arXiv subjects

Yu-Liang Xin

Publications and source records attributed to Yu-Liang Xin.

13 recordsLinked to original sources

arXiv2024

Evolution of High-energy Electron Distribution in Pulsar Wind Nebulae

In this paper, we analyze the spectral energy distributions (SEDs) of 17 powerful (with a spin-down luminosity greater than $10^{35}$ erg s$^{-1}$) young (with an age less than 15000 yrs) pulsar wind nebulae (PWNe) using a simple time-independent one-zone emission model. Our aim is to investigate correlations between model parameters and the ages of the corresponding PWNe, thereby revealing the evolution of high-energy electron distributions within PWNe. Our findings are as follows: (1) The electron distributions in PWNe can be characterized by a double power-law with a superexponential cutoff; (2) As PWNe evolve, the high-energy end of the electron distribution spectrum becomes harder with the index decreasing from approximately 3.5 to 2.5, while the low-energy end spectrum index remains constant near 1.5; (3) There is no apparent correlation between the break energy or cutoff energy and the age of PWNe. (4) The average magnetic field within PWNe decreases with age, leading to a positive correlation between the energy loss timescale of electrons at the break energy or the high-energy cutoff, and the age of the PWN. (5) The total electron energy within PWNe remains constant near $2 \times 10^{48}$ erg, while the total magnetic energy decreases with age.

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arXiv2019

Discovery of a spatially extended GeV source in the vicinity of the TeV halo candidate 2HWC J1912+099: a TeV halo or supernova remnant ?

Observations by HAWC and Milagro have detected spatially extended TeV sources surrounding middle-aged ($t\sim100-400 \,{\rm kyr}$) pulsars like Geminga and PSR B0656+14, which have been named "TeV Halos", representing very extended TeV pulsar wind nebulae (PWNe) powered by relatively old pulsars. A few more HAWC-detected sources have been suggested to be TeV halo candidates. In this paper, we search for possible GeV counterparts of three TeV halo candidates with Fermi Large Area Telescopes. We detect a new spatially extended GeV source in the vicinity of the TeV halo candidate 2HWC J1912+099, which is also detected by HESS (HESS J1912+101). We find that the size of the GeV source is significantly larger than that of the TeV emission measured by HESS, and a spatial template characteristic of a PWN can fit the GeV data. We suggest that the GeV source is an extended PWN powered by the central middle-aged pulsar PSR J1913+1011. This discovery favors the TeV halo scenario for the TeV source 2HWC J1912+099 (HESS J1912+101), although the possible shell-like morphology measured by HESS challenges this interpretation. Alternatively, the TeV emission could be dominated by a supernova remnant (SNR) via the hadronic process. Future multi-wavelength observations of the source and more precise measurements of the spatial profile of the TeV emission will be useful to distinguish between the two scenarios.

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arXiv2019

Circinus Galaxy Revisted with 10 Years of Fermi-LAT Data

Circinus galaxy is a nearby composite starburst/AGN system. In this work we re-analyze the GeV emission from Circinus with 10 years of {\it Fermi}-LAT Pass 8 data. In the energy range of 1-500 GeV, the spectrum can be well fitted by a power-law model with a photon index of $Γ$ = $2.20\pm0.14$, and its photon flux is $(5.90\pm1.04) \times 10^{-10}$ photons cm$^{-2}$ s$^{-1}$. Our 0.1-500 GeV flux is several times lower than that reported in the previous literature, which is roughly in compliance with the empirical relation for star-forming and local group galaxies and might be reproduced by the interaction between cosmic rays and the interstellar medium. The ratio between the $γ$-ray luminosity and the total infrared luminosity is near the proton calorimetric limit, indicating that Circinus may be a proton calorimeter. However, marginal evidence for variability of the $γ$-ray emission is found in the timing analysis, which may indicate the activity of AGN jet. More {\it Fermi}-LAT data and future observation of CTA are required to fully reveal the origin of its $γ$-ray emission.

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arXiv2018

HESS J1640-465 -- a Gamma-ray emitting pulsar wind nebula ?

HESS J1640-465 is an extended TeV $γ$-ray source and its $γ$-ray emission whether from the shell of a supernova remnant (SNR) or a pulsar wind nebula (PWN) is still under debate. We reanalyze the GeV $γ$-ray data in the field of HESS J1640-465 using eight years of Pass 8 data recorded by the Fermi Large Area Telescope. An extended GeV $γ$-ray source positionally coincident with HESS J1640-465 is found. Its photon spectrum can be described by a power-law with an index of $1.42\pm0.19$ in the energy range of 10-500 GeV, and smoothly connects with the TeV spectrum of HESS J1640-465. The broadband spectrum of HESS J1640-465 can be well fit by a leptonic model with a broken power-law spectrum of electrons with an exponential cut-off at $\sim$ 300 TeV. The spectral properties of HESS J1640-465 are broadly consistent with the characteristics of other sources identified as PWNe, such as the correlations between high-energy luminosity ratios and the physical parameters of pulsar, including spin-down luminosity $\dot{E}$ and characteristic age $τ_c$. All these pieces of evidence support that the $γ$-ray emission of HESS J1640-465 may originate from the PWN powered by PSR J1640-4631 rather than the shell of the SNR G338.3-0.0.

