New Astronomy Reviews 52 (2008) 450–453
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Probing cosmic chemical evolution with GRBs Dieter H. Hartmann * Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
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Article history: Available online 26 June 2008 PACS: 98.70.Rz Keywords: Gamma Ray Bursts Cosmology
a b s t r a c t The association of long-duration Gamma Ray Bursts with the ﬁnal stages of the evolution of massive stars has been established with photometry, via late ‘‘bumps” in their optical afterglows, and several cases of direct spectroscopic evidence. The link to massive stars offers their bright afterglow emission as a perfect tool for absorption line spectroscopy of their host galaxy environments and any material along their lines of sight. With typical redshifts of z 2, and a present record of z = 6.3, it is clear that their afterglows offer a very powerful tool to probe cosmic chemical evolution, to the earliest epochs of star formation, to the epoch of reionization by population III stars and accretion onto rapidly growing black holes. Fast afterglow decline does require a rapid response with very sensitive spectrometers on large aperture telescopes, which is a challenge for current astronomical resources – but rewards are correspondingly high. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Investigations of the formation and evolution of cosmic structures and the chemical abundances of stars and the interstellar- and intergalactic medium are at the frontier of current observational astrophysics. Not too long ago, the most distant objects known were quasars. However, sensitive surveys with space- and ground-based telescopes now allow us to ﬁnd ordinary galaxies beyond the farthest quasars. The race is on for the most distant stars and their explosive deaths events that drive the cosmic cycle of chemical enrichment. Tomography of the structured universe with bright background light sources, such as quasars and Gamma Ray Bursts (GRBs), probes the large-scale distribution and physical state of coupled gas–star–dark matter systems underlying the basic building blocks of the universe. Only about half of the baryons in the local universe are currently accounted for. The rest probably resides in warm–hot tenuous gas tracing underlying dark matter. Studies of cosmic chemo-dynamics are advanced with sensitive X-ray studies of the diffuse baryonic components in the warm– hot-ionized medium (WHIM) in clusters, and between clusters in the amazing structures of the cosmic web. The dark side of the universe and its more familiar luminous side are coupled. Probing the links between these aspects leads to a better understanding of how the universe transitioned from the dark ages to the present. Radioactive isotopes like 26Al and 60Fe, ejected during the explosions and prior by strong winds of their progenitors, trace the local, galactic star formation activity. On a larger scale, c-rays from Supernovae produce a cumulative background in the MeV regime, which can * Tel.: +1 864 656 5298. E-mail address: [email protected]
1387-6473/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2008.06.028
be used to investigate the global star formation history of the universe. Directly tracing cosmic chemical evolution back in time to the very ﬁrst stars is an even more daunting task. Gamma Ray Bursts offer a unique way to accomplish this goal. The long-soft class of GRBs is believed to be produced during the ﬁnal stages of the evolution of some massive stars, when an accretion disk forms around a black hole and enormous energy release in its vicinity drives ultra-relativistic jets through the star along its rotation axis. Short-hard GRBs are also believed to be associated with a black hole – accretion disk – jet system, but with merging binary systems of compact stars (e.g., a neutron star binary) as progenitors. Both of these source classes trace, more or less directly, the cosmic star formation history, and thus offer a unique probe of the high-redshift Universe. Regardless of the nature of their central engines, and progenitors, their high luminosities allow easy detection, even at very large distances, and their optical afterglows provide a (rapidly decaying) source of light that offers unique opportunities for spectroscopic studies of the intervening intergalactic gas. 2. Gamma Ray Bursts Gamma Ray Bursts (GRBs) and their afterglows at lower photon energies are amongst the most luminous electromagnetic signatures of black hole formation. GRBs occur at a global, cosmic rate of about one per day, which should be contrasted with the global rate of a few core-collapse Supernovae (ccSNe) per second. Their very large ﬂuxes are easily detected above the Earth’s atmosphere, and if localized accurately and rapidly their low-energy afterglows can be studied from the ground even with small aperture (sub 1 m) telescopes. Afterglows can serve as powerful tools to probe
