Lower Bounds on Intergalactic Magnetic Fields from Simultaneously Observed GeV-TeV Light Curves of the Blazar Mrk 501

We derive lower bounds on intergalactic magnetic fields (IGMFs) from upper limits on the pair echo emission from the blazar Mrk 501, that is, delayed GeV emission from secondary $e^{-}e^{+}$ pairs produced via interactions of primary TeV gamma rays with the cosmic infrared background. Utilizing only simultaneous GeV-TeV light curves observed by VERITAS, MAGIC and {\it Fermi}-LAT during a multiwavelength campaign in 2009 that included a TeV flare, bounds are deduced on the IGMF strength of $B \gtrsim 10^{-20} {\rm G}$ at 90% confidence level for a field coherence length of 1 kpc. Since our analysis is based firmly on the observational data alone and nearly free of assumptions concerning the primary TeV flux in unobserved periods or spectral bands, our evaluation of the pair echo flux is conservative and the evidence for a non-zero IGMF is more robust compared to previous studies.