This paper presents searches for flaring sources of neutrinos using the
IceCube neutrino telescope. For the first time, a search is performed over the
entire parameter space of energy, direction and time looking for neutrino
flares of 20 microseconds to a year duration from astrophysical sources among
the atmospheric neutrino and muon backgrounds. Searches which integrate over
time are less sensitive to flares because they are affected by a larger
background of atmospheric neutrinos and muons that can be reduced by the time
constraint. Flaring sources considered here, such as active galactic nuclei,
soft gamma-ray repeaters and gamma-ray bursts, are promising candidate neutrino
emitters. We used mainly data taken between April 5, 2008 and May 20, 2009 by a
partially completed configuration of IceCube with 40 strings. For the presented
searches an unbinned maximum likelihood method is used with a time-dependent
prior to test several different source hypotheses. An "untriggered" search
covers any possible time-dependent emission from sources not correlated to any
other observation using other astrophysical messengers such as photons.
Moreover, a similar time scan is performed for a predefined catalogue of
sources that exhibit intense photon flares. Searches triggered by
multi-wavelength information on flares from blazars and soft gamma-ray
repeaters are performed using the 40 string data and also the data taken by the
previous configuration of 22 strings in operation between May 31, 2007 and
April 5, 2008. Flares for which extensive and continuous monitoring is
available from Fermi-LAT and SWIFT and flares detected by imaging Cherenkov
telescopes with shorter time-scale monitoring are considered. The results from
all searches are compatible with a fluctuation of the background.Comment: 37 pages, 14 figures. Accepted to the Astrophysical Journa