arXiv subjects

Zhen-Feng Sheng

Publications and source records attributed to Zhen-Feng Sheng.

5 recordsLinked to original sources

arXiv2026

The $γ$-ray-emitting blazar B3 1239+376 at $z$ = 3.82 identified in a multi-wavelength context

Among thousands of extragalactic $γ$-ray emitters, only a handful of distant ($z >$ 3) sources are detected. Yet, they are crucial probes shedding light on the cosmic evolution of jets of active galactic nuclei and the initial phase of mass growth of supermassive black holes. Here, we report on a multi-band study of the radio quasar B3 1239+376 with $z$ = 3.82. By analyzing the Fermi-LAT data, a significant (globally 7.7$σ$) $γ$-ray source in its direction, with an estimated association probability of 0.91, is observed in a half-year period of 2025. The analysis also reveals the emergence of co-spatial $γ$-ray residues in prior epochs. Moreover, the $γ$-ray and infrared light curves obtained from WISE and SPHEREx observations are likely correlated, as we observe that the emissions in both bands peak at the same time. The temporal coincidence establishes a firm association relationship between the $γ$-ray source and the quasar. Therefore, B3 1239+376 is proposed as the third most distant $γ$-ray detected blazar to date. Benefiting from the multi-wavelength observations, broadband spectral energy distributions in different flux states are compiled and reproduced by the classic one-zone leptonic radiation model to investigate the jet properties. Considering the recent brightening in $γ$ rays, prompt follow-up observations are encouraged, especially radio interferometry observations which may catch the potential ejection of a new jet blob.

astro-ph.HE↗

arXiv2025

Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. V. Dramatic Variability in High-Ionization Broad Emission Lines

We present a systematic search for changing-look (CL) quasars at high redshift z > 0.9 by cross-matching the spectroscopic datasets from the Dark Energy Spectroscopic Instrument Data Release 1 and Sloan Digital Sky Survey Data Release 18. We identify 97 CL quasars showing significant variability in high-ionization broad emission lines, including 45 turn-on and 52 turn-off events, corresponding to a detection rate of approximately 0.042%. This rate is lower than that found for low-ionization CL quasars, likely due to both selection effects and physical differences in high-ionization lines. Based on the CL quasar sample, we find that CL quasars generally exhibit lower accretion rates compared to typical quasars, with average Eddington ratios of log lambda_Edd approximately -1.14 in the bright state and approximately -1.39 in the dim state, compared to approximately -0.65 for typical quasars. While high-ionization lines in CL quasars follow the Baldwin effect on a population level, some individual sources show inverse Baldwin trends. We also find a positive correlation between the variability of high-ionization lines such as Mg II and C III] and the change in bolometric luminosity. In addition, we estimate a characteristic rest-frame timescale of approximately 3 years for CL transitions, with no significant difference between turn-on and turn-off cases. Taken together, these results support an accretion-driven origin for the CL phenomenon and provide new insights into the variability of high-ionization emission lines.

astro-ph.GA↗

arXiv2024

Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

AT2018cow is the most extensively observed and widely studied fast blue optical transient to date; its unique observational properties challenge all existing standard models. In this paper, we model the luminosity evolution of the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum of AT2018cow with a magnetar-centered engine model. We consider a two-zone model with a striped magnetar wind in the interior and an expanding ejecta outside. The soft and hard X-ray emission of AT2018cow can be explained by the leakage of high-energy photons produced by internal gradual magnetic dissipation in the striped magnetar wind, while the luminous thermal UV/optical emission results from the thermalization of the ejecta by the captured photons. The two-component energy spectrum yielded by our model with a quasi-thermal component from the optically thick region of the wind superimposed on an optically thin synchrotron component well reproduces the X-ray spectral shape of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in order to explain the very short rise time to peak of the thermal optical emission, a low ejecta mass $M_{\rm ej}\approx0.1~M_\odot$ and high ejecta velocity $v_{\rm SN}\approx0.17c$ are required. A millisecond magnetar with $P_0\approx3.7~\rm ms$ and $B_p\approx2.4\times10^{14}~\rm G$ is needed to serve as the central engine of AT2018cow.

astro-ph.HE↗

arXiv2022

GB6 J2113+1121: A multi-wavelength flaring gamma-ray blazar temporally and spatially coincident with the neutrino event IceCube-191001A

A radio-emitting tidal disruption event (AT2019dsg) is proposed as a likely counterpart of the IceCube neutrino event IC-191001A. In this work we have revisited the {\it Fermi}-LAT data in the direction of the neutrino and confirmed no signal at the site of AT2019dsg. Instead, at the edge of the 90\% confidential level error region of this neutrino there is a $γ$-ray transient source associated with a blazar GB6 J2113+1121. In May 2019, GB6 J2113+1121 was undergoing an unprecedented $γ$-ray flare since the start of the {\it Fermi}-LAT operation, with a variability amplitude about 20-fold. Similar violent flares of GB6 J2113+1121, unobserved before, have been also detected observed in optical bands. Moreover, the blazar remained in a high flux state in the infrared bands when IceCube-191001A arrived, though its $γ$-ray and optical activities has temporally ceased. Motivated by the spatial and temporal coincidence, we suggest that GB6 J2113+1121 is a candidate of the counterpart of IC-191001A. The jet properties of GB6 J2113+1121 are investigated, which are found to be comparable with that of the neutrino-emitting blazars (candidates). A specific analysis of archival IceCube data in this direction and future observations would put a further constraint on the origin of the neutrino.

astro-ph.HE↗

arXiv2020

Detections of simultaneous brightening of gamma-ray and optical emissions of a distant blazar GB 1508+5714 at redshift 4.3

GB 1508+5714 is a high-redshift blazar ($z$ = 4.3) and a spectrally soft $γ$-ray source has been detected in its direction. By analyzing 11.4-yr {\it Fermi}-LAT data, significant long-term variability of the $γ$-ray source is confirmed. More importantly, a $γ$-ray emission flare appeared in an epoch of several tens of days in year 2018, when the flux is about four times of the value from the global fit. Meanwhile, optical flares displayed in both $r$- and $i$-bands from the {\it Zwicky} Transient Facility light curves. Detections of the simultaneous $γ$-ray and optical brightening provide a decisive evidence to pin down the association between the $γ$-ray source and GB 1508+5714, which makes it the {\it first} identified $γ$-ray blazar beyond redshift 4. A broadband spectral energy distribution in the high flux state is constructed and the origin of the multiwavelength brightening is also briefly discussed. Upcoming wide-deep-fast optical telescopes together with the $γ$-ray sky surveyors will shed lights on the role that the AGN jets play in the early cosmic time.

astro-ph.HE↗