High Energy Astrophysics Gamma-ray Bursts Giampaolo Pisano Jodrell Bank Centre for Astrophysics - University of Manchester giampaolo.pisano@manchester.ac.uk May 2011
Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par.7.1-7.2 - Melia - Par.11.2 - Seward & Charles 2 nd ed. (2010) - Chap.17
GRB Observations1/12 - The discovery 1963 - Vela satellites launched during the cold war by USA to monitor nuclear tests explosions in space (even behind the moon) 1967 - Unexpected flash of gamma-rays not attributable to an atomic bomb (direction reconstructed by arrival times) 1973 - First scientific result on the discovery of GRB from random directions in the sky at random intervals (16 events in 3 yrs) Theoretical models ranging from neutron star glitches, meteoric impacts on neutron stars to super-massive black holes
GRB Observations 2/12 - Properties & theories - The observed GRB light curves timescale variations were of the order: t 1ms - The source of the explosion is probably a compact object: D c t D 300km Neutron star or a Black Hole -There was a joke at the time: The number of theories for their origins exceeded the number of bursts detected - Most of consensus was on neutron stars in our Galaxy: Waiting more accurate satellites to confirm concentration on galactic plane
- All sky distribution GRB Observation 3/12 C-GRO BATSE (1991) - Isotropic distribution across the sky - Several bursts per day observed on Earth - Early hypothesis: Spherical Halo surrounding the Milky way Distributed in space as normal galaxies
GRB Observations 4/12 - Large diversity in the GRB light curves - Long times between two pulses - Isolated spikes (<1sec) - FREDs (Fast Rise Exponential Decay) - Sequence erratic sub-bursts
GRB Observations 5/12 - GRBs durations Bimodal distribution Two types of Gamma-ray Bursts Two different types of sources or same source in two operating modes?
- Classification GRB Observations 6/12 - There are two types of Gamma-ray Bursts: Long Duration GRBs - Duration: ~ 2-1000 sec before to fade away (typically in low-mass galaxies with active star formation) Short Duration GRBs - Duration: from a few 10-3 - 2 sec (in all types of galaxies) Higher energy photons Unlike X-ray Bursts both types of GRBs emit only one burst in their history (The emission is non-thermal: possibly Synchrotron alone or combined with ICS)
GRB Observations 7/12 - Distances - GRB measured fluxes are of the order of: S 10 7 Wm -2 - Assuming isotropic emission, the GRB luminosity is: 2 L iso = 4πd S - We can estimate the GRBs luminosity for different distances: L iso = 4πd 2 S ~ 2 10 3 10 1 10 35 36 45 W for W for W for d d d = 15kpc = 50kpc = 1Gpc - Galactic radius - Galactic Halo radius - Ex of cosmological distance If GRBs extragalactic Very high energy involved Exotic objects
GRB Observations 8/12 - Afterglows - To find a distant counterpart is necessary to select the source from many faint objects within the field of view (γ-ray resolution was too low) - The burst of energy in GRBs should involve material ejected at high velocities that interact with the ambient medium: Heated material will radiate energy for some time after the burst - There is indeed a long-lived component associated to GRBs: Afterglow To identify GRBs: γ-ray detection and rough localisation (~arcmin) must be followed by optical localisation (~arcsec) of the faint rapid afterglows
- Crucial X-ray afterglow observation GRB Observation 9/12 GRB 970228 Beppo SAX (1997) - First GRB observed simultaneously at: Low energy γ-rays (detection) X-rays (localised within one arcmin) Search for the optical counterpart
GRB Observation 10/12 - Optical afterglow GRB970228 van Paradijs et al 1997-21 hours later an optical afterglow was located (La Palma telescopes) - It was the first ever GRB detected from ground The GRB was associated with a faint distant galaxy However, it was too faint to detect its spectrum and so the distance
