1. Gamma Ray Burst Afterglows and Host Galaxies James E. Rhoads Space Telescope Science Institute

2. The Quest for Counterparts Most classes of astronomical object can be studied at a range of wavelengths. For 24 years GRBs were an exception. Difficulties due to the difficulty of deriving accurate positions from gamma rays, especially in short times. Potential payoff… Large, as we shall see.

3. Fast Debris from GRBs GRBs release a lot of energy (about 10 52 ergs, corresponding to 1% of the Suns rest mass energy) very quickly (0.1 to 100 sec). Their gamma ray brightness varies strongly in milliseconds… implying sizes < 100 km. This much power in so small a region requires fast outflow. In fact, the ejecta are highly relativistic, with Γ > 100.

4. The Prediction of Afterglows GRBs require relativistic ejecta. Sometime, these ejecta must encounter an ambient medium…. => Afterglow! This argument led to the prediction of afterglows years before they were observed (Paczyński & Rhoads 1993, Katz 1994, Mészáros & Rees 1997).

5. Probable Sequence of GRB Events The central engine emits a large amount of energy (in almost any form). Most of that energy accelerates a small mass (about the mass of the Earth) to speeds > 99.99% of lightspeed. Collisions between different shells of ejected debris creates the gamma rays. Collisions between ejected debris and interstellar gas create the afterglow.

6. Afterglows are decoupled from the central engine The prediction of GRB afterglows requires only that there be highly energetic, highly relativistic ejecta. It does not matter what the original source of the energy is. Also, it natural that a large part of the GRB energy goes into kinetic energy of ejecta, given the sizes and energies involved.

7. Possible Central Engines The Central Engine refers to the monster in the middle… the ultimate source of the burst. Current interest focuses on two possibilities: The collapse of a massive, rotating star into a central black hole; or Two neutron stars that merge in a cataclysmic explosion. Either way, the immediate progenitor is a rotating black hole surrounded by a massive ring of matter. This can provide sufficient energy for the burst.

8. The Central Engine (speculative!) Afterglow properties are decoupled from the central engine. Martin Reess best buy model. Can tap gravitational, rotational, or magnetic energy.

9. Precise GRB Positions and the First GRB Counterparts The Italian-Dutch satellite BeppoSAX began providing accurate GRB locations in 1997 (good to a few arcminutes and available within a few hours). This led to the first counterparts at X-ray (GRB 970111), optical (GRB 970228), and radio (GRB 970508) wavelengths. These have been named afterglows.

10. How Afterglows are Found Gamma ray detectors see the burst. Fairly good positions come from gamma ray instruments (via triangulation) and/or X-ray images within hours of the burst. These positions are searched for variable objects using X-ray, optical, and/or radio telescopes. The brightest optical afterglow yet peaked at 9th magnitude, during the GRB itself!

11. Example: GRB 990123 BeppoSAX X-ray images of GRB 990123

12. GRB 990123 in Visible Light ROTSE optical images of GRB 990123

13. GRB 990123: Hubble Images Hubble Space Telescope images of GRB 990123 and its host galaxy at 16, 59, and 380 days after the gamma ray burst.

14. Triumphs of Afterglow Studies Afterglows have demonstrated that: Fireball models describe GRBs reasonably well; GRBs are at cosmological distances; GRB ejecta move relativistically; GRBs occur in galaxies. GRBs may be associated with the deaths of some (but not all!) high mass stars; GRBs may be collimated (search lights rather than flood lights).

15. Afterglow Models: Ingredients Initial conditions: Energy, ejecta mass, Lorentz factor, jet opening angle. Ambient medium density profile Relativistic shock physics: Distribution of energy among radiating electrons and magnetic fields in highly relativistic shocks Note, this cannot be tested in the lab!

16. Fireball Models & Basic Results Blast wave structure: Reverse Contact Forward shock discontinuity shock Unshocked shocked material unshocked ejecta Γ ~ 2 Γ ~ 100 ambient gas Predict broken power law spectra and light curves, with a predicted relation between spectral and light curve slopes.

17. Afterglow Spectral Energy Distribution Spectral energy distribution for the simplest afterglow model. Measurement of whole spectrum can determine the energy, ambient density, and some shock physics parameters. (Figure from Sari, Piran, & Narayan 1998)

18. Afterglow Light Curve for GRB 970508 The R c band light curve of GRB 970508.

19. Prompt Optical Followup Several experiments (LOTIS, ROTSE, TAROT, …) Only one detection. Several GRBs have low optical to gamma ratios. Implications for initial Lorentz factor Fig. 2 of Akerlof et al 2000: ROTSE data rescaled by GRB fluence.

20. The GRB Distance Scale (GRB 970508) Metzger et al 1997, Nature 387, 878 GRB 970508: z >= 0.835

21. Afterglow Distance Determinations HST spectrum of GRB 000301C. (From Smette et al 2000.)

22. Proof of Relativistic Speeds Interstellar gas in our Galaxy causes small radio sources to twinkle (like stars seen in visible light through our atmosphere). Larger sources do not twinkle (like planets). Measuring the time when an afterglow stops twinkling at radio wavelengths reveals its speed of expansion to be near light speed.

