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Gamma-Ray Bursts The Brightest Explosions Since the Big Bang

Gamma-Ray Bursts The Brightest Explosions Since the Big Bang. H. A. Bethe Lecture March 6, 2002 . Electromagnetic radiation – of which ordinary visible light is an example – can be characterized by its wavelength. Gamma-rays have

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Gamma-Ray Bursts The Brightest Explosions Since the Big Bang

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  1. Gamma-Ray Bursts The Brightest Explosions Since the Big Bang H. A. Bethe Lecture March 6, 2002

  2. Electromagnetic radiation – of which ordinary visible light is an example – can be characterized by its wavelength. Gamma-rays have the shortest wavelengths of all and are very penetrating. They pass easily through a block of wood but not through the Earth’s atmosphere. Consequently cosmic sources of gamma-rays can only be seen by satellites, rockets, and high flying balloons that go above most of the Earth’s atmosphere.

  3. Nuclear Test Ban Treaty, 1963 First Vela satellite pair launched 1963 Velar – to watch The Vela 5 satellites were placed in orbit by the Advanced Research Projects of the DoD and the AEC. Launched on May 23, 1969 into high earth orbit (118,000 km), this pair of satellites and their predecessors, Vela 4, discovered the first gamma-ray bursts. The discovery was announced by Klebesadel, Strong, and Olson (ApJ, 182, 85) in 1973.

  4. First Gamma-Ray Burst The Vela 5 satellites functioned from July, 1969 to April, 1979 and detected a total of 73 gamma-ray bursts in the energy range 150 – 750 keV (n.b, radiation is sometimes defined by its energy measure in electron volts. 0.3 to 30 keV approximately is x-rays. Greater than 30 keV is gamma-rays)

  5. Typical durations are 20 seconds but there is wide variation both in time- structure and duration. Some last only hundredths of a second. Others last thousands of seconds.

  6. A Cosmic Gamma-Ray Burst, GRB for short, is a brief, bright flash of gamma-rays lasting typically about 20 seconds that comes from an unpredictable location in the sky. Some, in gamma-rays, are as bright as the planet Venus. Most are as bright as the visible stars. It is only because of the Earth’s atmosphere and the fact that our eyes are not sensitive to gamma-rays that keeps us from seeing them frequently. With appropriate instrumentation, we see about one of these per day at the Earth. They seem never to repeat from the same source.

  7. A problem in 1973 was – and still is – that gamma-ray detectors(CsI and NaI) do not give much information on directect. Yes, there are gamma-raysbut I don’t know where they came from..... but I do know when ...

  8. One can get information on directionality from timing if there are more than one detectors. BANG ! 1 2 If “1” hears the sound later than “2” the sound is somewhere to the right.

  9. Interstellar warfare • Primordial black hole evaporation • Flares on nearby stars • Distant supernovae • Neutron star quakes • Comets falling on neutron stars • Comet anti-comet annihilation • Thermonuclear explosions on neutron stars • Name your own .... uncertainty in distance – a factor of one billion.

  10. In 1993 there were 135 models nb. 27, 42, 105, 126

  11. Most of them involved neutron stars in our own Galaxy (quakes, comets falling, thermonuclear runaways, etc.) The expected distribution on the sky if Galactic neutron stars were responsible should center around the Galactic equator. Plane of the MilkyWay Galaxy

  12. So basically we wandered in the wilderness for twenty years ... (1973 – 1993).

  13. Compton Gamma-Ray Observatory April 5, 1991 – June 4, 2000 BATSE Module

  14. Observed Expected if haven’t reached any edge yet log number of sources log sensitivity

  15. This posed a problem for the models that had GRBs in our own Galaxy .. * * * * * * * * * * * * * * * * * * X * * * * * * * * * We are not at the center of our galaxy so we should see more bursts towards the center than in the opposite direction, * * * * * * * * *

  16. Isotropy could mean three things: • Very nearby bursts centered on the Earth – e.g., • the Oort cloud of comets • A very extended spherical halo around the Galaxy – much bigger than the distance from here to the center of the Galaxy • Bursts very, very far away – billions of light years What we really needed was source identifications.

  17. High Energy Transient Explorer (HETE-1)

  18. HETE-1 conceived July, 1981, Santa Cruz born Nov 4, 1996 died Nov 5, 1996

  19. BeppoSax (1996-2002) (Italian-Dutch X-ray astronomy mission) MEC S and LECS (medium and lowenergy x-ray sensors, 1 arc min positions) (2-30 keV; 20x20 degree FOV angular resolution 5 arc min) The scintillator anti-coincidence shields of the Phoswich detector are able to detect gamma-rays 60-600 keV and get crude angular information

  20. BeppoSax GRB 970228 (discovered with WFC) Feb 28, 1997 (8 hr after GRB using MECS) March 3, 1997 (fainter by 20) Each square is about 6 arc min or 1/5 the moon’s diameter

  21. GRB 970228 William Hershel Telescope Isaac Newton Telescope Groot, Galama, von Paradijs, et al IAUC 6584, March 12, 1997

  22. Wagner, Foltz, and Hewet IAUC 6581 using the MMT get a crude spectrum of the host galaxy. Estimate z = 0.5 March 10, 1997

  23. Later .... Spectrum of the host galaxy of GRB 970228 obtained at the Keck 2 Telescope. Prominent emission lines of oxygen and neon are indicated and show that the galaxy is located at a redshift of z = 0.695. (Bloom, Djorgovski, and Kulkarni (2001), ApJ, 554, 678. See also GCN 289, May 3, 1999.

