DETECTABILITY AND MODELING OF HIGH-REDSHIFT GRBS

Transcription

1 The Pennsylvania State University The Graduate School Department of Astronomy and Astrophysics DETECTABILITY AND MODELING OF HIGH-REDSHIFT GRBS A Thesis in Astronomy and Astrophysics by Li-Jun Gou c 007 Li-Jun Gou Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 007

2 ii The thesis of Li-Jun Gou was reviewed and approved* by the following: Peter Mészáros Eberly Chair of Astronomy & Astrophysics and Professor of Physics Thesis Adviser Chair of Committee Derek B. Fox Assistant Professor of Astronomy and Astrophysics David Burrows Professor of Astronomy and Astrophysics Gordon Garmire Evan Pugh Professor of Astronomy and Astrophysics Richard Robinett Professor of Physics Lawrence Ramsey Professor of Astronomy and Astrophysics Head of the Department of Astronomy and Astrophysics *Signatures are on file in the Graduate School.

3 iii Abstract Gamma-ray bursts are the brightest, and probably the most energetic, explosions in the Universe. As both the burst and its subsequent afterglow are bright events shining from cosmological distance, GRBs are also a promising tool to trace the star formation history of the Universe and investigate its properties and structure, out to the highest redshifts. Before extending GRB research to these immense distances, however, one should ask whether these high-redshift burst, and their afterglows, can be detected by current and future instruments? Next, one should ask how will we know when such events are detected can the redshift of the burst be determined by means other than the traditional method of optical spectroscopy? In this thesis, we have investigated these two questions with detailed numerical studies. In addition, we have carefully modeled the physical properties of the redshift z = 6.9 GRB , which has the highest confirmed redshift for any GRB and so may serve as a prototype of high-redshift GRBs. We begin in the framework of the afterglow model, including both forward and reverse shocks, by calculating the observed flux as a function of redshift and observer time for typical GRB afterglows. We consider two possibilities, based on current theoretical ideas, for the redshift dependence of the gas properties in typical locations of primordial star formation. We calculate fluxes in the X-ray and near-infrared bands for comparison to the sensitivities of different facilities including Chandra, XMM-Newton, Swift, and JWST. Using standard assumptions, we find that Chandra, XMM, and Swift can potentially detect GRBs in the X-ray band out to very high redshifts z 30. In the infrared K and M bands, JWST and ground-based telescopes are potentially able to detect GRBs even one day after the trigger out to z 16 and 33. Next, we calculate the GeV spectra of GRB afterglows produced by inverse- Compton scattering of the sub-mev emission of these objects. Improving on earlier treatments by using refined afterglow parameters and incorporating new model developments motivated by Swift observations, we evaluate the limiting redshift to which afterglows can be detected by the GLAST LAT as well as AGILE. We demonstrate that the limiting redshift is z 0.8 for GLAST LAT and z 0.5 for AGILE, for GRBs with typical parameters and integration times t seconds. The limiting redshifts decrease for delayed responses by either mission. Next, since X-ray afterglows are detected for roughly twice as many bursts as are optical afterglows, we have investigated the detectability of iron line emission in afterglow X-ray spectra, which would enable a direct X-ray measurement of the GRB redshift. We calculate the significance levels for iron line detection as a function of source redshift and response time, for the Swift XRT, Chandra ACIS and XMM-Newton Epic detectors. For bursts with standard luminosities and decay rates, and assuming a constant equivalent width of 1 kev for the iron line, we find that Swift will be able to detect such line emission up to z 1.5 with >3-σ significance, with exposures of t < 104 s. The same line would be detectable with > 4σ significance at z < 6 by Chandra, and at z < 8 by XMM, for response times of t < 105 s. For similar bursts with a variable equivalent width which peaks at 1 kev between 0.5 and 1 days in the source frame, Swift achieves

4 the same significance level for z 1 at t 1 day, while Chandra reaches the previous detection significances around t 1 days for z 4, that is, the line is detectable near time of peak equivalent width, and is undetectable at earlier or later times. For high-luminosity X-ray afterglows, which may be observed for pop. III bursts, similar significance levels are obtained out to substantially higher redshifts. These results were originally presented prior to the launch of the Swift mission; the absence of iron line detections in Swift XRT observations to-date thus demonstrates that iron lines of this strength are extremely rare, if not absent, among GRB afterglows. Last, we explore the physical parameters of the burst GRB , which is the first gamma-ray burst to be identified from beyond the epoch of reionization. Since the progenitors of long gamma-ray bursts have been identified as massive stars, this event offers a unique opportunity to investigate star formation environments at this epoch. Apart from its record redshift, the burst is remarkable in two respects: first, it exhibits fast-evolving X-ray and optical flares that peak simultaneously at t 470 s in the observer frame, and may thus originate in the same emission region; and second, its afterglow exhibits an accelerated decay in the near-infrared (NIR) from t 10 4 s to t s after the burst, coincident with repeated and energetic X-ray flaring activity. We make a complete analysis of available X-ray, NIR, and radio observations, utilizing afterglow models that incorporate a range of physical effects not previously considered for this or any other GRB afterglow, and quantifying our model uncertainties in detail via Markov Chain Monte Carlo analysis. In the process, we explore the possibility that the early optical and X-ray flare is due to synchrotron and inverse Compton emission from the reverse shock regions of the outflow. We suggest that the period of accelerated decay in the NIR may be due to suppression of synchrotron radiation by inverse Compton interaction of X-ray flare photons with electrons in the forward shock; a subsequent interval of slow decay would then be due to a progressive decline in this suppression. The range of acceptable models demonstrates that the kinetic energy and circumburst density of GRB are well above the typical values found for low-redshift GRBs. iv

