MultiMessenger Studies with Gravitational Wave Events Challenges and

Multi-Messenger Studies with Gravitational Wave Events Challenges and Opportunities Jochen Greiner Max-Planck Institut für extraterrestrische Physik, Garching Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

“LIGO: A discovery that shook the world” Ø GW 170817 was breakthrough in GW astrophysics Ø Quality and quantity of EM counterpart observations is staggering https: //www. youtube. com/watch? v=-Yt 5 Em. Egz 2 w Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

Contemporaneous GW & γ-ray Detection Nomenclature: the event was on 17. August 2017 GRB 170817 A GW 170817 AT 2017 gfo (kilonova) 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 Ø no LIGO auto-detection due to noise glitch Abbott et al. 2017, Ap. J Ø manual LIGO check after automatic GBM trigger after 27 min: the coincidence was recognized New era had begun! …and at least 1000 astronomers Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 knew it at this time

Multi-wavelength follow-up observations automatic multi-directional info exchange via GRB emailnetwork: To+25 s: first Fermi/GBM email To+27 min: LIGO info (NS) 192 emails followed 1/3 of all observational astronomers worldwide were involved optical Radio γ-rays, X-rays Neutrinos Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Abbott et al. 2017, PRL § §

Combination of different astrophysics Stellar evolution – NS-NS merger – Neutrino emission - Accretion – Jets – Gamma-ray burst – Nucleosynthesis – Chemical evolution Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Tsujimoto+2014 4 different observational phenomena Ø Gravitational waves (GW) Ø Short Gamma-ray burst (GRB) Ø Kilonova Ø GRB afterglow Plan of talk Ø what did we learn? Ø what else can we learn? Ø prospects

Short GRBs • due to NS-NS merger (Eichler+1989) • typical jet opening angles ~5 -10 o • closest known short GRB: 080905 at z=0. 1218 (570 Mpc) • thus, for any short GRB within the a. LIGO horizon we expected (1) 100 -1000 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! Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

Gamma-Ray Burst 170817 A Ø Ø 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 10. 000 x! Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

GRB jet structure and viewing geometry Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Troja et al. 2018 Mooley et al. 2018, Nature • no afterglow during first 10 d: slow rise suggest off-axis geometry • superluminal motion of compact radio emission from afterglow • together with flux evolution: very narrow jet • jet opening angle: 4 o observed off-axis • off-axis angle of observer: 20 o • consistent with LIGO limit of <28 o for inclination of orbital plane (under assumption of jet being perpendicular to NS-NS orbit)

The GRB prompt emission spectrum Constraints on synchrotron emission models a. Structured or on-axis top-hat jet Begue, JG et al. 2017, Ap. J Lorentz factor of accelerated e- is γe = κmp/me κ parametrizes uncertainty of acceleration ξ < 1 fraction of accelerated eα < 1 fraction of E turned into radiation αξ extremely tiny for GRB 170817 A: impossible ξ « 1 small efficiency α « 0. 01 incompatible with relativ. jet b. off-axis top-hat jet Γ<20 at θ~10 o for Epeak difference incompatible with compactness Emission mechanism is very unlikely synchrotron Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 1047 erg/s 1052 erg/s

Kilonova AT 2017 gfo Kasen et al. 2017, Nat 551, 80 produced by successive n-capture reactions (with intermittant β-decay) early blue component late red component from outflow from very n-rich ejecta needs n-rich process NS Merger Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

Nucleosynthesis from merging NSs Barnes+2016 Smartt, JG+2017 which isotopes? • heavy (up to A~195) ? ? • lighter (A<130) ? ? (both provide acceptable fits!) Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Rosswog+2018 1. light curve and color evolution of KN are consistent with r-process But no, we have not seen gold

Radioactivity from Kilonova Relatively slow decay of optical light curve of kilonova: Potentially, gamma-ray line measurements could decide between radioactivity or (e. g. Dai+2017) pulsar/magnetar power, but likely not in near future (1 Mpc limit vs. GRB distances) SPI ~Me. V Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Hotokezaka+ 2016

