ICEHAP Seminar, Chiba University, Jan 19th 2022
March 4th 2022
April 6th 2022

Source models of UHECR nuclei
Shunsaku Horiuchi

Image credit: NASA/ESA

Today’s contents•

Introduction to ultra-high energy cosmic rays (UHECR)
Observations
Mass composition•

Origins of nuclei
Nucleosynthesis in massive stars•

Source models for UHECR nuclei: gamma-ray bursts
Initial loading
In-situ nucleosynthesis
Entrainment (time allowing)•Concluding remarks

Shunsaku Horiuchi

2Cosmic rays
15

10~EeV1E1911-1912

Much history:
100+ years since Victor Hess’s
first discovery. Millikan coined
the name cosmic rays.

0~1

5 e
7.

1E

Sources: Galactic
supernovae?

0~1V5 e8.

VSources: ??

Blumer et al 2009

Much progress:
Since those times, we have much data. But we still are grappling with
fundamental questions: what is (are) their source(s)?
Shunsaku Horiuchi

3

The most extreme energies
Ultra-high energy cosmic rays (UHECR)•The most energetic (known) particles in
the Universe

Macroscopic energies in
microscopic particles…!

•UHECRLHC

Energy frontier, extreme
sources, very interesting

Shunsaku Horiuchi

4Why is it so difficult?
Charged particles bend in magneticfields• Magnetic field is ubiquitousin the Universe???

Difficult to model• Requires some of the mostextreme astrophysics andparticle physics

Shunsaku Horiuchi

5

The most extreme sources
Magnetars

Only the most extreme objects canaccelerate to UHECR energies

GRBs

The mostluminous
explosions

AGNs

The most
massive blackholes

Clusters

The largest
boundobjects

rL < R

Hillas plot: required, but not sufficient,condition for UHECR sourcesShunsaku Horiuchi

The strongest
B-fields

Which of these are the source(s) ???
6

Why is it so difficult?
Charged particles bend in magneticfields• Magnetic field is ubiquitousin the Universe???

Difficult to model• Requires some of the mostextreme astrophysics andparticle physics
Difficult to measure & interpret• UHECR are extremely rare
~ 1 /m2/Myr ~ 1 /km2/yr• Beyond terrestrial probes
Shunsaku Horiuchi

7

Extensive air showers
Detect UHECR through extensive air showers.
Giant detectors!
• Pierre Auger Observatory:~3,000 km2 @Argentina• Telescope Array (TA):~680 km2 @Utah à TA x4

è Three observables:
Energy, arrival direction, mass composition
Shunsaku Horiuchi

8

Hints from energy spectrum• After consideration for detector systematics, mostly in agreement• Cut-off seen just before 1e20 eV
Maximum acceleration energy or GZK cutoff?

Shunsaku Horiuchi

ICRC (2019)

9

GZK cutoff
CR with E > ~1018 eV suffer energy losses during propagation, aka “GZK”

p+
N+Allard et al. (2006)

CMB !

+

CMB ! (N! n + ⇡+Greisen (1966)Zatsepin and Kuz’min (1966)

1) + p/n

Allard et al. (2006)• Energy loss during propagation limits the UHECR horizon to ~100 Mpc• Ongoing debate on GZK effect vs maximum energy
Shunsaku Horiuchi

10

Hints from anisotropies
Large scale
Dipole anisotropy shows
extragalactic origin, not
Galactic

E > 8 EeVPAO (2017)

Galactic

Hotspots
Growing hints, eg the TA
hotspot.
Coverage includes eg M82,
Mrk 180, 421.
However still unclear.
Shunsaku Horiuchi

TA (2014)

Equatorial

11Hints from mass composition
Shower development is composition dependentSome observables to probe thecomposition:
1.

<Xmax> : mean maximum of theshower longitudinal profile
smaller for nuclei

1.

rms<Xmax>: the shower toshower variance is smaller fornuclei
FDSD
From Anchordoqui 2011
Shunsaku Horiuchi

12

Heavy composition
Gradually transitions to heavier composition above ~ankle

heavylightintermediate
heavy?