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properties of the gas along the line of sight, but requires spectroscopy to be carried out quickly after the trigger in the c-band. The need for rapid target of opportunity response and sustained monitoring distinguishes GRB follow-up programs from the usual procedures of the astronomical community, and requires a dedicated worldwide effort. After their discovery in the 1960s (Klebesadel et al., 1973) with the Vela satellites, the following decades provided much insight into their prompt c-ray properties (spectra and light curves) and established their isotropic angular distribution on the sky. However, they remained undetected at wavelengths other than those of the c-ray regime. The lack of accurate and rapid directional information proved to be the bottleneck. The vibrant era of the Compton Gamma-Ray Observatory (1991–1999), with eight dedicated instruments (BATSE) and several other detectors (OSSE, COMPTEL, and EGRET) covering the keV–GeV range of energies, demonstrated that bursts are in fact highly isotropic on the sky, strongly suggesting their cosmological origin (Meegan et al., 1991), and conﬁrmed some earlier indications of a bimodal duration distribution, suggesting multiple model classes (Kouveliotou et al., 1993). Breakthroughs came with BeppoSAX (1996–2002), for the ﬁrst time providing rapid, accurate localization (hours after the event, and less than a few arcminutes). In March 1997, BeppoSAX discovered the X-ray afterglow of GRB970228 (Costa et al., 1997), which led to the discovery of the ﬁrst optical transient (van Paradijs et al., 1997) (for a review see van Paradijs et al., 2000). These developments paved the way for measurements of actual distances (via redshift) and identiﬁcation of host galaxies. Their large ﬂuences and distances imply that GRBs release tremendous amounts of energy (100 B; where 1 B = 1 Bethe = 1051 ergs). This is the right scale for core-collapse Supernovae (ccSNe), but there most of this energy is in the form of neutrinos and gravitational waves (both forms of energy have not yet been associated with GRBs by direct observations). For GRBs comparable amounts of energy appear to be involved, but emerging in the gamma band! This ‘‘energy challenge” found its resolution in the fact that GRBs do not emit isotropically, but are highly beamed. The sudden release of such large amounts of energy leads to a ﬁre-‘‘ball”, which is opaque to its own radiation and is accelerated to highly relativistic outﬂow speeds (Lorentz factors C 100). Internal dissipation of energy via shocks of colliding ‘‘shells” lead to the production of c-rays, observed as the prompt GRB phase, but a signiﬁcant fraction remains in the form of bulk motion (kinetic energy) of cold material. Eventually these coasting ejecta interact with a circum-GRB medium via external shocks, and thereby generate afterglow emission that can be detected for a long time. The hydrodynamics of these relativistic outﬂows with the assumption of synchrotron (or jitter-) radiation being the dominant radiation process, led to a simple afterglow model for the observed ﬂux density; fm / ta, where the power law index is usually around a = 1 and time (t) is measured in days since the GRB (for reviews of the basic scenarios, see, Meszaros, 2002, 2006; Piran, 2004; Zhang, 2006). This emission covers a broad range of the spectrum and thereby provides a powerful background light source for absorption studies of gas along the line of sight. The eventual break in the time evolution (a jet-break, resulting from a combination of outﬂow geometry and hydrodynamics) yields information on the jet opening angle, and thus yields a correction of the isotropic energy release, Eiso, inferred from the ﬂuence and the luminosity distance, DL(z). The jet opening angles turned out to be small, of order of degrees, which results in a smaller energy scale of order 0.5 B, with a relatively narrow distribution function (Frail et al., 2001). The ‘‘simple model” of relativistic, jetted outﬂows has been conﬁrmed with the observation of achromatic jet-breaks across the optical-IR regime,
but the lack of expected corresponding breaks in the X-ray regime, or their very late appearance (Burrows and Racusin, 2007), has challenged this picture and is likely to induce reﬁnements and modiﬁcations of the models, and perhaps even a paradigm shift (Dado et al., 2008). However, regardless of the details of the models for afterglow emission, analysis of their infrared-optical-UV properties allow us to probe the interstellar medium in the vicinity of the GRBs and, on a larger scale, the ISM of the host galaxies as well as the intergalactic medium along their lines of sight. Spectroscopy of GRB afterglows has shown that many site lines show evidence for large neutral hydrogen column densities (>1020 cm2, which classiﬁes them as Damped Lyman Alpha, DLA, systems). Metal absorption lines in the spectra indicate abundances that are typically less than solar and provide some evidence for a trend of cosmic chemical evolution (Savaglio, 2006). However, it is important to note that