Neutron star mergers and multi-messenger astronomy
Adapted from lecture by Koutarou Kyutoku (KEK, IPNS) GW170817
2017/11/17 KEK 2 Contents
1. Introduction 2. Gravitational waves from binary black holes 3. Gravitational waves from binary neutron stars 4. Short gamma-ray burst 5. r-process and the kilonova/macronova 6. Future prospect and summary
2017/11/17 KEK 3 1. Introduction
2017/11/17 KEK 4 Nightly sky as our eyes see
http://www.eso.org/public/images/potw1333a/ 2017/11/17 KEK 5 Multi-wavelength sky
2017/11/17 KEK 6 Toward multi-messenger astronomy strong / opaque Strong interaction: (strong but short-range)
Electromagnetic interaction: electromagnetic waves Signals are strong but also hidden very easily Weak interaction: neutrino Another interesting messenger, particle physics
Gravitational interaction: gravitational waves Signals are weak but extremely penetrating weak / transparent 2017/11/17 KEK 7 The first event: GW150914
Merger of two black holes
http://apod.nasa.gov/apod/ap160211.html 2017/11/17 KEK 8 Neutron star binary coalescence
• Gravitational waves - test of the theory of gravitation in a non-vacuum - high-density matter signature: equation of state • Formation of a hot massive remnant (star/disk) - central engine of short gamma-ray bursts • Mass ejection of neutron-rich material - r-process nucleosynthesis - radioactively-driven “kilonova/macronova”
2017/11/17 KEK 9 Short gamma-ray burst
About 10 erg/s explosions - the sun is ~4×10 erg/s Long-soft GRB: ≥ 2s deaths of massive stars
Short-hard: ≤ 2s neutron star binary merger? rigorous confirmation needs gravitational waves
http://www.daviddarling.info/images/gamma-ray_bursts.jpg 2017/11/17 KEK 10 R-process nucleosynthesis
Synthesize heavy, neutron-rich elements (Au, Pt…) R = rapid: neutron capture faster than beta decay
need very dense and neutron-rich matter supernova explosions now seem to fail to achieve r-process Sneden+ (2008)
2017/11/17 KEK 11 2. Gravitational waves from binary black holes
2017/11/17 KEK 12 Black hole
Object with the strongest gravity
very bright in astronomy - gamma-ray burst - active galactic nuclei quantum phenomena? - Hawking radiation? - information loss?
2017/11/17 KEK 13 LIGO detector
Measures the propagating strain of spacetime, i.e., gravitational waves, with the precision of < 10
Location of Noise level two LIGO at the first detectors observing run
Schematic picture of Michelson-Morley interferometer 2017/11/17 KEK 14 GW150914
Observed data Agreement at two locations!
Theoretical model
Residual
LIGO&Virgo (2016), filtered to 35-350Hz 2017/11/17 KEK 15 Parameters of GW150914
• Masses of individual stars are measured - even at 400Mpc (Milky way is only ~10kpc) • The luminosity distance is measured directly
1Mpc ~ 3 million light years ~ 3 x 10^24 cm Obtained from the luminosity distance using Planck cosmology … not important 2017/11/17 KEK LIGO&Virgo (2016) 16 Summary of binary black holes
We saw many heavier-than-expected black holes
- low metal pop I/II? isolated? dynamical? - pop III? - primordial black hole? We now also have 12+7Msun GW170608 http://www.ligo.org/detections/images/BHmassChartGW170814.jpg 2017/11/17 KEK 17 Test of general relativity (skip today)
This event probes the strong and dynamical gravity
Small curvature scale (quantum somewhere) � = � = 1 assumed
Yunes+ (2016) Yunes+ (2016)
Weak gravity Strong gravity Dynamical Static Large curvature scale (classical)
2017/11/17 KEK 18 Gravitational-wave detector network
http://gwcenter.icrr.u-tokyo.ac.jp/wp-content/themes/lcgt/images/img_abt_lcgt.jpg KAGRA (Kamioka, Japan)
Advanced LIGO (Hanford, USA) another at Livingston https://www.advancedligo.mit.edu/graphics/summary01.jpg Advanced Virgo (Pisa, Italy)