Shunsaku Horiuchi

Batista et al (2019)

Interpretation
uncertainties, fromhadronic showermodels that need to
be extrapolated
PAO (2019)13• UHECR are extragalactic, but we haven’t revealed thesource(s) yet.
• More data is needed! But spectrum is generic and manysources follow the matter distribution.
• Interesting possibility for nuclei UHECR. Quantitatively
still unknow, but it leads us to ask: what does composition
imply for sources?

Shunsaku Horiuchi

14

The check list
1. Source must be able to accelerate UHECR
2. Source must be energetic & common enough
3. Source must have a nuclei origin story

Shunsaku Horiuchi

15Origins of nuclei
Q: where are nuclei?
• Nucleosynthesis in stars• Explosive nucleosynthesis
in supernovae:– Up to Fe elements via
α captures– Heavier nuclei via fastnucleon captures• Rapid neutron captures inNS-NS mergers

è i.e., massive stars

Shunsaku Horiuchi

Big bang

Ye = 0.88

a Elements
Ye ≈ 0.5

protonYe ⌘nucleon

Neutron-capture peaks
Ye < 0.5

Fusion Captures

16

Stellar Nucleosynthesis
Nucleosynthesis
Massive stellar nucleosynthesis
results in the famous onion-shellstructure:

Iron nuclei abundance
The final Fe core mass is ~1.5 Msun withradius of 108 cm or soCore mass

+ : Fe core mass

Initial mass
Woosley et al (2002)

Shunsaku Horiuchi

17

Explosive Nucleosynthesis
Propagation of shock wave
through the core & envelope

Compression
and heating

Energy injection

Mass cut defines
final remnant

Explosivenucleosynthesis

Expansion
Shockcompression
and heating

The rest
is ejected

However, how much heavy nuclei is released is model dependent:
1. Some amount of energy injection
2. At some location
3. With some mass cut
Shunsaku Horiuchi

18

How much is possible?
A lot of 56Ni (potentially)
With large CO core, large explosion energies, and small mass cut, up to ~10 Msun~109 cmUmeda & Nomoto (2008)
Core collapses typically observed ~0.1 Msun of 56Ni
But can be more, e.g., hypernovae, superluminous supernovae (x100 56Ni
needed*)Shunsaku Horiuchi*if powered by nickel decay

19r-process nucleosynthesis
Generates neutron-rich nuclei
R-process network• Neutron captures• Photo-disintegrations• Decays (weak, fission)

Shunsaku Horiuchi

20

Massive stars collapse spectacularlyGRB(long)030329Supernova

GRBs are rare:GRB⇠ 10 3
SN
Stanek et al (2003)

Shunsaku Horiuchi

21

Inside a gamma-ray burst
GRBs, Low-luminosity GRB
Massive stars launching relativistic outflow (jets) upon gravitational collapse
Collapse to a
BH or NS

Energy deposited as some combinationof radiation and magnetic energy

GRB: internal dissipation of jetenergy (KE or mag -> heat)

Afterglow: moredissipation of jetenergy by interaction
with ambient
medium (cf SNR)
Meszaros 2002

Shunsaku Horiuchi (VT)22

UHECR source candidate
1. UHECR energy can be reached• Acceleration by shocks and/ormagnetic reconnection.
• Energy loss by photodisintegration
typically most important for nuclei

2. Energetics can be explained• Satisfied if CR luminosity > photonluminosity• Depends on CR spectral index andminimum CR energyrL < R

Shunsaku Horiuchi

Batista et al (2019)24

1. Initial loading
2. In-situ nucleosynthesis
3. (Entrainment – if time)

COMPOSITION OF JETS

Shunsaku Horiuchi

25

1. Initial loading
External composition• Radial dependence: stellar nuclei distribution• Time dependence: supernova shock, explosive nucleosynthesis

Shunsaku Horiuchi(20Msun)26

Can initial nuclei survive?
Nuclei survival
Optical depths of destructive
processes must be small
Photodisintegration
Thermal temperatures T0
Spallation
Target ion/nucleons
thermalized to T0survive

à Low-luminosity better forsurvival. Anotherscenario is magneticdominated outflows.
Shunsaku Horiuchi

