Tibet AS-gamma Experiment

Present status of our research activities:

Detection of TeV gamma rays from celestial point sources.

We successfully observed TeV gamma-ray signals from the Crab Nebula for the first time in the world as an air shower array (Fig. 6, 7). TeV gamma-ray signals from active galactic nuclei Markarian 501 were observed in their flaring states in spring 1997 (Fig. 7,10). Recently we detected TeV gamma-ray signals from another active galactic nuclei Markarian 421 which were very active during some period between the year 2000 and 2001 (Fig. 8,11,12). Our air shower array is suitable for observing an active celestial point source emitting high-energy gamma rays, as it constantly gives a wide view (pi steradian) sky survey, very weakly dependent on weather condition.

Fig. 6 Fig. 7 Fig. 8
Fig. 6 Contour map of the weights of excess event densities around the Crab Nebula centered in the figure. The contour lines are drawn with a step of 1 sigma. Angular distance is measured from the direction of the Crab Nebula along the right ascension (abscissa) and the declination (ordinate). Fig. 7 Contour map of the weights of excess event densities around Markarian501 centered in the figure. The contour lines are drawn with a step of 1 sigma. Angular distance is measured from the direction of Mrk501 along the right ascension (abscissa) and the declination (ordinate). Fig. 8 Contour map of the weights of excess event densities around Markarian421 centered in the figure. The contour lines are drawn with a step of 1 sigma. Angular distance is measured from the direction of Mrk421 along the right ascension (abscissa) and the declination (ordinate).

Fig. 9 Fig. 10
Fig. 9 Energy spectra of gamma rays
from the Crab Nebula.
Fig. 10 Energy spectra of gamma rays
from the Markarian 501.

Fig. 11 Fig. 12
Fig. 11 Time profile of our gamma-ray signals
(31-day moving average) from Markarian 421.
Also plotted is the correlation between our
observation and those by X-ray satellite RXTE
and Cherenkov TeV gamma-ray detectors.
Fig. 12 Energy spectrum of gamma-ray signals
from the Markarian 421.