5 v Table of Contents List of Tables List of Figures viii ix Acknowledgments xi Chapter 1. Introduction History of GRB Research GRB Classification and Progenitors Physics of GRBs and Afterglows Light Curve of Afterglows X-ray Light Curve Optical and Radio Light Curve GRB Cosmology Motivation of the Thesis Outline of the Thesis Chapter : Detectability of Long GRB Afterglows from Very High Redshifts Chapter 3: GLAST Prospects for Swift-Era Afterglows Chapter 4: Detectability of GRB Iron Lines by Swift, Chandra and XMM Chapter 5: Modeling GRB : Autopsy of a Massive Stellar Explosion at z = Chapter 6: Conclusion Chapter. Detectability of Long GRB Afterglows from Very High Redshifts Introduction Afterglow characteristics Forward Shock Reverse Shock GRB density environment Intergalactic and galactic absorption Initial Conditions and Numerical Results Light Curve Infrared Flux Redshift Dependence X-ray Flux Redshift Dependence Summary and Conclusions Chapter 3. GLAST Prospects for Swift-Era Afterglows Introduction Afterglow Synchrotron-inverse Compton Spectra at GeV Energies.. 9

6 3.3 Detectability of GRB Afterglows with GLAST and AGILE Discussion Chapter 4. Detectability of GRB Iron Lines by Swift, Chandra and XMM Introduction Model and Procedure Simulation Results: Line Detection Significance Discussion Chapter 5. Modeling GRB : Autopsy of a Massive Stellar Explosion at z = Introduction Observations and Theoretical Frameword Burst and Afterglow Observations Afterglow Modeling in the Swift Era Two Different Scenarios (A) Forward Shock Fit Only (B) Forward Shock and Flares Fit Synchrotron and Inverse Compton Afterglow Model Forward Shock Synchrotron Formulae Reverse Shock Synchrotron Formulae Inverse Compton Effects, Jet Break and Non-relativistic Case Host Galaxy Extinction and Lyman-α Damping Absorption Fitting Data and Procedure Observational Data and Constraints Parameters and Methodology Numerical Results Analysis of the Results J-Band Light-Curve and IC Suppression Radio Light-curve Density and Energy Constraints Burst Energetics and Efficiency Discussion Conclusions Chapter 6. Conclusions Appendix A. GRB Afterglow Flux Evolution A.1 Forward Shock A.1.1 Reverse Shock Appendix B. Inverse-Compton Flux Ratio vi

7 Appendix C. Self Absorption Frequency and Inverse Compton Spectrum C.1 Self-Absorption Frequency C. Inverse Compton Spectrum C..1 ν c < ν a < ν m C.. ν c < ν m < ν a C..3 ν m < ν c < ν a C..4 ν m < ν a < ν c C.3 Derivation of Radiative Correction Factor C.4 Kinetic Energy References vii

8 viii List of Tables 5.1 Reverse and Forward Shock Observation Data Points Other observational constraints for the fitting The best fit values and parameter ranges for models (A) and (B)

9 ix List of Figures.1 Typical light curves, for a redshift z = Combined forward and reverse shock observed flux as a function of redshift. 4.3 Combined forward and reverse shock observed flux as a function of redshift for the two density profiles n = n 0 = constant ( without symbols) and n = n 0 (1 + z) 4 (symbols) with n 0 = 1 cm Combined forward and reverse shock observed flux as a function of redshift for the two density profiles n = n 0 = constant (without symbols) and n = n 0 (1 + z) 4 (symbols) with n 0 = 0.01 cm Observed X-ray fluxes for GRB afterglows at different redshifts Detectability of redshift limits for the standard, energy injection, and evolving parameter models The partial fluence curves for a set of more typical parameters Comparison of GLAST and AGILE detectability Comparison of detectability for hyper-energetic GRBs and GRB like GRBs Detetability limits for GLAST for stanardard models but with different Compton Y parameters Effective area of the Swift XRT, Chandra-ACIS and XMM-Epic detectors Fe K-α detection significance level contour plot for Swift Chandra and XMM Bursts of higher initial luminosity L X,0 = ergs s 1 observed by Swift Variable equivalent width case, for a standard luminosity burst seen with Swift (left), a higher luminosity burst seen with Swift and a standard luminosity burst seen with Chandra Swift ability to distinguish two lines either with different central energy or with different widths Chandra ability to distinguish two Fe lines XMM-Newton ability to distinguish two Fe lines The combined light curves of GRB in the BAT, XRT, J and I bands, in the observer frame Illustration of the multiple mechanisms contributing to the observed flux at early times Posterior distribution of all parameters for model (A) Theoretical light curves corresponding to the best-fit parameters for model (A) Posterior distribution of all parameters in model (B) Theoretical light curves corresponding to the best-fit parameters in model (B) Joint confidence regions for three chosen parameters from our model fits. 89

