  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.
