OUTLINE:

What we know about GW170817? - Electromagnetic (EM) counterpart - Host galaxy NGC4993 - GW170817 vs neutron stars in the Milky Way

What we speculate about GW170817? - Formation mechanisms - Merger rate evolution - Host galaxies of binary neutron stars (BNSs)

What next? - O3 and next LIGO – Virgo observing runs - 3G: Einstein Telescope / Cosmic Explorer

Michela Mapelli Venice, March 18 – 22 2019 Credits: Riccardo Ciolfi + LVC e.g. Abbott+ 2017, ApJ, 848, L13

Michela Mapelli Venice, March 18 – 22 2019 What we know about GW170817? GW170817: EM counterpart

FERMI satellite:

Faint short GRB ~ 1.7 s after merger

→ First direct evidence that BNS mergers are associated with short GRBs

Abbott+ 2017, ApJ, 848, L13 Goldstein+ 2017, ApJ, 848, L14

Michela Mapelli Venice, March 18 – 22 2019 What we know about GW170817? GW170817: HOST GALAXY

- Early-type (S0) galaxy NGC 4993 - Mostly old stars (~ 10 Gyr)

- z ∼ 0.0098 (Levan et al. 2017)

- stellar mass ~ 1010 – 11 Msun (Im et al. 2017)

Coulter+ 2017, Sci, 358, 1556 Abbott+ 2017, ApJ, 848, L12 Abbott+ 2017, ApJ, 848, L13 Blanchard+ 2017, ApJ, 848, L22

Michela Mapelli Venice, March 18 – 22 2019 What we know about GW170817? GW170817: MASSES

Michela Mapelli Venice, March 18 – 22 2019 What we know about GW170817? GW170817: MASSES

MASSES of GW170817 consistent with other measured NS masses

Michela Mapelli Venice, March 18 – 22 2019 What we know about GW170817? GW170817: MASSES

Consistent with masses of Galactic binary neutron stars (BNSs)

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

GW170817: FORMATION MECHANISMS

VERY COMPLEX EVOLUTION:

* Roche lobe overflow

* Two supernovae ( core collapse? electron capture?)

* One common envelope

* ???

From Tauris et al. 2017, ApJ, 846, 170

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

Supernova kicks and BNSs: A massive-star binary can become a BNS only if it is not unbound by SN kicks

WHY KICKS? ejecta compact * asymmetry in mass ejection object during core collapse

* asymmetry in neutrino emission during core collapse

* symmetric mass loss in a binary: breaks the binary only if pre-SN mass > companion mass (Blaauw mechanism, Blaauw 1961)

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

Supernova kicks and BNSs: Hobbs et al. (2005): kicks from proper motions of 233 pulsars → Maxwellian distribution with sigma ~ 265 km/s Beniamini & Piran (2016), Verbunt et al. (2017): significantly lower kicks

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

GW-INFERRED vs THEORETICAL MERGER RATES Predicted binary neutron star merger rate density in comoving frame

LVC

MM & Giacobbo 2018, MNRAS, 479, 4391

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

EXPECTED HOST GALAXIES of BNSs

BH binaries form mostly in <1010 M⊙ NS binaries form mostly in 109 – 1012 M⊙ galaxies and merge in both small and galaxies and tend to merge where form large galaxies → match GW170817 and short GRB hosts

MM et al. 2018, MNRAS, 481, 5324

Michela Mapelli Venice, March 18 – 22 2019 What we speculate about GW170817?

PROBABILITY OF HOSTING BNSs DEPENDS ON MASS and SFR?

Simulations predict strong correlation of number of merging DNSs qith stellar mass and SFR of host galaxy NGC is massive but very low SFR: is this a problem?

Artale et al. 2019, arXiv1903.00083

Michela Mapelli Venice, March 18 – 22 2019 What next?

Advanced LIGO + Virgo + KAGRA + LIGO India:

redshift < 0.05

Credits: LVC

Michela Mapelli Venice, March 18 – 22 2019 What next?

Advanced LIGO + Virgo + KAGRA + LIGO India: Expected number of detections in O3: ~ 0 – few (based on GW170817 and MW BNSs, Pol et al. 2019) Expected number of detections per year at design sensitivity: ~ 5 – 200 (assuming merger rate 110 – 3840 Gpc– 3 yr– 1, Abbott+ 2018) Only local Universe (z < 0.05)

What can we do with ~ 5 – 200 detections at z < 0.05? → characterize EM emission (properties of GRB, kilonova) → study metal enrichment ( “r-process”) → constrain mass of BNSs → constrain LOCAL merger rate (to a factor of few) → study local sample of host galaxies: spirals / ellipticals? → measure Hubble constant [now H0 = 70 (+12, -8) km/s/Mpc, Abbott+ 2017, Nature, 551, 85] → neutrino emission associated with BNS merger?

Michela Mapelli Venice, March 18 – 22 2019 What next?

3 G DETECTORS (Einstein Telescope + Cosmic Explorer):

?

see Michele Punturo’s talk on Friday!

Michela Mapelli Venice, March 18 – 22 2019 What next?

3 G DETECTORS (Einstein Telescope + Cosmic Explorer): Expected number of detections per year with ET + 2 CE: ~ 990k Maximum redshift: z ~ 2 (peak of cosmic star formation) See GWIC 3G science books https://gwic.ligo.org/3Gsubcomm/documents.shtml

What can we do with ~ 1 M detections at z < 2 ? → constrain merger rate EVOLUTION with redshift → study host galaxy evolution with redshift → constrain mass of BNSs to very small uncertainty

→ measure Hubble constant to very high precision

→ UNDERSTAND THE FORMATION MECHANISMS OF BNSs: * constrain supernova mechanisms and kicks * constrain binary evolution (comparison with models)

Michela Mapelli Venice, March 18 – 22 2019 CONCLUSIONS: What do we know about GW170817? ° EM counterpart: short GRB, kilonova, radio jet ° Host galaxy: massive, low SFR elliptical galaxy NGC4993 ° NS masses consistent with other BNSs in the MW

What do we speculate about GW170817? ° Formation through evolution of binary star with common envelope and supernova explosions ° GW-inferred local merger rate favors small supernova kicks ° Merger rate density grows with redshift up to z ~ 2 ° Local host galaxies of BNSs predominantly massive (109 – 1012 Msun)

What next? ° 5 – 200 BNSs / yr with Advanced LIGO – Virgo – Kagra ° ~ 1 M BNSs / yr up to z ~ 2 with Einstein Telescope – Cosmic Explorer

→ complete understanding of BNS formation mechanisms

Michela Mapelli Venice, March 18 – 22 2019 THANK YOU!