Central Bureau for Astronomical Telegrams
IAUC 6592: Mkn 501
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Circular No. 6592
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
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Phone 617-495-7244/7440/7444 (for emergency use only)
MARKARIAN 501
A. C. Breslin, P. Boyle, J. Buckley, M. Catanese, F. Krennrich,
J. McEnery, J. Quinn, J. Rose, R. Srinivasan, and T. C. Weekes
report for the Whipple Observatory Gamma Ray Collaboration: "TeV
emission from the BL Lac object Mkn 501 has been in a high state
since Feb. 14, when its flux was observed to be comparable to that
from the Crab nebula. Thirty hours of observations were made with
the Whipple Observatory 10-m reflector (+ 151-pixel camera) at an
energy threshold of 300 GeV. Since Mar. 5, the emission level has
increased, reaching a peak on Mar. 13 at 2-2.5 times that of the
Crab in a 4-hr observation. By observing over a range of
elevations, we observed a broad spectrum of energies; the spectrum
is steeper than that of the Crab and Mkn 421 above 2 TeV, and it
extends at least to energies of 5 TeV (5-sigma significance). The
flux is variable on a timescale of hours to days, but on average
from Feb. 10 through Mar. 15 is comparable to the Crab at the
threshold energy; at discovery in 1995, the flux was 0.08 times
that of the Crab (Quinn et al. 1996, Ap.J. 456, L83)."
B. Degrange for the CAT group [Cerenkov Array at Themis,
incorporating the Centre d'Etudes Nucleaire de Bordeaux-Gradignan;
Centre d'Etude Spatiale des Rayonnements, Toulouse; Laboratoire de
Physique Corpusculaire, College de France; Laboratoire de Physique
Nucleaire de Hautes Energies at Ecole Polytechnique and Paris;
Service d'Astrophysique, Centre d'Etudes de Saclay; and
Universities of Perpignan, Charles, and Olmouc], operating a very-
high-energy (VHE) gamma-ray detector in the French Pyrenees, have
observed Mkn 501 in a high state for 18.55 hr (and a control region
for 4.54 hr) on Mar. 7-13 and 17-18. The maximum of emission was
observed on Mar. 13, with a flux at a level of 4 times that of the
Crab Nebula, for which the emission was seen over the energy range
from 0.3 to > 5 TeV; on this particular night, the emission seemed
stable at this high level during the observation time (1.71 hr).
Also, variations on a half-hour timescale were seen on Mar. 10.
D. Kranich, E. Lorenz, D. Petry, and G. Rauterberg report that
the High Energy Gamma Ray Array (HEGRA) collaboration, operating a
cosmic-ray detector complex for VHE and ultra-high-energy gamma-ray
astronomy on La Palma, has observed unusually strong VHE gamma-ray
emission from Mkn 501 with a 5-mE2 air Cerenkov telescope on every
night during Mar. 10-17. A mean flux of about 1.5 x 10E-11 cmE-2
sE-1 was observed above 2.5 TeV. The flux is about 10 times higher
than observed in a 200-hr observation in 1996. The spectrum
extends above 7 TeV. Observations are continuing with the full
HEGRA array of six telescopes.
(C) Copyright 1997 CBAT
1997 March 19 (6592) Daniel W. E. Green
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