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Space-study of the re-ionization epoch of young Universe by using gamma-ray bursts and their afterglow
One of the most compelling problems facing modern astrophysics in 21-st century is to understand the early Universe, and in particular the epoch nicknamed as the Cosmic Dawn (Blandford et al. 2010). This is the epoch when starlight from the first generation of stars start to re-ionize the neutral intergalactic medium, and when the structure of Universe starts to form which is prevailing until present. Looking back in the time, this re-ionization was completed around redshift z~6 (Fan et al. 2006).
The Universe at such high redshift is a challenge to study, but progress was steady in recent years, and the redshift record is now around of ~9.4 involving observations of GRB (Cucchiara et al. 2011).
Fig.1. Reionization history calculated by de Souza, Yoshida & Ioka (2011). Blue line is model proposed by de Souza et al. (2011), while black dotted line is a result of calculation by using code CAMB, developed by Antony Lewis & Anthony Challinor (http://camb.info).
Each of the potential probes of the Cosmic Dawn, like high-z quasars, galaxies and gamma-ray bursts (GRBs) have their strong pro- and contra-points. Quasars are extremely luminous and can be bright sources even at high redshifts, but the high UV and X-ray flux from the central engine strongly affects the properties of the host galaxy, making it atypical. Moreover, density of quasars drops rapidly after z~6.
Normal galaxies can be found at very high redshifts (Bouwens et al. 2010) in the Hubble Ultra Deep Field through their photometric dropouts. But these dropouts are not reliable, as those can be produced by dust extinction. Noteworthy, that those galaxies represent the bright end of the luminosity distribution, but still are so faint that it makes their spectroscopy unreliable, and largely beyond the capabilities of existing instruments.
Fig.2. Left: prediction of the SFR by Barkana and Loeb (2000), and Right: actually measured SFR by Hubble from Bouwens et al. (2010).
GRBs, on the other hand, probe star formation regions in all types of galaxies, are intrinsically bright, that can be used to probe the interstellar gas and dust of host galaxies, even those can be very faint themselves.
The first stars in the Universe are thought to play a crucial role in the early cosmic evolution, by emitting first light and producing the first heavy elements (Bromm et al. 2009). The standard cosmological model predicts that the first stars were formed when the age of the Universe was less than a few million years, and that they were predominantly massive (Abel et al. 2002; Omukai & Palla 2003; Yoshida et al. 2006).
There has not been yet a single direct observation of these first massive stars of the so-called Population III (Pop III). Recently it was proposed that massive PopIII stars may produce gamma-ray bursts, whose total isotropic energy could be Eiso≥1055-57 ergs, i.e., ≥ 2 orders of magnitude larger than average (Komissarov & Barkov 2010; Toma et al. 2010; Meszaros & Rees 2010; Barkov 2010; Suwa & Ioka 2011).
Fig. 3. Depiction of different probes of the high-redshift Universe, from Lamb et al. (2001).
Even if the PopIII stars have a supergiant hydrogen envelope, the GRB jet can break out the first stars because of the long-lasting accretion of the envelope itself (Suwa & Ioka 2011; Nagakura et al. 2011).
The observations of the spectral peak energy - peak luminosity relation also imply that GRBs actually occur at high refshift, z≥10 (Yonetoku et al. 2004).
Observations of such energetic GRBs at very high redshifts will provide a unique probe of the young Universe.
In this proposal we want to concentrate on the use of GRBs and their afterglow as a major source (probes) to gain information on the evolution of young Universe shortly before and during the re-ionization epoch.
Gamma-ray bursts (GRBs).
Gamma-ray bursts (GRB) are the most luminous sources on the sky, and thus act as signposts throughout the Universe. The long-duration sub-group of GRBs is believed to be produced by the explosion of massive stars, while short-duration GRBs likely originate during the merger of compact stars. Both types are major electromagnetic emission, neutrino and gravitational wave sources. Being stellar size objects at cosmological scales, they connect different branches of research and thus have a broad impact on present-day astrophysics. NASA's dedicated GRB-mission Swift is a good example of such an impact, helping dramatically to improve present understanding of GRB afterglows, environments and host galaxies. GRB are the most energetic events in the universe
Identifying and understanding objects at high redshifts of z>6 has become one of the main goals and drivers of modern observational cosmology, but this goal turned out to be difficult for the traditional approach. For example, 600(!) orbits long HST observation of the Hubble ultra-deep field resulted in only a few z>7 candidates (Windhurst et al. 2006), none of which has been confirmed.
