoff power-law model, with a photon index of ~1.9 and a cutoff energy at ~3.8 GeV. By comparing to 47 Tuc, we suggest that the observed gamma-ray emission is associated with millisecond pulsars, and is either from the magnetospheres or inverse Compton scattering between the relativistic electrons/positrons in the pulsar winds and the background soft photons from the Galactic plane. Furthermore, it is suggestive that the distance to Terzan 5 is less than 10 kpc and > 10 GeV photons can be seen in the future.

astro-ph.HE↗
arXiv2009

Diffuse X-ray emission in globular cluster cores

The unresolved X-ray emission in the cores of 10 globular clusters hosting millisecond pulsars is investigated. Subtraction of the known resolved point sources leads to detectable levels of unresolved emission in the core region of M28, NGC 6440, M62, and NGC 6752. The X-ray luminosities in the 0.3-8 keV energy band of this emission component were found to lie in the range $\sim 1.5 \times 10^{31}$ ergs s$^{-1}$ (NGC 6752) to $\sim 2.2 \times 10^{32}$ ergs s$^{-1}$ (M28). The lowest limiting luminosity for X-ray source detections amongst these four clusters was $1.1 \times 10^{30}$ ergs s$^{-1}$ for NGC 6752. The spectrum of the unresolved emission can be fit equally well by a power-law, a thermal bremsstrahlung model, a black body plus power-law, or a thermal bremsstrahlung model plus black body component. The unresolved emission is considered to arise from the cumulative contribution of active binaries, cataclysmic variables, and faint millisecond pulsars with their associated pulsar wind nebulae. In examining the available X-ray data, no evidence for any pulsar wind nebular emission in globular clusters is found. It is shown that the X-ray luminosity contribution of a faint source population based on an extrapolation of the luminosity function of detected point sources is compatible with the unresolved X-ray emission in the cores of NGC 6440 and NGC 6752. Adopting the same slope for the luminosity function for M62 as for NGC 6440 and NGC 6752 leads to a similar result for M62. For M28, the contribution from faint sources in the core can attain a level comparable with the observed value if a steeper slope is adopted. The characteristics on the faint source population as constrained by the properties of the unresolved X-ray emission are briefly discussed.

astro-ph.HE↗
arXiv2008

Exploring the X-ray emission properties of the supernova remnant G67.7+1.8 and its central X-ray sources

We have studied the supernova remnant G67.7+1.8 with the Chandra X-ray observatory. The remnant's X-ray morphology correlates well with the double-arc structure seen at radio wavelength. The X-ray spectra of the northern and southern rim of G67.7+1.8 exhibit emission line features of highly ionized metals, which suggests that most of the observed X-rays originate in a thermal plasma. We find magnesium, silicon, and sulphur are overabundant relative to the solar values. Gaussian emission lines at $\sim4$ keV and $\sim7$ keV are detected. The $\sim4$ keV line is consistent with K-emission lines from $^{44}$Ca and/or $^{44}$Sc whereas the $\sim7$ keV line feature may arise from unresolved Fe-K lines. Chandra's sub-arcsecond angular resolution allowed us to detect four faint point sources located within $\sim1.5$ arc-minutes of the geometrical remnant center. Among these objects, CXOU195424.75+312824.9 and CXOU195429.82+312834.1 do not have optical counterparts, leaving them as candidates for a possible compact stellar remnant.

astro-ph↗
arXiv2008

Resolving the bow-shock nebula around the old pulsar PSR B1929+10 with multi-epoch Chandra observations

We have studied the nearby old pulsar PSR B1929+10 and its surrounding interstellar medium utilizing the sub-arcsecond angular resolution of the Chandra X-ray Observatory. The Chandra data are found to be fully consistent with the results obtained from deep XMM-Newton observations as far as the pulsar is concerned. We confirm the non-thermal emission nature of the pulsar's X-radiation. In addition to the X-ray trail already seen in previous observations by the ROSAT and XMM-Newton X-ray observatories, we discovered an arc-like nebula surrounding the pulsar. We interpret the feature as a bow-shock nebula and discuss its energetics in the context of standard shock theory.

