arXiv subjects

Tan-Zheng Wu

Publications and source records attributed to Tan-Zheng Wu.

5 recordsLinked to original sources

arXiv2025

X-ray Emission Properties of a Compact Symmetric Object Sample

We present a comprehensive analysis of the X-ray observations obtained from \xmm\, and \chandra\, for a sample of bona-fide Compact Symmetric Objects (CSOs) to investigate their X-ray emission properties. Ultimately, we obtain 32 effective X-ray observational spectra from 17 CSOs. Most spectra can be well described by an absorbed single power-law model, with the exception of 6 spectra requiring an additional component in the soft X-ray band and 2 spectra exhibiting an iron emission line component. The data analysis results unveil the diverse characteristics of X-ray emission from CSOs. The sample covers X-ray luminosity ranging within $10^{40}-10^{45}$ erg s$^{-1}$, intrinsic absorbing column density ($N_{\rm H}^{\rm int}$) ranging within $10^{20}-10^{23}$ cm$^{-2}$, and photon spectral index ($Γ_{\rm X}$) ranging within 0.75--3.0. None of the CSOs in our sample have $N_{\rm H}^{\rm int}$ > $10^{23}\rm~cm^{-2}$, indicating that the X-ray emission in these CSOs is not highly obscured. The distribution of $Γ_{\rm X}$ for these CSOs closely resembles that observed in a sample of radio-loud quasars and low-excitation radio galaxies (RGs). In the radio--X-ray luminosity panel, these CSOs exhibit a distribution more akin to FR I RGs than FR II RGs, characterized by higher luminosities. The positive correlation between $Γ_{\rm X}$ and the Eddington ratio, which has been noted in radio-quiet active galactic nuclei, is not observed in these CSOs. These findings suggest that although the contribution of the disk-corona system cannot be completely ruled out, jet/lobe radiation likely plays a dominant role in the X-ray emission of these CSOs.

astro-ph.HE↗

arXiv2024

Origin of the Very High Energy γ-rays in the Low-luminosity Active Galactic Nucleus NGC 4278

NGC 4278, a Low-luminosity active galactic nucleus (AGN), is generally classified as a low-ionization nuclear emission line region (LINER). Recently, it has been reported to be associated with a very high energy $γ$-ray source 1LHAASO J1219+2915 in the first Large High Altitude Air Shower Observatory source catalog. However, no associated counterpart has been detected by analyzing the data collected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. By analyzing its X-ray observation data from Swift-XRT, we find NGC 4278 is in a high-flux state on MJD 59546, with the X-ray flux more than one order of magnitude higher than that observed $\sim$ 11.7 year earlier by Chandra. Interestingly, this Swift-XRT observation was conducted during the active phase of the $γ$-ray source 1LHAASO J1219+2915. We propose that the detection of VHE $γ$-rays from NGC 4278 may be attributed to the presence of an active nucleus in its center. To reproduce the spectral energy distribution (SED) of NGC 4278, we employ a one-zone leptonic model, typically used for fitting broadband SEDs of BL Lacs, and find that a smaller magnetic field strength is required than that of typical TeV BL Lacs. Furthermore, NGC 4278 exhibits significantly lower luminosity in both radio and TeV bands when compared with typical TeV BL Lacs. In the radio-luminosity vs. Eddington-ratio plane, NGC 4278 shows greater similarity to Seyfert galaxies and LINERs rather than BL Lacs; however, it still roughly follows the extension towards lower luminosity seen in BL Lacs.

astro-ph.HE↗

arXiv2024

Study of γ-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 $γ$-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 $γ$-ray source, 4FGL J1010.0+1416, located at 0.24° 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 $γ$-ray source and find that PKS 1007+142 is in close proximity to the $γ$-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 6.6σ$). PKS 1007+142 exhibits a steep power-law spectrum in the 0.1--300 GeV band, with a photon spectral index ($Γ_γ$) 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 $γ$-ray emitting AGNs in both the $L_γ-Γ_γ$ and $L_γ-L_{\rm 1.4GHz}$ planes, it shows a softer $γ$-ray spectrum and lower luminosity compared to other $γ$-ray emitting CSSs. Furthermore, the possible origins of $γ$-ray in PKS 1007+142 are also discussed.

astro-ph.HE↗

arXiv2024

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 $Π_{\rm X}=21.9\%\pm1.9\%$ (MDP$_{99}\sim$6.0\%). An even higher polarization is observed in the 3--4 keV band, reaching $Π_{\rm X}=28.6\%\pm2.7\%$ (MDP$_{99}\sim$8.1\%) with a confidence level of 10.8$σ$. 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 $Π_{\rm X}=12.4\%\pm0.7\%$ (MDP$_{99}\sim$2.2\%), with an electric vector position angle (EVPA) of $ψ_{\rm X}=19.7^{\circ}\pm1.6^{\circ}$. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of $ψ_{\rm X}$, 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.

astro-ph.HE↗

arXiv2024

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

We report on the detection of extended $γ$-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$σ$, and the contribution of lobes accounts for more than 50\% of the total $γ$-ray flux. Furthermore, the $γ$-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 $γ$-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 $γ$-rays of core region are due to the synchrotron-self-Compton process while the $γ$-rays from northwest lobe are interpreted as inverse Compton emission of the cosmic microwave background.

astro-ph.HE↗