1. 1 Gamma-Ray Bursts: Early afterglows, X-ray flares, and GRB cosmology Zigao Dai Nanjing University

2. 2 Outline Shallow decay of X-ray afterglows  Observations  Popular models  Prediction on high-energy emission X-ray flares in early afterglows  Observations  Late internal shock model  Prediction on high-energy emission  Model for X-ray flares of short GRBs Gamma-ray burst cosmology Summary

3. 3 What are GRBs?

4. 4 Spectral features: broken power laws with E p of a few tens to hundreds of keV Temporal features: diverse and spiky light curves. Light Curves and Spectra

5. 5 How to understand?

6. 6 Six eras 1) “Dark” era (1973-1991): discovery Klebesadel, Strong & Olson’s discovery (1973) 2) BATSE era (1992-1996): spatial distribution Meegan & Fishman’s discovery (1992), detection rate: ~1 to 3 /day, ~3000 bursts 3) BeppoSAX era (1997-2000): afterglows, redshifts van Paradijs, Costa, Frail’s discoveries (1997) 4) HETE-2 era (2001-2004): origin of long bursts Observations on GRB030329/SN2003dh 5) Swift era (2005-): early afterglows, short-GRB afterglows, high-redshift GRBs, GRB cosmology 6) Fermi era (2008-): high-energy gamma-rays

7. 7 Swift : Gehrels et al. (2004) Launch on 20 Nov 2004 Burst Alert Telescope: 15-150 keV X-Ray Telescope: 0.2-10 keV Ultraviolet/Optical Telescope: (5-18)  10 14 Hz Which satellites detect now?

8. 8 Fermi: Launch on 11 June 2008 Two instruments: Fermi Burst Monitor (GBM) 10 keV-25 MeV, dedicated to detecting GRBs; Large Area Telescope (LAT) 20 MeV-300 GeV.

9. 9 Discoveries and studies in the Swift-Fermi era (2005 － ) 1.Prompt emission and very early afterglows in low-energy bands 2.Early steep decay and shallow decay of X-ray afterglows 3.X-ray flares from long/short bursts 4.Highest-redshift (z=8.2) GRB090423 5.Afterglows and host galaxies of short bursts 6.Some particular bursts: GRB060218 / SN2006aj, GRB060614 / no supernova, GRB080109 / SN2008D, GRB080319B, … 7.High-energy gamma-ray radiation by Fermi 8.Classification and central engine models 9.GRB cosmology

10. 10 I. Shallow decay of X-ray afterglows Cusumano et al. 2005, astro-ph/0509689 t -5.5 ν -1.6  0.22 GRB050319 t -0.54 ν -0.69  0.06 t -1.14 ν -0.80  0.08

11. 11 See Liang et al. (2007) for a detailed analysis of Swift GRBs: ~ one half of the detected GRB afterglows. Why shallow decay? ─ big problem!

12. 12 Popular models Initial steep decay: High-latitude emission from relativistic shocked ejecta, e.g. curvature effect (Kumar & Panaitescu 2000; Zhang et al. 2006; Liang et al. 2006): flux density  (t-t 0 ) -(2+β) with the t 0 effect. Shallow decay: Continuous energy injection (Dai & Lu 1998a, 1998b; Dai 2004; Zhang & Meszaros 2001; Zhang et al. 2006; Fan & Xu 2006) or initially structured ejecta (Rees & Meszaros 1998; Sari & Meszaros 1998; Nousek et al. 2006) …… Normal decay: Forward shock emission (e.g., Liang et al. 2007) Final jet decay in some cases

13. 13 Injected energy = E/2

14. 14 F ollowing the pulsar energy-injection model, numerical simulations by some groups (e.g., Fan & Xu 2006; Dall’Osso et al. 2010) provided fits to shallow decay of some GRB afterglows with different slopes.