astro-ph↗
arXiv2007

Radio and X-ray nebulae associated with PSR J1509-5850

We have discovered a long radio trail at 843 MHz which is apparently associated with middle age pulsar PSR J1509-5850. The radio trail has a length of ~7 arcmin. In X-rays, Chandra observations of PSR J1509-5850 reveal an associated X-ray trail which extends in the same orientation as the radio trail. Moreover, two clumpy structures are observed along the radio trail. The larger one is proposed to be the supernova remnant (SNR) candidate MSC 319.9-0.7. Faint X-ray enhancement at the position of the SNR candidate is found in the Chandra data.

astro-ph↗
arXiv2007

A search for X-ray counterparts of the millisecond pulsars in the globular cluster M28 (NGC 6626)

A recent radio survey of globular clusters has increased the number of millisecond pulsars drastically. M28 is now the globular cluster with the third largest population of known pulsars, after Terzan 5 and 47 Tuc. This prompted us to revisit the archival Chandra data on M28 to evaluate whether the newly discovered millisecond pulsars find a counterpart among the various X-ray sources detected in M28 previously. The radio position of PSR J1824-2452H is found to be in agreement with the position of CXC 182431-245217 while some faint unresolved X-ray emission near to the center of M28 is found to be coincident with the millisecond pulsars PSR J1824-2452G, J1824-2452J, J1824-2452I and J1824-2452E.

astro-ph↗
arXiv2007

X-ray emission properties of the old pulsar PSR B2224+65

Using archival Chandra data we studied the X-ray emission properties of PSR B2224+65 and its environment. Albeit limited by photon statistics the spectral analysis suggests that the bulk of the emission from PSR B2224+65 is non-thermal. Fitting a power-law to the observed energy spectrum yields a photon index of $Γ=1.58^{+0.43}_{-0.33}$. The possible origin of the non-thermal pulsar emission is discussed in the context of the outer-gap model. We did not find any evidence for a compact nebula around PSR B2224+65 though the Chandra data reveal the existence of an extended feature which appears to be associated with PSR B2224+65. It extends from the pulsar position about 2 arcmin to the north-west. Its orientation deviates by $\sim118^{\circ}$ from the pulsar's proper motion direction. Investigating its energy spectrum shows that the emission of this extended feature is much harder than that of the pulsar itself and is non-thermal in nature.

astro-ph↗
arXiv2006

A Fast-Moving Central Compact Object in Puppis-A

Utilizing two Chandra High Resolution Camera (HRC-I) observations with an epoch separation of somewhat more than five years, we have measured the proper motion of the central compact object, RX J0822-4300, in the supernova remnant Puppis-A for the first time. The position of RX J0822-4300 is found to be different by 0.574 +/- 0.184 arcsec, implying a proper motion of 107.49 +/- 34.46 mas/yr with a position angle of 241 degree +/- 24 degree. For a distance of 2.2 kpc, this proper motion is equivalent to a recoil velocity of 1121.79 +/- 359.60 km/s. Both the magnitude and the direction of the proper motion are in agreement with the birth place of RX J0822-4300 being near to the optical expansion center of the supernova remnant. Although the positional shift inferred from the current data is significant at a ~ 3 sigma level only, one or more future HRC-I observations can obtain a much larger positional separation and further constrain the measurement.

astro-ph↗
arXiv2006

Discovery of an X-ray Nebula associated with PSR J2124-3358

We report the discovery of an X-ray nebula associated with the nearby millisecond pulsar PSR J2124-3358. This is the first time that extended emission from a solitary millisecond pulsar is detected. The emission extends from the pulsar to the northwest by ~ 0.5 arcmin. The spectrum of the nebular emission can be modeled by a power law spectrum with photon index of 2.2 +/-0.4. This is inline with the emission being originated from accelerated particles in the post shock flow.