10 B.1 The synchrotron and inverse Compton spectra for two Compton parameters at t = seconds x

11 xi Acknowledgments I am certainly most indebted to my advisor, Prof. Peter Mészáros, for his direction and teaching throughout my time at Penn State. He has provided me with much support and helpful counsel, on research and on life. He has been deeply involved in every paper collected in this thesis, from discussing the problems and framing the questions, to catching typos, responding to referee reports, and improving my written English, for all of which I am deeply grateful. Without his help, I surely would not have finished the thesis you are reading now. Meanwhile, I extend additional appreciation to Prof. Derek B. Fox, who partly supervised me during the last two years. He has introduced me to the field of data analysis and statistical modeling, and also provided me with useful assistance and advice in work and in life. Particularly during the days when I was upset and worried about my future, he has provided many suggestions and much encouragement. I also wish to thank the members of my dissertation committee: Prof. Gordon Garmire, Dr. David Burrows, and Prof. Richard Robinett. They have provided countless fruitful suggestions and advice along the road to completing this thesis. I also thank the faculty members and staff in the department for their countless and useful assistance. I would also like to thank my fellow astrograds, including those who have already graduated, and my friends. Shared meals, celebrations, discussions, and just hanging out have made these days into joyful memories I will carry with me. Finally, I acknowledge the constant and steadfast love of my parents and wife, who have supported me throughout these years.

12 1 Chapter 1 Introduction 1.1 History of GRB Research Gamma-ray bursts (GRBs) are short-duration bursts of gamma radiation that occur at an average rate of a few per day throughout the Universe. Since the serendipitous discovery of Gamma-Ray Bursts (GRBs) by the Vela satellites in 1967, 40 years have now elapsed. The history of GRB research during this period can be divided into 4 stages (Mészáros 006): Prior to the Compton Gamma-Ray Observatory (CGRO); between CGRO and Beppo-SAX; between Beppo-SAX and Swift; and post-swift. I will now briefly discuss each of these stages in turn. Stage One: Before the launch of Compton Gamma Ray Observatory ( ). GRBs were first discovered in 1967, and first publicly announced in 1973 (Klebesadel et al. 1973). When these mysterious gamma-ray flashes were discovered, it was determined that they were neither from the Earth s direction nor from the Sun s direction, and the suspicion that they were the emissions of an advanced extraterrstrial civilization was also soon abandoned. Rather, it was realized that this was a new and extremely puzzling cosmic phenomenon. In the following years, the gamma-ray flashes were observed but disppeared quickly, leaving no traces and yielding only very rough locations on the sky. Even so, this mysterious phenomenon stimulated great interest among astrophysicists, leading to numerous conferences and hundreds of publications on the subject, including a spectacular proliferation of theories. More than 100 different possible models were proposed; however, none could be ruled out directly by observations. Stage Two: From the launch of Compton Gamma Ray Observatory to the launch of Beppo-SAX ( ). The first significant steps in understanding GRBs started with the launch of the Compton Gamma-Ray Observatory, whose discoveries are summarized in Fishman & Meegan (1995). The all-sky monitoring of the BATSE instrument showed that bursts were isotropically distributed, yet not as a homogeneous population in Euclidean space, strongly suggesting a cosmological distribution. At cosmological distances the observed GRB fluxes imply enormous energies, which, from the fast time variability, must arise in a small volume in a short time. This leads to the formation of e ± γ fireball, which expands relativistically. The main difficulty with this scenario was that a expanding fireball would convert most of its energy into kinetic energy of accelerated baryons (rather than into photon energy) and would produce a quasi-thermal spectrum. This situation stimulates the emergence of the fireball shock scenario (Rees & Mészáros 199), that is, when the fireball ejecta runs into the external circumburst medium, after the fireball has become optically thin, it will reconvert the expansion kinetic energy into non-thermal energy. In this scenario, the complicated light curves can also be understood in terms of internal shocks generated by collision of different

13 shells moving with high relative velocities. Another contribution at this stage was the identification of the Band function of GRB burst spectra. Finally, the classification of bursts into short and long populations was confirmed definitively within this period. Stage Three: From the launch of Beppo-SAX to the Swift satellite ( ). The Italian-Dutch satellite Beppo-SAX was launched in April 1996, and made its most important contribution beginning in early 1997, when it began regularly to detect the X-ray counterparts to gamma-ray bursts, localize them to few-arcmin precision, and then follow-up with focusing X-ray telescopes to refine the positions and discover, and study, their fading X-ray afterglows. These discoveries, and the much-improved positions for GRBs that they derived, led to follow-up at optical and radio wavelengths (van Paradijs et al. 1997). This facilitated the determination of the distances, the identification of candidate host galaxies, and the confirmation that GRBs are indeed coming from cosmological distances (Djorgovski et al. 1998; Kulkarni et al. 1998a). In addition, the study of afterglows has given a strong confirmation for the fireball shock model of GRBs. This model led to a prediction, in advance of the observations, of the quantitative nature of the afterglows, which were found to agree with the observational data. Shortly following the Beppo-SAX discoveries, the launch of the HETE- satellite provided burst positions comparable in quality to those of Beppo-SAX, but on a much more rapid timescale. Both satellites contributed to the characterization of a new class of sources called X-ray Flashes (Heise et al. 001), which appear to be a less-luminous and lower-redshift population than the traditional GRBs. HETE- was responsible for the discovery of GRB (Hjorth et al. 003), which resulted in the first unambiguous association of a GRB with a supernova, SN003dh, demonstrating finally that long-duration GRBs are associated with the deaths of massive stars without hydrogen envelopes. Stage Four: After the launch of multi-wavelength satellite Swift (004 present). As mentioned above, typical response times for GRB follow-up observations, prior to Swift, were in the range of hours, especially at X-ray energies. Swift, a multi-wavelength satellite with burst-detection (BAT) and afterglow observation capabilities (the X-ray and UV/optical telescopes) onboard a single satellite, was designed to accurately localize the burst, observe the afterglow within minutes, and provide a cascade of progressively more-accurate positions over the course of the first ten minutes after each burst. Indeed, Swift has been a great success. Swift observations revealed unusual afterglow behavior, including bright X-ray flaring activity, in the previously unexplored regime during the first hours after the burst, and enabled detailed studies of the transition from prompt to afterglow emission. It detected the first afterglow of a short GRB, a milestone for short burst research. Finally, it detected GRB050904, the most distant cosmic explosion and the first GRB from beyond the epoch of reionization. The many other accomplishments of Swift are too many to list here; the interested reader is referred to recent reviews for more details (Mészáros 006; Zhang 007a). 1. GRB Classification and Progenitors One of the basic questions to GRBs is the classification, i.e. how many intrincially different categories they have. From the burst sample collected by BASTE onboard the Compton Gamma-Ray Observatory (CGRO), a clear bimodal distribution of bursts