the present-day sample is still very small and that the quality of the spectra is often insufﬁcient for accurate abundance measurements. Furthermore, to fully utilize GRBs as probes of cosmic chemical evolution the community must signiﬁcantly improve the resources (the network of telescopes and their associated imagers and spectrographs) allocated to this task. For Swift the available resources are often insufﬁcient to provide appropriate follow-up, so with new generations of GRB detectors, e.g., EXIST (Grindlay, 2006), the trigger rate could rise to about one per day, which current facilities (and human resources) would not be able to fully take advantage of. Of the GRBs detected by Swift, more than 80% were localized with the onboard X-ray telescope (XRT), and for 40% of these events optical afterglow observations led to a measured redshift, i.e., for only a quarter of the entire burst sample. After discovery of the ﬁrst afterglows and follow-up of their light curves, there was the expectation that afterglows could be detectable at R 20 within a day of the trigger. But it turned out that the ﬁrst optically identiﬁed afterglows were near the bright end of the brightness distribution. There is still no consensus on the reason(s) for the non-detections of an optical afterglow for a large fraction of GRBs. Possible explanations include a broad luminosity function of the afterglows, absorption in the interstellar medium within the GRB host, and a signiﬁcant fraction of high-z GRBs (Lamb and Reichart, 2000; Bromm and Loeb, 2002), thus being affected by Lyman absorption in the optical bands by intervening intergalactic clouds. In the latter case ‘‘dark” GRBs may simply represent a very distant population. Swift is detecting GRBs at hzi 2.7 (Jakobsson et al., 2006) and it seems that the z-distribution is roughly consistent with models where the GRB rate traces the cosmic star formation rate (Jakobsson et al., 2005). To understand whether bursts are dark due to extinction, Kann et al. (2006) investigated the SEDs of a signiﬁcant sample, and showed that typically only a modest amount of extinction (<0.4 mag) is present in hosts. This is surprising, as the association of long-duration GRBs with massive stars, Woosley and Bloom (2006) suggests that their afterglows should be affected by extinction due to dust in the star forming region in which they occur. Observations with HST (Fruchter et al., 2006) show that GRB sites are indeed associated with star forming regions in small, sub-luminous, irregular galaxies. After nearly 10 years of afterglow research, the highest redshift burst (in which the Lyman drop-out in the optical bands was established) is GRB 050904 at z = 6.29 (Kawai et al., 2006), which was originally identiﬁed as a high-redshift candidate based on multi-color observations (Haislip et al., 2006). More recently, GRB060927 (z = 5.47) provided another example of a GRB population that exploded near the end of the re-ionization epoch, when the Universe ﬁnally emerged fully from the dark ages (Fan et al., 2006). Finding high-z GRBs beyond z = 6 is the goal of special ground-based projects, like the multi-band photometer
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GROND (Greiner et al., 2008), and future GRB missions, like EXIST (Grindlay, 2006) and CASTER (McConnell, 2006). 3. Tracing – CCE with GRBs While Supernovae and galaxies are bright enough to be traced deep into the universe, the diffuse gas in their vicinity is not luminous enough to be detectable in emission at cosmological distances, but can be probed with absorption spectroscopy. GRBs are unique and complementary beacons for this task. In particular, GRBs associated with massive stars (SNIbc) track star formation with little time delay (but not 1–1), which may allow us to reach Pop III stars of ‘‘zero” metals. Their enormous brightness gives us the opportunity to measure abundances in proto-galaxies, and trace their changes from z 1 to 3 (where GRBs are most common) into the era of re-ionization beyond z 6. In contrast to QSOs, GRBs are brief transients that affect their local environment only (D < 100 pc), so their afterglow probe the physical conditions of both the host galaxy and the intervening IGM. The proposed Xenia mission (Kouveliotou et al., 2008) would carry out a follow-up with high resolution X-ray spectroscopy, to study CCE with GRB light. As the afterglows fade quickly, Xenia would have a rapid re-pointing capability, with a spectrometer pointing at the source within 60 s. GRBs from the earliest generation of stars, occurring before galaxies were assembled, would provide a spectacular glimpse into the pre-galactic era of the universe (see Fig. 1). Studies of star formation in the early universe (Bromm and Loeb, 2002) suggest a bias of the Initial Mass Function (IMF) to larger stellar masses with decreasing metal content. This affects the mass-loss properties of stars and their subsequent explosive fates (Heger et al., 2003) While most massive stars today explode as Type II core-collapse Supernovae (ccSNe or SNII), a Pop III star of 150 t Mo more likely dies by pair instability (Heger et al., 2003)