astro-ph↗
arXiv2006

Probing the Proper Motion of the Central Compact Object in Puppis-A with the Chandra High Resolution Camera

By making use of the sub-arcsecond angular resolution of the High Resolution Camera (HRC-I) aboard the Chandra X-ray satellite we have examined the central compact object RX J0822-4300, in the supernova remnant Puppis-A for a possible proper motion. Using data which span an epoch of 1952 days we found the position of RX J0822-4300, different by $0.574\pm0.184$ arcsec, implying a proper motion of $μ=107.49\pm34.46$ mas/yr. For a distance of 2.2 kpc, this proper motion is equivalent to a recoil velocity of $1121.79\pm 359.60$ km/s. The position angle is found to be $241^\circ \pm 24^\circ$. Since both the magnitude and direction of the proper motion are in agreement with the birth place of RX J0822-4300, being near to the optical expansion center of the supernova remnant, the result presented in this letter is a promising indication of a fast moving compact object in a supernova remnant. Although the positional shift inferred from the current data is significant at a $\sim3σ$ level only, one or more future HRC-I observations can obtain a much larger positional separation and further constrain the measurement.

astro-ph↗
arXiv2006

Exploring the Central Compact Object in the RX J0852.0-4622 Supernova Remnant with XMM-Newton

The properties of the presumably young galactic supernova remnant (SNR) RX J0852.0-4622, discovered by ROSAT, are still uncertain. The data concerning the distance to the SNR, its age, and the presence of a compact remnant remain controversial. We report the results of several XMM-Newton observations of CXOU J085201.4-461753, the central compact source in RX J0852.0-4622. The currently prefered interpretation of CXOU J085201.4-461753 being a neutron star is in line with our analysis. The Chandra candidate pulsation periods are not confirmed; actually no period was found down to a 3-sigma upper limit for any pulsed fraction. The spectrum of CXOU J085201.4-461753 is best described by either a two blackbody spectrum or a single blackbody spectrum with a high energy power law tail. The two blackbody temperatures of 4 MK and 6.6 MK along with the small size of the emitting regions with radii of 0.36 and 0.06 km invalidate the interpretation that the thermal radiation is cooling emission from the entire neutron star surface. The double blackbody model suggests emission from the neutron star's hot polar regions. No X-ray lines, including the emission feature previously claimed to be present in Chandra data, were found.

astro-ph↗
arXiv2006

X-ray observations of RX J0822-4300 and Puppis-A

Based on observations with the X-ray observatories Chandra and XMM-Newton we present results from a detailed spectro-imaging and timing analysis of the central compact X-ray source RX J0822-4300 in the supernova remnant Puppis-A. The superior angular resolution of Chandra allows for the first time to pinpoint the point source nature of this object down to $0.59\pm0.01$ arcsec (FWHM) and to determine its position:{RA=$08^{\rm h}21^{\rm m}57.40^{\rm s}$, Dec=$-43^{\circ}00^{'}16.69^{''}$ (J2000)} with sub-arcsecond accuracy. Spectral fits based on Chandra and XMM-Newton data provide a tight constraint on the emission properties of RX J0822-4300. Most of its X-ray emission seems to be of thermal origin. A model spectrum consisting of two blackbody components with $T_{1}\simeq 2.6\times10^{6}$ K, $T_{2}\simeq 5.0\times10^{6}$ K and $R_{1}\simeq 3.3$ km, $R_2 \simeq 0.75$ km for the blackbody temperatures and the size of the projected emitting regions, respectively, provides the best model description of its spectrum. A search for X-ray pulsations from RX J0822-4300, revealed an interesting periodicity candidate which, if confirmed, does not support a scenario of steady spin-down.

astro-ph↗