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Gravitational-Wave and Multi-Messenger Astronomy Gravitational-wave and multi-messenger astronomy Koutarou Kyutoku Department of Physics, Kyoto University 2019/8/19 Strings and Fields 2019 1 Contents 1. Introduction 2. Binary black hole merger 3. Binary neutron star merger 4. Future prospect 5. Summary 2019/8/19 Strings and Fields 2019 2 1. Introduction 2019/8/19 Strings and Fields 2019 3 Gravitational-wave detector network http://gwcenter.icrr.u-tokyo.ac.jp/wp-content/themes/lcgt/images/img_abt_lcgt.jpg KAGRA (Kamioka, Japan) under development… 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 2019/8/19 Strings and Fields 2019 4 The first event: GW150914 Merger of two black holes http://apod.nasa.gov/apod/ap160211.html 2019/8/19 Strings and Fields 2019 5 What we learned from 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 2019/8/19 Strings and Fields 2019 LIGO&Virgo (2016) 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 2019/8/19 Strings and Fields 2019 7 Multi-messenger event: GW170817 2019/8/19 Strings and Fields 2019 8 Era of gravitational-wave astronomy 10 binary black holes, 1 binary neutron stars (2018) 2019/8/19 Strings and Fields 2019 9 Black-hole mass distribution Light ones are dominant (detection rate ∝ ℳ2.5) Mass of the primary (=heavier member) Mass ratio (lighter divided by heavier) LIGO&Virgo 1811.12940 2019/8/19 Strings and Fields 2019 10 Black-hole spin distribution 휒eff = 푚1휒1,∥ + 푚2휒2,∥ / 푚1 + 푚2 (near-)extremal spins are not likely to be typical Saying more is difficult Report of a rapid spin by non-LVC analysis [Zackey+ 2019] LIGO&Virgo 1811.12907 If true, found more soon 2019/8/19 Strings and Fields 2019 11 LIGO-Virgo Observation Run 3 (O3) More than those found during LIGO/Virgo O1, O2 See https://gracedb.ligo.org/latest/ for the list 2019/8/19 Strings and Fields 2019 12 Example: S190412m BBH: binary black hole BNS: binary neutron star Terrestrial ~ noise on the earth 2019/8/19 Strings and Fields 2019 13 S190814bv The classification changed from MassGap to NSBH What is “MassGap”? What does “NSBH” mean? Aug 14 21:31 Aug 14 22:58 Aug 15 10:16 2019/8/19 Strings and Fields 2019 14 LIGO-Virgo classification scheme https://emfollow.docs.ligo.org/userguide/_images/content-1.svg Mass-based labeling Mass of the lighter Black hole: > 5푀⊙ Neutron star: < 3푀⊙ 푀⊙: solar mass If either star has 3푀⊙ < 푚 < 5푀⊙ -> MassGap Mass of the heavier 2019/8/19 Strings and Fields 2019 15 2. Binary black hole merger 2019/8/19 Strings and Fields 2019 16 Black hole Object with the strongest gravity in the universe From Wikipedia very bright in astronomy - gamma-ray burst - active galactic nuclei quantum phenomena? - evaporation? - information loss? 2019/8/19 Strings and Fields 2019 17 Observation of Galactic black holes Black holes are invisible Electromagnetic observations have relied on emission from black-hole activity associated w/ accretion from companions e.g., accretion disk, jet …. 2019/8/19 Strings and Fields 2019 18 Mass of (near-)Galactic black holes We have observed up to ~20 solar-mass black holes with many objects having very weak constraints Ozel+ (2012) 2019/8/19 Strings and Fields 2019 19 Mass gap? The maximum mass of neutron stars ∼ 2 − 3푀⊙? - determined by the high-density equation of state The minimum mass of black holes ∼ 5 − 7푀⊙? Kreidberg+ (2012) - relying on observations - matter of vigorous debate Nothing in between? 2019/8/19 Strings and Fields 2019 20 Supernova: birth of a compact object When the massive star dies, a supernova explosion could occur and leave a black hole or a neutron star (Two outcomes may be distinguishable w/ neutrinos for nearby [Galactic] supernovae) 2019/8/19 Strings and Fields 2019 21 Mass after supernovae? Massive stars do not necessarily end in black holes due to complexity associated with stellar evolution (but supernova simulations are not mature enough) Ugliano+ (2012) Red: explode -> neutron star Gray: not explode -> black hole = initial stellar mass 2019/8/19 Strings and Fields 2019 22 Compact object before GW150914 mass (Primordial black hole?) ~1푀⊙: neutron-star minimum mass Neutron star ∼ 2 − 3푀⊙: neutron-star maximum mass Absence of compact objects - Mass gap? ∼ 5 − 7푀⊙: astro. black-hole minimum mass Stellar-mass black hole ~15 − 20푀⊙: astro. black-hole maximum mass (some researchers have predicted heavier: e.g., Kinugawa+ 2014) 