Neutrino and Gamma-RayDiffuse Backgrounds

fromGalaxy & Cluster Mergers

Peter Mészáros

Pennsylvania State University

NOAC, Beijing, March 2018

Collaborators: CC Yuan, K. Murase, D. Jeong, K. Kashiyama

Data on theastrophysical VHE

neutrino background

D. Cowen/Penn State IceCube and Tau Neutrinos3

Amundsen-Scott South Pole station

South PoleOld Dome

Summer camp

road to work

landing str

ip

Passenger terminal

↙ IceCube

IceCube Observatory

Schematic

The IceCube (IC) neutrino observatory is located at the Antarctic pole and has been at full operating capacity since 2011.

Neutrinos produce charged particles when they interact with ice molecules. The Cherenkov radiation from these particles are observed by the optical sensors.

Sensitive to two types of signals:

Charged current (CC) muon interactions are seen as track-like events

CC electron and tau interactions, and all neutral current (NC) interactions are seen as cascades

IceCube

1 GTon instrumented volume, US$ 300M = 30c/Ton

“cheap” instrument:

Measuring the light from neutrinos interacting with ice or rock particles

• In track events, the Cherenkov angle determines the energy

• Time of flight arrival to different PMTs determines the direction

Muon tracks

Two types of events:

TRACKS:long: from muons

CASCADES:blobby: electrons and tau

Muon track

Electron Cascade

tau double cascade

IceCube capabilities

IceCube (South, complete 2011)

• Ice, volume 1 km3 (~1 Gton)

• 86 strings (1 Km long) x 60 sensors (DOM)

• DOM spacing 17m vertical x 100m horizontal

• Ice: λabs~100m, λscat~20m

• Water, volume 3 to 4 km3 (3-4 Gtons)

• Water: λabs~15m, λscat~100m

KM3NeT (North, planned)(Mediterranean)

Neutrinos light up the ice

μ

Muon track simulation(all photons expected to be produced)

Muon track detection(photons actually detected, “Strangepork” event)

Cascade simulation(all photons expected to be produced)

Cascade detection(photons actually detected, “Ernie” event)

There is strong evidence for a diffuse, astrophysical flux of neutrinos with energies between 25 TeV and 2.8 PeV.

The measured flux is well fit (at the 3.8σ level) by a soft power-law with index -2.50 ± 0.09 and an all-flavor flux of ~ 7 x 10-8 GeV cm-2 s-1 sr-1 at 100 TeV.

Sources of the neutrino flux are unknown.

4Aartsen, M.G. et al. ApJ 809, 98 (2015)IceCube results

(Halzen, 2017, TeVPA)

IceCube diffuse astrophysical neutrino background

(maybe two components?)

Other neutrino backgrounds:In broader context:

Big Bang ν

10-4 eV

(obs.)

(obs.)

There is increasing evidence for an extra-galactic origin for the observed neutrinos

The measured flavor ratio (νe:νμ:ντ) is consistent with oscillation over cosmological distances ( >100 Mpc)

22

The neutrino arrival directions are consistent with isotropically distributed sources

→No obvioussources!

NEUTRINO PRODUCTION

Astrophysical neutrinos are produced by CR interactions with ambient light or matter (pγ or pp interactions, respectively)

VHE neutrinos and γ-rays are produced with ~0.05% and ~0.1% of the initial CR energy respectfully.

For neutrinos with energy 25 TeV–5 PeV, CRs with energy ~50–100 PeV are needed

To find the maximum CR energy achievable in our source models, we compare the acceleration time with the various energy-loss (cooling) timescales

5

p+ p/� ! N + ⇡± + ⇡0 + ...

(Tan-background slides credit: Nick Senno)

8

⇡� ! µ� + ⌫̄µ ,µ� ! e� + ⌫̄e + ⌫µ

⇡+ ! µ+ + ⌫µ ,µ+ ! e+ + ⌫e + ⌫̄µ K+ ! µ+ + ⌫µ

n ! p+ e� + ⌫̄e

⇡0 ! � + �

p+ p/� ! N + ⇡± + ⇡0 + ...

