arXiv subjects

Paul K. H. Yeung

Publications and source records attributed to Paul K. H. Yeung.

16 recordsLinked to original sources

arXiv2026

A common four-beam geometry reveals altitude-stratified GeV pulses in canonical young pulsars

Despite the diversity and energy dependence of $γ$-ray pulse morphologies in Crab, Vela and Dragonfly, the phaseograms of these three canonical young pulsars can be organised within a single four-beam geometric template. Using \textit{Fermi} Large Area Telescope data, we fit the 60~MeV--3~GeV phaseograms with a mechanism-agnostic, geometry-first parametric model that incorporates phase-dependent Doppler shifts and constrains the three-dimensional locations and bulk motions of four emission sites. In each pulsar, the phaseogram admits a decomposition into two altitude-separated beam pairs. The lower-altitude pair is produced by plasma with bulk motion close to azimuthal corotation, sharpening the main peaks. The higher-altitude pair shows a radially outward bulk-motion component, suggestive of inertial effects in a toroidally dominated magnetic field, and contributes bridge/shoulder emission and ripple-like modulations overlapping the main peaks. As a posteriori, the lower-altitude pair is consistent with curvature-dominated outer-magnetospheric emission, while the higher-altitude pair is consistent with synchrotron-dominated emission from a current-sheet-like outflow. Higher-altitude site heights vary from $\simeq 0.7$ (Crab, $\approx 1$~kyr) to $\simeq 1.1$--$1.4$ light-cylinder radii (Vela and Dragonfly, $\approx 10$~kyr). This unified four-beam, observation-driven geometry maps an altitude-dependent azimuthal tilt of pulsed $γ$-ray emission, providing an observationally anchored framework amenable to systematic tests and readily extensible to other young pulsars.

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arXiv2025

A mechanism-independent methodology for modeling γ-ray phaseograms of pulsars in the framework of north-south symmetry

Fermi-LAT observations revealed that each GeV phase-folded light-curve (aka. phaseogram) of the Crab, Geminga, Dragonfly and Vela pulsars consists of two pulses (P1 & P2) and a "Bridge" between them. There is clearly a "bump" at the Bridge phase of Vela's pulse profiles, that could also be regarded as the third pulse (P3). Differently, the Crab's, Geminga's & Dragonfly's Bridges relatively resemble a "valley floor". Despite such an apparent difference, it is interesting to investigate whether their Bridge emissions are still within the same general picture as Vela's. Assuming the north-south symmetry, we would expect the fourth component (Bridge2/P4) to exist as well. However, such a hypothetical Bridge2/P4 is not intuitively identified on $γ$-ray phaseograms of the Crab, Geminga, Dragonfly and Vela pulsars. It is also intriguing to hint at the rationale for the non-discovery of Bridge2/P4. Our prototypical toy model is free of assumptions on emission regions or radiation mechanisms. Instead, it assumes a north-south symmetric geometry and one circularly symmetric beam per hemisphere, while taking into account Doppler shifts (the most innovative element), time delays and energy-dependent beam shapes. Tentative compatibility of our fitting results with wind models is reported. Notably, for the Crab pulsar, we found a preliminary qualitative correlation between our model predictions and the IXPE results on X-ray polarisation. The softer $γ$-ray pulsation of the Geminga pulsar is found to span over its full phase. Prompted by systematic evaluations, we outline some potential improvements for our toy model.

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arXiv2024

Examining a hadronic $γ$-ray scenario for the radiative shell & molecular clouds of the old GeV supernova remnant G298.6$-$0.0