Horiuchi et al (2012)27

Simulations
Initial loading simulations
2D relativistic MHD simulation of jet
induced collapse and jet acceleration• Fireball: heavy nuclei dissociated• Magnetic (partially): partial
dissociation of nuclei

Fireballmagnetization

à Magnetic models (and low-Lrad)better for nuclei composition

Magnetic

L~1e51 erg/sTime evolution [s]Shunsaku Horiuchi

Shibata & Tominaga (2015)28

2. In-situ jet nucleosynthesis

Nucleosynthesis inside the jet• Protected• Critically depends on thermodynamic conditions
Personally, a very interesting possibility

Shunsaku Horiuchi

29

Fireball scenario
Composition of GRB fireball:• Initial radiation temperatures of a few MeV
à nuclei are dissociated• Large entropy (ng/np ~ 105)
• Rapid expansion time scales (~0.1 ms)• n/p fraction probably close to equal (but uncertain)

Mass fraction

Hed“Deuterium bottleneck”

Pruet et al (2002),
Lemoine (2002),
Beloborodov (2003)
Shunsaku Horiuchi

Radius [cm]à Very few heavy
nuclei are made(cf BBN)
30

Alternative scenarios
Importance of entropy and expansion timescale
Lower entropy and larger timescale are conducive to nucleosynthesisà Deuterium bottleneck can be prevented
1. Magnetic scenario
Lrad can be low if the GRB is powered magnetically, e.g., for rapidly rotating
neutron stars:
2Ė ⇠ 1049 Pms4 B15
R66 erg s 1

Other models, e.g., magnetized disk winds or BH powered

e.g., Usov (1992)
Thompson (1994)
e.g., Blandford &Znajek (1977)

2. Low-luminosity GRB and dirty (baryon) jets
With lower Lrad by default, or with large baryon loading by default

Shunsaku Horiuchi

31

Proto-magnetar

c=0: aligned
c=p/2: oblique

Proto-magnetar model:
The birth of magnetars. Consider atwo-component model:
1. Neutrino-driven wind
2. magnetic outflow
The ratio sets the magnetizationof the outflow<latexit sha1_base64="a/8FteIzBD1hcC650xlTTrutNhA=">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</latexit>

Ė(B, P )0 (B, P ) =Ṁ c2
Interpretations of X-ray light curvesof GRBs, suggests certain ranges of(B,P) are likely to be causing GRBs

Shunsaku Horiuchi

Lu & Zhang (2014)32

Analytic study: nucleosynthesis

Shunsaku Horiuchi

GRB phasecp -r ihnegil,adTime since core bounce [s]Pair wind phase

Jet break out

Especially for:• Initially n-rich matter• Oblique rotators(receive less nheating, thus haslower entropies)

A ≥56 nuclei mass fraction

Freezeout yields
Estimate the mass fraction of A≥56 (Xh) with analytic wind nucleosynthesis
Roberts et al (2010)• Ye: electron fraction• texp: timescale
Metzger, Giannios, Horiuchi (2011)(B=5e15G,P=2ms)• s: entropy
n-richà Freezeout
composition can beueqilb
heavy-dominatedo,p-richduring the GRB phase33

Analytic study: acceleration
Acceleration:
Demand acceleration isfaster than cooling &expansion timescales:(B=5e15G,P=2ms)
Collision case: Rd ~ 1015-16 cm

GRB phase

Cooling limit
Expansion limited
Survival:Calculate the optical depthof photodisintegration basedon the Band function for thephoton spectrum
Demanding this is = 1 (or afew, allowing for a few
destructions) defines theUHECR phase

Nuclei UHECR
phase

Metzger, Giannios, Horiuchi (2011)à There remains a window for UHECR nuclei generation
Shunsaku Horiuchi

34

Analytic study: acceleration
Acceleration:
Demand acceleration isfaster than cooling &expansion timescales:
Cooling limit
Expansion limited(B=5e15G,P=2ms)
Magnetic reconnection case: Rd ~ 1012-13 cm

GRB phase
Nuclei UHECRphase

Survival:Calculate the optical depthof photodisintegration basedon the Band function for thephoton spectrum
Demanding this is = 1 (or afew, allowing for a few
destructions) defines theUHECR phase

