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GW170817 – GRB 170817A – At2017gfo: the Birth of Multi- Messenger Astronomy Carlo Ferrigno University of Geneva

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GW170817 – GRB 170817A – At2017gfo: the Birth of Multi- Messenger Astronomy Carlo Ferrigno University of Geneva GW170817 – GRB 170817A – AT2017gfo: the birth of multi- messenger astronomy Carlo Ferrigno University of Geneva 07.06.2019 MM astronomy 1 Outline of this talk Recap on Pulsars and GRBs Gravitational waves from coalescence mergers Three electromagnetic components • The initial burst of gamma-rays • The kilonova (UV to IR) • The afterglow (X-rays to radio) 07.06.2019 MM astronomy 2 Binary evolution track(s) 07.06.2019 MM astronomy 3 The Hulse-Taylor pulsar Evidence for gravitational wave radiation • Discovered in 1974 • Nobel Prize for physics in 1993 07.06.2019 MM astronomy 4 Gamma-ray bursts (recap) jet aperture <2s ~10 Relativistic jet n ~100-1000 o i t a r -> Strong u D beaming of radiation >2s -> we see a small fraction of events 07.06.2019EWASS- 4 July 2016 GW150914MM astronomy - C. Ferrigno 5 Fireball model • Prompt gamma-rays from internal shocks • Fast decaying afterglow with non-thermal spectrum from interaction with local medium. 07.06.2019 MM astronomy 6 Gravitational waves 07.06.2019EWASS- 4 July 2016 GW150914MM astronomy - C. Ferrigno 8 Basics • For conservation of mass and momentum, only quadrupole signals are possible • We measure the fractional change in gravity gradient 07.06.2019 MM astronomy How “big”? • h is the strain. The first term is roughly the size of a black hole of mass M. The last term is sizable only in extreme systems as binary black holes or neutron stars or in the collapse of star interior core. • h approaches unity when standing in the immediate vicinity of black holes moving about at the speed of light or neutron stars. • The strongest waves we expect to observe passing the Earth will have h ∼ 10 10-21 or less. This distorts the shape of the Earth by 10-13 m. By contrast, the tidal field of the Moon raises a tidal bulge of about 1 m on the Earth's oceans. 07.06.2019 MM astronomy Interferometers 07.06.2019 MM astronomy A global network • Only LIGO and Virgo are powerful enough. Multiple detectors assure a high confidence for simultaneous signals, localization and polarization measurement. 07.06.2019 MM astronomy Sensitivity • This is the sensitivity at the time of GW170814 • Lines are resonances of external sources as electric power, seismic etc. LVC 2017a 07.06.2019 MM astronomy Sensitivity 07.06.2019 MM astronomy The three phases of a signal • Observable for massive binary black holes 07.06.2019 MM astronomy GW170814 LVC 2017a • Two black holes of 30 and 25 Msol at 540 Mpc 07.06.2019 MM astronomy Observing runs • Advanced LIGO active from September 2015 to March 2016 for the first observing run (O1) • The second Advanced LIGO run (O2) began on November 30, 2016 and ended on August 25, 2017. The run was suspended for one month (May 8 - June 8) for in vacuum refurbishement activities. • The average reach of the LIGO network for binary merger events has been around 70 Mpc for 1.4+1.4 Msun, 300 Mpc for 10+10 Msun and 700 Mpc for 30+30 Msun mergers, with relative variations in time of the order of 10%. • Virgo joined on August 1, 2017 and it was fundamental for two events: GW170814 and GW170817 • O3 started on 1 April 2019 and it detected already 1 NS-NS binary merger, one possible NS-BH and about ten BH-BH. 07.06.2019 MM astronomy Results (O1-O2) 07.06.2019 MM astronomy Localizations • With two detectors, only poor localization. • Only tens of squared degrees with three detectors 07.06.2019 MM astronomy I segnali 07.06.2019 MM astronomy Tutte le fusion di buchi neri 07.06.2019 MM astronomy Other sources ? • Every fast (~c) movement that is not spherically symmetric could produce GW. Impulsive events could be generated by Hypernovae collapses • Impulsive events could be originated by neutron star starquakes • Steady GW emission could come from spinning NS with asymmetries for instance due to accretion of matter. 07.06.2019 MM astronomy Results from the Thegravitational GW results signal 07.06.2019 MM astronomy GW170817: NS+NS merger • Identified by matched filtering • A long signal of ~100 s gives a precise chirp mass • The loudest signal in GW ever detected 07.06.2019 MM astronomy GW170817 • No merging signal due to limited band width • Loose limits on equation of state 07.06.2019 MM astronomy GRB170817A The Gamma-ray burst (1.7 s after the GW) 07.06.2019 MM astronomy 26 GW170817 – GRB170817A Binary Neutron Star merger, discovered by Fermi/ GBM and LIGO, independently observed by INTEGRAL/SPI-ACS, in good agreement with Fermi/GBM Despite an unfavorable soft GRB spectrum and moderately favorable orientation, INTEGRAL achieved a confident detection LVC+Fermi+INTEGRAL 2017 07.06.2019 MM