spatially coincident icecube neutrinos and fermi … · three columbia professors, leon lederman,...

S PAT I A L LY C O I N C I D E N T I C E C U B E N E U T R I N O S A N D

F E R M I - L AT 𝛾 - R AY S O U R C E S

H A N N A H S E Y M O U R - B A R N A R D C O L L E G E

1

This summer, I worked with the Fermi gamma ray telescope to search for gamma ray point sources that could be the origin of mu neutrinos detected by IceCube

• Multi-messenger Astronomy Search for Cosmic-Ray Sources

• IceCube Neutrino Observatory and Fermi Gamma-Ray Observatory

• Neutrino Events and Fermi Blazars

• Implications of Data

Spatially Coincident IceCube Neutrinos and Fermi-LAT Gamma Ray Sources-2

I will discuss the motivation for neutrino and gamma ray astronomy, as well as the structure of the IceCube detector and the Fermi telescope. Finally, I will present my data and discuss the sources detected and the implications

N E U T R I N O / M U LT I -M E S S E N G E R A S T R O N O M Y

• Neutrinos mostly interact via weak force

• Cross-section is very low

• Cosmic rays-mainly charged particles and nuclei

• Can be scattered by B-fields

• Don’t “point back” to source

• Cosmic rays thought to be produced by astrophysical nuclear processes

• Charged pion decay

• Same processes produce neutrinos and gamma rays


The electron, muon, and tau neutrinos are extremely light particles that are only weakly interacting, so, despite the cosmic flux of neutrinos being extremely high, their interaction cross section is very low. This works both to our disadvantage and potentially advantage-despite being extremely difficult to detect, neutrinos are not scattered by intergalactic magnetic fields like charged particles, making them a potential way to trace the sources of cosmic rays. Cosmic rays are mostly composed of charged particles and light ions. The processes occurring in extreme astrophysical environments could produce these and also neutrinos, via charged pion decay, which can also produce gamma rays.

N E U T R I N O A S T R O N O M Y-P O T E N T I A L S O U R C E S

• TeV-PeV neutrinos most likely produced in extreme astrophysical environments

• Proton interaction with gas

• Blazars and other AGN

• Starburst galaxies

• PeV dark matter decay


Very high energy neutrinos detected by IceCube almost certainly are astrophysical in origin. Muon neutrinos can be produced via the decay of charged pions, which can subsequently also produce other particles like charged particles and other neutrinos via muon decay. Several environments have conditions in which these interactions can occur-namely, blazars and other AGN, which I will explore in more detail in this talk. Starburst galaxies, which contain a great deal of superhot gas, are also a potential candidate. A more exotic scenario is PeV dark matter decay.

I C E C U B E N E U T R I N O O B S E R VAT O R Y

• >2km of ice near South Pole

• 86 strings of 60 PMTs and electronics

• Detects muon events as tracks, electron events as showers (in case of charged-current interaction)

• Much better angular resolution for muons

• Compare ~1º to ~20º


The IceCube neutrino observatory is located in Antarctica, near to the south pole. IceCube consists of approximately 2.5km deep of ice. Drilled into the ice are strings of PMTs and associated electronics, which detect photons from Cherenkov radiation. IceCube can observe muon, electron, and tau neutrinos, although at this point it is not possible to distinguish between tau and electron events. Charged-current interaction between neutrinos and the ice produces the neutrino’s partner lepton, which travels through the ice and produces Cherenkov radiation. The higher energy muons travel in a long straight track, allowing for much better angular resolution than electron events, which produce a shower.

M U O N N E U T R I N O D E T E C T I O N

• Interaction with ice produces muon

• Cherenkov radiation

• Select for events going through Earth

• Neutrinos rarely interact

• Avoids confusion with atmospheric muonsSpatially Coincident IceCube Neutrinos and Fermi-LAT Gamma Ray Sources-6

Again, interaction with the ice produces a muon via charged-current interaction. The high energy of the muon allows it to travel further along the ice before decaying, as opposed to the lighter electron, which quickly loses energy, and the heavier tau, which decays very fast. By selecting for events whose light begins inside the detector, we avoid confusion with muons that are produced in the atmosphere, which do not come from the extremely high energy neutrinos we are studying.

M U O N N E U T R I N O D I S C O V E R Y

• Leon Lederman, Melvin Schwartz, and Jack Steinberger

• 1962 at Brookhaven

• 1988 Nobel Prize

• Columbia professors

• Worked on Nevis synchro-cyclotron and other experiments

Spatially Coincident IceCube Neutrino Events and Fermi-LAT Gamma Ray Sources-7

The muon neutrino was discovered by three Columbia professors, Leon Lederman, Melvin Schwartz, and Jack Steinberger at Brookhaven national lab, using the Alternating Gradient Synchrotron to accelerate protons to produce charged pions, which would subsequently decay and produce muons and muon neutrinos. They observed the impact of muon neutrinos on aluminum plates, which produced muon tracks. Leon Lederman also worked at the Nevis synchro-cyclotron.


E A R L I E S T L I G H T

L AT E S T L I G H T

Here is an example of a muon and electron event. The reddest light is the earliest light detected, bluest is the latest. The round dots are PMTs which lit up. Note the reddest light is well inside the detector.

B I G B I R D 2 P E V

7 1 T E V

I C E C U B E S K Y M A P

W H E R E A R E T H E

P O I N T S O U R C E S ?


While IceCube has seen slight excesses, the neutrino flux it sees is mostly isotropic and thus it has not identified any neutrino point sources. So, we move on to multi-messenger astronomy.

