1 Models for early afterglows (shallow decay & X-ray flares) and implications for progenitors Zigao Dai Nanjing University 2008 Nanjing GRB Conference

2 Collaborators Tan Lu, Daming Wei, Yongfeng Huang, Xiangyu Wang  Nanjing GRB group, K. S. Cheng Zhuo Li, Xuefeng Wu, Yizhong Fan, Yuanchuan Zou, Lang Shao, Yunwei Yu, Dong Xu, Lei Xu, Fayin Wang, Dong Zhang …… Bing Zhang, Enwei Liang, Peter Meszaros, Zong-Hong Zhu, Xinmin Zhang

3 Outline Shallow decay of X-ray afterglows  Observations  Popular models  Prediction on high-energy emission  Other models X-ray flares in early afterglows  Observations  Late internal shock model  Prediction on high-energy emission  Model for X-ray flares of short GRBs Implications for the progenitors Summary

4 Swift : Gehrels et al. (2004) Launch on 20 Nov 2004 Burst Alert Telescope: keV X-Ray Telescope: keV Ultraviolet/Optical Telescope: (5-18)  Hz

5 Discoveries in the Swift era (2005 － 2008) 1.Prompt optical-IR emission and very early optical afterglows 2.Early steep decay and shallow decay of X-ray afterglows 3.X-ray flares from long/short bursts 4.High-redshift (z=6.295) GRB Afterglows and host galaxies of short bursts 6.Some particular bursts: GRB / SN2006aj, GRB / no supernova, GRB / SN2008D, GRB080319B 7.GRB cosmology?

6 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

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

8 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 ejected shells with different Lorentz factors (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

9 Generally, (Zhang & Meszaros 2001, Zhang et al. 2006) or Kerr black holes

10 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

11 Liang, Zhang & Zhang (2007): energy-injection index q and closure relations after shallow decay.

12 A variant energy-injection model ─ relativistic wind bubbles The energy sources of bursts are highly- magnetized, spinning compact stars based on:  most of the current energy-source models, e.g., assuming a Kerr black hole (or a newborn microquasar) (Woosley 1993; Meszaros & Rees 1997; Paczynski 1998; Vietri & Stella 1998; MacFadyen et al. 2001; Lee et al. 2000) or a highly-magnetized millisecond pulsar (Usov 1992; Duncan & Thompson 1992; Kluzniak & Ruderman 1998; Dai & Lu 1998a,b; Spruit 1999; Wheeler et al. 2000)

13 A fter GRB, this star continues to produce an energy outflow. An interaction of this outflow with an outward-expanding fireball implies a post-burst energy injection. F or the Crab Nebula (see Figure), the successful models were proposed by Rees & Gunn (1974) and Kennel & Coroniti (1984). Kennel & Coroniti (1984)

14 As in the Crab Nebula, a continuous outflow from the Crab pulsar may be dominated by the energy flux of electron-positron pairs. Similarly, in an afterglow, an ultrarelativistic pair wind from a Kerr black hole or a millisecond pulsar is expected and its interaction with a post-burst fireball or jet produces a relativistic wind bubble (Dai 2004).

15 Reverse shock (S2) Forward shock (S1) 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) Relativistic wind bubble Black hole (Dai 2004, ApJ, 606, 1000)

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

17 Structured ejecta model: initially ejected shells with different Lorentz factors

18 Yu, Liu & Dai (2007, ApJ, 671, 637) High-energy emission in energy injection models Radially structured ejecta model

19 Relativistic wind bubble model: E w =2*10 52 ergs Structured ejecta model: E ej = 2.1*10 52 ergs

20 Yu, Liu & Dai (2007, ApJ, 671, 637)

21 Other models for shallow decay  Dust-scattering-driven emission (Shao & Dai 2007; Shao, Dai & Mirabal 2008)  Scattering off forward shock photons by later- time ejected shells (Panaitescu 2007)  Central engine afterglows (Ghisellini et al. 2007)  ……

22 Dust scattering (Shao & Dai 2007, ApJ, 660, 1319)

23 Shao, Dai & Mirabal (2008, ApJ): more detailed modeling. Advantages: chromatic light curves and massive-star origins

24 Shao, Dai & Mirabal (2008): evolution of the hardness ratio of echo emission with different shell distances by assuming a delta-like pulse with a power-law spectrum (“disadvantage”).

