Galactic diffuse gamma-ray and neutrino emission in multi-TeV energy range
Andrii Neronov
Astroparticle and Cosmology laboratory, Paris & University of Geneva

Galactic diffuse gamma-ray and neutrino emission in multi-TeV energy range

1) Neutrino and gamma-ray sky
2) Gamma-ray diffuse Galactic emission at TeV with Fermi/LAT
3) Multi-TeV Galactic diffuse emission with Cherenkov telescopes

Neutrino sky

105

IceCube detects “astrophysical” neutrino signal in the 0.03-10 PeV
energy range. Origin of the signal is uncertain.
Neutrino event distribution is consistent with being isotropic, which
points to extragalactic origin (?)

IceCubeastrophysicalneutrinosignal

Neutrino sky!
IceCube Collab. ICRC2019

IceCube astrophysical neutrino signal is detected in several“channels”: through-going muon tracks, “High-Energy Startingevents” (HESE), cascades.
Spectral measurements with different techniques show slight
discrepancies (less than 3 sigma).
Spectral measurements with different techniques sample different
parts of the sky.

!, e, $

Extragalactic neutrinos from AGN
PG 1553+113

Proton-proton interactions

Proton-photon interactions

sensitivity

sensitivity

Radio-loud active galactic nuclei and more specifically blazars are considered as viable extragalactic source
candidates. High-energy proton interactions in the AGN jet with– low-energy protons from the accretion flow or circumnuclear medium and– low-energy radiation fields from the accretion disk or jet
Result in production and decays of pions with emission of neutrinos and gamma-rays.

Biermann, Mannheim, Stecker et al, ‘90th,……..
Figure from Neronov, Ribordy ‘09

Neutrino and gamma-ray production in high-energy proton interactions

Pion production in proton-proton interactions is characterized by energy threshold !"#$ ≃ 100 MeV. Typical energy ofneutrinos is !+ ∼ 0.03 !"#$ . Total power emitted in neutrinos is comparable to the total power output in electromagnetic
component (electrons plus gammas). Spectra of gamma and neutrinos nearly repeat the spectrum of parent protons.
Kelner, Aharonian 2006

Extragalactic neutrinos from AGN (?)
IceCube Collab. ’18
MAGIC Collab. ‘18

3-sigma evidence for neutrino emission from a blazar (TXS0506+056) has been reported.
Gamma-ray emitting AGN which dominate Fermi/LAT extragalactic
sky are not responsible for the production of the bulk of theIceCube signal.
It is still possible that the “neutrino loud” blazar AGN population
exists, but neutrino flux from an AGN is not proportional to its
gamma-ray flux.

TXS 0506+056

Galactic neutrino sources (?)

Mild evidence for anisotropy of neutrino signal exists: there are more events at lower Galactic latitudes. Deficit of neutrino events athigh Galactic latitude and excess at low Galactic latitude amount to 3" evidence post-trial for anisotropy (with 5 year HESE event set).
A 2.3" excess is found in the IceCube likelihood sky template fitting (KRA% model of Galactic pion decay emission) in cascade events.
Neronov, Semikoz ’16
IceCube Collab. arXiv:1907.06714

Gamma-ray sky
Fermi/LAT E>1 GeV

Pion decay

Galactic diffuse emission is dominated by pion decay flux from cosmic ray
interactions in the interstellar medium.

Neutrino and gamma-ray production in cosmic ray interactions

Pion production in proton-proton interactions is characterized by energy threshold !"#$ ≃ 100 ()* . Typical energy ofneutrinos is !+ ∼ 0.1 !"#$ . Total power emitted in neutrinos is comparable to the total power output in electromagnetic
component (electrons plus gammas). Spectra of gamma and neutrinos nearly repeat the spectrum of parent protons.
Kelner, Aharonian 2006

Gamma-ray + neutrino sky?

The strongest gamma-ray pion decay flux is from the inner Galactic Disk. Its
extrapolation to 100 TeV range is below the observer sky-averaged neutrinoflux.

Galactic (?) Gamma-ray + neutrino sky!"∝ # %&.(!#All-sky gamma-ray and neutrino spectra are consistent with each other.
Is there a Galactic component in the astrophysical neutrino signal?
AN, Semikoz, 2014

Gamma-ray + neutrino sky!"∝ # %&.(!#Conventional models of pion decay emission from cosmic ray interactions would assume the spectral slope -2.7 of the locally observed
cosmic ray spectrum (e.g. Fermi diffuse emission model of 2012).
Ackermann et al. (2012)

Average Galactic cosmic ray spectrum
Fermi LAT Collab. ‘12E-2.4

E-2.7

AN, Malyshev ’15Fermi LAT Collab. ’16
Yang, Aharonian ‘16

Fermi/LAT gamma-ray data suggest that average cosmic ray spectrum in theGalactic Disk is different from the locally measured one. Average spectrum of!"cosmic rays residing in the Galactic Disk has the slope !# ∝ %&' , Γ =2.4 … 2.5, rather than Γ ≃ 2.7.
Similar slope is found for cosmic rays residing in the Large Magellanic Cloud.