Based on the 13.7~yr Fermi-LAT data, Yeung et al. (2023) claimed detection of two $γ$-ray sources (Src-NE and Src-NW) associated with the supernova remnant (SNR) G298.6$-$0.0, and interpreted it as an old GeV SNR interacting with molecular clouds (MCs). In this follow-up study, we refine the flux measurements below 2~GeV with Fermi-LAT event types of better angular reconstruction. Then, we report our cosmic-ray phenomenology in a hadronic scenario, considering both the shell and MC regions of SNR G298.6$-$0.0. We confirm that both the $γ$-ray spectra of Src-NE and Src-NW exhibit spectral breaks at $1.50_{-0.50}^{+0.60}$~GeV and $0.68_{-0.11}^{+0.32}$~GeV, respectively. Src-NW has a harder broadband photon index than Src-NE, suggesting an appreciable difference between the physical separations of their respective emission sites from SNR G298.6$-$0.0. The cosmic-ray spectrum responsible for Src-NE starts with a minimum energy $E_\mathrm{CR,min}=1.38_{-0.16}^{+0.47}$~GeV, and has a proton index $Γ_\mathrm{CR}=2.57_{-0.21}^{+0.18}$ below the exponential cutoff energy $E_\mathrm{CR,max}=240_{-150}^{+240}$~GeV. Accordingly, we argue that Src-NE is dominated by the SNR shell, while only a minor portion of lower-energy emission is contributed by the MCs interacting with the SNR. The cosmic-ray population for Src-NW starts at a higher energy such that the $E_\mathrm{CR,min}$ ratio of Src-NW to Src-NE is $\gtrsim$2. The high $E_\mathrm{CR,min}$, as well as the high cosmic-ray energy density required ($\sim$26~eV~cm$^{-3}$), supports the interpretation that Src-NW is predominantly the $γ$-ray emission from the farther MCs being bombarded by protons that had earlier escaped from SNR G298.6$-$0.0. By comparing the high-energy features of G298.6$-$0.0 with those of analogical SNRs, especially SNR W28 and SNR W44, we further constrain the age of SNR G298.6$-$0.0 to be 10--30~kyr.

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arXiv2023

Multiwavelength studies of G298.6$-$0.0: An old GeV supernova remnant interacting with molecular clouds

Hadronic $γ$-ray sources associated with supernova remnants (SNRs) can serve as stopwatches for the escape of cosmic rays from SNRs, which gradually develops from highest-energy particles to lowest-energy particles with time. In this work, we analyze the 13.7~yr \emph{Fermi}-LAT data to investigate the $γ$-ray feature in/around the SNR G298.6$-$0.0 region. With $γ$-ray spatial analyses, we detect three point-like components. Among them, Src-NE is at the eastern SNR shell, and Src-NW is adjacent to the western edge of this SNR. Src-NE and Src-NW demonstrate spectral breaks at energies around/below 1.8~GeV, suggesting an old SNR age of $>$10~kyr. We also look into the X-ray emission from the G298.6$-$0.0 region, with the Chandra-ACIS data. We detected an extended keV source having a centrally filled structure inside the radio shell. The X-ray spectra are well fit by a model which assumes a collisional ionisation equilibrium of the thermal plasma, further supporting an old SNR age. Based on our analyses of the NANTEN CO- and ATCA-Parkes HI-line data, we determined a kinematic distance of $\sim$10.1~kpc from us to G298.6$-$0.0. This distance entails a large physical radius of the SNR of $\sim$15.5~pc, which is an additional evidence for an old age of $>$10~kyr. Besides, the CO data cube enables us to three-dimensionally locate the molecular clouds (MCs) which are potentially interacting with SNR G298.6$-$0.0 and could account for the hadronic $γ$-rays detected at Src-NE or Src-NW. Furthermore, the multiwavelength observational properties unanimously imply that the SNR--MC interaction occurs mainly in the northeast direction.

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arXiv2022

Spatially decomposed $γ$-ray features surrounding SNR Kes 79 & PSR J1853+0056

There have been substantial improvements on Fermi Large Area Telescope (LAT) data and analysis tools since the last analysis on the mid-aged supernova remnant (SNR) Kes 79 (Auchettl et al. 2014). Recent multi-wavelength studies confirmed its interaction with molecular clouds. About $0.36\degr$ north from Kes 79, a powerful pulsar -- PSR J1853+0056 also deserves our attention. In this work, we analyse the 11.5-year Fermi-LAT data to investigate the $γ$-ray feature in/around this complex region. Our result shows a more significant detection ($\sim$34.8$σ$ in 0.1--50 GeV) for this region. With $\ge$5 GeV data, we detect two extended sources -- Src-N (the brighter one; radius $\approx0.31\degr$) concentrated at the north of the SNR while enclosing PSR J1853+0056, and Src-S (radius $\approx0.58\degr$) concentrated at the south of the SNR. Their spectra have distinct peak energies ($\sim$1.0 GeV for Src-N and $\lesssim$0.5 GeV for Src-S), suggesting different origins for them. In our hadronic model that includes the leaked cosmic-rays (CRs) from the shock-cloud collision, even with extreme values of parameters, SNR Kes 79 can by no means provide enough CRs reaching clouds at Src-N to explain the local GeV spectrum. We propose that the Src-N emission could be predominantly reproduced by a putative pulsar wind nebula (PWN) powered by PSR J1853+0056. On the other hand, our same hadronic model can reproduce a majority of the GeV emission at Src-S with typical values of parameters, while the three known pulsars inside Src-S release a total power that is too low to account for half of its $γ$-ray emission.