http://virgopisa.df.unipi.it/sites/virgopisa.df.unipi.it.virgopisa/files/banner/virgo.jpg 2017/11/17 KEK 19 GW170814 (not 170817)
Simultaneous detection by LIGO twins and Virgo
Weak … coincidence w/ LIGO was crucial
2017/11/17 KEK LIGO&Virgo (2017) 20 3. Gravitational waves from binary neutron stars (GW170817)
2017/11/17 KEK 21 Neutron star
Remnant of massive stars (mass range is uncertain)
Mostly consists of neutrons 1.4 solar mass, ~10km The density is higher than
nuclear saturation values Lattimer (2014) “a huge nucleus” Arena for nuclear physics
2017/11/17 KEK 22 Neutron star equation of state
We want to know the realistic equation of state, that uniquely determines the mass-radius relation Equation of state: Nuclear physics Mass-Radius relation: Astrophysics Özel-Freire (2016)
Özel-Freire (2016)
2017/11/17 KEK 23 Encoded physics Remnant massive NS: Early inspiral: mass, spins… extreme temperature/density Late inspiral and merger: (H/S) Ringdown: GR tidal deformation, NS EOS MNS
Hotokezaka, KK+ (2011)
2017/11/17 KEK 24 Merger dynamics of NS-NS
�total < �max
�total < �crit Stable NS � > � : Massive, total crit hot NS prompt collapse Hypermassive: (unlikely) Supramassive: ~100ms �total < �rot,max ~spin-down time
Small-mass disk Large-mass disk Small-mass disk? [See e.g., Hotokezaka+KK+ (2013)] 2017/11/17 KEK 25 GW170817 http://www.ligo.org/detections/GW170817.php https://pbs.twimg.com/media/DOyV7PVVoAAkRZN.jpg
The longest BH-BH waveform is only ~2s (so far)
The NS-NS, GW170817, was detected for >1min
2017/11/17 KEK 26 Spectrogram
LIGO twins observed clear “chirp” signals, i.e., gravitational waves with increasing frequency and amplitude in time But Virgo did not see… -> the source should be at Virgo’s blind spot!
2017/11/17 KEK LIGO&Virgo (2017) 27 Sky map and localization accuracy
Improved with Virgo!
Improved with Virgo! http://www.ligo.org/detections/GW170817/images-GW170817/O1-O2-skymaps-white.jpg 2017/11/17 KEK 28 Parameters of GW170817
Low-spin: limiting to spin values observed for Galactic binary neutron stars that merge within the Hubble time (with some safe margins) High-spin: as far as GW models may be applicable
2017/11/17 KEK 29 Constraints on parameters
The NS radius may be smaller than ~13-14km - this can be made tighter with better waveforms
Blue: astro. realistic
Red: conservative
mass tidal deformability
LIGO&Virgo (2017) 2017/11/17 KEK 30 Shape of mass constraints
Gravitational waves tightly constrain the chirp mass / / � � / / ℳ = / = � � � + � But the mass ratio (e.g., � = � /� < 1) tends to be degenerated with the spin of components, �� � = � = 1,2 �� The error appears large particularly for nearly equal-mass systems like binary neutron stars
2017/11/17 KEK 31 Quadrupolar tidal deformability
Leading-order finite-size effect on orbital evolution (strongly correlated with the neutron-star radius) � 2 � � Λ = �� = � ∝ � �� 3 �� �~0.1: (second/electric) tidal Love number
External deformed � = −�ℰ field
1 � Φ � ≡ � � � − � � � � ℰ ≡ 3 �� �� 2017/11/17 KEK 32 Representative example
Different EOSs predict different tidal deformabilities
For the EOS models computed here � 1.35�⊙ = 13.7,13.0,12.3,11.6,11.0km Λ 1.35�⊙ = 1211,863,607,422,289
2017/11/17 KEK 33 Electromagnetic followup
2017/11/17 KEK 34 Transient and host galaxy
First found by Swope Supernova Survey
Faded -> transient!
Soares-Santos+ (2017) 2017/11/17 KEK 35 4. Short gamma-ray burst (GRB 170817A)
2017/11/17 KEK 36 Short-hard gamma-ray burst
Is this really caused by neutron star mergers? http://www.icrr.u-tokyo.ac.jp/~cta/images/GRB.jpg
2017/11/17 KEK 37 LIGO,Virgo, Fermi Quite normal appearance INTEGRAL (2017)
Fermi and INTEGRAL agree each other though relatively weak The 1.7s delay from GWs - jet launch - jet propagation in the ejected material - onset of transparancy
2017/11/17 KEK 38