14 was identified (Kouveliotou et al. 1993). The primary criterion is the duration which is based on the bimodal distribution of the GRB sample collected by BASTE onboard CGRO. The dividing line at seconds was adopted to separate the double-hump duration distribution of the BATSE bursts. Hardness is another criterion to distinguish these two different classes, defined as the hardness ratio of the two energy bands of the detector. On average, the short GRBs are harder, while the long GRBs are softer. So these two distinct populations of bursts are long-soft GRBs and short-hard GRBs. When it comes to the progenitor problem, we should naturally expect two different progenitors for each kind of GRBs. Afterglow observations also revealed the nature of these two disctinct class of bursts. Beginning in 1997 with the localization of longwavelength counterparts, it has become clear that long-duration GRBs were associated with the young stars in distant actively star-forming galaxies, or irregular dwarf galaxies. In addition, among the observed long GRBs with a number of more than 00 up to now, several unambiguous associations between GRBs and Type Ib/c supernovae have been detected, which include GRB 98045/SN 1998bw at z = (Galama et al. 1998; Kulkarni et al. 1998b), and GRB 03039/SN 003dh at z = (Hjorth et al. 003), etc. These evidence strongly suggests that most, even if not all, long GRBs are produced by the collapse of massive stars, dubbed collapsar. For short GRBs, Swift observations have revealed a different picture due to a series of discoveries of short GRB afterglows (GRB B at z=0.6; GRB at z=0.1606; GRB at z=0.58). Up to now, a total number of more than 10 afterglows for short bursts has been discovered. The common message infered from these observations is that short GRBs are different from long GRBs. Most of the afterglows are either in elliptical galaxies or far away from the star forming regions even if two are found to be associated with the star-forming galaxies. These observaitons are consistent with long-held speculation that short GRBs are associated with mergers of compact objects, such as neutron-star - neutron star (NS/NS) mergers, or neutron star- black hole (NS/BH) mergers. No smoking gun has been discovered for the progenitor of short GRBs. However, merger of compact objects can produce strong gravitational wave during the phase of coalescence, the ongoing instrument LIGO II and the upcoming space instrument LISA might possibly detect these radiation and provide a direct test to the model. 1.3 Physics of GRBs and Afterglows The leading model to explain the current GRB prompt emission and afterglow emission is the generic fireball shock model which involves a relativistically expanding ejecta. According to the model, the ejecta is composed of many shells with a wide range of bulk Lorentz factors. Due to the Lorentz factor variations, i.e. velocity variations, internal shocks are produced before the global fireball (mini-shells are merged into one big global shell) is decelerated by the circumburst medium, and they are believed to be responsible for the observed prompt emission. The fireball is later decelerated at a larger distance after accumulating enough material whose intertia is not negligigle and the blashwave hence enters a self-simlar evolution regime (Blandford & Mckee 1976). Upon deceleration, a pair of shocks, reverse and forward shocks, are produced. In the rest frame of the shell, the reverse shock is moving backward and the forward shock is 3

