Upper limits on the solar-neutron flux at the Yangbajing neutron monitor from BATSE-detected solar flares

Aims. The purpose of this work is to search the data from Yangbajing neutron monitor obtained between 1998 October and 2000 June for the solar neutrons associated with solar flares. Methods. Using the onset times of 166 BATSE-detected flares with the GOES peak flux (1-8 angstrom) higher than 1.0 x 10(-5) Wm(-2), we prepared a light curve of the Yangbajing neutron monitor for each flare, spanning +/-1.5 h from the BATSE onset time. Based on the light curves, a systematic search for solar neutrons in energies above 100 MeV from the 166 flares was performed. No statistically significant signals due to solar neutrons were found. Therefore, we put upper limits on the >= 100 MeV solar-neutron flux for 18 events consisting of 2 X and 16 M class flares. The calculation assumed a power-law shaped neutron energy spectrum and three types of neutron emission profiles at the Sun. Results. Compared with the other positive neutron detections associated with X-class flares, typical 95% confidence level upper limits for the two X-class flares are found to be comparable to the lowest and second lowest neutron fluxes at the top of the atmosphere. In addition, the upper limits for M-class flares scatter in the range of 10(-2) to 1 neutrons cm(-2) s(-1). This provides the first upper limits on the solar-neutron flux from M-class solar flares, using space observatories, as well as ground-based neutron monitors.