The origin of ultra-high-energy cosmic rays (UHECRs; ≳ 10 EeV) is unknown. Gamma-rays and neutrinos produced in CR-induced hadronic interactions can serve as the smoking gun pointing back to sources. Motivated by the fact that IceCube-measured diffuse TeV neutrino flux is comparable to Waxman-Bahcall bound derived from the detected UHECR flux, we assume a common origin of UHECRs and TeV neutrinos, and expect TeV hadronic gamma-rays associated with UHECRs as well, the detection probability of which depends on UHECR source density. Here we use LHAASO-WCDA to search for TeV gamma-rays associated with UHECRs. A detailed data analysis based on LHAASO-WCDA sky map and UHECR events detected by Telescope Array results in non-detection of gamma-ray signals. A lower limit is put on the source number density, 𝑛 𝑠 > 10 −3.5 Mpc −3 , with 95% C.L.

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With high duty cycle and wide aperture, the Large High Altitude Air Shower Observatory-Water Cherenkov Detector Array(LHAASO-WCDA) can conduct an unbiased gamma-ray sky survey in the energy range from a few hundred GeV to 100 TeV. The sensitivity of WCDA is as high as a few percent of Crab units, which allow us to monitor the VHE variability of blazars. The LHAASO Collaboration has developed an online monitoring program to monitor the extragalactic VHE flare in the WCDA's field of view. Once a flare exceeding the threshold is detected, an alert will be sent to other instruments automatically, and a follow-up multiwavelength observation could be carried out. In this paper, the current status of the monitor, the candidate sources, background estimation method, data reduction, testing running result are presented.

The Water Cherenkov Detector Array (WCDA) is an important component of Large High Altitude Air Shower Observatory (LHAASO). The main scientific target of WCDA is continuously surveying the northern sky at the level of few percent of Crab unit. In this proceeding, based on its first year data of WCDA-1 data, the result about the Crab Nebular observation and its energy spectrum measurment have been presented. And with WCDA full array data taking since Mar. 2021, some preliminary results have been reported.