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arXiv2018

Search for GeV flare coincident with the IceCube neutrino flare

Recently the IceCube collaboration and 15 other collaborations reported the spatial and temporal coincidence between the neutrino event IceCube-170922A and the radio-TeV activity of the blazar TXS 0506+056. Their further analysis on 9.5 years of IceCube data discovered neutrino flare between September 2014 and March 2015, when TXS 0506+056 is however in "quiescent" state. We analyze the Fermi-LAT data in that direction, and find another bright GeV source PKS 0502+049, which is at an angle of $1.2^{\circ}$ from TXS 0506+056, with strong activties during the neutrino flare. No other bright GeV source was detected in the region of interest. Though PKS 0502+049 is $1.2^\circ$ separated from TXS 0506+056, it locates within the directional reconstruction uncertainties of 7 neutrinos, out of the 13 neutrino events during the neutrino flare. Together with the observed high flux of the $γ$-ray flare, it may be unreasonable to fully discard the (partial) contribution of PKS 0502+049 to the neutrino flare. The single source assumption used in the neutrino data analysis might need to be revisited.

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arXiv2018

Active galactic nuclei with GeV activities and the PeV neutrino source candidate TXS 0506+056

On 2017 September 22 the IceCube neutrino observatory detected a track-like, very-high-energy event (IceCube-170922A) that is spatially associated with TXS 0506+056, a quasar at a redshift of $z=0.3365$. This source is characterized by the increased acitivies in a very wide energy range (from radio to TeV) during these days. To investigate the possible connection of the PeV neutrino emission with the GeV activity of blazars, in this work we select 116 bright sources and analyze their lightcurves and spectra. We focus on the sources displaying GeV activities. Among these blazars, TXS 0506+056 seems to be typical in many aspects but is distinguished by the very strong GeV activties. We suggest to search for neutrino outburst in the historical data of IceCube, as recently done for TXS 0506+056, from the directions of these more energetic and harder blazars with strong GeV activities.

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arXiv2018

Fermi Large Area Telescope detection of gamma-ray emission from the direction of supernova iPTF14hls

The remnant of supernova explosion is widely believed to be the acceleration site of high-energy cosmic ray particles. The acceleration timescale is, however, typically very long. Here we report the detection of a variable $γ$-ray source with the Fermi Large Area Telescope, which is positionally and temporally consistent with a peculiar supernova, iPTF14hls. A quasi-stellar object SDSS J092054.04+504251.5, which is probably a blazar according to the infrared data, is found in the error circle of the $γ$-ray source. More data about the $γ$-ray source and SDSS J092054.04+504251.5 are needed to confirm their association. On the other hand, if the association between the $γ$-ray source and the supernova is confirmed, this would be the first time to detect high-energy $γ$-ray emission from a supernova, suggesting very fast particle acceleration by supernova explosions.

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arXiv2017

Detection of GeV $γ$-ray emission in the direction of HESS J1731-347 with Fermi-LAT

We report the detection of GeV $γ$-ray emission from supernova remnant HESS J1731-347 using 9 years of {\it Fermi} Large Area Telescope data. We find a slightly extended GeV source in the direction of HESS J1731-347. The spectrum above 1 GeV can be fitted by a power-law with an index of $Γ= 1.77\pm0.14$, and the GeV spectrum connects smoothly with the TeV spectrum of HESS J1731-347. Either a hadronic-leptonic or a pure leptonic model can fit the multi-wavelength spectral energy distribution of the source. However, the hard GeV $γ$-ray spectrum is more naturally produced in a leptonic (inverse Compton scattering) scenario, under the framework of diffusive shock acceleration. We also searched for the GeV $γ$-ray emission from the nearby TeV source HESS J1729-345. No significant GeV $γ$-ray emission is found, and upper limits are derived.

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arXiv2017

The SNR Puppis A Revisited with Seven Years of Fermi Large Area Telescope Observations

Puppis A is a very famous and extensively studied supernova remnant (SNR) that shows strong evidence of shock-cloud interaction. We re-analyze the GeV $γ$-ray emission of it using seven years Pass 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT). The morphology of the $γ$-ray emission is more compatible with that of the thermal X-ray and IR emissions than the radio image, which suggests a possible correlation between the gamma-ray emitting region and dense clouds. The $γ$-ray spectrum in the energy range of 1-500 GeV shows a break at $7.92\pm1.91$ GeV with the photon indices of $1.81\pm0.08$ below the break and $2.53\pm0.12$ above the break, which can naturally explain the lack of TeV $γ$-ray emission from Puppis A. The multi-wavelength observations favor a hadronic origin for the $γ$-ray emission.