In contrast, GRBs offer a promising opportunity to identify high-z objects, and moreover allow us to investigate the host galaxies at these redshifts. GRB are a factor of 105-7 brighter than quasars during the first hour after explosion, and a favourable relativistic k-correction implies that they do not get fainter beyond z~3. Present and near-future ground- and space-based facilities sensitivity limits the measurement of redshifts at z~13 (as H-band drop-outs), because GRB afterglows above 2.5 μm are too faint by many magnitudes for 8-10 m telescopes. The use of GRBs as probes of distant galaxies has boomed since the launch of the Swift satellite (Gehrels et al. 2004), which is capable to provide rapid arcsecond positions of GRBs. But, after six years of operation, Swift has found only 3 GRBs with spectroscopic redshift exceeding z~6. The paucity of the high redshift GRBs found by Swift probably stems from several factors, with one of those being the long delay (typically a few hours) which is needed to obtain the first indication of GRB being a high redshift event (Cusumano et al. 2006; Kawai et al. 2006; GCN Circ #4545; Greiner et al. 2009; Tanvir et al. 2009; Cucchiara et al. 2011).
Fig. 4. Fast variability of the prompt gamma-emission of GRB detected on September 16, 2008 by Anticoincidence Shielding (ACS) of SPI on-board INTEGRAL.
Thus a completely different strategy is asked for to step beyond redshift ~10, to size the epoch when the first stars were formed. The key strategy to finding more high-z bursts is to design an instrument suite optrimized to detect high-z GRBs and to measure their redshifts within minutes, if not seconds.
Therefore, here we propose to combine an X-ray trigger, near-IR imaging instruments that are part of Ultra Fast Flash Observatory -100 (UFFO-100, UFFO collaboration: http://uffo.skku.edu), with a survey type (broad FoV) MeV-gamma monitor to get sizable sample of the high-redshift GRBs, with measured redshift via peak-energy-z relation, as well, as by using gamma-resonance absorption of the GRB prompt gamma emission (Iyudin et al. 2005; Greiner et al. 2009).
GRB's jet with a very narrow open angle (artist's impression).
( from Iyudin et al. (2010))
Fig. 5. Gamma-ray absorptions for a jet open angle of 0o-0.18o , that fired behind the absorbing curtain with a: 1 > Nh=6.0 1025 cm-2; 2 >Nh=1.2 1026 cm-2; 3 > Nh=1.8 1026 cm-2.
For a narrow open angle jets, while looking along the jet one might get lucky to measure a narrow gamma-absorption resonance trough for a PopIII GRBs, that will provide an observer with a redshift of the jet origin site, and even will give the metallicity of the site.
Analysis of GRB's prompt gamma-emission spectrum will reveal an absorption trough properties, from which we can gain information on the object's redshift, it obscuration, and the metallicity of the obscuring matter. Spectroscopy of the host galaxy can be done with a prior knowledge of the redshift by a facility similar to JWST in its capabilities. The drawback of the resonance absorption method is that it works only for quite high absorbing columns (>1025 cm2) on the sightline towards the object, which can be GRB, like in picture below, or blazar, or even micro-quasar (μQSO).
Fig. 6. This artist's impression shows two galaxies in the early Universe. The brilliant explosion on the left is a gamma-ray burst. The light from the burst travels through both galaxies on its way to Earth (outside the frame to the right). Analysis of observations of the light from this gamma-ray burst made using ESO's Very Large Telescope have shown that these two galaxies are remarkably rich in heavier chemical elements. (Image courtesy: ESO/L. Calcada)
In both approaches, using UFFO-100, and/or gamma-ray monitors, like "Gammascope" and "GROME-S", it is important to have a fast response, good timing and moderate energy resolution instruments to allow for measurements of the bulk Lorentz factor, rise and delay times in multi-messenger events with fast variability (see for example Fig. 4), as well, as the spectrum of GRB prompt emission.
Additionally, synergy of UFFO-100 approach with MeV-energy range gamma-monitor might help to calibrate nearby SNae of type Ia as a standard candle by filling in existing till present gap in understanding how much quantity of 56Ni produced in SN Ia varies, depending on the host galaxy properties, etc...
While UFFO-100 is considered to be at mature state, the inclusion of gamma-monitor into instrumental suite might demand quite some consideration to the gamma- monitor development during the time of the laboratory scientific activity under leadership of Prof. Smoot.
The work will start with a concept of UFFO-100 at hand, following with a development of the gamma-ray monitor "Gammascope", and advance it to the next stage of fusing in instrumental suite of UFFO-100 a MeV-emission gamma-monitor named "GROME-S".
There are variety of GRBs like, long and short (see above), but also varied by the brightness of the prompt emission and of the afterglow.