23. Gamma Ray Burst Host Galaxies Optical and radio afterglow observations can pinpoint GRB locations to an accuracy of < 10,000 light years. This is smaller than a typical galaxy. In most cases, a galaxy is indeed seen where the afterglow is found. These galaxies are reasonably typical of distant, star forming galaxies.

24. Locations of GRBs in their Host Galaxies Not all gamma ray bursts occur at the nucleus of their host galaxies. This rules out quasars and related objects (i.e., the central black holes of galaxies) as the origin of GRBs. (Figure from Bloom et al 1999)

25. GRB Host Galaxies are Blue Colors and magnitudes (brightness) of galaxies in the Hubble Deep Field and of three GRB Host Galaxies. Bluer is down, fainter is to the right. (From Fruchter et al 1999.)

26. GRBs in Obscured Starbursts? Recently, two GRB host galaxies have been shown to have unusually high submillimeter wavelength brightness. This suggests strong star formation activity hidden by dust.

27. Gamma Ray Bursts and the Deaths of Massive Stars Afterglow data suggests that GRBs occur… in galaxies with active star formation, often in regions with a lot of gas, which is where new stars form and where the most massive stars spend their entire brief lives. However, GRBs are so rare that only a tiny fraction of massive star deaths could produce them.

28. The GRB-Supernova Connection A nearby supernova, SN 1998bw, was found by searching the error box of GRB 980425. Some other GRBs show evidence for late time bumps in the light curve, often red in color…These could be supernovae also. The smoking gun will be spectroscopy of a light curve bump.

29. Are Gamma Ray Bursts Searchlights? The extreme energy needed to produce a GRB could be reduced dramatically if the bursts are collimated searchlights. Three predictions for collimated GRBs: There should be orphan afterglows, and Afterglows of collimated GRBs should fade more rapidly at late times. Afterglow light may be polarized. There are now several likely observations of rapid late time fading.

30. Afterglows that faded fast: GRB 990510

31. Afterglows that faded fast: GRB 000301C

32. Collimation Corrected Energies Gamma ray energies before and after collimation correction. From Frail et al (2001) See also Kumar and Panaitescu 2002.

33. Orphan Afterglows During the evolution of a GRB remnant, The ejecta slow down; The characteristic photon frequency drops; Collimation of the photons decreases. So, the observed transient rate should increase with wavelength if GRBs are collimated. (Rhoads 1997; Perna & Loeb 1998)

34. Polarization Afterglows are thought to be produced by synchrotron emission, which is typically polarized. By symmetry, a spherically symmetric burst should have no net polarization. There is no corresponding argument for collimated bursts… and net polarization is expected.

35. GRBs at Extremely High Redshifts GRBs and their afterglows could be detected at very high redshifts (at least to z>>5) if GRBs occur there. GRBs should occur at high redshift, if they are really associated with the deaths of massive stars. => Probe of the earliest stars and the universe in which they formed.

36. GRB Redshift Distribution From Bloom, Frail, & Sari 2001

37. Effects of Gamma Ray Bursts on their Environments Gamma ray bursts are not nice neighbors. The high energy photons they produce can destroy interstellar dust grains up to 100 pc away, and ionize interstellar gas at similar distances. The ionized gas will fluoresce as it gradually recombines, and can be used to look for GRB remnants in nearby galaxies.

38. The Shape of Things to Come Swift, a NASA MidEx mission, is approved and should fly in 2003. Yield: 300 good positions per year? Swift will have hard X-ray, soft X-ray, and optical/ultraviolet instruments on board. Response time for the optical: 20 to 70 seconds. This will open the way for systematic study of afterglows, including the still-mysterious short bursts.

39. The Niche for Amateurs in the Swift Era How can amateur observers complement the onboard optical capability of Swift? Red wavelength observations Light curve monitoring during Earth occultation of Swift Polarization information? (Hard…) Orphan afterglow followup? (Faint…) Monitoring of candidate lensed GRBs?

40. Triumphs of Afterglow Studies Afterglows have enabled explosive growth in GRB studies. In particular, they have shown that: GRBs are at cosmological distances; GRBs are likely collimated (search lights rather than flood lights). => GRB energy scale is determined, 10 52 ergs. GRBs occur in star-forming galaxies; GRBs may be associated with supernovae; => Progenitors are probably massive stars of some kind.

41. Tomorrows Questions Upcoming space missions, better coordinated followup, and ongoing theoretical work all promise continued rapid progress in GRBs. Specific areas of enquiry: Do the short GRBs have afterglows? Host galaxies? Do all GRBs have associated supernovae? Are all GRBs associated with massive stars, or are some caused by merging neutron stars? And, ultimately…. What is the source of the Gamma Ray Bursts?