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

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 What are GRBs?

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 How to understand?

6 Six eras 1) “Dark” era ( ): discovery Klebesadel, Strong & Olson’s discovery (1973) 2) BATSE era ( ): spatial distribution Meegan & Fishman’s discovery (1992), detection rate: ~1 to 3 /day, ~3000 bursts 3) BeppoSAX era ( ): afterglows, redshifts van Paradijs, Costa, Frail’s discoveries (1997) 4) HETE-2 era ( ): 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 Swift : Gehrels et al. (2004) Launch on 20 Nov 2004 Burst Alert Telescope: keV X-Ray Telescope: keV Ultraviolet/Optical Telescope: (5-18)  Hz Which satellites detect now?

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 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) GRB Afterglows and host galaxies of short bursts 6.Some particular bursts: GRB / SN2006aj, GRB / no supernova, GRB / SN2008D, GRB080319B, … 7.High-energy gamma-ray radiation by Fermi 8.Classification and central engine models 9.GRB cosmology

10 I. Shallow decay of X-ray afterglows Cusumano et al. 2005, astro-ph/ t -5.5 ν -1.6  0.22 GRB t ν  0.06 t ν  0.08

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 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 Injected energy = E/2

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

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

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

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

23 Tests of energy injection models: 2. Gravitational radiation

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

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

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

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

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 Basic features of short GRBs 1. low-redshifts (e.g., GRB050724, z=0.258; GRB050813, z=0.722) 2. E iso ~ – 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 Rosswog et al., astro-ph/

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 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 III. GRB cosmology

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

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

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

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 Wang, Dai & Zhu (2007, ApJ)

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> Parameter w(z) deviates from GRBs Constraints on evolution of w(z) (Wang, Qi & Dai 2011)

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 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 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.