G A M M A R AY A S T R O N O M Y

• Can travel far without absorption by gas/dust

• Produced in extreme environments

• Supernovae

• Active Galactic Nuclei

• Neutron Stars

• Gamma-Ray Bursts

• Typically absorbed by Earth’s atmosphere

• Low-energy must be observed from space


Gamma rays are the most energetic electromagnetic waves. They are produced in the most violent, hot, energetic environments, such as AGN, supernovae, and neutron stars. The vast majority of gamma rays are absorbed by Earth’s atmosphere, and as such, the lowest-energy gamma rays must be observed from space.

F E R M I S PA C E T E L E S C O P E

• Sensitive to energies up to 1 TeV

• LAT sees 20% of sky

• Covers entire sky in 3hrs

• Gamma ray -> e-e+ pair

• Si measures path to determine direction

• Calorimeter measures energy


The Fermi Gamma-Ray Observatory is such a space-based telescope. The Large Area Telescope, LAT, is capable of seeing 20% of the sky at any given time and sweeps the entire sky in 3hrs, giving it an advantage or VERITAS and MAGIC in that it does not have to point at a small area of sky for a very long time to achieve meaningful results. A gamma ray entering the detector passes through an anti-coincidence detector, then through tungsten sheets in which it produces an e+e- pair. The direction of the pair is measured by si tracking strips, and the energy is measured by a calorimeter.

S K Y, M O D E L , R E S I D U A L S M A P S F O R E A C H N E U T R I N O E V E N T• Fermi analysis of a 7ºx7º region around 16 IceCube track events

• 1-300 GeV

• Search for Fermi sources with ~1º radius of neutrino

• 3 events with one source, 1 event with two

• Analysis using Fermi Science Tools, P7REP_SOURCE_V16

• Updated to Pass 8 data one day after maps finished

• Light curves use Pass 8


I went looking for Fermi sources within one degree of 16 IceCube muon neutrino events. I used the Fermi Science Tools, a software developed by the collaboration. Of the 16, I identified five named sources within one degree of four neutrinos. 3 events contained one source, one contained two. I produced count, model, and residuals maps for each area of sky around the neutrinos, using Pass 7 data.


Neutrino Event 11

Count Model Residual


Neutrino Event 12



Neutrino Event 17



Neutrino Event 19


• One-week before neutrino event, one after

• 14 one-day bins

• Used flux values for TS>4, upper limits for TS<4

• Sqrt(TS)~Significance

• Most bins had very low TS values

• Neutrino events seemingly not correlated with significant excess flux


The light curves were produced using data from an unbinned analysis of a two week period-one week before the neutrino event and one week after. Using the test statistics produced by the likelihood analysis, I placed upper limits for flux points with a ts<4. Most bins had very low TS values, which leads me to believe the neutrino events are not correlated with a flare of the source. In addition, none seem to be correspondent with a local excess in flux.

I N T E R P R E TAT I O N

• Value of isotropic neutrino flux obtained from power law

• Flux of each bin/Neutrino flux

• 10e-3 to 10e-2

• Probability of spatial coincidence

• Constant declination, random right ascension

• IceCube positions dependent on declination

• Compare to entire 3FGL catalog

• Repeat 10000x

• >95% probability of one or more random positions being spatially coincident


I compared the isotropic neutrino flux as obtained from the power law to the flux of each bin, getting a ratio on the order of 10-2 or 10-3. This means that at least 100 or more sources would be needed to account for the isotropic flux. This is obviously not seen, and also indicates that no one source can account for the entire flux. In order to determine how often sources are randomly spatially coincident with 3FGL sources, I generated random numbers 10000 times, assigned them each to one of the 16 declinations, and compared them to the 3FGL catalog. The vast majority contained at least one source within the error radius of 1 degree.

• Probability for all 107,569 muon events from 59-string configuration

• Most events likely not astrophysical

• Constant DEC, random RA

• Compared to 3FGL catalog 100x

• Different than first simulation

• How often is a single neutrino closer than the error radius?


This is a slightly different test than the first. Instead of looking at all 16 neutrinos at once (16 positions is one trial) it looks at one neutrino at a time and compares it to the 3FGL catalog. I am hoping to make an energy cut to eliminate the lower energy neutrinos and refine this trial.

B L A Z A R S A S P O T E N T I A L S O U R C E S

• Sources are mostly BL Lac objects and one FSRQ

• FSRQs more likely to be neutrino-bright

• Optically thick accretion disk/strong external radiation field

• Expect ~1e-6 neutrinos detected by IceCube for Mrk 501 flare

• ~30% probability of detecting neutrinos from flare, few per year

• Could detect a few neutrinos from flares (i.e., 3C 279)

21Spatially Coincident IceCube Neutrinos and Fermi-LAT Gamma Ray Sources-21

Neutrino 11 contains an FSRQ (Flat spectrum radio quasar) and the rest are BL Lac objects, with neutrino event 17 containing an unclassified source. FSRQs are typically brighter in gamma rays and are also more likely to be neutrino bright, as BL Lac objects lack certain features like broad emission lines, suggesting a thick accretion disk and a large external radiation field capable of inducing the kind of photopion reactions necessary. It is suspected that during a flare of an FSRQ, like the one pictured, IceCube could detect a few neutrinos, and could possibly detect neutrinos during a quiescent period. It is unlikely the neutrino flux from a quiescent BL Lac object would be detectable.

F U T U R E D I R E C T I O N S

• Energy cut on 59-string data

• Fermi analysis of upcoming muon events

• VERITAS analysis of point sources

• Better understanding of AGN and cosmic rays?

W H E R E A M I ?


M A R C O S S A N TA N D E R J O H N PA R S O N S M I K E S H A E V I T Z

T H E N AT I O N A L S C I E N C E F O U N D AT I O N A L L O F T H E R E U S T U D E N T S

Acknowledgements


spatially coincident icecube neutrinos and fermi … · three columbia professors, leon lederman,...

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