Inner Galaxy

LMC-80080
Gal. longitude, deg

Galactic (?) Gamma-ray + neutrino sky

IceCube and ANTARES telescopes have reported tight limits on Galactic contribution to the astrophysical neutrino flux.
The analysis is based on “non-observation of the excess” along the Galactic Plane.
The analysis method uses a template of Galactic neutrino flux derived from modelling of cosmic ray propagation in the Galaxy.
IceCube Collab. ’17, ’18 ANTARES Collab. 17, ’18 Denton, Marfatia, Weiler ’17

Galactic (?)

All-sky multi-messenger (neutrino +gamma-ray) spectrum

Mismatch?
What is the dominantComponent of Galactic diffuse flux
In multi-TeV band then?

KRA! model

IceCube Collab. ’17, ’18 ANTARES Collab. 17, ’18 Denton, Marfatia, Weiler ’17
AN, Semikoz, 2014

Galactic (?)

All-sky multi-messenger (neutrino +gamma-ray) spectrum

Is it possible to close the gap between
Gamma-ray and neutrino measurements?

IceCube Collab. ’17, ’18 ANTARES Collab. 17, ’18 Denton, Marfatia, Weiler ’17
AN, Semikoz, 2014

All-sky neutrino signal in multi-TeV band

Data points down to 1 TeV in the “cascade” event sample have been shown in a recent IceCube publication.
The astrophysical neutrino flux is detectable down to TeV energy.
IceCube Collab. 1907.06714

All-sky neutrino signal in multi-TeV band

Statistics of IceCube signal in the TeV band is high enough.
The errorbars (not from the quoted IceCube paper) in the TeV band are dominated by the uncertainty of the dominant atmospheric
neutrino flux component. Progress is possible via improved measurement / modelling of the atmospheric neutrino flux.
IceCube Collab. 1907.06714

All-sky gamma-ray signal in multi-TeV band
Fermi/LAT E>1 TeV

Mrk 421

Diffuse gamma-ray emission
Or residual cosmic ray background?

Galactic disk

Pion decay emission ?

Cygnus region
CrabResidual cosmic ray background in Fermi/LAT data

Anti-coincidence detector (ACD)(charged particles « click » here)
Bruel et al. ‘18

Electromagnetic calorimeter (CAL)
Cosmic ray showers appear wider here)
Starting points of residual events leaking throughthe ACD tiles in the SOURCE class event selection

Residual cosmic ray background in Fermi/LAT data
Bruel et al. ‘18

Comparison of photon dominated (red) and “high Galactic latitude diffuse”
event sample signals in the ACD tiles (!"#$% ) through which the event shouldhave passed in SOURCE event class (Bruel et al. ‘18).
Comparison of statistics of “pure photon” (0.3 degree circles around 4FGL
catalog sources, the Earth limb and the Galactic Ridge) events in the SOURCE
and new SOURCEVETO class, and of events in the “High latitude diffuseemission” (HLD) (outside 0.3 degrees around 4FGL sources, Galactic latitude>20 degrees) regions allows to……..
estimate of the residual cosmic ray background fraction in SOURCEVETO class(Neronov & Semikoz ‘19)

All diffuse emission region eventsResidual cosmic rays

Neronov & Semikoz ‘19

Galactic Plane in multi-TeV band

The residual cosmic ray background is largely sub-dominant compared to the multi-TeV gamma-ray signal all along the Galactic Plane(inner and outer galaxy).
Neronov, Semikoz, arXiv:1907.06061 (A&A accepted)

Galactic Plane in multi-TeV band

total
4GFL sources
Diffuse signal
Diffuse signal

4GFL sources

The residual cosmic ray background is largely sub-dominant compared to the multi-TeV gamma-ray signal all along the Galactic Plane(inner and outer galaxy).
Neronov, Semikoz, arXiv:1907.06061

Galactic Plane in multi-TeV band

Galactic Plane is visible
well able 3 TeV
Spectrum is not shownAt higher energy because
instrument response isnot known

The residual cosmic ray background is largely sub-dominant compared to the multi-TeV gamma-ray signal all along the Galactic Plane(inner and outer galaxy).
Neronov, Semikoz, arXiv:1907.06061

Galactic Plane in multi-TeV band

LAT measurements of the Galactic Plane signal at TeV are systematically higher than those of HESS (which should be considered aslower bound on the flux, as stated in the HESS paper).

