Detection of multi-TeV gamma rays from Markarian 501 during an unforeseen flaring state in 1997 with the Tibet air shower array

Amenomori, M Ayabe, S Cao, PY",Danzengluobu,"Ding, LK Feng, ZY Fu, Y Guo, HW He, M Hibino, K Hotta, N Huang, Q Huo, AX Izu, K Jia, HY Kajino, F Kasahara, K Katayose, Y",Labaciren,"Li, JY Lu, H Lu, SL Luo, GX Meng, XR Mizutani, K Mu, J Nanjo, H Nishizawa, M Ohnishi, M Ohta, I Ouchi, T Ren, JR Saito, T Sakata, M Sasaki, T Shi, ZZ Shibata, M Shiomi, A Shirai, T Sugimoto, H Taira, K Tan, YH Tateyama, N Torii, S Utsugi, T Wang, CR Wang, H Xu, XW Yamamoto, Y Yu, GC Yuan, AF Yuda, T Zhang, CS Zhang, HM Zhang, JL Zhang, NJ Zhang, XY",Zhaxisangzhu,Zhaxiciren,"Zhou, WD

M. Amenomori et al., Detection of multi-TeV gamma rays from Markarian 501 during an unforeseen flaring state in 1997 with the Tibet air shower array, ASTROPHYS J, 532(1), 2000, pp. 302-307

16

INGLESE

Article

Space Sciences

ASTROPHYSICAL JOURNAL

0004-637X → ACNP

532

1

2000

1

302 - 307

ISI

0004-637X(20000320)532:1<302:DOMGRF>2.0.ZU;2-B

In 1997, the BL Lac object Mrk 501 entered a very active phase and was the brightest source in the sky at TeV energies, showing strong and frequent ba ring. Using the data obtained with a high-density air shower array that has been operating successfully at Yangbajing in Tibet since 1996, we searched for gamma-ray signals from this source during the period in 1997 from Febr uary through August. Our observation detected multi-TeV gamma-ray signals a t the 3.7 sigma level during this period. The most rapid increase in the ex cess counts was observed between April 7 and June 16, and the statistical s ignificance of the excess counts in this period was 4.7 sigma. Among severa l observations of flaring TeV gamma-rays from Mrk 501 in 1997, this is the only observation using a conventional air shower array. We present the ener gy spectrum of gamma-rays, which will be worth comparing with those obtaine d by imaging atmospheric Cerenkov telescopes.