Both νe and νμ are produced by charged pion decay,

γ-ray photons are produced by neutral pion decay

Secondary leptonic pairs also up-scatter ambient photons to GeV–TeV energies

8

⇡� ! µ� + ⌫̄µ ,µ� ! e� + ⌫̄e + ⌫µ

⇡+ ! µ+ + ⌫µ ,µ+ ! e+ + ⌫e + ⌫̄µ K+ ! µ+ + ⌫µ

n ! p+ e� + ⌫̄e

⇡0 ! � + �

p+ p/� ! N + ⇡± + ⇡0 + ...

Both νe and νμ are produced by charged pion decay,

γ-ray photons are produced by neutral pion decay

Secondary leptonic pairs also up-scatter ambient photons to GeV–TeV energies

expect a corresponding γ-ray background !

27

Fermi

28

• LAT: high energy det., 20 MeV-800 GeV

• 10 layers of 0.5 rad. length converter (Pb)

• 12 layers XY Si strips • ΔE/E~10% @1 GeV;

FoV = 2.5 sterad. (1/5 of all-sky simultan.)

• GBM: “Low” energy det. 10 keV-30 MeV, FoV = 4π sterad,

• 2.5 ton , 518 W power

LAT

GBM

FERMI

Fermi EGB & IGB

• EGRB: Extragalactic “gamma-ray” background (incl. everything, incl, point sources, etc)

• IGRB: Isotropic gamma-ray bkg. (incl. unresolved sources, or truly diffuse) : ~14% of

EGB EGBIGB

Observed:

EGB

11

VHE γ–rays are expected to accompany neutrinos. They are related via:

ϵ2γΦϵ ≃ 2s−1ενΦε

!!εν=0.5ϵγ

A fraction of γ–rays are attenuated by extra-galactic background light (EBL)

The resulting spectrum is universal for large distances

Finke et al. ApJ 712, 238 (2010)

⇠ 1� e�⌧��

(injection spectrum similar)

BUT:

( γγ→e+e- cascades )

High energy γ-ray propagation in

intergalactic space• γh +γs → e+ + e-

• Threshold: Eγh > (mec2)/Eγs

• Target photons Eγs : diffuse IR bkg, from starlight + CMB

• Multiple γγ cascades until below threshold

• MC simulations, or kinetic equ’s →universal final spectrum Bechtol+16, arXiv 1611.00688

ΥΥ cascades of injected HE Υ on EBL ΥFinke+10, ApJ 712:238

EBL

IGBINB

Cold it be due to Galaxy mergers?

Origin of nu-bkg:

Galaxy mergers

• Mergers happen!

• Their gas components will undergo a strong shock

• Shock →particles will be Fermi accelerated→CRs

• Dense gas →ample targets for pp

Galaxy merger shocks

shock

M✸~1011 M⊙, Mgas~1010 M⊙

vs ~ 3-5x107 km/sCosmic ray energy input into Universe:

Shock CR acceleration in gal. merg. shock

lacc < lrad M >> 1, r ~ 4i.e. strong shock

Kashiyama & Mészáros ’14, ApJL 790:L14

pp →π+ →ν from gal. merger shock

Galaxy mergers, INB & IGB• Every galaxy merged at least

once in the last Hubble time

• Major mergers → Egms~1058.5 erg, R~10-4 Mpc-3 Gyr-1 vs~107.7 cm/s Qcr,gms~3x1044 erg Mpc-3 yr-1 εcr,max ~1018.5 Z eV

• pp → PeV νs, 100 GeV γs

• ν: Individual GMS: 10-2 μ/yr, INB: 20-60% IC3 obs.flux

• γ: Individual GMS flux: ~3.10-13 erg/cm2/s→CTA? IGB ~10-8

GeV/cm2/s/sr , about 10-30% Fermi IGB

• Minor mergers: uncertain, could add up to 70-100%

• Good, at least for PeV nus !(Kashiyama & Mészáros ’14, ApJL 790:L14)

37 (However: after 2014, new data on TeV !)

