Jian-Kang Li

Building on prior FAST targeted and blind SETI campaigns toward 33 exoplanet systems, we introduce a wavelet-integrated search pipeline for narrowband technosignature candidates in dynamic radio spectra. At its core, the pipeline uses a Multiscale Wavelet Net to produce an interpretable multiresolution representation, followed by a lightweight parameter estimator for endpoint localization. Rather than relying solely on hard-threshold drift searches, the pipeline reframes narrowband detection as wavelet-guided feature extraction followed by endpoint regression, morphology-aware filtering, raw-data signal-to-noise ratio validation, and multibeam anticoincidence veto. Applied to real FAST data, the pipeline recovers representative events from prior analyses and produces a compact set of veto-ready candidates for downstream inspection. The resulting workflow preserves interpretability, low-regression complexity, and auditable threshold control, making it readily transferable to other radio surveys and large-scale technosignature searches.

The Five-hundred-meter Aperture Spherical Telescope (FAST), the world’s largest single-dish radio telescope, lists the search for extraterrestrial intelligence (SETI) as one of its key scientific objectives. In this work, we present a targeted SETI observation for seven nearby active stars utilizing the FAST L-band multibeam receiver, employing an observational strategy that combines position switching with multibeam tracking to balance on-source integration time with the accuracy of the beam response. Using both pulsar and SETI backends, we perform a comprehensive search for narrowband drifting signals with Doppler drift rates within diversified ranges and channel-width periodic signals with periods between 0.12 and 100 s and duty cycles between 10% and 50%. No credible radio technosignatures were detected from any of the target systems. Based on this null result, we place constraints on the presence of transmitters at a 95% confidence level, ruling out narrowband transmitters with EIRP above 3.98 × 10 ⁸ W and periodic transmitters with EIRP above 1.80 × 10 ¹⁰ W within the observation band.

Electromagnetic waves (EMWs) can be generated by gravitational waves (GWs) within a magnetic field via the Gertsenshtein effect. The conversion probability between GWs and EMWs can be enhanced by inhomogeneities in the electron density and magnetic field within the magnetized plasma of both the Milky Way (MW) and the intergalactic medium in the expanding Universe. Polarized GWs can induce polarized EMWs, and the polarization properties of these EMWs can be altered by Faraday rotation as they propagate through magnetized plasma. Additionally, the polarization intensity of the EMWs may be weakened due to depolarization effects. In this study, we calculate the enhanced GW–EMW conversion in inhomogeneous magnetized plasma during the propagation of GWs through the Universe and our galaxy. We analyze the polarization states of the EMWs generated by polarized GWs and discuss the depolarization effects induced by the medium’s irregularities, as well as the differential Faraday rotation occurring in multilayer polarized radiation. Our work provides alternative methods for detecting GWs and exploring their polarization states, and potentially constraining the parameters of the possible GW sources, especially the primordial black hole, contributing to the advancement of very-high-frequency GW detection and research.

In this paper, we propose a novel method for distinguishing extraterrestrial intelligence (ETI) signals from radio frequency interference (RFI) by leveraging polarization features. We exploit the sinusoidal variation of the linearly polarized components of Stokes parameters with the parallactic angle as a characteristic signature of ETI signals, while such linearly polarized components remain relatively stable for terrestrial RFI. Typically, a minimum of 4–8 hr of observation time is required to detect these sinusoidal variations. The polarization approach in the search for extraterrestrial intelligence also enables us to study the radio stellar bursts emitted by M-type stars as ancillary science, which is relevant to assessing the habitability of exoplanets. Compared to the frequency drift method, the polarization method effectively reduces the required observation time for signal identification while improving the signal identification process.

The Doppler shift of a radio signal is caused by the relative motion between the transmitter and receiver. The change in frequency of the signal over time is called drift rate. In the studies of radio SETI (Search for Extraterrestrial Intelligence), extraterrestrial narrowband signals are expected to appear "chirped" since both the exoplanet and the Earth are moving. Such planet rotation and orbital revolution around the central star can cause a non-zero drift rate. Other relative motions between the transmitter and receiver, such as the gravitational redshift and galactic potential, are negligible. In this paper, we mainly consider the common cases that the drift rate is contributed by the rotations and orbits of the Earth and exoplanet in celestial mechanics perspective, and briefly discuss other cases different from the Earth-exoplanet one. We can obtain the expected pseudosinusoidal drifting result with long-term observations, shorter orbital periods of exoplanets. Exoplanets with higher orbital eccentricities can cause asymmetric drifting. The expected result should be intermittent pseudosinusoidal curves in long-term observations. The characteristics of pseudo-sinusoidal curves, as another new criterion for extraterrestrial signals, can be applied to long-term SETI reobservations in future research.

The Doppler shift of a radio signal is caused by the relative motion between the transmitter and receiver. The change in frequency of the signal over time is called the drift rate. In the studies of radio search for extraterrestrial intelligence (SETI), extraterrestrial narrowband signals are expected to appear “chirped” since both the exoplanet and Earth are moving. Such planet rotation and orbital revolution around the central star can cause a nonzero drift rate. Other relative motions between the transmitter and receiver, such as the gravitational redshift and galactic potential, are negligible. In this paper, we mainly consider the common cases in which the drift rate is contributed by the rotations and orbits of Earth and the exoplanet in the celestial mechanics perspective, and we briefly discuss other cases different from the Earth–exoplanet one. We can obtain the expected pseudosinusoidal drifting result with long-term observations and shorter orbital periods of exoplanets. Exoplanets with higher orbital eccentricities can cause asymmetric drifting. The expected result should be intermittent pseudosinusoidal curves in long-term observations. The characteristics of pseudosinusoidal curves, as another new criterion for extraterrestrial signals, can be applied to long-term SETI reobservations in future research.