  24. From the red shift a distance can be inferred – billions of light years. Far, far outside our galaxy. From the distance and brightness an energy can be inferred. 1.6 x 1052 erg in gamma rays alone This is 13 times as much energy as the sun will radiate in its ten billion year lifetime, but emitted in gamma-rays in less than a minute. It is 2000 times as much as a really bright supernova radiates in several months.

  25. GRB 990123 This shows the light curve as seen by BATSE starting at 9:46:56 UT on January 23, 1999. The burst was seen simultaneously by the WFC on BeppoSax. The position was promptly determined to a few degrees by BATSE and shortly thereafter to 5 arc min by BeppoSax.

  26. The Robotic Optical Transient Search Telescope (ROTSE-1) by Akerlof et al, located at Los Alamos, images a piece of the sky 16.5 degrees across. It is able to slew anywhere in the sky in about 10 s. It was notified of GRB 990123 within 4 s of the onset and was taking data 22 s into the event. The first three ROTSE points are shown superimposed on the BATSE light curve.

  27. m = 8.95 47 s, 5 s exposure m = 11.82 22 s, 5 s exposure m =10.08 72 s, 5 s exposure m = 13.22 259 s, 75 s exposure m = 14.0 447 s, 75 s exposure m = 14.53 612 s, 75 s exposure

  28. And Palomar finds a light where no light was before. This ID was made 3 hours after the burst following a 5AM “wake-up” call to Palomar from Italy GRB 990123

  29. BeppoSax observations of GRB 990123 6 to 33 hours after the burst and 34 to 64 hours after the burst. The x-ray source is clearly fading. Each grid box is 12 x 15 arc min. The error in position is then about 1.5 arc min.

  30. Two HST images of GRB 990123. The image on the left was taken February 8, 1999, the one on the right March 23, 1999. Each picture is 3.2 arc seconds on a side. Three orbits of HST time were used for the first picture; two for the second – hence the somewhat reduced exposure.

  31. The spectrum of host galaxy (Kelson et al, IAUC 7096) taken using the Keck Telescopes gives a redshift of 1.61. Given the known brightness of the burst (in gamma-rays) this distance implies an energy of over several times 1054 erg. About the mass of the sun turned into pure energy. Had this burst occurred on the far side of our Galaxy, at a distance of 60,000 light years, it would have been as bright – in gamma-rays – as the sun. This is ten billion times brighter than a supernova and equivalent to seeing a one hundred million trillion trillion megaton explosion.

  32. As of February, 2002, 129 GRBs had been promptly localized by various satellites (starting in 1997). X-ray afterglows had been detected from 40 and optical afterglows from 28. Red shifts (distances) were determined for about 21. Typical values are z ~1. The largest is 3.42 (GRB 971214) corresponding to emission when the universe was about 15% its present age.

  33. The typical energy is 1053 erg or about 5% of the mass of the sun turned to pure energy according to E = mc2

  34. But are the energies required really that great? Earth If the energy were beamed to 0.1% of the sky, then the total energy could be 1000 times less Earth Nothing seen down here

  35. But then there would be a lot of bursts that we do not see for every one that we do see. About 1000 in fact.

  36. Earth

  37. Quasar 3C 175 as seen in the radio Quasar 3C273 as seen by the Chandra x-ray Observatory Artist’s conception of SS433 based on observations Microquasar GPS 1915 in our own Galaxy – time sequence

  38. It also turns out that in order to get the spectrum and time scales for GRBs correct the beam must be “relativistic”, that is it must travel at very close to the speed of light.Otherwise the bursts would be much softer and last much longer.

  39. It is a property of matter moving close to the speed of light that it emits its radiation in a small angle along its direction of motion. The angle is inversely proportional to the Lorentz factor This offers a way of measuring the beaming angle. As the beam runs into interstellar matter it slows down. Measurements give an opening angle of about 5 degrees.

  40. Frail et al. Nature, (2001), for 17 GRBs with known redshifts and afterglow light curves. Using the angles inferred from this sort of analysis of the afterglows, Frail et al find that an inverse correlation exists between the apparent energy of the burst and this angle. Correcting for the fact that the burst is beamed to a small part of the sky, they find a typical energy in gamma-rays is 5 x 1050 erg. For a reasonable conversion efficiency between explosion energy and gamma-rays (20%), the total jet energy is about 2 x 1051, not so different from an ordinary supernova.

  41. Requirements on the Central Engine and its Immediate Surroundings • Provide adequate energy to material moving close to the speed of light (2 x 1051 erg) • Collimate the emergent beam to approximately 5 degrees • Last approximately 10 s • Make bursts in star forming regions

  42. Merging Neutron Stars

  43. Merging neutron star - black hole pairs Strengths: a) Known event b) Plenty of angular momentum c) Rapid time scale d) High energy e) Well developed numerical models Weaknesses: a) Outside star forming regions b) Beaming and energy may be inadequate for long bursts But this model may still be good for a class of bursts called the “short hard” bursts for which we have no counterpart information yet.

  44. The Collapsar Model (aka the “Hypernova”)

  45. Usually massive stars make supernovae. Their iron core collapses to a neutron star and the energy released explodes the rest of the star. But what if the explosion fizzled? What if the iron core collapsed to an object too massive to be a neutron star – a black hole. A star without rotation would then simply disappear.... But what if the star had too much rotation to all go down the (tiny) black hole? If supernovae are the observational signal that a neutron star has been born, what is the event that signals the birth of a black hole?

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