Quinn, J. and Bond, I.H. and Boyle, P.J. and Bradbury, S.M. and Breslin, A.C. and Buckley, J.H. and Burdett, A.M. and Bussons Gordo, J. and Carter-Lewis, D.A. and Catanese, M. and Cawley, Michael F. and Fegan, D.J. and Finley, J.P. and Gaidos, J.A. and Hall, T. and Hillas, A.M. and Krennrich, F. and Lamb, R.C. and Lessard, R.W. and Masterson, C. and McEnery, J.E. and Moriarty, P. and Rodgers, A.J. and Rose, H.J. and Samuelson, F.W. and Sembroski, G.H. and Srinivasan, R. and Vassiliev, V.V. and Weekes, T.C. (1999) The Flux Variability of Markarian 501 in Very High Energy Gamma Rays. Astrophysical Journal, 518 (693). ISSN 0004-637X

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Abstract

The BL Lacertae object Markarian 501 was identified as a source of γ-ray emission at the Whipple Observatory in 1995 March. Here we present a flux variability analysis on several timescales of the 233 hr data set accumulated over 213 nights (from March 1995 to July 1998) with the Whipple Observatory 10 m atmospheric Cerenkov imaging telescope. In 1995, with the exception of a single night, the flux from Markarian 501 was constant on daily and monthly timescales and had an average flux of only 10% that of the Crab Nebula, making it the weakest very high energy source detected to date. In 1996, the average flux was approximately twice the 1995 flux and showed significant month-to-month variability. No significant day-scale variations were detected. The average γ-ray flux above ~350 GeV in the 1997 observing season rose to 1.4 times that of the Crab Nebula—14 times the 1995 discovery level—allowing a search for variability on timescales shorter than 1 day. Significant hour-scale variability was present in the 1997 data, with the shortest, observed on MJD 50,607, having a doubling time of ~2 hr. In 1998 the average emission level decreased considerably from that of 1997 (to ~20% of the Crab Nebula flux), but two significant flaring events were observed. Thus the emission from Markarian 501 shows large amplitude and rapid flux variability at very high energies, as does Markarian 421. It also shows large mean flux level variations on year-to-year timescales, behavior that has not been seen from Markarian 421 so far.

Item Type:	Article
Additional Information:	Citation: J. Quinn et al 1999 ApJ 518 693
Keywords:	BL Lacertae objects: individual; Markarian 501; gamma rays : observations;
Academic Unit:	Faculty of Science and Engineering > Experimental Physics
Item ID:	9947
Identification Number:	https://doi.org/10.1086/307329
Depositing User:	Dr. Michael Cawley
Date Deposited:	18 Sep 2018 15:23
Journal or Publication Title:	Astrophysical Journal
Publisher:	American Astronomical Society
Refereed:	Yes
URI:	http://iopscience.iop.org
Use Licence:	This item is available under a Creative Commons Attribution Non Commercial Share Alike Licence (CC BY-NC-SA). Details of this licence are available here

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