15 moving forward. The forward shock gives rise to the long-term afterglow covering from gamma-ray, to the X-ray and optical bands, and a short-lived reverse shock is responsible for a possible optical/ir flash and radio flare. Two basic radiation mechanisms have been considered existing in prompt and afterglow emission: (1) Synchrotron. If one looks closely at the spectrum of GRB prompt emission or afterglow, the spectra are clearly non-thermal. The most likely mechanism is synchrotron, i.e., the radiation from the relativistic electrons gyrating in random magnetic fields. Although there is some debate on whether the GRB prompt emission is produced by synchrotron radiation (please note that the thermal component has been found existing in the prompt emission spectra for some bursts), there is no doubt that the dominant radiation mechanism for the afterglow is synchrotron, especially at sub- MeV energies. The synchrotron radiation predicts that the expected spectrum should be composed of several broken power-law segments, and the observed lower-energy (e.g. X-ray, optical) spectra indicates that this is indeed the case. () The inverse-compton scattering. The electrons are relativistic in the shell, and the seed synchrotron photons can interact with electrons, resulting in a high-energy emission. Inverse Compton process might be a dominant mechanism in the GeV or even higher energy band. Up to now, there are only a limited number of observations of prompt and delayed afterglow emission in GeV band from the EGRET onboard CGRO satellite. The only running space telescope in GeV band is AGILE, which is a Italian project and is launched in April 007. AGILE hasn t had observed any high energy emission from either GRB itself or afterglows since then. 1.4 Light Curve of Afterglows X-ray Light Curve The Swift observations show that the GRB X-ray light curves are much more complicated that before, and many new features have been found. Below is a complete description of the light curve, and it consists of six segments (also see Zhang 007a): (1) the prompt emission, which is produced by the internal shocks; () the sharp decay phase, which is the decaying phase of prompt phase. At this point, the central engine has stopped, but high latitude emission on the same shell continue to radiate but comes at a later time. (3) the shallow decay phase; it is still not clear about the mechanism of generating the radiation in this phase although several leading models are proposed. One of the models is the energy injection model, and the other one is the parameter evolving model, which argues that the electron equipartition parameter ǫ e starts from a smaller value and keep increasing until some value and stay the same after the end of the shallow decay phase. (4) the normal decay phase; the reason why we call it normal is because it is one of the expected behaviors and has been seen in the BeppoSAX afterglows again and again. The radiation in this phase is thought to be from the real afterglow and its behavior has been well studied since the first afterglow was detected. (5) the jet break phase; it is the other one of the two expected phases from the BeppoSAX era. Even though, the existence of the phase is challenged by the Swift observations because up to end of 006, more than 00 bursts have been observed and only less than 10% of the 4

16 bursts show the jet break signature and most of them don t show any jet break at all even if the Swift X-ray observations usually last several days after the burst. This is also called the jet break paucity problem (Burrows & Racusin 007). (6) the X-ray flares. After the prompt emission phase, some X-ray fluctuations have been observed as well, mixing with Phases () and (3) most time. The fluctuations are normally attributed to be the continuous activity of the central engine Optical and Radio Light Curve Before Swift, most of the optical/radio light curves are pretty simple, usually consisting two segments: normal decay phase and the jet break phase. After the launch of Swift the onboard UVOT telescope has been pusing hard to collect the optical photons 100 seconds after the burst for most GRBs, however, most bursts have very dim or undetectable optical afterglows (Roming et al. 006). In some cases, early-time flares were observed in optical and radio bands (e.g. GRB 99013, Akerlof et al. 1999; GRB 01004, Fox et al. 003b; GRB 0111 Fox et al. 003a,Li et al. 003), which were usually interpreted as the emission from the reverse shock(sari & Mészáros 000; Mészáros & Rees 1999; Kobayashi & Zhang 003; Zhang et al. 003). Because it shares the similar temproal behavior, to some extent, with the wiggles and bumps seen in both X-ray and optical afterglows which is due to the late-time central engine activity, the efforts to model optical flares using the internal shock have been carried out recently as well (Wei et al. 006a; Wei 007). 1.5 GRB Cosmology GRBs are cosmological events and there is a bright future for applications of GRBs to the comological study (Bromm & Loeb 007). First, GRBs provide probes of the star formation rate and metallicity history in the high-redshift universe due to several advantages over the traditional cosmic sources such as quasar: (1) The GRB afterglow flux at a given observed time after the burst is not expected to decay substantially with increasing redshift, since higher redshift means the earlier time in the source frame, i.e., time-dilation effect, during which the afterglow is intrinsically brighter. For standard afterglow light curves and spectra, the increase in the luminosity distance with redshift is compensated by this cosmological time-stretching effect, or k correction factor. () GRB afterglows have smooth and simple continuum spectra unlike quasars which show strong spectral features that complicate the extraction of IGM absorption features. (3) GRBs are expected to originate in the star-forming galaxies, which is most time low mass at a given redshift in contrast to the host galaxies of quasars (Barkana & Loeb 004). The low-mass host galaxies only have a weak ionization effect on the surrounding IGM, namely there is no proximity effect. Hence the observed Lyα damping wing would be closer to the unperturbed IGM and detailed spectral shape will be easier to interpret (Miralda-Escudé 1998). GRBs also provide great oportunity to discover proto-galaxies at high redshifts. In the standard CDM cosmology, galaxies form hierarchically, starting from small masses and increasing their average mass with cosmic time. Hence, the typical mass of a galaxy 5