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arXiv2021

Investigation of gamma-ray variability and glitches of PSR J1420-6048

PSR J1420-6048 is a young gamma-ray pulsar with recurrent glitches. Utilizing long-term monitoring data obtained from the Fermi Gamma-ray Space Telescope, we found that PSR J1420-6048 has shown gamma-ray flux variation and we also detected four glitches between 2008 and 2019. Two of the glitches are previously unknown, and their gamma-ray spectrum also shows variability between each glitch. Since the results might be contaminated by background sources, we discuss whether the observed changes in flux and spectra were caused by artificial misallocations of photons from a nearby pulsar wind nebula (HESS J1420-607) and a pulsar (PSR J1418-6058), or a change of the emission geometry from the target pulsar itself. We examine the correlation of the flux changes and the alternating pulse structure to investigate whether the emission geometry in the outer magnetosphere was changing. By assuming the observational features were not totally resulted from the background environment, we compare our results with similar phenomena observed in other gamma-ray pulsars and propose that a strong crust crack can cause timing anomaly of a neutron star, which can affect the particle accelerations or pair creation regions resulting in the changes of emission behaviors.

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arXiv2021

Inferring the origins of the pulsed gamma-ray emission from the Crab pulsar with 10-year Fermi LAT data

Context: The Crab pulsar is a bright $γ$-ray source detected at photon energies up to $\sim$1 TeV. Its phase-averaged and phase-resolved $γ$-ray spectra below 10 GeV exhibit exponential cutoffs while those above 10 GeV apparently follow simple power-laws. Aims: We re-visit the $γ$-ray properties of the Crab pulsar with 10-year \emph{Fermi} Large Area Telescope (LAT) data in the range of 60 MeV--500 GeV. With the phase-resolved spectra, we investigate the origins and mechanisms responsible for the emissions. Methods: The phaseograms are reconstructed for different energy bands and further analysed using a wavelet decomposition. The phase-resolved energy spectra are combined with the observations of ground-based instruments like MAGIC and VERITAS to achieve a larger energy converage. We fit power-law models to the overlapping energy spectra from 10 GeV to $\sim$1 TeV. We include in the fit a relative cross-calibration of energy scales between air-shower based gamma-ray telescopes with the orbital pair-production telescope of the Fermi mission. Results: We confirm the energy-dependence of the $γ$-ray pulse shape, and equivalently, the phase-dependence of the spectral shape for the Crab pulsar. A relatively sharp cutoff at a relatively high energy of $\sim$8 GeV is observed for the bridge-phase emission. The $E>$10 GeV spectrum observed for the second pulse peak is harder than those for other phases. Conclusions: In view of the diversity of phase-resolved spectral shapes of the Crab pulsar, we tentatively propose a multi-origin scenario where the polar-cap, outer-gap and relativistic-wind regions are involved.

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arXiv2021

The Energy-dependent $γ$-ray Morphology of the Crab Nebula Observed with the Fermi Large Area Telescope

The Crab Nebula is a bright emitter of non-thermal radiation across the entire accessible range of wavelengths. The spatial and spectral structures of the synchrotron nebula are well-resolved from radio to hard X-ray emission. The un-pulsed emission at GeV to TeV energies is mostly produced via inverse-Compton scattering of energetic electrons with the synchrotron-emitted photons. The spatial structure observed at these energies provides insights into the distribution of electrons and indirectly constrains the so-far unknown structure of the magnetic field in the nebula. Analyzing the LAT data accumulated over $\sim$9.1 years with a properly refined model for the Crab pulsar's spectrum, we determined the 68\% containment radius ($R_{68}$) of the Crab Nebula to be $({0.0330\pm0.0025_{stat}}{^{+0.0012}_{-0.0075}}_{sys})^\circ$ (${1.98'\pm0.15'_{stat}}{^{+0.07'}_{-0.45'}}_{sys}$) in the 5--500 GeV band. The estimated systematic uncertainty is based on two factors: (1) different analysis methods, morphological models and event types, and (2) the point-spread-function evaluated with observations of Mkn 421. When comparing the \emph{Fermi} LAT and H.E.S.S. results on the spatial extension, we find evidence for an energy-dependent shrinking of the Crab Nebula's $γ$-ray extension ($R_{68}\propto E_\mathrm{IC}^{-α}$ where $α=0.155\pm0.035_{stat}{-0.037}_{sys}$).