Also : galaxy mergers are only an intermediate step in a continuum process:

• Universe’s large scale structure (LSS) formation proceeds in hierarchical manner

• Dark matter (DM) halos have a minority of mass in baryons which cool down toward the center to make small protogalaxies.

• Bottom-up scenario; small galaxies merge to make larger galaxies, then clusters, then clusters merge to make larger clusters….

CC Yuan, P. Mészáros, K. Murase, D. Jeong, ApJ, 2018, in press (1712.09754)

Large Scale Structure (LSS) formation in LCDM Universe

• DM halos collapsing out of Hubble flow→virialize

• Baryons (gas) collapse inside the virialized halos →galaxies: stars + ISM

• Smaller DM halos merge → baryonic galaxies merge →shocks in galactic ISM

• Larger DM halos→Clusters: multiple galaxies + IGM

• Cluster-Cluster IGM shock + galaxy-galaxy ISM shocks

LSS formation (cont.)

• DM halo mass function dN/d lnM , e.g. from N-body simulations and ← Seth- Tormen’97 fit

• This halo mass function i.e. number of halos per unit comoving volume within log. mass interval d lnM can be analytically approximated using Press & Schechter ’74

Press-Schechter approximation

where

ρc,0= 3H02/(8πG) = critical density

Number N of halos of mass M at redshift z is:

P(k) ~ kn ~ (2π/λ)n = matter power spectrum (λ=λ[M])

top-hat filter function,

δ= δρ/

(variance of power spectrum)

δ= δρ/ = density contrast

Growth of density contrast in LCDM:

where

→collapse

Seth-Tormen fit to f(ν), (with A=0.322, a=0.707, p=0.3 fit to num. sim. )

and

Merger Rate & CR Luminosity

3 relevant timescales for CR luminosity production:

where

, where

= dimensionless merger rate per unit halo mass ratio ζ

with Rg(z)=galaxy radius, vs(z)=shock velocity, λ~1 is a geometry parameter)

with these,

= probability that halo of mass Mmerges within age of Universe (z)

Comoving CR energy input rate / ln εp

Gas mass fraction vs. z(𝝌* = from obs.)

= gas mass fraction

z-evolution of mean gas mass fraction given by↓→

(also plot shock velocity →)

vs ξg

Shock velocity evolution- Peculiar velocity of the gas in a cluster ~O(virial velocity)- Estimate shock velocity from pairwise velocity dispersion

- Locally (z=0): 2-point correlation fcn.:

where- Combine 3-pt corr. fcn. with cosmic virial theorem → shock velocity

where

- Stable clustering hypothesis (Groth-Peebles): →

- Thus, for varying z → ⇒ vs (z)

( shock veloc. in previous plot )

Gas density and B evolutioncollapse if : ⇢(z) � 1.686D(z)

�1⇢c(z) ; where ⇢c(z) = 3H2

(z)/8⇡G

virialized gas density : ⇢g(z) = �c(z)⇢c(z) ' 178⌦0.45m

where : ⌦m = ⌦m,0(1 + z)3/[⌦m,0(1 + z)3 + 1� ⌦m,0].

In general, virialized gas : ⇢gas(z) = g(z)ngas,0mp

where : g(z) =�c⇢c(z)�c,0⇢c(0)

= (1 + z)1.35[⌦m,0(1 + z)3 + 1� ⌦m,0]0.55

In shocks, usual equipartition argument : B2/8⇡ ⇡ 1

2

✏Bngmpv2s / ⇢v2

s

B ⇡q

4⇡✏Bng,0mpg(z)v2s ⇡ 14 ✏1/2

B,�2n1/2g,0 g(z)1/2 ⇥ (

vs

300km s�1) µG

CR acceleration & diffusion

"max

p

⇠ 32eB

s

h(vs

/c) ' 1.3⇥ 1016 eV(B

s

30 µG)(

h

3 kpc)(

vs

300 km s�1)

- Adopt gas disk radius and scale height Rg(z) ∝ hg(z) ∝ (1+z)1.10 (HST obs.)

