Correlation of IceCube neutrinos with the 2MASS Redshift Survey
CCorrelation of IceCube neutrinos with the 2MASSRedshift Survey
The IceCube Collaboration ∗ http://icecube.wisc.edu/collaboration/authors/icrc19_icecubeE-mail: [email protected] Since the detection of high energy astrophysical neutrinos in IceCube, there has been a searchfor their sources. Although recent evidence of neutrinos from a flaring blazar could explainsome of the neutrino flux, sources for the remainder are still unknown. This analysis searchesfor neutrinos produced via interactions between diffuse intergalactic Ultra-High Energy CosmicRays (UHECR) and matter. In this work the local galaxy density serves as the target for cosmicray interactions, thus neutrinos produced from these interactions are expected to trace thegalaxies spatially. The spatial distribution of galaxies within the local universe (z < 0.10) as seenin the 2MASS Redshift Survey (2MRS) is anisotropic. Here we present an analysis that searchesfor the spatial correlation between the arrival directions of neutrinos observed at the IceCubeneutrino observatory and the directions of high galaxy density in the local universe. No suchcorrelation was found and this analysis presents limits on the flux of neutrinos from the localuniverse.
Corresponding authors:
Stephen Sclafani † , Naoko Kurahashi Neilson Drexel University36th International Cosmic Ray Conference -ICRC2019-July 24th - August 1st, 2019Madison, WI, U.S.A. ∗ For collaboration list, see PoS(ICRC2019) 1177. † Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ a r X i v : . [ a s t r o - ph . H E ] A ug orrelation of IceCube neutrinos with 2MASS Redshift Survey Stephen Sclafani
1. Introduction
IceCube, a cubic kilometer detector at the geographic south pole, has discovered a flux ofastrophysical neutrinos [1, 2], and more recently presented compelling evidence for one neutrinosource [3, 4]. However the sources for the majority of the astrophysical neutrino flux remain unde-fined. Existing IceCube searches have focused on intrinsic neutrino emission of bright point-likesources, such as blazars [9], or diffuse emission of neutrinos from our own galaxy [7]. This work,instead, searches for neutrinos produced in interactions between the matter of nearby galaxies andan isotropic intergalactic diffuse flux of UHECR. If such interactions were responsible for signif-icant neutrino production, the spatial distribution of arrival directions of neutrinos would followthe local matter distribution and therefore directional correlation would be observable between theoverdensities of the local universe and excesses of astrophysical neutrinos.
2. 2MRS
The 2MASS redshift survey (2MRS) is an all-sky catalog of local galaxies with associatedredshift measurements, and is the most extensive and unbiased survey up to redshift of z < . K s band magnitude of 44,000 of thenearest galaxies, including galaxies out to a redshift of z ≈ .
10. The survey is mostly complete toa redshift of z = .
03, after which it becomes magnitude-limited. The catalog density as a functionof direction and redshift is shown in Figure 1. This analysis uses the 2MRS galaxy catalog to tracethe distribution of matter in the local universe.