What did we learn? Answers to… …expected, old questions: Ø short GRBs are indeed due to merging NS Ø GRB jets are narrow Ø theoreticians have grossly predicted the KN signal correctly (duration; luminosity ~1000 x nova) Ø 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: Ø GRBs emit off-axis γ-rays Ø the γ-ray emission mechanism in short GRBs is different than previously thought Øthere are indeed ‘red’ and ‘blue’ components in kilonovae: but they are not mutually exclusive; instead, they come together Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

Open questions Ø what is the emission mechanism in short GRBs? How does off-axis emission (luminosity, spectrum, variability, polarization) scale with Γ? Ø 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? ? Ø 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? Ø final merger product: NS or black hole? Ø how will BH-NS merger look like? Ø need better atomic data for (light) r-process elements Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Metzger et al. 2017, Science

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 10 o, beyond power-law decline up to 90 o Janka et al. 2006 Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 at faint flux levels, local off-axis GRBs dominate

NS-NS merger rate • LIGO detection suggests rate of 1. 5+3. 2 -1. 2 yr-1 (per 100 Mpc 3) • 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 Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Belczynski et al. 2018, A&A

Predictions II: GW detectors Ø Advanced LIGO / Virgo Plan now 2/2019 ~2020 ~2022 ~2024 ~2032? ? Ø Next few years will bring hundreds of of GW triggers Ø GW localizations will improve, but generally 10 -50 x larger than that of GW 170817 Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 upgrade O 3 Start, incl. Virgo full design-sensitivity KAGRA (Japan) LIGO India LISA

Predictions III: multi-λ instruments Wavelength # src FOV / □o Sensitivity instruments γ-rays 0 ++ - Fermi/GBM, INTEGRAL/ACS X-rays 2 + -- Swift (tiling), MAXI UV 10 -- - - Optical/NIR 1000 + - many IR 50 -- -- - Radio 2 - - LOFAR Largest progress possible: with new, more sensitive γ-ray detector Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

How would a new gamma-ray detector help? Ø Ø 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 Ø Measure nuclear lines in nearby SN explosions to understand the basics (unlikely to detect nuclear lines from NS-NS merger even with next generation Me. V telescope) Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

Predictions IV: GW counterparts & science Ø for majority (large off-axis angles): no GRB, no afterglow, possibly kilonova within LIGO volume Ø for few: GRB, KN, but no AG Ø for very few: GRB, AG, KN (as 170817) Likely: Ø Kilonova behavior / statistics (not necessarily r-process details) Ø BH-NS merger Ø Unexpected new phenomenon Ø cosmology: Ho (needs a few dozen GRBs) Possibly: Ø Off-axis radiation (process) of short GRBs / Jet physics Ø Possibly inferences on NS radii and equation of state Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018

BH-BH and BH-NS merger Ø Several BH-BH mergers seen by a. LIGO § None has shown a convincing EM counterpart (claim for EM of first merger, GW 150914, disproven) this is consistent with many earlier predictions, though 150914 sparked the phantasy of some theoreticians § But: we should keep open-minded, and continue searching Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018 Greiner+2016 Connaughton+2016 Ø BH-NS merger: still un-observed by a. LIGO/Virgo case for EM counterpart completely open

Summary Ø New era (multi-messenger & multi-wavelength) Ø more new questions than solved problems (nice!) Ø Urgent need for better § γ-ray mission (sensitivity & localization) § Atomic data of (light) r-process elements Ø Many expectations: KN, off-axis emission mechanism, BH-NS merger, likely also unexpected surprises § Pessimistic view: GW 170817 was unique event; no NS-NS merger(s) in O 3 § Optimistic view: off-axis emission and 100 x more frequent NSNS binaries are the rule, then handful of new NS-NS mergers in O 3 (2019) Astroparticle Physics in Germany, Mainz, 17. -19. 9. 2018