高能暂现源 · 每日文献综述

We present a detailed study of X-ray spectral properties observed in four classical TeV (teraelectronvolt) photon-emitting high synchrotron-peaked BL Lacertae (BL Lac) objects using the simultaneous data of the Neutron Star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array Mission (NuSTAR) satellites. We analyzed 13 spectra in total from four BL Lac objects: Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304. We fitted all the spectra using the absorbed log-parabolic (LP) model first. While seven spectra were fitted well using the absorbed LP model, we observed that four spectra of Mrk 421 and two spectra of Mrk 501 were not fitted satisfactorily using the absorbed LP model. The investigation of the flux states of the sources revealed that Mrk 421 was in a moderate- to low-flux state during the four epochs and Mrk 501 was in a low-flux state during the two epochs. We concluded that there was a contribution from the disk in these six spectra. The moderate- to low-flux state can justify the contribution of disk emission in the X-ray spectra. In the case of four spectra of Mrk 421, we observed a Gaussian feature between 1.42 and 1.70 keV.

Changing-look active galactic nuclei (CL-AGNs) provide a unique opportunity to probe the coupling between accretion flows and relativistic jets in supermassive black holes. We investigate the long-term radio behavior of CL-AGNs over ∼20 yr by combining FIRST and VLASS observations with quasi-simultaneous optical spectroscopy and photometry. From a parent sample of 1092 CL-AGNs, we identify 58 sources with radio detections. Radio-detected CL-AGNs exhibit systematically higher radio kinetic efficiency, quantified by P<SUB>j</SUB>/L<SUB>bol</SUB>, than both typical radio-detected AGNs and radio transients, consistent with their preference for low Eddington ratios. At the population level, the expected anticorrelation between radio emission and accretion rate is weak. However, a clear source-by-source anticorrelation emerges in a small subset of CL-AGNs with continuous multiepoch coverage. We further identify four radio transients, including both radio turn-on and turn-off events, and one source exhibiting a multiwavelength flare that may be indicative of tidal disruption event-like activity. These results suggest that radio activity in CL-AGNs is not governed by instantaneous accretion state changes but is instead regulated by long-term accretion history and jet evolution, with additional stochastic or transient channels contributing in rare cases.