Daily average light curves for Mrk 501 from April 17 to May 5, 2009. Each dotted horizontal line represents a constant line fit for each instrument involved. Top: OVRO at 15 GHz (black filled circles) and Metsähovi at 37 GHz (red filled diamonds); Second: Mitsume in g band (blue filled diamonds) and GASP in R band (black filled circles). Third: Swift-UVOT in the ultraviolet, with three different bands, UVW1 (260 nm, blue diamonds), UVM2 (220 nm, black circles) and UVW2 (193 nm, red squares). Fourth: X-ray: Swift-XRT 0.3−2 keV. Fifth: X-ray: RXTE-PCA (blue squares) and Swift-XRT (black circles) 2−10 keV (nightly average); Sixth: HE gamma-ray: Fermi-LAT (E> 300 MeV; 3-day average); Bottom: VHE γ-rays: Whipple (E> 400 GeV, normalized to E> 300 GeV; red filled stars) and VERITAS (E> 300 GeV; black filled circles). The different dotted lines are the constant fit for the low and high state.