1. Multi-Messenger Studies with Gravitational Wave Events Challenges and Opportunities
Jochen Greiner Max-Planck Institut für extraterrestrische Physik, Garching

2. “LIGO: A discovery that shook the world”
GW was breakthrough in GW astrophysics
Quality and quantity of EM counterpart observations is staggering

3. Contemporaneous GW & γ-ray Detection
GRB A GW
AT 2017gfo (kilonova)
Nomenclature: the event was on 17. August 2017
To be fair:
Without GW detection this GRB would be just another among 2000 Fermi/GBM GRBs with nothing than γ-ray data: No kilonova, no afterglow, no distance, no jet structure and geometry details, no wondering about low luminosity
 truly multi-messenger event
no LIGO auto-detection due to noise glitch
manual LIGO check after automatic GBM trigger  after 27 min: the coincidence was recognized
New era had begun!
Abbott et al. 2017, ApJ
…and at least 1000 astronomers knew it at this time

4. Multi-wavelength follow-up observations
automatic multi-directional info exchange via GRB -network:
To+25 s: first Fermi/GBM
To+27 min: LIGO info (NS)
192 s followed
1/3 of all observational astronomers worldwide were involved
optical
γ-rays, X-rays
Radio
Neutrinos
Abbott et al. 2017, PRL

5. Combination of different astrophysics
Stellar evolution – NS-NS merger – Neutrino emission - Accretion – Jets – Gamma-ray burst – Nucleosynthesis – Chemical evolution
4 different observational phenomena
Gravitational waves (GW)
Short Gamma-ray burst (GRB)
Kilonova
GRB afterglow
Plan of talk
what did we learn?
what else can we learn?
prospects
Tsujimoto+2014

6. Short GRBs
due to NS-NS merger (Eichler+1989)
typical jet opening angles ~5-10o
closest known short GRB: at z= (570 Mpc)
thus, for any short GRB within the aLIGO horizon we expected
(1) LIGO detections of NS-NS mergers without a GRB (2) monster-bright γ-ray emission
 Instead, the first LIGO detection of a NS-NS merger comes with a weak GRB! Total surprise! Shows our incomplete/biased knowledge!

7. Gamma-Ray Burst A
Both Gamma-ray instruments developed at MPE Garching
Fermi/GBM localization consistent with LIGOs
INTEGRAL/ACS’ temporal structure sharper
With D~40 Mpc: GRB is underluminous by x!

8. GRB jet structure and viewing geometry
no afterglow during first 10d: slow rise suggest off-axis geometry
superluminal motion of compact radio emission from afterglow
together with flux evolution:
jet opening angle: 4o
off-axis angle of observer: 20o
consistent with LIGO limit of <28o for inclination of orbital plane (under assumption of jet being perpendicular to NS-NS orbit)
very narrow jet observed off-axis
Mooley et al. 2018, Nature
Troja et al. 2018

9. The GRB prompt emission spectrum
Constraints on synchrotron emission models
Begue, JG et al. 2017, ApJ
a. Structured or on-axis top-hat jet
Lorentz factor of accelerated e- is γe = κmp/me
κ parametrizes uncertainty of acceleration
ξ < 1 fraction of accelerated e-
α < 1 fraction of E turned into radiation
αξ extremely tiny for GRB170817A: impossible
ξ«1 small efficiency α«0.01 incompatible with relativ. jet
1047 erg/s
1052 erg/s
b. off-axis top-hat jet
Γ<20 at θ~10o for Epeak difference
incompatible with compactness
 Emission mechanism is very unlikely synchrotron

10. produced by successive n-capture reactions (with intermittant β-decay)
Kilonova AT 2017gfo
Kasen et al. 2017, Nat 551, 80
produced by successive n-capture reactions (with intermittant β-decay)
needs n-rich process  NS Merger
early blue component from outflow
late red component from very n-rich ejecta

11. Nucleosynthesis from merging NSs
Barnes+2016
Smartt, JG+2017
But no, we have not seen gold
which isotopes?
heavy (up to A~195) ??
lighter (A<130) ??
(both provide acceptable fits!)
Rosswog+2018
light curve and color evolution of KN are consistent with r-process

12. Radioactivity from Kilonova
Relatively slow decay of optical light curve of kilonova:
Potentially, gamma-ray line measurements could decide between radioactivity or pulsar/magnetar power,  but likely not in near future (1 Mpc limit vs. GRB distances)
(e.g. Dai+2017)
SPI
~MeV eV
~MeV
~MeV
Hotokezaka+ 2016