15. 15 Generally, (Zhang & Meszaros 2001; Zhang et al. 2006) Variants of the pulsar energy-injection model: 1. Luminosity as a power-law function of time

16. 16 GRB060729: Grupe et al. (2007, ApJ, 662, 443) GRB070110: Troja et al. (2007, ApJ, 665, 599) GRB050801: De Pasquale et al. (2007, MNRAS, 337, 1638) q=0 millisecond pulsars

17. 17 Termination shock (TS) External shock (ES) Contact discontinuity Ambient gas (zone 1) A relativistic e - e + wind A relativistic e - e + wind (zone 4) Shocked wind (zone 3) Shocked ambient gas (zone 2) Variants of the pulsar energy-injection model: 2. Relativistic wind bubble (RWB) Black hole Dai (2004, ApJ, 606, 1000)

18. 18 Yu & Dai (2007, A&A, 470, 119) Dai 2004

19. 19 Variants of the pulsar energy-injection model: 3. RWB with a Poynting-flux component Mao, Yu, Dai et al. (2010): TS-dominated and ES-dominated types for different σ = η σ* (where σ* ~ 0.05). ~ const.

20. 20 Structured ejecta model: initial ejecta with a distribution of Lorentz factors Structured ejecta model: protonic-component-dominated energy injection

21. 21 Yu, Liu & Dai (2007, ApJ, 671, 637) Tests of energy injection models: 1. High-energy emission Structured ejecta model

22. 22 GeV flux: Yu, Liu & Dai (2007, ApJ, 671, 637)

23. 23 Tests of energy injection models: 2. Gravitational radiation

24. 24 Summary: Shallow Decay of Afterglows Several explanations for the shallow decay of early X-ray afterglows: energy injection models (electronic- and protonic-component- dominated), and so on. Detections of high-energy emission (by Fermi) and gravitational radiation (by advanced- LIGO) are expected to test energy injection models.

25. 25 II. X-ray flares from long bursts Burrows et al. 2005, Science, 309, 1833 Explanation: late internal shocks (Fan & Wei 2005; Zhang et al. 2006; Wu, Dai, Wang et al. 2005), implying a long-lasting central engine.

26. 26 Chincarini et al. (2007, ApJ, 671, 1903): ~ one half of the detected GRB afterglows.

27. 27 Short GRB050724: Barthelmy et al. 2005, Nature, 438, 994

28. 28 Lazzati & Perna (2007): Flare duration vs. occurrence time in different dynamical settings as a function of the spectral index. The shaded area represents the observed distribution of Δt/t from Chincarini et al. (2007). Why internal dissipation models?

29. 29 Why internal dissipation models? Liang et al. (2006) tested the curvature effect of X-ray flares and showed that t 0 is nearly equal to t pk.

30. 30 Central Engine Relativistic Wind The Internal-External-Shock Model How to produce X-ray flares? External Shock Afterglow Internal Shocks GRB Late Internal Shocks XRFs

31. 31 Late-internal-shock model for X-ray flares Two-shock structure: Reverse Contact Forward shock (S2) discontinuity shock (S1) unshocked shocked materials unshocked shell 4 3 2 shell 1 Gamma_3 = Gamma_2 P_3 = P_2 Dynamic s

32. 32 Yu & Dai (2008): spectrum and light curve

33. 33 Energy source models of X-ray/optical flares How to restart the central engine? 1.Fragmentation of a stellar core (King et al. 2005) 2.Fragmentation of an accretion disk (Perna Armitage & Zhang 2005) 3.Magnetic-driven barrier of an accretion disk (Proga & Zhang 2006) 4.Magnetic activities of a newborn millisecond pulsar (for short GRB) (Dai, Wang, Wu & Zhang 2006) 5.Tidal ejecta of a neutron star-black hole merger (Rosswog 2007)

34. 34 Basic features of short GRBs 1. low-redshifts (e.g., GRB050724, z=0.258; GRB050813, z=0.722) 2. E iso ~ 10 48 – 10 50 ergs; 3. The host galaxies are very old and short GRBs are usually in their outskirts.  support the NS-NS merger model ！ 4. X-ray flares challenge this model!

