arXiv subjects

Yu-Wei Yu

Publications and source records attributed to Yu-Wei Yu.

8 recordsLinked to original sources

arXiv2026

Fast Optical Variability of the TeV Blazar PKS 1725+123 Observed by SVOM-VT and Insights from Multi-wavelength Follow-up Observations

PKS 1725+123 is a flat-spectrum radio quasar (FSRQ) with a redshift of $z=0.586$. The detection of this object in the TeV band was reported by the MAGIC telescopes and H.E.S.S. in August 2025. Subsequently, we promptly initiated Target-of-Opportunity observations using the Space-based multi-band astronomical Variable Objects Monitor (SVOM) satellite. By analyzing the observational optical data from SVOM-VT and comprehensively examining the Fermi-LAT and Swift-XRT observational data, it was found that the source is in a high-flux state across the optical, X-ray, and GeV $\gamma$-ray bands around the time of the TeV detections. Its optical flux reaches a historically unprecedented high level and shows significant variability on timescale as short as minutes. The variability is accompanied by changes in the color index, exhibiting a bluer when brighter behavior during the high-flux state. Based on the simultaneous multi-wavelength data, we construct the broadband spectral energy distribution (SED) of the source in the high-flux state. PKS 1725+123 demonstrates a remarkably high synchrotron peak frequency, which is distinctly different from that of other FSRQs. We propose a two-zone spine-sheath jet model to reproduce this SED. The optical--X-ray emission is generated by the synchrotron process of the relativistic electrons within a compact zone. The inverse Compton (IC) scattering processes of the same electron population contribute to the low-energy end of the Fermi-LAT spectrum, while the high-energy end of the Fermi-LAT spectrum is ascribed to the IC scattering of the synchrotron photons within the compact zone by the higher-energy electrons in an extended region.

astro-ph.HE↗

arXiv2026

Spectral Variations of $\gamma$-rays in Mrk 421

We present a comprehensive analysis of the 17-year Fermi-LAT observational data of Mrk 421 to investigate the spectral variations in the $\gamma$-ray bands. The light curve of the source in the 0.1--1000 GeV band with a 14-day time bin exhibits significant variability at a confidence level exceeding 5$\sigma$, which is accompanied by spectral variation, displaying a {\it harder-when-brighter} behavior. Moreover, its flux variation can reach up to one order of magnitude within one day, with a daily flux up to $(1.19\pm0.84)\times10^{-8}~{\rm erg~cm^{-2}~s^{-1}}$ on MJD 56152. The 17-year integrated spectrum of Mrk 421 necessitates a complex model for explanation, whereas its time-resolved spectra over one-day or several-day time intervals can be well fitted by a power-law model. We propose that the complex spectral shape of the 17-year integrated spectrum stems from the superposition of different spectral shapes in different flux states. By generating the GeV spectra that are simultaneously observed with the archived TeV observations and constructing the combined GeV--TeV spectra, we find that some combined GeV--TeV spectral shapes clearly imply different radiation origins for the GeV and TeV emissions, challenging the one-zone leptonic model. It is found that the flux follows a lognormal distribution, while the photon spectral index distributions can be well fitted by either a lognormal or a Gaussian functions. The possible nature of the $\gamma$-ray variability in Mrk 421 is discussed.

astro-ph.HE↗

arXiv2025

X-ray Polarization of the High-Synchrotron-Peaked BL Lac H 1426+428

We report the X-ray polarization properties of the high-synchrotron-peaked BL Lac H 1426+428, based on two-epoch observational data from the Imaging X-ray Polarimetry Explorer (IXPE). For the first observation, only an upper limit of polarization degree ($\Pi_{\rm X}$), $\Pi_{\rm X}<19.5\%$, at the $99\%$ confidence level (C.L.) is determined. In contrast, for the second observation, we derive $\Pi_{\rm X}=20.6\%\pm2.9\%$ with a polarization angle ($\psi_{\rm X}$) of $\psi_{\rm X}=116.1^{\circ}\pm4.1^{\circ}$ at a C.L. of 7.1 $\sigma$. The time-resolved and energy-resolved polarization analysis reveals no significant variation in $\psi_{\rm X}$ and no detectable polarization within narrower energy bins for the first observation, while the polarization during the second observation is dominated by low-energy photons. Furthermore, the X-rays during the second observation are found to be in a higher flux state with a harder spectrum compared to that observed during the first observation, consistent with a {\it harder-when-brighter} behavior. We propose that the enhanced X-ray emission observed during the second observation is produced by shock-accelerated electrons within an ordered magnetic field region via synchrotron radiation. Nonetheless, no significant detection of polarization during the first IXPE observation may be due to the limited number of detected photons.

