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The Fermi/LAT team is organizing a multi-wavelength campaign on the blazar Markarian 501, starting on March 2011 and finishing on August September 2011.
The goal for this time interval is to sample every 3 days (approx.) the peaks of the 2 components in the SED of Mrk501 (optical/X-ray and GeV/TeV)
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The instruments participating in the campaign, together with the (approximate) observation schedule can be found here here.