Neronov, Semikoz, arXiv:1907.06061

The challenge of observing diffuse emission with Cherenkov telescopes

Electromagnetic calorimeter (CAL)
Cosmic ray showers appear wider here)

The challenge of observing diffuse emission with Cherenkov telescopes

1907.08171

Residual cosmic ray backogrund in Cherenkov telescopes is much above
the diffuse sky flux level.

The challenge of observing diffuse emission with Cherenkov telescopes

HESS colaboration was able to partially overcome the problem of high residual cosmic ray backogrund by defining backgorund
estimate regions within wide enough Galactic Plane survey region.
These background estimate regions are, however, not signal-free.
HESS Collab. ‘14

High Galactic latitude signal in multi-TeV band
Fermi/LAT E>1 TeV

Mrk 421

Diffuse gamma-ray emission
Or residual cosmic ray background?

Neronov, Semikoz, arXiv:1907.06061

High Galactic latitude signal in multi-TeV band
-60o<l<60o-120o<l<240o

Residual cosmic ray background is also lower than the observed flux at high Galactic latitude. This means that large part of the highlatitude counts are photons.
Neronov, Semikoz, arXiv:1907.06061

High Galactic latitude signal in multi-TeV band

Diffuse signal
Diffuse signal

4FGL sources

Residual cosmic ray background is also lower than the observed flux at high Galactic latitude. This means that large part of the highlatitude counts are photons.
Neronov, Semikoz, arXiv:1907.06061

High Galactic latitude signal in multi-TeV band

High Galactic latitude emission in
multi-TeV region is of Galactic
origin, because extragalactic flux issuppressed by absorption on EBL.
The Galactic diffuse spectrum
appears to harden at the highestenergy.

Residual cosmic ray background is also lower than the observed flux at high Galactic latitude. This means that large part of the highlatitude counts are photons.
Neronov, Semikoz, arXiv:1907.06061

Local Galactic environment

Some 2 Myr ago a supernova shell hass passed through the Earth anddeposited radioactive 60Fe isotopes in the deep ocean sediments.
Knie et al ‘04

Local Galactic environment

Peculiarities of the local measurements of cosmic ray spectrum(positron and antiproton spectra, feature at 1Kachelriess, AN, Semikoz ’15, ‘17High-latitude Galactic gamma-ray and neutrino flux: possible models

High Galactic latitude flux

Vela Jr
Vela XCrabCosmic rays with total energy 1050 erg which have escaped nearby recent (within the escape time of PeV particles) source loose energy into
neutrino and gamma-rays on time scale tpp~108 (nISM/0.5 cm-3) yr.
This might result in very extended emission with a flux
F~10-10 (nISM/0.5 cm-3) (d/0.5 kpc)-2 erg/(cm2 s)~10-7GeV/(cm2 s)
ULTRACLEANVETO event selection
Neronov, Semikoz, Kachelriess 2018

High-latitude Galactic gamma-ray and neutrino flux: possible models
Simulated dark matter halo

Vela Jr
Vela XCrabDecays of heavy dark matter particles with lifetime ~1028 s into pions or directly into neutrinos and gamma-rays could provide the required
multi-messenger flux at high Galactic latitude.

Neronov, Semikoz, Kachelriess 2018

Measurement of Galactic diffuse emission at E>3TeV?

HERD?
Fermi/LAT

Inner Galactic Plane

10#$ %&'

Galactic diffuse emission at E>3TeV?

C herenkov te le s

1%systemco paticse reo f ths idu

e rea l cos idu

sm ial bc ray ba

ac k grounc k gro und?

d?

Re-using HESS approach: Galactic diffuse emission with CTA

HESS Galactic Plane survey-60o<l<60o

Background estimate regions

Galactic diffuse emission with CTA-60o<l<60o

SST FoV
SST FoV

Background estimate regions

Larger FoV of CTA SST will provide an improvement for the measurement of the Galactic Disk diffuse emission in multi-TeV range

Re-using HESS approach: Galactic diffuse emission with 20o FoV telescope-60o<l<60o1% systematic of residual CR
background suppressed by 102

20o FoV

Larger FoV telescopes would allow a measurement of the diffuse emission up to larger Galactic latitudes, via measurements of thegradient of event count rates along the Galactic latitude direction in long stacked images aligned along Galactic longitude.

Re-using HESS approach: Galactic diffuse emission with dedicated 20o FoV telescope

Residual CR background:
108 events / (50 hr × 1 km2 × 20∘ FoV) @ TeV? Cherenkovtelesco pe res idua l cosm ic ray bac k gro und?

CTA SST, 50 hr

3 hr

20o FoV, 10

Potentially detectable signal (ignoring systematics)
106 events / (1000 hr × 1 km2 × 20∘ FoV) @ TeV

Summary
Galactic diffuse emission spectrum is measured up to E=3 TeV
Fermi/LAT E>1 TeV

Measurement of multi-messenger (gamma+neutrino) signal in 10100 TeV band is possible.