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arXiv2020

Fermi Large Area Telescope Observations of the Fast-dimming Crab Nebula in 60-600 MeV

Context: The Crab pulsar and its nebula are the origin of relativistic electrons which can be observed through their synchrotron and inverse Compton emission. The transition between synchrotron-dominated and inverse-Compton-dominated emissions takes place at $\approx 10^9$ eV. Aims: The short-term (weeks to months) flux variability of the synchrotron emission from the most energetic electrons is investigated with data from ten years of observations with the Fermi Large Area Telescope (LAT) in the energy range from 60 MeV to 600 MeV. Methods: The off-pulse light-curve has been reconstructed from phase-resolved data. The corresponding histogram of flux measurements is used to identify distributions of flux-states and the statistical significance of a lower-flux component is estimated with dedicated simulations of mock light-curves. The energy spectra for different flux states are reconstructed. Results: We confirm the presence of flaring-states which follow a log-normal flux distribution. Additionally, we discover a low-flux state where the flux drops to as low as 18.4% of the intermediate-state average flux and stays there for several weeks. The transition time is observed to be as short as 2 days. The energy spectrum during the low-flux state resembles the extrapolation of the inverse-Compton spectrum measured at energies beyond several GeV energy, implying that the high-energy part of the synchrotron emission is dramatically depressed. Conclusions: The low-flux state found here and the transition time of at most 10 days indicate that the bulk ($>75$%) of the synchrotron emission above $10^8$ eV originates in a compact volume with apparent angular size of $θ\approx0.4" t_\mathrm{var}/(5 \mathrm{d})$. We tentatively infer that the so-called inner knot feature is the origin of the bulk of the $γ$-ray emission.

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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

Leaked GeV CRs from a broken shell: Explaining 9 years Fermi-LAT data of SNR W28

Supernova remnant (SNR)\,W28 is well known for its classic hadronic scenario, in which the TeV cosmic rays (CRs) released at the early stage of this mid-aged SNR are illuminating nearby molecular clouds (MCs). Overwhelming evidences have shown that the northeast of the SNR (W28-North) has already encountered with the MC clumps. Through this broken shell -- W28-North, we believe the CRs with energy down to $<$1\,GeV to be able to be injected into nearby MCs. To further testify this hadronic scenario, we first analyse the 9 years Fermi-LAT data in/around W28 with energy down to 0.3\,GeV. Our Fermi-LAT analysis display a 10-200 GeV skymap which spatially matches well with the known TeV sources -- HESS\,J1801-233 (W28-North), HESS\,J1800-240\,A,\,B\,\&\,C (240\,A\,B\,\&\,C). At low energy band, we has discovered a 0.5-1\,GeV blob located to the south of 240\,B\,\&\,C, and a low flux of 0.3-1\,GeV at 240\,A. A hadronic model is build to explain our analysis results and previous multi-wavelength observations of W28. Our model consists of three CR sources: The run-away CRs escaped from a strong shock; The leaked GeV CRs from the broken shell -- W28-North; And the local CR sea. Through modelling the SNR evolution, CR acceleration\,\&\,releasing, we have explained the GeV-TeV emission in/around SNR\,W28 (except for 240\,A) in one model. Both the damping of the magnetic waves by the neutrals and the decreased acceleration efficiency are taken into account in our model due to the mid-age of SNR W28.

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arXiv2017

A search for energy-dependence of the Kes 73/1E 1841-045 morphology in GeV

While the Kes 73/1E 1841-045 system had been confirmed as an extended GeV source, whether its morphology depends on the photon energy or not deserves our further investigation. Adopting data collected by Fermi Large Area Telescope (LAT) again, we look into the extensions of this source in three energy bands individually: 0.3-1 GeV, 1-3 GeV and 3-200 GeV. We find that the 0.3-1 GeV morphology is point-like and is quite different from those in the other two bands, although we cannot robustly reject a unified morphology for the whole LAT band.

