Gamma-Ray Burst XIANG DanFeng 2017.12.29 Summary • What is a GRB(Gamma-Ray Burst)? • Short and intense pulses of soft gamma rays with non-thermal spectra • The bursts last from a fraction of a second to several hundred seconds. • Narrowly beamed • Long lasting afterglow(in X-ray, optical, radio wavelengths) • The physical pictures • High energy physical processes • Relativistic effects • Synchrotron emission, inverse Compton scattering • Internal and external shocks • Stellar collapse & neutron star merger Observation features of GRB GRB: observation • Spatial distribution • Prompt Emission • Afterglow • Association with supernovae Spatial Distribution • BATSE (Burst and Transient Source Explorer) Isotropic Cosmic origin! For a constant peak luminosity: 푉/푉푚푖푛 = 0.5 But the observed value: 푉/푉푚푖푛 = 0.348 NOT homogeneous! GRB: observation • Spatial distribution: isotropic but not homogeneous, cosmic origin • Prompt Emission • Fast γ-ray emission, together with X-ray flash(XRF) and possible optical and radio emission(very rare) • Afterglow • Association with supernovae Prompt emission Spectrum • Non-thermal spectrum peaks at a few hundred keV, and many events have a long high-energy tail extending up to GeV. • An empirical fit for the spectrum: broken power- law • Peak Energy: • Break Energy: • Paucity of soft or harder events • Intrinsic or observational artifact? –Not clear Prompt emission: spectrum Hardness vs. duration -18 to 14 s • NHE(no high energy) bursts: • no emission above 300keV(very negative β) 14 to 17 s • Fainter than regular ones • Many bursts have NHE pulses along with regular pulses • High energy tail 47 to 80 s • GRB941017(González et al. 2003) • Remains roughly constant 80 to 113 s • The “tail”(10Mev-200MeV) contains more than 50 times energy than the main γ- ray(30keV-2MeV) energy 113 to 211 s • Low Energy tail • Some bursts have steeper low-energy Spectra of GRB 941017 power spectrum(α>1/3) Prompt emission: Temporal structure Prompt emission: Temporal structure • GRB duration: T90(T50) • the time in which 90% (50%) of the counts of the GRB arrive • Hardness: N(100–300 keV) / N(50–100 keV) • Hardness – duration correlation • Long & short • Ultra-long • Variable • Variability time scale(δt) much GRB920627 shorter than the burst duration • ~80% GRBs show variability structures • The rest have rather smooth light curves with a fast-rise exponential decay(FRED) • Variability luminosity correlation Prompt emission: Temporal structure • Pulses • The bursts are composed of series of individual pulses • Light curve of an individual pulse is a FRED with an average rise-to- decay ratio of 1:3 • The low-energy emission is delayed compared to the high-energy emission • The pulses’ low-energy light curves are wider compared to the high- energy light curves(width ~ E-0.4) • Width-symmetry-intensity correlation: High intensity pulses are (statistically) more symmetric (lower decay-to-rise ratio) and with shorter spectral lags • Hardness-intensity correlation: The instantaneous spectral hardness of a pulse is correlated to the instantaneous intensity GRB: observation • Spatial distribution: isotropic but not homogeneous, cosmic origin • Prompt Emission • Fast γ-ray emission, together with X-ray flash(XRF) and possible optical and radio emission(very rare) • Afterglow • slowly fading emission at longer wavelength • Association with supernovae Afterglow • X-ray • Optical and IR • Radio Venn diagram(till 2001) • X-ray −훽 −훼 • 푓휈(푡) ∝ 휈 푡 (α~1.4, β~0.9) • Normal distribution of the flux in 1-10keV, 11h after burst • Constant luminosity • Beam effect • Optical and IR afterglow • Power-law decay (~t-α), or broken power law −α1 • Fν(t)=f*(t / t*) {1−exp[−(t (α1−α2) (α1−α2) GRB 990510 / t*) ](t / t*) }. • Power-law spectrum(~ν-β) • Absorption lines • Providing information of the host galaxy: distance and redshift • Dark GRBs mag Observed • ~50% GRBs do not have