17 were expected to be smaller at higher redshifts, making these sources intrically fainter than the ones at low redshift. However, GRBs are believed to be originated from a stallar mass progenitor and so the intrinsic luminosity of their engine shouldn t depend on the mass of their host galaxy. GRB afterglows are thus expected to outshine their host galaxies by a factor that gets larger with increasing redshift. Once the afterglows fade away, one may search for their host galaxies. Hence GRBs may serve as a guide for the high-redshift dwarf galaxies that are too faint intrinsically to find them. Thirdly, GRBs can be used to constrain the cosmological parameters, provided they are standardizable candles which invokes a narrowly-clustered tight correlation between the peak energy of the burst and the isotropic/geometrically-corrected energy. Several tight correlations have been investigated (Ghirlanda et al. 004; Liang & Zhang 005) and the GRB cosmology is starting to make progress. 1.6 Motivation of the Thesis As mentioned above, it is believed that the progenitor of the long-duration GRBs are resulted from the collapse of massive stars, dubbed collapsar. First stars, form at z 0 are expected to be very massive stars whose typical mass is over 100 M. Therefore, provided that first stars can trigger GRBs, bursts and their afterglows are expected to be at very high redshift. At this point the interesting question naturally arises as to whether it is possible to detect these high-redshift bursts and afterglows with the current instruments or future satellites? That is our basic motivation to estimate the GRB afterglow detectability by X-ray and near-infrared telescopes. The UV/optical radiation has been totally absorbed by neutral hydrogen in IGM. Following the similar motivation, we calculate the GeV afterglow emission detectability by Agile and Glast because high energy emission tells us more information about the sites much closer to the progenitor. Although a similar work has been done by Zhang & Mészáros (001), the peak flux ratio of IC to synchrotron was overestimated by a factor of 10. Unlike the low energy part of the spectrum, the inverse-compton scattering will play an important role at the high end of the spectrum, rather than synchrotron. In addition, we have incorporated some new elements (e.g. energy injection) into our model. Detection of GRBs and afterglows is only the preliminary step. Without the redshift distance to a GRB, one can not extract useful information on the burst and the burst environment. The traditional method of determining a redshift is the optical spectroscopy. However, observation shows that 90% of the afterglow is detected in the X-ray and only half of the afterglows is seen in the optical. Therefore, although a burst itself can be easily detected through its prompt emission, there is a small chance that its redshift can be determined through its optical afterglow. There are several reports of possible Fe line, one of the strongest lines in X-ray band, from the BeppoSAX, ASCA, or Chandra observations. If one can use it as a standard method, the chances of determining the redshift will be greatly increased. This is our purpose of starting the project on iron line detectability as described in Chap 4. We also made a detailed modeling to GRB which has the highest redshift so far z = 6.9, exploring the parameter space with two independent models. Except its high redshift, the burst has the most complete data set so far which covers from the 6

18 X-ray to the NIR, to the radio, so it provides a great oppurtunity to investigate the properties of the burst, which can be served as a template for the high-redshift GRBs. In addition, it provides us a chance to compare the properties of high-redshift GRBs with the low-redshift ones. 1.7 Outline of the Thesis The Thesis consists of the theoretical prediction to the detectability of highredshift GRBs with different instrument, and the modeling to GRB First, we look into the detectability of long GRB afterglows from very high redshifts in the X- ray and near-infrared bands. Then, incorporating some new GRB features into the theoretical models, the possibility of detecting GeV emission from afterglows has been investigated. Thirdly, assuming a strong Fe emisson line superposed on the X-ray continuum, we investigate the possibility of determine redshift from the Fe line. Finally, GRB with the highest redshift is modelled and its parameter space is explored Chapter : Detectability of Long GRB Afterglows from Very High Redshifts Gamma-ray bursts are promising tools for tracing the formation of high redshift stars, including the first generation. At very high redshifts the reverse shock emission lasts longer in the observer frame, and its importance for detection and analysis purposes relative to the forward shock increases. We consider two different models for the GRB environment, based on current ideas about the redshift dependence of gas properties in galaxies and primordial star formation. We calculate the observed flux as a function of the redshift and observer time for typical GRB afterglows, taking into account intergalactic photoionization and Lyman-α absorption opacity as well as extinction by the Milky Way Galaxy. The fluxes in the X-ray and near IR bands are compared with the sensitivity of different detectors such as Chandra, XMM, Swift XRT and JWST. Using standard assumptions, we find that Chandra, XMM and Swift XRT can potentially detect GRBs in the X-ray band out to very high redshifts z > 30. In the K and M bands, the JWST and ground-based telescopes are potentially able to detect GRBs even one day after the trigger out to z 16 and 33, if present. While the X-ray band is insensitive to the external density and to reverse shocks, the near IR bands provide a sensitive tool for diagnosing both the environment and the reverse shock component Chapter 3: GLAST Prospects for Swift-Era Afterglows We calculate the GeV spectra of GRB afterglows produced by inverse Compton scattering of the sub-mev emission of these objects. We improve on earlier treatments by using refined afterglow parameters and new model developments motivated by recent Swift observations. We present time-dependent GeV spectra for standard, constant parameter models, as well as for models with energy injection and with time-varying parameters, for a range of burst parameters. We evaluate the limiting redshift to which such afterglows can be detected by the GLAST LAT, as well as AGILE. 7

