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Paper text:
in a neutrino point source analysis one focuses on obtaining the best angular resolution by observing muon tracks inside the detector volume .
the expected signal stems from charged - current interactions of muon neutrinos and a small fraction of tau neutrinos .
the background consists primarily of high energy muons , produced in the atmosphere in extended air showers , with a much lower but irreducible contribution of atmospheric neutrinos from the same interactions . in icecube
[ ] , cherenkov light from these events is detected in digital optical modules , with 60 of these arranged vertically on a string .
a total of 80 strings will be installed until 2011 , situated at a depth of 1.45 to 2.45  km inside the glacial ice at the geographic south pole . a standard method to suppress atmospheric muons , reaching the detector from above ,
is a cut on the reconstructed track direction at the horizon , which allows icecube to cover only the northern hemisphere , as presented in [ ] . inside the earth ,
the neutrino - nucleus cross section rises with energy and prevents the majority of up - going neutrinos with energies above a pev to reach the vicinity of the detector . in our extended analysis ,
down - going tracks are included and their energy is used to separate signal neutrinos from atmospheric background , based on the softer spectrum of the latter .
different zenith dependent cuts make it possible to perform a point source search above and below the horizon .
the analysis presented here used icecube data collected between may 2007 and april 2008 ( 276 days ) with 22 strings .
an on - line filter stream provided an all - sky muon track sample with a rate of @xmath0 hz , and elaborate direction reconstructions were performed off - line .
down - going events with declinations smaller than @xmath1 were rejected due the small interaction volume above the detector leading to very low signal expectations .
the steeply rising muon background above the horizon is dominated by multiple muons forming so - called bundles . to separate them from neutrino - induced single muon tracks ,
a selection cut based on the resolution of individual photon pulses in an optical module was applied .
the threshold values were a function of the reconstructed zenith angle and energy for each event .
employing parameters depending on the quality of the fitted track , similar to those in [ ] , the background was reduced further . by systematically testing combinations of threshold values in dependence of zenith angle
, these cuts were optimized for the best sensitivity considering two template spectra , a standard @xmath2 and a harder @xmath3 power law of the neutrino energy @xmath4 .
+ the resulting sample is composed of 1877 events between @xmath1 and @xmath5 .
simulations show that the down - going events from the southern hemisphere are dominated by atmospheric muons , while the up - going tracks are mainly induced by atmospheric neutrinos .
related to zenith dependent signal and background expectations , the cuts allow for an excess of events in the region just above the horizon . the neutrino signal efficiency , as estimated from simulations , improves with energy and effectively defines a threshold rising from the tev regime in the northern hemisphere to pev energies in the south . compared to previous point source analyses in icecube  [ ] ,
an improved direction reconstruction was used .
it is a likelihood algorithm that uses not only the timing information of the first pulse in each optical module , but also the total charge to account for the probability of multiple photoelectrons from individually scattered photons .
this is particularly important for neutrino energies above 100 tev , where photons from secondaries produced along the muon track are an important addition to the light yield .
the median angular resolution for the final event selection is @xmath6 ( @xmath7 ) under the assumption of an @xmath2 ( @xmath3 ) spectrum .
we performed a binned scan of the entire sky map and also tested a list of pre - defined directions , with a search bin radius of @xmath8 .
details on the method for the all - sky search are given in [ ] .
the major difference is that in this work we extended the scan to the southern sky .
no significant deviation from the background - only hypothesis was observed .
the fluctuation with the highest significance was found in the direction of dec .
@xmath9 and r.a .
@xmath10 ( 8 events observed , 1.2 expected ) . by performing searches on randomized sky maps
, we determined the probability for an equally or more significant excess anywhere in the sky due to a background fluctuation to be 37% .
+ in the second test a pre - defined list of source candidates was used , aiming at reducing the effects of trial factors . from the confirmed active galactic nuclei ( agn )
listed in the third egret catalog  [ ] we selected 23 bright blazars by imposing thresholds for average ( maximum ) photon fluxes of @xmath11 ( @xmath12 ) ph @xmath13s@xmath14 .
two additional blazars reported in november 2008 were also included , see [ ] , [ ] , as well as two close - by agn , m87 and centaurus a , and the center of our galaxy , sgr a*. no significant excess above background was found for any of these candidates .
the upward fluctuation with the highest significance occurs for pks 1622 - 297 with 1 event observed and 0.3 expected . from background simulation using randomized data , we expect a similar or more significant result from any of the sources in 98% of all cases .
the limits given in table  [ sourcelist ] , calculated according to  [ ] , are preliminary and do not yet include systematic uncertainties
. a previous point source search with better sensitivity for the northern sky has been performed , and it was decided a priori that it gives the final results for the up - going region ( dec .
@xmath15 ) [ ] .
a new approach to neutrino point source searches extends both below and above the horizon and increases the reach of icecube above pev energies and to the southern sky .
results from data taken with 22 strings are consistent with the background - only hypothesis .
details will be discussed in an upcoming publication .
icecube is taking data with 40 strings since april 2008 and has been recently extended to 58 strings , with better sensitivity to be expected for upcoming analyses .
point source searches with neutrino telescopes like icecube are normally restricted to one hemisphere , due to the selection of up - going events as a way of rejecting the atmospheric muon background . in this work 
 we show that the down - going region above the horizon can be included in the search by suppressing the background through energy - sensitive selection procedures . 
 this approach increases the reach to the eev regime of the signal spectrum , which was previously not accessible due to the absorption of neutrinos with energies above a pev inside the earth . 
 we present preliminary results of this analysis , which for the first time includes up - going as well as down - going muon events in a combined approach . 
 we used data collected with icecube in a configuration of 22 strings . 
 no significant excess above the atmospheric background is observed . 
 while other analyses provided results for the northern hemisphere , this new approach extends the field of view to a large part of the southern sky , which was previously not covered with icecube .