The IceCube Neutrino Observatory is a cubic kilometer detector at the geographic South Pole.We give an overview of searches for time-variable neutrino point-sources using

data taken from May 2008 to May 2010.

Time Variable Neutrino Point Source Searches with Time Variable Neutrino Point Source Searches with IceCubeIceCube

M. Baker (mfbaker@icecube.wisc.edu), J.A. Aguilar, J. Dumm, N. Kurahashi and T. Montaruli for the IceCube Collaboration

IceTop

InIce

Air shower detector

threshold ~ 300 TeV

86 Strings,

60 Optical Modules

Per String

AMANDA

19 Strings

677 Modules

Total

The IceCube DetectorThe IceCube Detector

IceCube is a high-energy neutrino observatory at the geographic South Pole. The detector is composed of 86 strings of 60 Digital Optical Modules (DOMs) each, deployed between 1500 and 2500m below the glacier surface. A six string Deep Core with higher quantum efficiency photomultipliers and closer DOM spacing in the lower detector enhances sensitivity to low energy neutrinos. Muons passing through the detector emit Cherenkov light allowing reconstruction with <1º angular resolution in the full detector.

All-Sky Flare Search with the 59 String Configuration DetectorAll-Sky Flare Search with the 59 String Configuration Detector

Time-Dependent Point Source Search with Fermi-LAT LightcurvesTime-Dependent Point Source Search with Fermi-LAT LightcurvesWe are interested in using a multi-messenger approach to search for neutrino sources. Information from high-energy astronomy experiments, such as Fermi, can be used to search for neutrinos in coincidence with high photon flux states, enhancing the potential for source discovery over a time-integrated search. The comprehensive sky coverage of Fermi is particularly interesting for this approach, as candidate sources such as blazars exhibit variability on the timescale of hours.

We have developed a method utilizing a Maximum Likelihood Block[6][7] method to denoise 1-day binnedLAT light curves. We then find a threshold level of the LAT light curve above which best describes theNeutrino emission. We find this substantially lowers the amount of E^-2 signal events needed for discovery on average compared to a time-integrated search.

A list of 23 sources with flaring seen with the LAT were tested using the combined data from 40 and 59 string configurations. The most significant source was PKS 1622-253 with a pretrial p-value of 8%. A more significant p-value is seen in 51% of scrambled samples, so this result is compatible with background.

An example of the Fermi LAT lightcurve, the Maximum Likelihood Blockreconstruction for the object 3C273.

We have used 348 days of livetime with the 59-string data (May 2009-10) [3] for a search for the most significant clustering ofevents in time and space. The method returns a best-fit signal fraction,spectrum, and the mean and width of a Gaussian in time.

The most significant flare in the sky comes from ra,dec=(21.35,-0.25), centered on March 4, 2010 with a FWHM of 13 days. An excess of 14 events is seen, but with a soft spectrum of E^-3.9 We find a spot with equal or greater significance than the most significant spot on this skymap in 1.4% of trials with scrambled data.

The same analysis was performed on the 40-string data [4], but the most significant flare did not stand out from background.

Pretrial significance map (celestial coordinates) of the flare search using59-string configuration data. The Galactic Plane is also marked.

We use the angular and energy distribution of events as information to characterize the signal with respect to the background using a maximum-likelihood ratio method [1]. In the analysis of the 22-string data [2] we use the number of hit DOMs in an event as an energy estimator, while for the 40-string configuration we use a more sophisticated energy estimator based on the photon density along the muon track. The analysis method returns a best-fit number of signal events and spectral index.

MethodMethod

Where σ is the angular uncertainty of the track direction, x

i and x

s the neutrino and source direction,respectively,

Ei the energy parameter of the event and γ the fit energy

spectrum and Ti is the time-dependent source term

developed in this work.

In the northern sky, the background for point source neutrino searches are the atmospheric neutrinos produced by cosmic ray air showers. In the southern sky the background is atmospheric muons, whose rate follow temperature variations at the South Pole. These backgrounds have known distributions in time, so if an astrophysical neutrino source has time-dependent behavior, that information can be used to enhance the potential for discovery.

Time-Dependent Motivation Time-Dependent Motivation

ReferencesReferences

[1] Braun. J. et al., Astropart. Phys. 29, 299 (2008).[2] Braun, J. et al., Astropart. Phys., 33, 175 (2010).[3] Aguilar, J.A. “Time-independent searches for

astrophysical neutrino sources with the combined data of 59 and 40 strings of IceCube”

[4] Abbasi, R. et al., arXiv:1104.0075[5] Abbasi, R. et al., ApJ, 732, 18 (2011)[6] Scargle, J. D. 1998, ApJ, 504, 405 (1998)[7] Resconi, E. et al,. A&A 502, 499-504 (2009)