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arXiv2016

HESS J1427-608: an unusual hard unbroken $γ-$ray spectrum in a very wide energy range

We report the detection of a GeV $γ$-ray source which is spatially overlapping and thus very likely associated with the unidentified very-high-energy (VHE) $γ$-ray source HESS J1427-608 with the Pass 8 data recorded by the Fermi Large Area Telescope. The photon spectrum of this source is best described by a power-law with an index of $1.85\pm0.17$ in the energy range of $3-500$ GeV, and the measured flux connects smoothly with that of HESS J1427-608 at a few hundred GeV. This source shows no significant extension and time variation. The broadband GeV-TeV emission over four decades of energies can be well fitted by a single power-law function with an index of 2.0, without obvious indication of spectral cutoff toward high energies. Such a result implies that HESS J1427-608 may be a PeV particle accelerator. We discuss possible nature of HESS J1427-608 according to the multi-wavelength spectral fittings. Given the relatively large errors, either a leptonic or a hadronic model can explain the multi-wavelength data from radio to VHE $γ$-rays. The inferred magnetic field strength is a few $μ$G, which is smaller than typical values of supernova remnants (SNRs), and is consistent with some pulsar wind nebulae (PWNe). On the other hand, the flat $γ$-ray spectrum is slightly different from typical PWNe but similar to that of some known SNRs.

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arXiv2016

Discovery of $γ$-ray emission from the radio-intermediate quasar III Zw 2: violent jet activity with intraday $γ$-ray variability

III Zw 2 is the prototype of radio-intermediate quasars. Although there is the evidence of possessing strong jet, significant $γ$-ray emission has not been reported before. In this work, we carry out a detailed analysis of the latest {\it Fermi}-LAT {\it Pass} 8 data. No significant $γ$-ray signal has been detected in the time-averaged 7-year {\it Fermi}-LAT data of III Zw 2, we however have identified two distinct $γ$-ray flares with isotropic luminosities of $\sim 10^{45}$ erg $\rm s^{-1}$. Multiwavelength data analysis (including also the optical photometric observations from Yunnan Observatories) are presented and the main finding is the simultaneous optical and $γ$-ray flares of III Zw 2 appearing in Nov. 2009. Violent $γ$-ray variability with doubling timescale of 2.5 hours was detected in another $γ$-ray flare in May 2010, for which the 3-hour $γ$-ray peak flux is $\sim 250$ times of the average flux in 7 years. Rather similar behaviors are observed in blazars and the blazar model can reasonably reproduce the spectral energy distribution of III Zw 2 in a wide energy range, strongly suggesting that its central engine resembles that of blazars. In view of its core which shares radio similarities with young radio sources together with the weak extended radio lobe emission, we suggest that III Zw 2 harbors a recurrent activity core and serves as a valuable target for investigating the fuelling and triggering of the activity in radio loud AGNs.

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arXiv2016

A GeV source in the direction of Supernova Remnant CTB 37B

Supernova remnants (SNRs) are the most attractive candidates for the acceleration sites of Galactic cosmic rays. We report the detection of GeV $γ$-ray emission with the Pass 8 events recorded by Fermi Large Area Telescope (Fermi-LAT) in the vicinity of the shell type SNR CTB 37B that is likely associated with the TeV $γ-$ray source HESS J1713-381. The photon spectrum of CTB 37B is consistent with a power-law with an index of $1.89\pm0.08$ in the energy range of $0.5-500$ GeV, and the measured flux connects smoothly with that of HESS J1713-381 at a few hundred GeV. No significant spatial extension and time variation are detected. The multi-wavelength data can be well fitted with either a leptonic model or a hadronic one. However, parameters of both models suggest more efficient particle acceleration than typical SNRs. Meanwhile, the X-ray and $γ$-ray spectral properties of CTB 37B show that it is an interesting source bridging young SNRs dominated by non-thermal emission and old SNRs interacting with molecular clouds.

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arXiv2015

Discovery of gamma-ray emission from a strongly lobe-dominated quasar 3C 275.1

We systematically analyze the 6-year {\it Fermi}/LAT data of the lobe-dominated quasars (LDQs) in the complete LDQ sample from 3CRR survey and report the discovery of high-energy $γ$-ray emission from 3C 275.1. The $γ$-ray emission of 3C 207 is confirmed and significant variability of the lightcurve is identified. We do not find statistically significant $γ$-ray emission from other LDQs. 3C 275.1 is the known $γ$-ray quasar with the lowest core dominance parameter (i.e., $R=0.11$). We also show that both the northern radio hotspot and parsec jet models can reasonably reproduce the $γ$-ray data. The parsec jet model, however, is favored by the potential $γ$-ray variability at the timescale of months. We suggest that some dimmer $γ$-ray LDQs will be detected in the future and LDQs could contribute non-negligibly to the extragalactic $γ$-ray background.

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