19 1.7.3 Chapter 4: Detectability of GRB Iron Lines by Swift, Chandra and XMM The rapid acquisition of positions by the upcoming Swift satellite will allow the monitoring for X-ray lines in GRB afterglows at much earlier epochs than was previously feasible. We calculate the possible significance levels of iron line detections as a function of source redshift and observing time after the trigger, for the Swift XRT, Chandra ACIS and XMM Epic detectors. For bursts with standard luminosities, decay rates and equivalent widths of 1 kev assumed constant starting at early source-frame epochs, Swift may be able to detect lines up to z 1.5 with a significance of > 3σ for times t< 104 s. The same lines would be detectable with > 4σ significance at z < 6 by Chandra, and at z < 8 by XMM, for times of t< 105 s. For similar bursts with a variable equivalent width which peaks at 1 kev between 0.5 and 1 days in the source frame, Swift achieves the same significance level for z 1 at t 1 day, while Chandra reaches the previous detection significances around t 1 days for z 4, i.e. the line is detectable near the peak equivalent width times, and undetectable at earlier or later times. For afterglows in the upper range of initial X-ray luminosites afterglows, which may also be typical of pop. III bursts, similar significance levels are obtained out to substantially higher redshifts. A distinction between broad and narrow lines to better than 3σ is possible with Chandra and XMM out to z and 6.5, respectively, while Swift can do so up to z 1, for standard burst parameters. A distinction between different energy centroid lines of 6.4 kev vs. 6.7 KeV (or 6.7 kev vs. Cobalt 7. kev) is possible up to z < 0.6, 1., and ( z < 1, 5, 7.5), with Swift, Chandra, and XMM respectively. For the higher luminosity bursts, Swift is able to distinguish at the 5σ level between a broad and a narrow line out to z < 5, and between a 6.7 kev vs. a 7. kev line center out to z < 5 for times of t< 104 s Chapter 5: Modeling GRB : Autopsy of a Massive Stellar Explosion at z = 6.9 GRB at redshift z = 6.9, discovered and observed by Swift and with spectroscopic redshift from the Subaru telescope, is the first gamma-ray burst to be identified from beyond the epoch of reionization. Since the progenitors of long gammaray bursts have been identified as massive stars, this event offers a unique opportunity to investigate star formation environments at this epoch. Apart from its record redshift, the burst is remarkable in two respects: first, it exhibits fast-evolving X-ray and optical flares that peak simultaneously at t 470 s in the observer frame, and may thus originate in the same emission region; and second, its afterglow exhibits an accelerated decay in the near-infrared (NIR) from t 10 4 s to t s after the burst, coincident with repeated and energetic X-ray flaring activity. We make a complete analysis of available X-ray, NIR, and radio observations, utilizing afterglow models that incorporate a range of physical effects not previously considered for this or any other GRB afterglow, and quantifying our model uncertainties in detail via Markov Chain Monte Carlo analysis. In the process, we explore the possibility that the early optical and X-ray flare is due to synchrotron and inverse Compton emission from the reverse shock regions of the outflow. We suggest that the period of accelerated decay in the NIR may be due to suppression of synchrotron radiation by inverse Compton interaction of X-ray flare photons with 8

20 electrons in the forward shock; a subsequent interval of slow decay would then be due to a progressive decline in this suppression. The range of acceptable models demonstrates that the kinetic energy and circumburst density of GRB are well above the typical values found for low-redshift GRBs Chapter 6: Conclusion We have summarized our main results of the Thesis in this chapter. 9

21 10 Chapter Detectability of Long GRB Afterglows from Very High Redshifts This chapter is reproduced from the published paper: L.J. Gou, P. Mészáros, T. Abel, and B. Zhang, The Astrophysical Journal, Vol. 604, P. 508 (004).1 Introduction Gamma-ray bursts are thought to be associated with the formation of massive stars (van Paradijs et al. 000). The evidence for this has been mainly in the class of long bursts, of γ-ray durations in excess of seconds, making up two-thirds of the GRB population, which are the only ones so far for which X-ray, optical, IR and radio afterglows, as well as redshifts, have been measured. The strongest evidence yet comes from the recently confirmed association of long GRBs with core-collapse supernovae (Stanek et al. 003; Hjorth et al. 003; Uemura et al. 003; Price et al. 003). Short bursts, of durations less than seconds, even if produced e.g. by neutron star mergers, would similarly be associated with massive star formation, and one expects the rate of occurrence of GRBs with redshift to follow closely the massive star formation rate. In currently favored ΛCDM cosmologies, star formation should start at redshifts higher than those where proto-galaxies and massive black holes at their centers develop (Miralda- Escudé 003). Thus, GRBs could trace the pre-galactic star formation era preceding quasars. Recent cosmic microwave background anisotropy data collected by WMAP reveal that the first objects in the Universe should be formed around z 18 (Bennett et al. 003). This is consistent with the theoretical modeling of the first star formation (Abel et al. 1998, 000, 00; Bromm et al. 1999). There is also indirect observational evidence for high-z GRBs. E.g., empirical relations have been found between the GRB luminosities and other measured quantities, such as the variability of the gamma-ray light curves (Fenimore & Ramirez-Ruiz 000) and spectral lags (Norris et al. 000). By extrapolating these empirical laws to a larger burst sample (e.g. the BATSE data), it is found that many BATSE bursts would be expected to have z > 6 (Fenimore & Ramirez-Ruiz 000). The discovery of the highest redshift quasars, such as the current record holder at z = 6.43 (Fan et al. 003), grows increasingly difficult because the quasar formation rate drops rapidly at higher redshifts, peaking between redshift and 3. Very few galaxies can be seen above z > 6, which is also consistent with the upper limit for the redshift of galaxy formation z gal 9 based on theoretical analysis (e.g. Padmanabhan 000). Although