astronomy 27 Joint localization • By comparing time of burst arrival to INTEGRAL, the GBM localization can be substantially improved. • LIGO/Virgo observation alone allows to measure distance to the GW source of 40 Mpc, exceptional for a short GRB, and well within LIGO BNS horizon 07.06.2019 MM astronomy 28 Fundamental consequences At least some short GRBs are associated to BNS mergers The 2 s delay comparing to 130 Mly distance implies that speed of gravity can be constrained to unprecedented precision: Such a consistency between GW speed and speed of light, implies stringent limits on Lorentz Invariance Violation 07.06.2019 MM astronomy 29 A very faint GRB Distance of 120 Mly is much less than ever measured for any GRB (short or long). This implies low luminosity, which is much less than that measured for other sGRB with known distances (Gamma- ray to GW ratio of <10-6 ). Why? 07.06.2019 MM astronomy 30 Could have we seen it? • There are GRBs with similar spectral properties in the GBM sample. GRB 101224A was the most probable candidate (located with doubts at 75 Mpc (?)) • The rates are expected to be low, have we been very lucky? • For INTEGRAL, predictions are more difficult at this flux level, as SPI-ACS is contaminated by short spikes. 07.06.2019 MM astronomy 31 AT2017gfo: the kilonova (observed from 11 hour to weeks after the GW) 07.06.2019 MM astronomy 32 Kilonova • Sub-relativistic outflows emitted from tidal interaction, collision and subsequent accretion disc. • They produce heavy elements like Gold, Platinum, Uranium, Thorium (which cannot be forged with the observed abundances in Supernovae) 07.06.2019 MM astronomy 33 Definition of kilonova • Kilonovae are day- to week-long supernova like thermal transients. They are powered by radioactive decay of heavy neutron-rich elements synthetized in the expanding ejecta. • The idea that NS-NS mergers are a source of heavy elements through r-processes dates back to 1980s and it was motivated by the abundance of heavy elements, which could not be produced only in Snae • Li and Pacynski (1998) proposed for the first time that the radioactive decay could produce an optical transient, but they overpredicted the peak luminosity to 1044 erg/s • The term “kilonova” was proposed by Metzger based on a peak luminosity 1000 times than a nova (the term macronova comes earlier nd refer to the same scale) • Berger et al found in 2013 a possible kilonova in the tail of a short GRB lightcurve Metzger (2018) arXiv 1610.09381v3 07.06.2019 MM astronomy Kilonova in GW170817 • Arcavi (2018) ApjL 07.06.2019 MM astronomy Time scale • Initially the flow is optically thick and radiation cannot escape • When the diffusion time scale • becomes • comparable to the expansion time, radiation can emerge. 07.06.2019 MM astronomy Ejecta • There are several sources of energy, including a central magnetar or residual accretion on the central black-hole • At poles, lighter material (blue) and at equator, heavier material (red) • Opacity is important to model the light curve 07.06.2019 MM astronomy 37 Kilonovae (predictions) Fernandez & Metzger (2016) • faster lanthanide-poor ejecta from polar regions • slower lanthanide-rich outflow from the equatorial IR belt • Evolution of spectra from UV Optical to IR over the first days. 07.06.2019 MM astronomy 38 Discovery of the optical transient • Twelve hours after the joint GRB-GW detection, an optical/ UV counterpart has been detected in NGC 4993, at distance (40 MpC) and redshift (0.0097) perfectly consistent with the GW: a kilonova ! 07.06.2019 MM astronomy 39 Light curves • A general agreement for mass of ejecta of 0.2 Msol • Better IR understanding than blue signal 07.06.2019 MM astronomy 40 Spectral evolution • General continuum consistent with black body with temperature decreasing from ~7000 to ~3000 K • 0.2 Msol mass of ejecta Broad features can be interpreted as blends of heavy element lines. Pian et al. 2017 07.06.2019 MM astronomy 41 Prediction = Observations ? Fernandez & Metzger (2016) • Not observed a normal GRB • Not seen the early afterglow • Radio signal from interactions of kilonova with ISM ? 07.06.2019 MM astronomy 43 The X-ray and radio afterglow 07.06.2019 MM astronomy 44 Late radio off-axis afterglow Troja et al 2017 • X-ray and Radio observations detected a source after 9 and 16 days and they can be joined in a synchrotron emission model. • Late appearance can be due to slowing down of jet (reduced Doppler beaming), but also to a structured emission pattern. 07.06.2019 MM astronomy 45 Break simulations • The light curves of an inhomogeneous jet observed from different angles (From Rossi et al. ). From the top theta0 = 0.5, 1, 2, 4, 8, 16°. https://ned.ipac.caltech.edu/level5/March04/Piran/Piran7_9.html • Acromatic break in optics afterglows let us estimate an opening angle between 5 an 10 degrees for most GRBs.
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