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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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arXiv2016

Formation of the Unequal-Mass Binary Protostars in L1551 NE by Rotationally-Driven Fragmentation

We present observations at 7 mm that fully resolve the two circumstellar disks, and a reanalyses of archival observations at 3.5 cm that resolve along their major axes the two ionized jets, of the class I binary protostellar system L1551 NE. We show that the two circumstellar disks are better fit by a shallow inner and steep outer power-law than a truncated power-law. The two disks have very different transition radii between their inner and outer regions of $\sim$18.6 AU and $\sim$8.9 AU respectively. Assuming that they are intrinsically circular and geometrically thin, we find that the two circumstellar disks are parallel with each other and orthogonal in projection to their respective ionized jets. Furthermore, the two disks are closely aligned if not parallel with their circumbinary disk. Over an interval of $\sim$10 yr, source B (possessing the circumsecondary disk) has moved northwards with respect to and likely away from source A, indicating an orbital motion in the same direction as the rotational motion of their circumbinary disk. All the aforementioned elements therefore share the same axis for their angular momentum, indicating that L1551 NE is a product of rotationally-driven fragmentation of its parental core. Assuming a circular orbit, the relative disk sizes are compatible with theoretical predictions for tidal truncation by a binary system having a mass ratio of $\sim$0.2, in agreement with the reported relative separations of the two protostars from the center of their circumbinary disk. The transition radii of both disks, however, are a factor of $\gtrsim$1.5 smaller than their predicted tidally-truncated radii.

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arXiv2016

Rotationally-Driven Fragmentation for the Formation of the Binary Protostellar System L1551 IRS 5

Either bulk rotation or local turbulence is widely invoked to drive fragmentation in collapsing cores so as to produce multiple star systems. Even when the two mechanisms predict different manners in which the stellar spins and orbits are aligned, subsequent internal or external interactions can drive multiple systems towards or away from alignment thus masking their formation process. Here, we demonstrate that the geometrical and dynamical relationship between the binary system and its surrounding bulk envelope provide the crucial distinction between fragmentation models. We find that the circumstellar disks of the binary protostellar system L1551 IRS 5 are closely parallel not just with each other but also with their surrounding flattened envelope. Measurements of the relative proper motion of the binary components spanning nearly 30 yr indicate an orbital motion in the same sense as the envelope rotation. Eliminating orbital solutions whereby the circumstellar disks would be tidally truncated to sizes smaller than are observed, the remaining solutions favor a circular or low-eccentricity orbit tilted by up to $\sim$25$^\circ$ from the circumstellar disks. Turbulence-driven fragmentation can generate local angular momentum to produce a coplanar binary system, but which bears no particular relationship with its surrounding envelope. Instead, the observed properties conform with predictions for rotationally-driven fragmentation. If the fragments were produced at different heights or on opposite sides of the midplane in the flattened central region of a rotating core, the resulting protostars would then exhibit circumstellar disks parallel with the surrounding envelope but tilted from the orbital plane as is observed.

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arXiv2016

Studying the SGR 1806-20/Cl* 1806-20 region using the \emph{Fermi} Large Area Telescope

The region around SGR 1806-20 and its host stellar cluster Cl* 1806-20 is a potentially important site of particle acceleration. The soft $γ-$ray repeater and Cl* 1806-20, which also contains several very massive stars including a luminous blue variable hypergiant LBV 1806-20, are capable of depositing a large amount of energy to the surroundings. Using the data taken with the \emph{Fermi} Large Area Telescope (LAT), we identified an extended LAT source to the south-west of Cl* 1806-20. The centroid of the 1-50~GeV emission is consistent with that of HESS J1808-204 (until now unidentified). The LAT spectrum is best-fit by a broken power-law with the break energy $E_\mathrm{b}=297\pm15$ MeV. The index above $E_\mathrm{b}$ is $2.60\pm0.04$, and is consistent with the flux and spectral index above 100 GeV for HESS J1808-204, suggesting an association between the two sources. Meanwhile, the interacting supernova remnant SNR G9.7-0.0 is also a potential contributor to the LAT flux. A tentative flux enhancement at the MeV band during a 45-day interval (2011 Jan 21 - 2011 Mar 7) is also reported. We discuss possible origins of the extended LAT source in the context of both leptonic and hadronic scenarios.

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