The most powerful explosions in the Universe :
Gamma-ray burst spectroscopy and polarization
Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the universe, with central engines which drive the outbursts in highly relativistic jets (Mészáros, 2006). They offer a unique possibility to shed light on, for example, the last stages of massive stellar evolution, on the physics of relativistic matter flows and on the formation of stellar-mass black holes. Their unique properties make them among the most promising astrophysical sources of non-photonic messengers (high energy neutrinos and cosmic rays, gravitational waves). In addition, given their very broad redshift distribution extending to z=8.1 and their huge luminosities (~1050 ergs/s), they have significant, as yet unrealised, potential use as tools for the investigation of the early universe and the testing of cosmological models. Long duration GRBs (>2 sec) are most likely the signatures of the deaths of rapidly rotating massive stars (‘collapsars’) and are connected to Type Ib/c supernovae, whereas GRBs < 2 sec are believed to be associated with ns-ns mergers.
Despite the significant progress that has been made in our understanding of the progenitors, afterglows, host galaxies and distances of GRBs since their discovery in the late 1960’s, fundamental characteristics of the bursts themselves still remain unexplained and, to a large extent, unexplored. Crucially, it is not yet established how the observed non-thermal spectrum is produced despite major theoretical efforts (e.g. Piran 2004). More generally the composition, magnetization and geometry of the relativistic outflow remain unknown. Until these fundamental issues have been disentangled using well-characterised spectral parameters from a large burst sample in the critical MeV energy range, the promise of GRBs as a tool to gain insight into supernovae mechanisms, ns-ns mergers, cosmic accelerators and cosmology, will remain unfulfilled.
GRB Spectroscopy with DUAL: The spectral output in GRBs peaks at a few hundred keV and continues as a ~-2.2 index power-law (Figure 4), consistent with the index expected from Fermi acceleration, extending up to GeV energies in some known cases. A physical picture of the prompt emission of GRBs has defied any simple explanation, despite the presence of rich observational material and substantial theoretical efforts. This is due in part to the lack of appropriate space experiments with sufficient sensitivity in the challenging MeV regime, where GRBs put out their peak power (Figure 4). Broadband spectroscopy combined with measurement of the polarization of the γ-ray radiation emitted during the prompt emission is crucial diagnostics for solving the prompt emission mechanism issue.
For a GRB with typical spectral parameters (i.e. a double power-law, smoothly connected function with α = -1, β = -2, E0 = 150 keV) the 6 σ detectable fluence in the 100 keV – 10 MeV energy band of ASCI in Compton mode (~100 cm2 effective area at 1 MeV) for the average incoming direction of 60° is ~1.5x10-6 erg/cm2 for long GRBs and ~4x10-7 erg/cm2 for short GRBs, comparable to BATSE and Swift-BAT (albeit in lower energy bands than DUAL).
In addition to the Compton mode, DUAL may be operated in a “Burst” mode which would extend the energy threshold down to ~20 keV by using the photons detected via photoelectric absorption and increase the effective area to ~300 cm2 per Ge layer. By considering the full-sky FOV of the DUAL/ASCI and its L2 orbit, and scaling rates from previous GRB missions to the DUAL energy range, about 600 GRBs/yr should be detected. For ~160 long and ~20 short GRBs per year DUAL offers an outstanding opportunity to remedy the current lack of understanding regarding burst emission mechanism, by providing detailed spectral constraints. DUAL will derive an unprecedented global picture of spectral characteristics for this sample over the mission lifetime which will inform and constrain future theoretical work in GRB acceleration and radiation mechanisms e.g. diffusive shock acceleration, proton acceleration and quasi-thermal Comptonisation.
GRB Polarization: The controversial claim of polarization in GRB 021206 with the RHESSI satellite (Coburn and Boggs, 2006) underscores the need for sensitive polarimeters in understanding -ray sources. No purpose-built polarimeter in this energy range has yet been launched. Constraining the magnetic field properties is crucial to understanding the driving mechanism of the outflow i.e. whether the GRB ejecta are dominated by kinetic (baryonic) or magnetic (Poynting flux) energy. A high percentage of polarization from a GRB not only suggests a synchrotron origin for the prompt GRB emission, but also requires a very strong, large-scale magnetic field produced by the central engine driving the explosion. Additional evidence for strong polarization has been found for prompt γ-ray emission from GRB041219a observed with INTEGRAL’s SPI and IBIS instruments by three independent groups (McGlynn et al., 2007, Kalemci et al., 2007, Gotz et al. 2009). In addition, two BATSE bursts were found to show evidence for high polarisation (P>35% and P>50%) through modelling of the Earth’s albedo (Willis et al. 2005). Establishing a significant statistical sample of prompt γ-ray polarization measurements in a large number of bursts will greatly aid our understanding by addressing questions about the role of magnetic fields in driving the relativistic outflows, in the GRB environments (e.g. large-scale, ordered fields vs. locally generated highly structured fields) and in the emission mechanisms (e.g. synchrotron vs. jitter radiation) of GRBs. Monte Carlo simulations performed by Toma et al. (2009) have shown that the statistical distribution of the polarization degree in GRBs is a highly discriminating criterion to distinguish between the different classes of theoretical models. DUAL will crucially allow systematics to be minimised and also be sensitive enough to perform time-resolved polarimetry in bright bursts. DUAL will be sensitive to a polarisation level of 50% for ~ 60 GRBs per year, and to a level of 10% for about 20 GRBs per year. A level of 10% is very constraining for models and is the level of optical polarisation which has been observed in a very few early afterglows.
Short GRBs: Due to its high sensitivity in the MeV range and its large FOV, DUAL will be very efficient for the detection of short hard GRBs which are spectrally harder that long GRBs and is likely to detect and localize at least 30 short hard GRBs per year. The capacity to detect and localize short GRBs over the whole sky will assume additional importance when next-generation experiments for the detection of gravitational waves (e.g. advanced-Virgo, advanced-LIGO, Einstein telescope, LISA) will be in operation. If short GRBs are associated with double neutron star mergers, then all mergers in a given volume will be detected by gravitational waves, but only a fraction of them as GRBs (due to beaming). Clearly, DUAL’s 4π sky coverage optimises the chance of a coincident γ-ray – gravitational wave detection.
The spectroscopic and polarization capabilities of DUAL, combined with the location accuracies <1° for at least the brightest 50% of GRBs and prompt transmission of GRB locations to ground, will transform GRB science by providing unique model-constraining data (including polarization) for a large burst sample, while involving a large community of ground-based observers, including amateur astronomers, making follow-up observations and redshift measurements.