Hillas criterion, tacc < tesc :

- Meson production efficiency: (1- e-fpp), where fpp ~ 𝞌pp σpp g(z)ni,0 cti

- Thereafter, CRs diffuse in merging galaxy system,

where σpp =σpp(εp) ~5x10-26 cm2, 𝞌pp~0.5 inelasticity, ti - residence time

ti = min[tdyn , tdiff ], tdyn~h/vs ~107yr (h/3kpc)(300km s-1/vs) , tdiff ~h(z)2/6 Dg

Diffusion time & neutrino production in galaxy mergers:

Diffusion coefficient in magnetic field - large and small angle scattering :

where D=Dc [ (ε/εc)1/2 +(ε/εc)2 ] ; with Dc=c rL(εc,g)/4, rL(εc,g)=lc/5

and rL =Larmor radius, B~30 μG, lc= B-field coherence length~30 pc,

so

where

and taking

get, for galaxy mergers:

i.e. → calorimetric for z≳ 1 gal. mergers

2

Diffusion & neutrino prod. in galaxy halo and the host gal. cluster

and with CR injection time tinj ~ tage(cluster), have

and, for low z also have Cluster-Cluster mergers

→ get formally similar fppcl-cl , with comparable numbers

- Take Cl-Cl mergers ocurring for Mcl ≳1013 M⊙ (=“HM”)

The combined all-flavor neutrino production rate is then

The 1st term (gal-gal mergers) and 2nd term (cl-cl mergers) dominate;3d term (w. η ≲ 0.1-0.2, gal-gal CRs escaping to cl.), is sub-dominant,

essentially because fppg (~calorimetric) > fppcl

Local CR input rate as fcn (z) →

& the resulting ν, γ areseen after they propagatethrough cosmic space↓

Thus,

and for γ-rays, additional effect:The locally produced γ-rays are degraded viaγγ interactions with infrared EBL photons

→γ cascades to lower energies→ universal final spectrum

Calculated ν and γ bkgs.

ChengChao Yuan, P. Mészáros, K. Murase, D. Jeong, ApJ, 2018, in press (1712.09754)

Both fits OK ✔

✔

No evolution, ξg=0.05 case

Overproduce γ-bkg !

Dependence on CR spectrum

• Adiabatic shock: expect index s=2

• But radiative shocks, expect s=(r+2)/(r-1), r=compression ratio, →harder CR spectra →harder ν-spectra

• γ-ray sp. unchanged (γγ-cascade leads to universal spectrum)

• could accomodate slopes s~2 or s~1.5

ChengChao Yuan, P. Mészáros, K. Murase, D. Jeong, ApJ, 2018, in press (1712.09754)

✔

✔

Conclusions• ≳ 50% of the IceCube neutrino bkg. could be produced by LSS

(cluster, galaxy) mergers, with ~ the right ν-spectrum.

• Can do this without violating the 14% of non-blazar diffuse γ-ray spectrum observed by Fermi

• Reduced γ-background is because most contributions come from high-z, where τpp and τγγ both larger

• Could also acommodate steeper slopes of s~1.5 resulting from, e.g., cooling shocks

• But any greater merger contribution to the ν-bkg. would violate the Fermi allowed non-blazar γ-bkg. (might the rest be blazars?)

• A possible feature (hump) at ~30 TeV remains unexplained - for this may require an extra component.

Thanks !

extra slides

Speaking generally: • Could pp sources

explain the PeV nu background ?

• In a model-indep. way, just assuming spectr. E-2 or E-2.2, the answer was yes - also suggesting a break @ few PeV

• This was as of 2013. But, now, w. newer data…not so easy!

Murase, Ahlers, Lacki 2013, PRD, 88:121301

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