Feedback on the environment from the ﬁrst generation of Supernovae is different than in a present-day galaxy, and star formation today takes place in dense molecular clouds, while in the early universe it was hampered by inefﬁcient cooling in pristine H–He environments. The ﬁrst generation of Supernovae quickly enriched the gas, and for GRBs from such environments we can determine the metallicity. The state of the universe when it was only a few seconds old seems to be well understood. The details have ﬁrmed up, and we make conﬁdent predictions about primordial neutrinos, and He and D synthesis. The way the universe cools, and recombines, and the evolution of linear perturbations that imprint angular structures on the microwave background, is well understood. This simplicity ends when primordial density contrasts evolve into the non-linear regime. The universe literally entered a dark age about 300,000 years after the big bang, when the primordial radiation cooled below 3000 K. Darkness persisted until the ﬁrst non-linear structures developed into bound systems, whose internal evolution gives rise to stars. The standard scenario of early star formation assumes cold dark matter (CDM) cosmology, in which small dark matter halos (M 106 Mo) experience star formation, producing UV-light responsible for some or all of the re-ionization transition. From the absence of the Gunn–Peterson effect one ﬁnds that the universe was fully ionized by z 6 (e.g., Fan et al., 2006). We do not yet know the temporal and spatial evolution of this transition. It may have started with Pop III stars at z > 20, and it may have undergone several episodes of re-ionization. Establishing the origin of the ionizing radiation is an open challenge in observational cosmology. Probably the most direct method to do so is the use of GRBs, for which the delay between the formation of the progenitor and its collapse (the burst) is short. As high-z GRBs are known to exist, a primary goal of astronomy should be the utilization of their bright X-ray emission to probe their sightlines for absorption by intervening matter.
Fig. 1. Much of our knowledge of galaxies and clusters of galaxies, stems from starlight. However, the diffuse gas in the interstellar-, intergalactic-, and intra-cluster medium is more than a bystander in the cosmic cycle of matter. Many nuclei forged in the cauldrons of the big bang never make it into compact stars, but cycle through the diffuse medium. Winds from massive stars and their Supernovae enrich and energize their surroundings. This feedback drives the structure and state of the ISM, affecting the global star formation process. If the supernova rate is high enough, this coupling between gas and stars affects galaxies even on large scales, as gas is driven into their halos and even escape the hosts completely. As many galaxies have assembled in a clustered environment, infall of primordial matter into a dark matter dominated cluster potential encounters galactic outﬂows. A complex network of gas–star–dark matter components evolves in the dynamic background of a dark-energy dominated universe.
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Many aspects of GRBs are not yet understood, but it is clear that their afterglows offer outstanding opportunities to probe cosmic chemical evolution with absorption line spectroscopy during the ﬁrst hours. Our understanding of the phenomenon has advanced tremendously, and the SN connection is well established (Woosley and Bloom, 2006). It has become apparent that one of the best ways to ﬁnd and study black hole formation in the early universe is through GRB detection. Presently Swift provides rapid and accurate localizations, leading to intensive observing campaigns at a rate of 100 per year. With future missions this rate could be much higher, and it is clear that astronomers must prepare new strategies to take advantage of and cope with these opportunities. The reward will be new ways to probe re-ionization, trace cosmic star formation and the chemical history of the universe to very early times including the epoch of ﬁrst stars. GRB research is now evolving towards their use as tools (Prochaska et al., 2007). Their transient nature, random positions, and rapid fading requires unique observational programs (space- and ground-based), to use them as laboratories of relativistic astrophysics, and to probe the evolving Universe with absorption spectroscopy of their multi-wavelength afterglow emission. From Vela to CGRO, HETE, BeppoSAX, and Swift, we have come a long way to unravel the GRB phenomenon. The next step requires improved detectors in space and a sophisticated global network on the ground. Probing the cosmic history of baryons is a frontier of modern observational astrophysics (Prochaska and Tumlinson, 2008), and here GRB afterglow observations are going to play an increasingly important role.
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