Metzger, Giannios, Horiuchi (2011)à There remains a window for UHECR nuclei generation
Shunsaku Horiuchi

35

Dependence on model parameters<latexit sha1_base64="a/8FteIzBD1hcC650xlTTrutNhA=">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</latexit>

Vary magnetic field (B) and rotation period (P):
Nucleosynthesis
Better for faster rotator, which impacts
outflow size & entropy

Shunsaku HoriuchiĖ(B, P )0 (B, P ) =Ṁ c2Emax
Assuming Fe composition:
At end of GRB phase
At breakout

Bhattacharya et al (2021)

36Numerical treatment: nucleosynthesis(B,P)

Semi-analytic
neutrino-driven
wind model(entropy, timescale)

Nucleosynthesis(abundances)

Electron fraction
Lippuner et al (2017)

Abundance(B=4e15G,P=1.5ms)(NB: when maximum particle acceleration takes place)Shunsaku Horiuchi

Ekanger et al (2022)

37

Epochs of the jet
Consider four epochs (or locations) along the jet
1. Initial launch
2. Breakout from progenitor (assumed Wolf-Rayet)
3. Beginning of when photodisintegration optical depth < 1
4. Last moment when maximum CR energy is > 1020 eV

Shunsaku Horiuchi

Epoch which
can havenuclei UHECR

38

Mean A

Ye = 0.45

Ye = 0.475

Ye = 0.5(i)Initial(ii)breakout(iii)
photodisintegration = 1(vi)End of 1020 eV

Broadly confirms analytic model
1. Neutron-rich (top row) much better
2. See synthesis even to late times (last column)
3. Period is more important than magnetic field• Impacts outflow steradian• Impacts n-heating• Lower periods conducive to synthesis

Ye = 0.55
Shunsaku Horiuchi

39

Gaining detailed pictureConsider four epochs (or locations) along the jet
1. Initial launch
2. Breakout from progenitor (assumed Wolf-Rayet)
3. Beginning of when photodisintegration optical depth < 1
4. Last moment when maximum CR energy is > 1020 eV

Abundance(B=4e15G,P=1.5ms)

Epoch which
can havenuclei UHECR

Shunsaku Horiuchi

Ekanger et al (2022)40

Ongoing studies
UHECR from population• (B,P) distributions, others
Detailed nuclei survival• Consideration of jet propagationin stars and collimation effects
Estimate time-integrated UHECR• Nucleosynthesis between
our 4 time epochs• Changes in the intermediate
photo-disintegration regime• Mass-weighted integral

n & g-ray signaturesof accelerated nuclei
UHECR propagation

Nick Ekanger

Jose Carpi

Detailed synthesis• Dependence onmodel parameters
Mukul Bhattacharya Kohta Murase
Shunsaku Horiuchi

41

3. Entrainment
Entrain

Entrainment of external nuclei into the jet
medium: do nuclei survive?
jetstarnuclei
Can it safely do this?
Shunsaku Horiuchi

Zhang et al. (2003)

42

Survival in entrainment
CocoonThe cocoon is made up of shockedstellar and shocked jet material andexpands non-relativistically into thestellar material.
3/8
1/2(1)⇠0.01Lrcke,50 9
3/16
1/2
Tc(1) ⇠ 100Lke,50 r9keV

Stellar matter(plenty of nuclei)

Jet has
G >> 1
nuclei-rich
Cocoonbc ~ 0.01

JETnuclei

cocoonjetcocoon

Stellar matterà Cocoon can be nuclei rich (mixing
aided by instabilities) e.g., Aloy (2002)

Jet headhas bh ~ 0.1

e±, γ,
somenuclei

Th

Can it safely do this?
Shunsaku Horiuchi

43Survival in entrainmentSurvival
Demand the nuclei velocity is always below the spallation thresholdà requires the nuclei to be thermalized FASTER than it takes to move up thevelocity gradient and its speed becomes too fastali
rmtioza(Collimated jets can tolerate a higher
gradient due to higher e- density)eth

Rapid thermalization for KEFe < 20 GeV
à If velocity
is aresmall,
Energyloss ongradientelectrons
fastnuclei(canthermalize
beforebyreaching
thewhen Tbe
further helped
pair e+espallationishigh) andthresholdspallation is slowernjet

cocoonnuclei

iontallaps

Conical jet(easier for collimated jet)

Can it safely do this?
Shunsaku HoriuchiHoriuchi
Horiuchietetalal(2012)(2012)

44

Summary tableèIn Fireball GRB, only entrainment remains (but likely is difficult)èIn magnetic or low-luminosity GRB, multiple options exist
Fireball GRB

Magnetic GRB

Low-luminosity GRB

Survives: initial loading?