Figure 1:
Left: Galaxy density of the complete 2MRS Survey. Right: Histogram of galaxies measuredredshift, showing the completeness and ultimate magnitude limit as a function of redshift
3. Analysis
A neutrino dataset consisting of 7 years of IceCube muon neutrino events previously used inmultiple neutrino source searches [6] is also used for this analysis. A Test Statistic (TS) is de-fined, describing the similarity between the spatial distribution of neutrinos to the spatial templateof galaxy densities obtained from 2MRS. The energies of the neutrino events are also considered inthe TS. Since atmospheric neutrinos follow an energy spectrum of E − . and astrophysical neutri-nos are assumed to follow E − , neutrinos with higher energies are more likely to be astrophysical2 orrelation of IceCube neutrinos with 2MASS Redshift Survey Stephen Sclafani in origin and assigned higher weight. The background events in IceCube, atmospheric neutrinosand muons, are isotropically distributed within a declination band due to the detector’s location atthe geographic South Pole. Thus the statistical significance of the correlation found in the data isquantified by comparing the TS of the true data to a distribution of TS values obtained from manybackground-only data sets where events are uniformly distributed in right ascension.The analysis TS defined in 3.1 is a likelihood ratio made up of likelihood L [8], which is afunction of n s the number of signal neutrinos. n s is free to vary between 0 and the total number ofobserved events, N. The entire dataset is fit for the most likely n s .TS = − (cid:18) L ( n s = ) L ( ˆ n s ) (cid:19) (3.1)where L ( n s = ) is the likelihood for the hypothesis corresponding to no correlation, and L ( ˆ n s ) isthe likelihood for the best fit n s . L ( n s ) = N ∏ i = (cid:16) n s N S i ( x i , σ i , E i ) + ( − n s N ) B i ( x i , E i ) (cid:17) (3.2)The likelihood is defined in 3.2. S i is the likelihood of event i being from the source, and B i is thelikelihood of event i being from the background, which are functions of event direction x i , energy E i and angular uncertainty σ i . The signal and background probability density functions (PDF), S i and B i , follow established methods applied previously in IceCube extended template analyses, with S i calculated using a fixed spectral index of E − . Detailed description of the method are describedin [7].
4. Signal Hypothesis a) Full 2MRS Catalog b) Full 2MRS Catalog, weighted by redshift
Figure 2:
Signal spatial PDF for a) Full 2MRS Catalog, and b) Full 2MRS Catalog with each galaxyweighted by its redshift. Galaxies located in the center of the map make up the Virgo Supercluster and areassigned larger weight when redshift information is included. orrelation of IceCube neutrinos with 2MASS Redshift Survey Stephen Sclafani
Three different signal hypotheses are tested in this analysis. These correspond to: • All galaxies in the 2MRS Catalog • All galaxies in the 2MRS Catalog with z < 0.03 • All galaxies in the 2MRS Catalog each galaxy weighted by its redshift distanceA template PDF is created for each signal hypothesis, examples of which are shown in Figure2. The template with a cutoff at z =0.03 corresponds to a close but relatively complete catalog. Theredshift-weighted catalog includes weighting for each galaxy proportional to the inverse square ofdistance where distance is calculated from the redshift. This does not include any corrections forgalaxy peculiar velocity effects or redshift uncertainty. The full catalog uses the complete survey.Galaxies are binned using HEALPix bins of 0.2 square degrees [10]. The PDF is based on thegalaxy density map, convolved with the detector acceptance for an E − hypothesis as a functionof event declination. The resulting map is then convolved with a Gaussian of width equal to eachevent angular uncertainty.
5. Results
The p-values reported in Table 1 denote the probability that the observed TS can be caused bya chance coincidence. All three scenarios are consistent with the no-correlation hypothesis. Giventhe absence of a significant correlation, limits are placed on the neutrino flux that originates frominteractions between diffuse intergalactic UHECR and matter in local galaxies. These limits arereported for each hypothesis.Template Test Statistic p-value Upper Limit Φ dN/dE (GeV − s − cm − )Full Catalog Template 0.0 1.0 2.89 × − z <0.03 Template 0.0 1.0 2.15 × − Full Catalog Templatewith redshift weighting 0.0 1.0 1.97 × − Table 1:
Analysis TS and p-value results. Fluxes are integrated over the full sky and parameterized as dN/dE= Φ × ( E TeV ) − GeV − cm − s − with 90% confidence level upper limits. These limits are calculated based on an assumed E − energy spectrum. They are also com-puted for a source energy distribution that follow different spectra. The 90% confidence limits forother spectra are shown in Figure 3 for each template.This analysis is limited by assumptions of the column density, distance, and completeness ofthe galaxies in the 2MRS survey. As additional surveys like the LSST provide a more completeview of the local universe, this analysis could be expanded to include other available information4 orrelation of IceCube neutrinos with 2MASS Redshift Survey Stephen Sclafani like galaxy luminosity, and provide better limits on the relationship between neutrinos and localmatter.
Figure 3:
Flux Upper limits for a range of source spectrum and templates. Flux greater than these pointscan be excluded with a 90% confidence level
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