High Energy Astrophysics

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1 High Energy Astrophysics Gamma-ray Bursts Giampaolo Pisano Jodrell Bank Centre for Astrophysics - University of Manchester giampaolo.pisano@manchester.ac.uk May 2011

2 Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par Melia - Par Seward & Charles 2 nd ed. (2010) - Chap.17

3 GRB Observations1/12 - The discovery Vela satellites launched during the cold war by USA to monitor nuclear tests explosions in space (even behind the moon) Unexpected flash of gamma-rays not attributable to an atomic bomb (direction reconstructed by arrival times) 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

4 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

5 - 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

6 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

7 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?

8 - Classification GRB Observations 6/12 - There are two types of Gamma-ray Bursts: Long Duration GRBs - Duration: ~ sec before to fade away (typically in low-mass galaxies with active star formation) Short Duration GRBs - Duration: from a few 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)

9 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 ~ 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

10 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

11 - Crucial X-ray afterglow observation GRB Observation 9/12 GRB Beppo SAX (1997) - First GRB observed simultaneously at: Low energy γ-rays (detection) X-rays (localised within one arcmin) Search for the optical counterpart

12 GRB Observation 10/12 - Optical afterglow GRB van Paradijs et al 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

13 GRB Observation 11/12 - Distance determination GRB (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

14 GRB Observations 12/12 - High redshift GRBs in cosmological context Lamb and Reichart 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

15 Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par Melia - Par Seward & Charles 2 nd ed. (2010) - Chap.17

16 Long GRBs: connection with Supernovae - Long GRBs as hypernovae GRB (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

17 - 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 = kg = m E grav ) G ( 43 = J - If the collapse happens in a few seconds, the luminosity will be: L W A factor 100 less than the observed luminosity inferred by isotropic emission

18 - 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

with accretion disk formed inside the star - Some material ejected in Jets along the rotation axis (Due to

that destroy the star (The jets last for ~ 20s: time required to the BH to accrete the inner core of the

19 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

20 Gamma-ray Bursts - Observations - Long-duration GRB - Short-duration GRB References: - Rosswog & Bruggen - Par Melia - Par Seward & Charles 2 nd ed. (2010) - Chap.17

21 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:

org - Two neutron stars orbit around each other and lose

they collide forming a Black Hole that is surrounded by a

$free-fall time of the matter flowing into the BH (fraction$

22 - The neutron stars merger model Short duration GRB 2/3: Model - 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

23 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

24 γ-ray Bursts Active Galactic Nuclei

Gamma-Ray Astronomy. Astro 129: Chapter 1a

Gamma-Ray Astronomy. Astro 129: Chapter 1a Gamma-Ray Bursts Gamma-Ray Astronomy Gamma rays are photons with energies > 100 kev and are produced by sub-atomic particle interactions. They are absorbed by our atmosphere making observations from satellites