25 Summary: Shallow Decay of Afterglows Several explanations for the shallow decay of early X-ray afterglows: energy injection models (electronic- and hadronic-component- dominated), scattering models, etc. High-energy emission could be used to distinguish between two types of energy injection models and other models by GLAST.

26 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 long-lasting central engine.

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

28 X-ray flare from short GRB Villasenor et al. 2005, Nature, 437, 855 光学余辉 : t t -2.8 射电余辉 : 上限 X-ray flare at t=100 s

29 GRB050724: Barthelmy et al. 2005, Nature, 438, 994

30 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). Evidence 1 for internal dissipation models

31 Evidence 2 for 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.

32 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

33 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

34 Yu & Dai (2008)

35 Constraints on model parameters from observations

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

37 Application to GRB B Yu, Wang & Dai (2008, arXiv: ): In the popular internal shock model, a forward and a reverse shock are generated simultaneously during collision of two relativistic shells. If these two shocks have very different Lorentz factors, their synchrotron emission could peak at two different energy bands. We show that such a two-component synchrotron scenario can account for the prompt optical and gamma-ray emissions of GRB 80319B (also see Yu’s talk). Racusin et al. (2008) Forward shock emission Reverse shock emission

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

39 Properties of short GRBs Fox, et al. 2005, Nature, 437, 845

40 Ages of the host galaxies Gorosabel et al. 2005, astro-ph/

41 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 old and short GRBs are usually in their outskirts.  support the NS-NS merger model ！ 4. X-ray flares challenge this model!

42 Rosswog et al., astro-ph/

43 Lattimer & Prakash (2007)

44 Ozel 2006, Nature, 441, 1115 Support stiff equations of state

45 Morrison et al. 2004, ApJ, 610, 941

46 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

47 Implications for the progenitors X-ray flares after some GRBs may be due to a series of magnetic activities of highly-magnetized, millisecond- period pulsars. The GRBs themselves may originate from transient hyperaccretion disks surrounding the neutron stars via neutrino or magnetic processes.

48 Hyperaccretion disks around neutron stars Usually, collapsars and NS-NS mergers as central engines: hyperaccretion disks surrounding stellar-mass black holes. Newborn pulsars have been invoked to be central engines of some GRBs in some progenitor/afterglow models.  X-ray flares of short GRBs are due to magnetic reconnection-driven events from millisecond pulsars (Dai et al. 2006).  Energy injection to forward shocks through magnetic dipole radiation from pulsars leads to flattening of afterglow light curves (Dai & Lu 1998a; Zhang & Meszaros 2001; Dai 2004).  Some progenitor models of GRBs: hyperaccretion disks of neutron stars (e.g., Kluzniak & Ruderman 1998; Dai & Lu 1998b; Wheeler et al. 2000; Wang et al. 2000; Paczynski & Haensel 2005). Hyperaccretion disks surrounding pulsars are likely to appear in type-II supernovae (Bethe 1990).

49 Advection-dominated outer disk Self-similar inner disk NS 1.Far from NS (outer disk), the disk is similar to the BH disk. 2.Near NS (inner disk), an energy balance between heating and cooling is built, leading to a self-similar structure (Zhang & Dai 2008, also see Zhang’s talk).

50 1.When the accretion rate is sufficiently low, most of the disk is advection-dominated, the energy is advected inward to heat the inner disk, and eventually released via neutrino emission in the inner disk. 2.If the accretion rate is large enough to make neutrino emission optically thick, then the effect of neutrino opacity becomes important. Zhang & Dai 2008, ApJ, in press

51 Density, pressure and temperature as functions of radius for three accretion rates (Zhang & Dai 2008, ApJ, in press).

52 Luminosity vs. accretion rate: solid line corresponds to the whole disk of a neutron star, dashed line to the inner disk, dotted line to the outer disk, and dash-dotted line to the black hole disk.

53 Summary of this talk Discoveries of early afterglows, X-ray flares, short GRB afterglows, and high-z GRBs by Swift. Mysteries due to Swift: e.g., multi-waveband light curves? central compact objects? classification? Future observations by Swift and GLAST could reveal models of GRBs and afterglows (e.g., in high energy bands and/or during shallow decay and X- ray flares). Hyperaccretion disks surrounding pulsars could provide an energy source for some explosions via neutrino/magnetic processes.

54 Thank you!