GRB Observation 11/12 - Distance determination GRB 970508 (Metzenger et al 1997) - BeppoSAX provided position for follow-up of afterglow with Kitt Peak Obs. - The distance was determined from the spectrum of host galaxy z=0.835 GRB at cosmological distances - The farthest event recorded so far z~6.39 (just 900Myr after Big Bang) GRBs at high redshifts to probe physical state of pre-galactic gas
GRB Observations 12/12 - High redshift GRBs in cosmological context Lamb and Reichart 2000 - First light'' of the Universe after the Dark ages probably at z=20: - first stars form; - UV light re-ionize the Universe. - GRBs expected up to z=20 (unlike QSOs) and their afterglow light passing through intergalactic gas and galaxies at lower z gives informations about: - the moment of "first light" - the star-formation history of the Universe - the elemental abundance history of the Universe - the reionization history of the Universe GRBs as unique and powerful probes of the early universe
Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par.7.1-7.2 - Melia - Par.11.2 - Seward & Charles 2 nd ed. (2010) - Chap.17
Long GRBs: connection with Supernovae - Long GRBs as hypernovae GRB030329 (Stanek et al 2003, Hjorth et al 2003) - Extremely bright long GRB was detected - After 6 days a bump in the light curve appeared: Spectroscopic investigations Unmistakable SN features Some long duration GRBs is related to SN explosions
- Long duration GRB Long GRBs: Maximum luminosity - Suppose the burst is associated with the collapse of a super massive star - The total gravitational energy of the star is: M G R E grav 2 Example M = 50M R = 50RΘ Θ = 10 32 kg = 3.5 10 10 m E grav 32 10 ) G 3.5 10 2 ( 43 = 2 10 10 J - If the collapse happens in a few seconds, the luminosity will be: L 10 43 W A factor 100 less than the observed luminosity inferred by isotropic emission
- Beaming GRBs Collimated outflow - Assuming radiation emitted in two cones of solid angle Ω: ϑ 2 2π ϑ ϑ Ω = 2 sin 4 (1 cos ) 4 ( << 1) 0 ϑ dϑdϕ = π θ π 0 2 ϑ Ω - The luminosity can be reduced by a large factor: Ω L true = L iso 4π 2 ϑ L true Liso = 2 L f iso b - True luminosity - We define: fb = 4π 2 = 2 Ω ϑ - Beaming factor Note that the GRBs true rate has to be larger by a factor f b Example - A beaming factor f b =100 in luminosity implies an angle: f b 2 = 2 ϑ 2 ϑ ϑ 10deg f b
Long duration GRB: Model - The collapsar model (hypernova) - A very massive star (M>30M ʘ ) has lost its H & He outer layers and it is rapidly rotating - At the end of thermonuclear reactions Core collapse Kerr BH with accretion disk formed inside the star - Some material ejected in Jets along the rotation axis (Due to magnetic field lines channelling) - The jets break through the surface of the star producing shock waves that destroy the star (The jets last for ~ 20s: time required to the BH to accrete the inner core of the star) - The relativistic particles in the jets radiates: If one of the jets point towards the Earth GRB www.nasa.gov
Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par.7.1-7.2 - Melia - Par.11.2 - Seward & Charles 2 nd ed. (2010) - Chap.17
Short duration GRBs 1/3 - Characteristics - Short duration GRBs (<2sec): - Proved in 2005 to occur also at cosmological distances - They appear dimmer by a factor 10 - Lower redshift than Long GRBs - Observed in all types of galaxies - Also in elliptical galaxies without star formation regions Similar to Type Ia Old progenitor - Have a large fraction of hard gamma-rays - Too fast to be explained with the collapsar model
- The neutron stars merger model Short duration GRB 2/3: Model www.eso.org - Two neutron stars orbit around each other and lose angular momentum radiating gravitational waves - Eventually they collide forming a Black Hole that is surrounded by a temporary debris torus - Accretion provides sudden release of gravitational energy - Duration depends only on the free-fall time of the matter flowing into the BH (fraction of sec) Note: Models with Neutron star - BH systems also proposed
Short duration GRBs 3/3: Supercomputer model (4/2011) Collision of two n-stars can naturally produce the magnetic structures thought to power the relativistic jets associated with Short GRBs NASA
γ-ray Bursts Active Galactic Nuclei