13. What did we learn? Answers to…
…expected, old questions:
short GRBs are indeed due to merging NS
GRB jets are narrow
theoreticians have grossly predicted the KN signal correctly (duration; luminosity ~1000x nova)
light curve and color evolution of KN are consistent with r-process blue component suggestive of merger into a magnetar; red component is likely combination of tidal material and disk wind
…unexpected, new questions/problems:
GRBs emit off-axis γ-rays
the γ-ray emission mechanism in short GRBs is different than previously thought
there are indeed ‘red’ and ‘blue’ components in kilonovae: but they are not mutually exclusive; instead, they come together

14. Open questions
what is the emission mechanism in short GRBs? How does off-axis emission (luminosity, spectrum, variability, polarization) scale with Γ?
If off-axis γ-ray emission is typical, there should be many local GRBs! Where are they? Why has Swift not seen a single one at <570 Mpc??
simultaneous blue/red components in kilonovae: How to explain the optical/NIR light curve? Do all KN have this blue/red components? What is the effect of viewing angle?
final merger product: NS or black hole?
how will BH-NS merger look like?
need better atomic data for (light) r-process elements
Metzger et al. 2017, Science

15. Predictions I: Rates of nearby GRBs
if GRBs also emit off-axis, then there will be many more of those
rate depends on relative luminosity ratio and jet opening angle
Burgess, JG et al. 2017
Nominal on-axis emission within 10o, beyond power-law decline up to 90o
 at faint flux levels, local off-axis GRBs dominate
Janka et al. 2006

16. NS-NS merger rate
LIGO detection suggests rate of yr-1 (per 100 Mpc3)
3 known channels
field binary evolution
globular clusters
nuclear clusters
highest rate is from classical isolated binary evolution: 10-2 yr-1
 Either rare event, or unknown binary channel with more frequent NS-NS mergers
Belczynski et al. 2018, A&A

17. Predictions II: GW detectors
Advanced LIGO / Virgo Plan
Next few years will bring hundreds of of GW triggers
GW localizations will improve, but generally 10-50x larger than that of GW
now upgrade
2/ O3 Start, incl. Virgo
~ full design-sensitivity ~ KAGRA (Japan) ~ LIGO India ~2032?? LISA

18. Predictions III: multi-λ instruments
Wavelength
# src / □o
FOV
Sensitivity
instruments
γ-rays ++ -
Fermi/GBM, INTEGRAL/ACS
X-rays 2 + --
Swift (tiling), MAXI UV 10
Optical/NIR
1000
many IR 50
Radio
LOFAR
 Largest progress possible: with new, more sensitive γ-ray detector

19. How would a new gamma-ray detector help?
Better localization
More accurate time difference between merger (GW) and GRB
Better spectroscopy to understand the emission process
Possibly measure γ-ray polarization to understand the emission process
Measure nuclear lines in nearby SN explosions to understand the basics (unlikely to detect nuclear lines from NS-NS merger even with next generation MeV telescope)

20. Predictions IV: GW counterparts & science
for majority (large off-axis angles): no GRB, no afterglow, possibly kilonova within LIGO volume
for few: GRB, KN, but no AG
for very few: GRB, AG, KN (as )
Likely:
Kilonova behavior / statistics (not necessarily r-process details)
BH-NS merger
Unexpected new phenomenon
cosmology: Ho (needs a few dozen GRBs)
Possibly:
Off-axis radiation (process) of short GRBs / Jet physics
Possibly inferences on NS radii and equation of state

21. BH-BH and BH-NS merger Several BH-BH mergers seen by aLIGO
None has shown a convincing EM counterpart (claim for EM of first merger, GW , disproven) this is consistent with many earlier predictions, though sparked the phantasy of some theoreticians
But: we should keep open-minded, and continue searching
BH-NS merger: still un-observed by aLIGO/Virgo case for EM counterpart completely open
Greiner+2016
Connaughton+2016

22. Summary New era (multi-messenger & multi-wavelength)
more new questions than solved problems (nice!)
Urgent need for better
γ-ray mission (sensitivity & localization)
Atomic data of (light) r-process elements
Many expectations: KN, off-axis emission mechanism, BH-NS merger, likely also unexpected surprises
Pessimistic view: GW was unique event; no NS-NS merger(s) in O3
Optimistic view: off-axis emission and 100x more frequent NS-NS binaries are the rule, then handful of new NS-NS mergers in O3 (2019)