22 young galaxies may exist at very high redshifts, they are likely to be too faint to obtain good spectra (Haiman & Loeb 1997). On the other hand, the extreme brightness of GRBs during their first day or so make them the most luminous astrophysical objects in the Universe. Thus, GRBs appear to be promising tools to explore the very high redshift Universe (Miralda-Escudé 1998). The natural question which needs to be quantified is the degree of detectability of GRBs with current or future detectors, if they occur at much higher redshifts than those currently sampled. Lamb & Reichart (000) used specific templates such as GRB 9708 observed at one day to estimate the highest redshifts at which such bursts could be observed using Swift. Ciardi & Loeb (000) calculated the flux evolution with redshift of common GRBs and discussed the flux change with redshift at several epochs in the infrared bands. These papers considered only forward shock radiation as known before 000 and some effects of the galactic mean density evolution but did not consider the primeval star-formation environment. In this Chapter we have calculated the flux evolution of typical GRBs based on current knowledge about GRB physics in a more realistic way. Among the refinements introduced are: (1) The contribution from reverse shocks is considered as a crucial element. This should be very important for the early afterglow in the rest-frame (which at high redshifts gets dilated to longer observed times). Therefore, we expect that at higher redshifts the possibility of observing the reverse shock is much increased. () We have taken up to date GRB parameters, e.g. incorporating new estimates of the typical magnetic equipartition parameter ǫ B about one order or more magnitude smaller than the electron parameter ǫ e in the forward shock, and a possibly higher ǫ B in the reverse shock. This has a significant effect on the GRB evolution. (3) We consider GRB external densities motivated both by views on the typical protogalaxy density evolution with redshift, and by views on the conditions around the first stars to form in the universe in the pre-galactic era. (4) We consider both the Lyman-α and photoionization absorption as well as our own galactic extinction. (5) We compare the expected fluxes in the X-ray and near IR bands to the sensitivity of various detectors such as Chandra, XMM, Swift XRT and JWST. In.1 and. we outline the basic forward and reverse shock flux calculations, the details of which are given in an appendix (see Appendix [A]). We discuss the GRB density environment in.3, and the intergalactic and galactic absorption effects are estimated in.4. In 3.1 and 3. we discuss the optical/ir and X-ray flux dependence on redshift, respectively, at various observer times, including the dependence on external density. We compare these to the Swift XRT, Chandra, XMM and JWST sensitivities for the detection of GRBs at different redshifts. We summarize the numerical results and discuss the implications in 4.. Afterglow characteristics..1 Forward Shock We assume that the shock-accelerated electrons have a power-law distribution of Lorentz factors γ e with a minimum Lorentz factor γ m : N(γ e )dγ e γ p e dγ e,γ e γ m. 11

23 We also define a critical Lorentz factor γ c above which the electrons cool radiatively on a time shorter than the expansion time scale (Mészáros et al. 1998). This leads to the standard (forward shock) broken power law spectrum of GRBs (Sari et al. 1998). In the fast-cooling regime, when γ m > γ c, all the electrons cool rapidly down to a Lorentz factor γ c and the observed flux at frequency ν is (ν/ν c,f ) 1/3 ν < ν c,f F ν = F ν,m,f (ν/ν c,f ) 1/ ν c,f ν < ν m,f (.1) (ν m,f /ν c,f ) 1/ (ν/ν m,f ) p/ ν m,f ν Hereafter the subscripts f and r indicate forward and reverse shock, respectively. In the slow-cooling regime, when γ c > γ m, only electrons with γ e > γ c cool efficiently, and the observed flux is (ν/ν m,f ) 1/3 ν < ν m,f F ν = F ν,m,f (ν/ν m,f ) (p 1)/ ν m,f ν < ν c,f (.) (ν c,f /ν m,f ) (p 1)/ (ν/ν c,f ) p/ ν c,f ν where F ν,m,f is the observed peak flux at the observed frequency ν = min(ν c,f,ν m,f ), while ν m,f and ν c,f are the observed frequencies corresponding to γ m and γ c, respectively. Synchrotron self-absorption can also cause an additional break at very low frequencies, typically about 5 GHz, in the radio range. Since here we focus on the IR and X-ray ranges, we will not consider this low-frequency regime in our calculations. For a fully adiabatic shock, the evolution of the typical frequency and peak flux are given by Sari et al. (1998): 1 ν c,f = ǫ 3/ B,f E 1/ 5 n 1 t 1/ 5 (1 + z) 1/ Hz, (.3) ν m,f = ǫ 1/ B,f ǫ e E1/ 5 t 3/ 5 (1 + z) 1/ Hz, (.4) F ν,m,f = ǫ 1/ B,f E 5n 1/ D 8 (z) (1 + z) µjy. (.5) Hereafter the quantities without the subscript s are in the observer frame, and the quantities with the subscript s are for the observer in the local frame of the source, which is connected with the observer frame quantities with a certain power of (1 + z). The source is assumed at a luminosity distance D L (z) = 10 8 D 8 (z) cm, and ǫ B and ǫ e are the fraction of the shock energy converted into energy of magnetic fields and accelerated electrons, respectively. The time is taken in units of t = 10 5 t 5 s ( 1 day), E 5 = E/10 5 ergs is the isotropic equivalent energy of the GRBs, and n is the particle density in units of cm 3 in the ambient medium around the GRB... Reverse Shock As GRBs are measured at increasingly larger redshifts, a given constant observer time corresponds to increasingly shorter source frame times. This is favorable for observing at very high redshifts the evolution of phenomena which happen only in the earliest