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arXiv2024

Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A

Pictor A is a $\gamma$-ray emitting radio galaxy and has a bright hotspot called WHS, located $\sim$4 arcmin away from the nucleus. In this work, we present an analysis of its 16-year Fermi-LAT data and report the Imaging X-ray Polarimetry Explorer (IXPE) observations for this source. Our analysis of the Fermi-LAT observations reveals evidence of two components in the average $\gamma$-ray spectrum of Pictor A, exhibiting a statistically significant hardening from $\Gamma_{\rm \gamma,1}=3.25\pm0.15$ to $\Gamma_{\rm \gamma,2}=1.81\pm0.07$ at a break energy of $2.46\pm0.09$ GeV. Notably, variability of $\gamma$-rays is evident in Pictor A, predominantly driven by the component below the break energy, while the component above the break energy remains stable. Furthermore, our analysis reveals that a power-law function provides an adequate fit for the high-flux-state spectrum, while a broken power-law function remains necessary to accurately model the low-flux-state spectrum. We suggest that the low-energy component originates from the nucleus, while the high-energy component primarily stems from WHS. The broadband spectral energy distributions of both nucleus and WHS can be well represented by a simple leptonic model, with both $\gamma$-ray components attributed to the synchrotron-self-Compton (SSC) process. Analysis of IXPE data provides upper limits on the polarization degree of $\Pi_{\rm X}<$6.6% for the nucleus and $\Pi_{\rm X}<$56.4% for the WHS within the 2--8 keV band. For the nucleus, this result aligns with X-ray emission originating from the SSC process. However, the upper limit of $\Pi_{\rm X}<$56.4% for WHS is insufficient to conclusively determine the X-ray emission mechanism in this region.

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arXiv2024

Study of {\gamma}-ray Emission from a Compact Radio Galaxy with the Fermi Large Area Telescope

The radio galaxy PKS 1007+142 is classified as a compact steep-spectrum source (CSS) and belongs to the class of young Active Galactic Nuclei (AGNs). In this paper, we investigate the $\gamma$-ray emission from this CSS by conducting a comprehensive analysis of the 15 yr Fermi Large Area Telescope (Fermi-LAT) observation data. The Fermi-LAT latest Source Catalog, 4FGL-DR4, includes an unassociated $\gamma$-ray source, 4FGL J1010.0+1416, located at 0.24{\deg} away from the radio position of PKS 1007+142. Using the 15 yr Fermi-LAT observation data, we re-estimate the best-fit position of the $\gamma$-ray source and find that PKS 1007+142 is in close proximity to the $\gamma$-ray source and falls within its 68% error circle. Therefore, we conclude that PKS 1007+142 is the most plausible counterpart to the unassociated LAT source with a detection test statistics (TS) $\sim$43.4 ($\sim 6.6\sigma$). PKS 1007+142 exhibits a steep power-law spectrum in the 0.1--300 GeV band, with a photon spectral index ($\Gamma_{\gamma}$) of $2.86\pm0.17$. The average flux in the considered time interval is $\rm (2.14\pm0.34)\times10^{-12}\ erg\ cm^{-2}\ s^{-1}$. Comparing PKS 1007+142 with other $\gamma$-ray emitting AGNs in both the $L_{\gamma}-\Gamma_{\gamma}$ and $L_\gamma-L_{\rm 1.4GHz}$ planes, it shows a softer $\gamma$-ray spectrum and lower luminosity compared to other $\gamma$-ray emitting CSSs. Furthermore, the possible origins of $\gamma$-ray in PKS 1007+142 are also discussed.

