Slide 1

Gamma-Ray Bursts Energy problem and beaming * Mergers versus collapsars GRB host galaxies and locations within galaxy Supernova connection Fireball model * Swift Afterglows of short bursts Slide 2 Energy Problem GRB 990123 required a total energy, if isotropic, of 3.4 10 54 erg = 1.9 M c 2. GRB energy source is almost certainly gravitational need few M collapsed into region not more than 100 km across. Energy density U = T 4 /c T ~ 10 12 K kT ~ 100 MeV This is high enough to produce e+e- pairs. Slide 3 Fireball Consider pure energy confined within a sphere, describe with E, R, T (Goodman 1986) Radius of sphere R = (3E/11 T 4 ) 1/3 Optical depth from center to edge Edge of sphere (photosphere) will expand at a speed close to c as long as kT > m e c 2 If baryons are added, most energy goes into accelerating baryons to ~ E/Mc 2 Slide 4 Fireball Optical depth from center to edge for burst which varies over time scale t with a sepctrum such that a fraction f p of the photon pairs can pair produce. Very high optical depth is inconsistent with non- thermal spectrum at high energies Slide 5 Relativistic outflow In a relativistic outflow, the observed photon energy is a factor (= Lorentz factor of bulk motion) higher than the photon energy in the rest frame. For a spectrum with an energy index this reduces the number of photon pairs above the electron-positron threshold by 2 Also the size of the emitting area can be larger by a factor 2 Need ~ 100 to solve the problem. Slide 6 Evidence for Jet Afterglow of GRB 990123 shows a break Slide 7 Observer Beaming Because of relativistic motion, radiation is beamed with an opening angle ~ 1/ Therefore, observer can see only a limited piece of an expanding shell Slide 8 R Area visible to an observer = (R ) 2 At Late time: At Early time: R Area visible to an observer = (R/ ) 2 = jet angle Slide 9 Monochromatic Jet Break Slide 10 Jet Breaks Jet opening angle is related to time at which break in light curve occurs Beaming fraction is determined by jet opening angle = 1 cos 2 /2 Energy required is reduced by a factor 2 /2 Slide 11 Frail et al. 2001 Jet Energy Slide 12 Burst Models Collapsing WDs Stars Accreting on AGN White Holes Cosmic Strings Black Hole Accretion Disks I) Binary Mergers II) Collapsing Stars Slide 13 Mergers Binaries must evolve before merger and binaries have non-zero speeds due to kicks in compact object formation. Thus, GRBs can occur in outskirts of or even far from host galaxy. Slide 14 Massive Star Collapse Beamed Explosion, accompanying supernova-like explosion, GRBs should be associated with young, massive stars. Slide 15 Holland 2001 Host Galaxies Hosts are similar to star-forming galaxies at similar redshifts. High star formation rates. Slide 16 Location of GRB within Host Slide 17 Distribution Follows Stellar Distribution The environments of GRBs show higher gas densities, higher metallicities, and higher dust content than random locations in host galaxies. Suggests that GRBs occur in star forming regions. Location of GRB within Host Slide 18 GRB Locations GRB hosts are star-forming galaxies GRBs trace the stellar distribution (in distance from galaxy center) GRBs occur in dense environments (probably star forming regions) Suggests collapsar model over merger model Slide 19 Supernova connection SN 1998bw was found in the 8 error circle of GRB 980425 in observations made 2.5 days after the burst. A slowly decaying X-ray source was subsequently found in the same galaxy (z = 0.0085) and identified with the GRB. However, the GRB was very underluminous and the SN was very usual with parculiar line emission (no H, no He, no Si at 615 nm. Radio emission a few days after GRB indicated relativistic outflow with energy ~ 3 10 50 erg. Thought to be oddball GRB and SN. Slide 20 GRB030329 and SN 2003dh Clear spectroscopic signature of a SN, broad emission lines, found after decay of afterglow of GRB030329. Smoking gun linking GRBs and SNe. Slide 21 SN 2003dh versus SN 1998bw Slide 22 SN Bumps Slide 23 GRB - Supernova Only a tiny fraction of SN are observed to be GRBs Slide 24 GRB 060218 = SN 2006aj Slide 25 Fireball Model Initial event accelerates baryons in bulk Later on, internal shocks re-accelerate particles produce GRB Even later, external shocks produce afterglow Slide 26 GRB 990123 990123 reached 9 th magnitude for a few moments! First optical GRB afterglow detected simultaneously Slide 27 Internal-External Shock Model Afterglow External Shocks Internal Shocks GRB ISM Central Engine Slide 28 3. Baryonic mass content of the jet ~ 2x10 -7 6x10 -6 M o Burst (as Jet) Properties Baryon mass is ~ 10 -5 M Jet opening angle means that we observe only one of each 1000 GRBs in the Universe, most are pointed away. The means that GRB rate is about 1% of SN rate. Slide 29 Swift BAT CZT detector with 5200 cm 2 area sensitive in 15-150 keV band. Coded aperture imaging of 1.4 steradian field with 4 arcmin resolution suing 32768 pixels. After detecting a burst, Swift autonomously repoints bringing the burst into view of the XRT and UVOT, often within 90 seconds. XRT focusing X-ray telescope in 0.5-6 keV band, 2.5 arcsecond source location accuracy. UVOT focusing UV/optical telescope. Slide 30 Swift Results Launched in 2004. Detects about 100 bursts/year More afterglow detections than all previous satellites combined GRB with redshift of z = 6.29 Average redshift = 2.7 compared to pre-Swift = 1.2 Expect 40 GRB with z > 5 and 4 with z > 8. Slide 31 Afterglow of short GRB GRB 050509b associated with elliptical galaxy. HETE-II GRB 050724 also associated with elliptical.