2019/8/19 Strings and Fields 2019 23 GW150914 Observed data Agreement at two location! Theoretical model Residual LIGO&Virgo (2016), filtered to 35-350Hz 2019/8/19 Strings and Fields 2019 24 Mass of observed binary black holes Massive black holes are ubiquitous in the universe LIGO&Virgo 1811.12907 2019/8/19 Strings and Fields 2019 25 Metallicity as the key to the mass Metallicity: fraction of elements other than H/He Stellar winds should have reduced the mass during the stellar lifetime… LIGO&Virgo (2016) Massive BHs challenge Upper limit of formed BH mass the stellar evolution (unit: solar mass) Low-metallicity + weak stellar wind high metal (normal) low metal Abundances of metal compared to solar values 2019/8/19 Strings and Fields 2019 26 Question about black holes What is the origin of these massive black holes? Metal-deficient, old stars must be important • Born as a binary and evolve via a suitable path? • Assembled into a binary in dense environments? Other possibilities (somehow popular in Japan) • First stars(=BBN composition) in the universe? • Primordial black hole? The mass/spin distribution will be important 2019/8/19 Strings and Fields 2019 27 Remark on classification We have not yet understood the minimum mass of the stellar-mass, astrophysical black holes (as well as the maximum mass … another topic) Whether a 2.5푀⊙ object (say) is a black hole or a neutron star cannot be answered with confidence unless we observe electromagnetic counterparts But it is treated as an “NS” in the LVC classification, e.g., “NSBH” at least formally … be careful! 2019/8/19 Strings and Fields 2019 28 Test of general relativity This event probes the strong and dynamical gravity Small curvature scale (quantum somewhere) 퐺 = 푐 = 1 assumed Yunes+ (2016) Yunes+ (2016) Weak gravity Strong gravity Dynamical Static Large curvature scale (classical) 2019/8/19 Strings and Fields 2019 29 Basic strategy Compute post-Newtonian correction terms, say 푑푓 푑푓 2 3 = 1 + 푎2푣 + 푎3푣 + ⋯ 푑푡 푑푡 quadrupole Usually, an expansion parameter is taken to be 2/3 푣 2 퐺휋푓푀 푥 = ≡ 푐 푐3 A term at 푣2푛 or 푓2푛/3 relative to the leading term is called the 푛-th post-Newtonian correction 2019/8/19 Strings and Fields 2019 30 Post-Newtonian coefficient Combination of all events improves the constraint - every terms are consistent with general relativity - these constraints will keep becoming tighter (may not kill general relativity in the near future) LIGO&Virgo 1903.04467 Merger-part obtained by numerical-relativity sims. Note: The 0PN term = quadrupolar emission is much strongly constrained by binary pulsars 2019/8/19 Strings and Fields 2019 31 Did we constrain modified gravity? Not very severely constrained model parameters regarding wave generation mechanisms… One reason is simply that merger simulations have not been performed for many of such theories! Yunes+ (2016) 2019/8/19 Strings and Fields 2019 32 Graviton Compton wavelength/mass Massive gravitons travel slower than the light 2 2 4 2 2 푣푔 푚푔푐 ℎ 푐 2 ≃ 1 − 2 = 1 − 2 2 푐 퐸 휆푔퐸 Gravitational waveforms are distorted 13 The Compton wavelength 휆푔 > 1.6 × 10 km −23 Then the graviton mass 푚푔 < 7.7 × 10 eV - nearly strongest model-independent bounds Note: “graviton mass” is a highly nontrivial quantity 2019/8/19 Strings and Fields 2019 33 3. Binary neutron star merger 2019/8/19 Strings and Fields 2019 34 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 nucleous” Interior is not understood 2019/8/19 Strings and Fields 2019 35 Diversity of neutron stars Radio pulsar (rotation powered) X-ray binary ©NASA (gravitation powered) ©NASA Central compact object (heat powered) Magnetar ©NASA (magnetic-field powered) ©NASA 2019/8/19 Strings and Fields 2019 36 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” 2019/8/19 Strings and Fields 2019 37 GW170817 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! 2019/8/19 Strings and Fields 2019 LIGO&Virgo (2017) 38 Sky map and localization accuracy Improved with Virgo! Improved with Virgo! http://www.ligo.org/detections/GW170817/images-GW170817/O1-O2-skymaps-white.jpg 2019/8/19 Strings and Fields 2019 39 Short gamma-ray burst About 1051erg/s explosions - the sun is ~4 × 1033erg/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 2019/8/19 Strings and Fields 2019 40 Gamma rays at 1.7s after merger © LIGO/Virgo; Fermi; INTEGRAL; NASA/DOE; NSF; EGO; ESA.
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