optical afterglow • Observational artifact? Days after the burst Absorption, higher z, or intrinsically fainter? • Radio afterglow GRB 970508 • ~50% GRBs have radio afterglow, among which ~80% have optical counterparts Afterglow of short GRB GRB 130603B GRB 050724 GRB: observation • Spatial distribution: isotropic but not homogeneous, cosmic origin • Prompt Emission • Fast γ-ray emission, together with X-ray flash(XRF) and possible optical and radio emission(very rare) • Afterglow • slowly fading emission at longer wavelengths • Only Long bursts • Association with supernovae • Related to stellar death • Association with GRB 090618 supernovae(SNe) • A SN bump in the afterglow • Only the long bursts • The GRB SNe are very different from normal type Ib/c supernovae Physical Processes in GRB Physical Process: Relativistic Motion • For gamma photons to produce e+e- pairs: • We get a optically thick source! • Considering relativistic motion, the source is optically thin! • Relativistic time effect 푅 −푅 푅 −푅 • 훿푡 = 2 1 − 2 1 ≈ (푅 − 푅 )/2푐Γ2 푣 푐 2 1 • 푅 = 2훿푡푐Γ2 Physical Process: Relativistic shocks • Hugoniot shock jump conditions(when the upstream matter is cold): Physical Process: Particle acceleration • How the electrons been accelerated? • diffuse shock acceleration model • The role of magnetic filed • The acceleration resulted in a power law spectrum in form of : 푁 퐸 d퐸 ∝ 퐸−푝d퐸 • With: Physical Process: Synchrotron • Energy source for the prompt emission and afterglow • To study the synchrotron emission, you need to consider the motion of the electron and the source • In observer’s frame: • Power( in local frame): • Cooling time(in observer’s frame): Physical Process: Synchrotron • Synchrotron spectrum(optical thin) 1/3 • Sum of power law: 퐹휈 ∝ 휈 • Peak power at 휈푠푦푛(훾푒): • The overall spectrum: sum of emission of all electrons • Self absorption: 휈푎 • For intermediate frequency: Cooling of electrons • Fast cooling(훾푒,푚푖푛 > 훾푒,푐) • Slow cooling(훾푒,푚푖푛 < 훾푒,푐) Synchrotron spectrum • Slow cooling(훾푒,푚푖푛 < 훾푒,푐) • Fast cooling(훾푒,푚푖푛 > 훾푒,푐) • Jets Considering a spherical shell with constant velocity • Time delay for light from angle θ: 푅(1 − cos 휃)/푐 ≈ 푅휃2/2푐 • Relativistic beaming: 휃~1/Γ • Jet angle: 휃푗~1/Γ −훼 • For an instantaneous flash of power law spectrum 푓휈 ∝ 휈 , the observed flux will decay as power law with 푡−(2−훼) at late times Physical Process: Inverse Compton scattering • The photons earn energy through interaction with electrons • Comptonization parameter 휖푒 • 푌 = 푖푓 푈푒 ≪ 푈퐵 푈퐵 • 푌 = 푈푒/푈퐵 푖푓 푈푒 ≫ 푈퐵 2 • Add ultrahigh energy component to the spectrum(훾푒 ) • Speed up cooling, shorten cooling time tsyn(by a factor of Y) Physical Mechanics and Progenitors Core-Collapse of massive stars • Association with supernovae SN1998bw SN2002dh etc. Core-collapse > central engine > Merger of compact stars : kilonova sGRB prompt emission & afterglow Kilonova Gravitational wave & sGRB GW170817/GRB170817A/AT2017gfo Multi- Messenger Astronomy!! Tidal disruption events GRB 110328A(Swift J2058.4+0516) Unanswered questions • what is the composition of jet/ejecta (baryonic, e± or magnetic outflow)? • how are γ-rays, particularly of energy less than ∼10 MeV, produced? • is a black hole or a rapidly rotating, highly magnetized, neutron star (magnetar) produced in GRBs? • what is the mechanism by which relativistic jets are launched? • what are the properties of long and short duration GRB progenitor stars? References • Kumara, P., Zhang B. The physics of gamma-ray bursts & relativistic jets. PhR, 561, 1-109(2015) • Piran T. The physics of gamma-ray bursts. RvMP, 76, 1143- 1210(2004) Thanks GRB energetics • Isotropic luminosity function(dlnL) • Related to star formation rate • NOT isotropic but beamed 휃2 • 퐸 = 퐸 훾 2 훾,푖푠표 Afterglow Associated with prompt emission • Central engine late central • Shocks engine activities • Jet forward shock External shock Jet break X-ray afterglow.