NYY

Source: jet nucleosynthesis

NY

Maybe

Survives: entrainment?

gradient

gradient

gradient

Survives: n-collisions?

YYY“Y” means possible for canonical parameters;“N” is not possible;

Shunsaku Horiuchi

45

Concluding remarks• UHECR origins remain a mystery• Supernova/related explosions are stores of heavy nuclei– Through stellar nucleosynthesis, explosive nucleosynthesis, andin-situ jet nucleosynthesis• Models for UHECR nuclei– Magnetar models for GRBs, low-luminosity GRBs, and baryonrich jets are especially conducive to heavy nuclei UHECR• Future works– Ongoing studies of the impacts of jet propagation insideprogenitors, regime of partial photo-disintegration, UHECR
propagation and connections to multi-messenger astronomy• Aiming to create self-consistent composition predictions!
Thank you for your attention!
Shunsaku Horiuchi

46

BACKUP SLIDES

Shunsaku Horiuchi

47

Multi-messenger considerations
At a minimum, UHECR propagation creates multi-messenger signatures(additional signals possible from sources themselves)

Batista et al (2018)
Shunsaku Horiuchi

48

Various observatories
Fluorescence from space:• EUSO balloon (‘13)• JEM-EUSO (proposed)

Hybrids:• Pierre Auger Observatory (‘04 ~)• Telescope Array (’09 ~)Fluorescence from ground:• Fly’s eye• HiRes

Surface array ofparticle detectors,
eg plastic scintillator,
Cerenkov detector• AGASA• Yakutsk• …many more

è Three observables:
Energy, arrival direction, mass composition
Shunsaku Horiuchi

49

Cutoff in more detail
Allowing for systematics
brings datasets in agreement,but only below ~1e19.7 eV
Systematics only at highest
energies or source physics?
Eg focusing only on
overlapping region of the sky
shows better agreement (atcost of statistics).
• TA shows declination
dependence.
• TA coverage includeseg M82, Mrk 180, 421.
Shunsaku Horiuchi

PAO (2018)50

Example: proto-magnetar scenario

Fast rotating core
à B-field generation
à Rapid mass-loss
à Evades BH
formation

Shunsaku Horiuchi

M-out

Having the right
angular momentum
distribution is key:
M-in

Central mass

Need a neutron star remnant:M-out > M-in

51

Shunsaku Horiuchi

A ≥56 nuclei mass fraction

A ≥56 nuclei mass fraction

Other model dependences

Bhattacharyna et al (2021)

52

Comparison
Fraction of heavy (Fe and above) nuclei:
Quantitative predictions require numerical treatment
Analytic

Numerical (SkyNET)

Shunsaku Horiuchi

Ekanger et al (2022)

53More simulations
Aligned rotator simulations
Force-free approximation, aligned dipole field 1014-15 G, SkyNet nuclear reaction
network. Shows r-process nucleosynthesis across a range of rotation periods
Approximately 40% He, trace n and p

The rest: abundance-weighed mean A

alphaneutronproton

Shunsaku Horiuchi

Vlasov et al (2017)54

4. Collisions with neutrons
Neutrons are collisionally coupled
to the accelerating plasma:
EMcoulomb strong$e$p$nBut they lag behind if tcollision > tacc•

Make sure the relative velocity
1 3
˜ ⌧coll⇠⌧acc/Lr ⌘does not exceed the spallationthreshold•Or, the neutrons decouple
before the spallation thresholdis reached

Conical jet(easier for collimated jet)

à Nuclei survive unless h is very large
Shunsaku Horiuchi

Horiuchi et al (2012)
Horiuchi, Murase, et al (2012)55