35. 35 Rosswog et al., astro-ph/0306418

36. 36 Dai, Wang, Wu & Zhang 2006, Science, 311, 1127: a differentially- rotating, strongly magnetized, millisecond pulsar after the merger. Kluzniak & Ruderman (1998) Lazzati (2007) 1. Many flares after a GRB 2. Spectral softening of flares 3. Average flare-L decline

37. 37 Implications for central engines X-ray flares after some GRBs may be due to a series of magnetic activities of highly-magnetized millisecond pulsars. The GRBs themselves may result from hyperaccretion disks surrounding the pulsars via neutrino or magnetic processes (Zhang & Dai 2008, 2009, 2010).

38. 38 III. GRB cosmology

39. 39 Disadvantages in SN cosmology: 1.Dust extinction 2.Z MAX ~ 1.7 z T ~0.5

40. 40 Two advantages of GRBs relative to SNe  GRBs can occur at very high redshifts and thus could be more helpful in measuring the slope of the Hubble diagram than SNe Ia.  Gamma rays are free from dust extinction, so the observed gamma-ray flux should be a direct measurement of the prompt emission energy. So, GRBs are an attractive and promising probe of the universe.

41. 41 The afterglow jet model (Rhoads 1999; Sari et al. 1999; Dai & Cheng 2001 for 1<p<2):

42. 42 Ghirlanda et al. (2004a); Dai, Liang & Xu (2004): a tight correlation with a slope of ~1.5 and a small scatter of  2 ~0.53, suggesting a promising and interesting probe of cosmography.  M =0.27,   =0.73 Ghirlanda correlation

43. 43 The Hubble diagram of GRBs is consistent with that of SNe Ia. Dai, Liang & Xu (2004, ApJ, 612, L101) Concordance cosmology Red: GRB Blue: SNIa

44. 44 Dai, Liang & Xu (2004) assumed a cosmology-independent correlation.

45. 45 Recent works  Schaefer (2007): 69 GRBs including Swift bursts + 5 correlations  Li et al. ( 2007), Wright (2007), Liang et al. (2008): GRBs + some other probes, D L calculated for the concordance cosmology or SNe  Wang, Dai & Zhu (2007): 69 GRBs + more other probes, D L by simultaneous fitting of 5 correlations for any given cosmology GRBs provide a much longer arm for measuring changes in the slope of the Hubble diagram than SNe Ia.

46. 46 Wang, Dai & Zhu (2007, ApJ)

47. 47 1.The addition of GRBs leads to a stronger constraint on w(z) at the 3rd redshift bin. 2.EOS of dark energy w(z)>0 at z>1.0. 3.Parameter w(z) deviates from -1. 115 GRBs Constraints on evolution of w(z) (Wang, Qi & Dai 2011)

48. 48 Explosions SNe IaGRBs Astrophysical energy sources Thermonuclear explosion of accreting white dwarfs Core collapse of massive stars Standardized candles Colgate (1979): L p constant Frail et al. (2001): E jet constant More standardized candles Phillips (1993): L p ~Δm 15 (9 low-z SNe Ia) Ghirlanda et al. (2004a): E jet ~E p (14 high-z bursts) Other correlationsRiess et al. (1995); Perlmutter et al. (1999) … Liang & Zhang (2005), Schaefer (2007) … Recent observations37 HST-detected SNe Ia up to z~1.7 (Riess et al. 2007) A large Swift-detected sample up to higher z~8.2 Comments on research status From infancy to childhood (1998) to adulthood (SNAP) At babyhood (to childhood by future missions?) Comparison of Two Cosmological Probes

49. 49 Summary on GRB cosmology Finding: There have been >150 papers on GRB cosmology, which show that GRBs might provide a complementary and promising probe of the early universe and dark energy. Advantages: 1) GRBs can occur at very high redshifts; 2) Gamma rays are free from dust extinction. Disadvantages: The correlations have not been calibrated with low-z bursts (but also Liang, N. et al. 2008). Status: The current GRB cosmology is at babyhood. Prospect: In the future, the GRB cosmology could progress from its infancy to childhood, if a larger sample of GRBs (or some subclass) and a more standardized candle are found.

50. 50 Summary of this talk Shallow decay of early afterglows and X-ray flares seem to imply a long activity of the central engine (e.g., highly-magnetized millisecond pulsars). Future detections by Fermi and advanced-LIGO are expected to test this implication. We expect possible progress in GRB cosmology in the Swift, Fermi, SVOM … eras.