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arXiv2024

Multiwavelength Polarization Observations of Mrk 501

Mrk 501 is a prototypical high-synchrotron-peaked blazar (HBL) and serves as one of the primary targets for the {\it Imaging X-ray Polarimetry Explorer} ({\it IXPE}). In this study, we report X-ray polarization measurements of Mrk 501 based on six {\it IXPE} observations. The detection of X-ray polarization at a confidence level exceeding 99\% is achieved in four out of the six observations conducted across the entire energy range (2--8 keV) of {\it IXPE}. The maximum polarization degree ($\Pi_{\rm X}$) is measured to be $15.8\%\pm2.8\%$, accompanied by a polarization angle ($\psi_{\rm X}$) of $98.0\deg\pm5.1\deg$ at a confidence level of $5.6 \sigma$. During the remaining two observations, only an upper limit of $\Pi_{\rm X}<$12\% could be derived at the 99\% confidence level. No temporal variability in polarization is observed throughout all six {\it IXPE} observations for Mrk 501. A discernible trend of energy-dependent variation in the polarization degree is detected in optical spectropolarimetry; however, no analogous indication is observed in $\Pi_{\rm X}$. The chromatic behavior of $\Pi$ and the consistent values of $\psi$ across different frequencies from X-rays to radio waves, along with the agreement between $\psi$ and jet position angle, strongly support the interpretation of the energy-stratified model with shock-accelerated particles in the jet of Mrk 501. Additionally, the possibility of the presence of a global helical magnetic field in the jet of Mrk 501 is discussed.

astro-ph.HE↗

arXiv2024

Fermi Large Area Telescope Detection of Gamma-Rays from NGC 6251 Radio Lobe

We report on the detection of extended $\gamma$-ray emission from lobes in the radio galaxy NGC 6251 using observation data of Fermi Large Area Telescope (Fermi-LAT). The maximum likelihood analysis results show that a radio morphology template provides a better fit than a point-like source description for the observational data at a confidence level of 8.1$\sigma$, and the contribution of lobes accounts for more than 50\% of the total $\gamma$-ray flux. Furthermore, the $\gamma$-ray energy spectra show a significant disparity in shape between the core and lobe regions, with a curved log-parabola shape observed in core region and a power-law form observed in lobes. Neither the core region nor the northwest lobe displays the significant flux variations in the long-term $\gamma$-ray light curves. The broadband spectral energy distributions of both core region and northwest lobe can be will explained with a single-zone leptonic model. The $\gamma$-rays of core region are due to the synchrotron-self-Compton process while the $\gamma$-rays from northwest lobe are interpreted as inverse Compton emission of the cosmic microwave background.

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arXiv2023

X-Ray Polarization Variability of High Spectral Peak BL Lacertaes: Cases of 1ES 1959+650 and PKS 2155-304

The high-energy-peaked BL Lacertae objects (HBLs) are the main targets of the Imaging X-ray Polarimetry Explorer (IXPE) for investigating the mechanisms of radiation and particle acceleration in jets. In this paper, we report the first IXPE observations of two HBLs, 1ES 1959+650 and PKS 2155--304. Both sources exhibit X-ray polarization with a confidence level exceeding 99\%, as well as significant variability in polarization across different time intervals and energy ranges. Notably, PKS 2155--304 demonstrates the highest X-ray polarization among all blazars detected by IXPE within its entire energy band (2--8 keV), with a polarization degree of $\Pi_{\rm X}=21.9\%\pm1.9\%$ (MDP$_{99}\sim$6.0\%). An even higher polarization is observed in the 3--4 keV band, reaching $\Pi_{\rm X}=28.6\%\pm2.7\%$ (MDP$_{99}\sim$8.1\%) with a confidence level of 10.8$\sigma$. Furthermore, no polarization is detected above 5 keV energy band. For 1ES 1959+650, the highest detected polarization degree in the 2--8 keV band is $\Pi_{\rm X}=12.4\%\pm0.7\%$ (MDP$_{99}\sim$2.2\%), with an electric vector position angle (EVPA) of $\psi_{\rm X}=19.7^{\circ}\pm1.6^{\circ}$. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of $\psi_{\rm X}$, flux, spectrum, and energy bin. We discuss possible implications of these observational findings, including the variability in polarization, rotation of EVPA, and transition between synchrotron and synchrotron-self-Compton. We speculate that the X-rays observed during different IXPE observations originate from distinct regions in the jet and may involve diverse mechanisms for particle acceleration.

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