Gamma-ray burst science in the era of the Cherenkov Telescope Array
Introduction
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe after the Big Bang, liberating as much as erg of isotropic-equivalent energy during a brief period of 0.01–1000 s, primarily as MeV–band gamma rays. They are also the most violent explosions, manifesting rapid and irregular variability on timescales down to sub-millisecond levels. Since their discovery in 1967, research on GRBs has steadily intensified, witnessing particularly rapid progress during the last 10–20 years, driven by observational results from satellite instruments such as the Burst And Transient Source Experiment (BATSE) and the Energetic Gamma Ray Experiment Telescope (EGRET) onboard the Compton Gamma-Ray Observatory (CGRO), the High Energy Transient Explorer (HETE-2), BeppoSAX, Swift, and most recently the Large Area Telescope (LAT) and Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope. We now know with confidence that: (1) They occur at cosmological distances, typically at redshifts of a few. (2) They are generated by (likely collimated) outflows with ultrarelativisic bulk velocities. (3) Their prompt, MeV-band emission is accompanied by afterglows that span the radio to X-ray bands and gradually decay over hours to days or more, most likely emitted by high-energy electrons accelerated in the blastwave resulting from the interaction of the outflow with the ambient medium. (4) Those with durations longer than 2 s (“long” GRBs) exhibit properties systematically different from those with shorter durations (“short” GRBs). (5) At least some long GRBs are associated with the core-collapse supernova events of massive stars. (For recent reviews on GRBs, see e.g. [1], [2], [3], [4], [5].)
However, many other basic aspects are still unknown or unclear, such as the identity and nature of the central engine, the formation mechanism of the ultrarelativistic jet, the physical mechanisms of energy dissipation and particle acceleration therein as well as the prompt and early afterglow emission, their cosmological evolution, the progenitors of short GRBs, etc. Thus they remain one of the most enigmatic phenomena in the Universe, and their origin is among the most important unsolved problems in modern-day astrophysics.
GRBs are also of potentially great importance for other fields of physics and astrophysics. From model-independent considerations, they are thought to be one of the leading candidates for the sources of ultra-high-energy cosmic rays (UHECRs) with energies up to eV, the highest energy particles known to exist in the Universe today and whose origin is yet unknown [6]. The production of UHECRs in GRBs may also induce observable fluxes of high-energy neutrinos. GRBs are also crucial probes for observational cosmology, as they are known to occur and are observable out to extremely high redshifts, including the epochs of cosmic reionization and the earliest star formation [7]. Indeed, the recently detected GRB at is one of the most distant and hence most ancient astrophysical objects known to humankind [8], [9]. Finally, they can serve as valuable beacons for testing fundamental physics, particularly in searching for possible violations of Lorentz invariance [10], [11].
The latest observational advances in GRBs have been brought forth by Fermi [12]. The Fermi LAT instrument has revealed intense emission in the GeV band from a sizable number of GRBs of both the long and short classes. The lack of apparent high-energy spectral cutoffs has allowed important new constraints to be derived on the bulk Lorentz factor of the emitting region. Some notable, common characteristics have also been discerned for the GeV emission compared to the MeV emission, such as the slightly delayed onset, occasionally distinct hard spectral components, temporally extended emission, etc., for which numerous theoretical explanations have already been proposed. However, the generally limited statistics of high-energy photons detected by Fermi LAT (only a few photons above 10 GeV even in the best cases) have so far prevented firm conclusions to be drawn on the nature of the high-energy emission from GRBs.
In order to stimulate further progress, observations with higher sensitivity over a wider energy band are strongly desirable. Compared to Fermi, ground-based, imaging atmospheric Cherenkov telescopes (IACTs) have a large advantage in terms of sensitivity for gamma rays above several tens of GeV because of their much larger effective area, although their field of view and duty cycle are more limited [13], [14]. Past and ongoing follow-up efforts of GRB alerts by current IACTs such as HESS, MAGIC and VERITAS have yet to uncover signals, but their present operational threshold energies of 50–100 GeV and the potential attenuation by the extragalactic background light (EBL) [15], [16] in this band at the typical distances of GRBs could be hindering their detection.
The Cherenkov Telescope Array (CTA), an advanced, next generation ground-based facility,1 is planned to be two sets of mixed arrays of large-size, mid-size and small-size telescopes (LSTs, MSTs and SSTs, respectively), one each situated in the northern and southern hemispheres,2 which when combined will cover the entire sky over a broad energy range from tens of GeV up to hundreds of TeV, with a sensitivity considerably better than existing instruments [17], [18]. The most critical component for GRB observations will be the LSTs, primarily responsible for the lower energy bands. Compared to current IACTs, they will feature: (i) appreciably lower threshold energy (30 GeV, possibly down to 15 GeV in some cases), and (ii) even larger effective area at multi-GeV energies ( times larger than Fermi LAT at 30 GeV) [19]. In addition, they are designed with (iii) rapid slewing capability (180 degrees azimuthal rotation in 20 s), comparable to MAGIC-II, allowing the observation of some long GRBs during their prompt phase, and many others in the early afterglow phase. By acquiring high-quality (i.e. high photon statistics) measurements of time-resolved spectra and energy-dependent variability at multi-GeV energies that was not possible with Fermi, some important science goals that can be addressed with CTA include the following. (1) Determine or more robustly constrain the bulk Lorentz factor of the emission zone. (2) Determine the emission mechanisms of prompt GRBs and early afterglows. (3) Reveal hadronic signatures accompanying the production of UHECRs and neutrinos (4) Probe the extragalactic background light at high redshifts, beyond the reach of blazar active galactic nuclei (). (5) Probe Lorentz invariance violation with better precision.
This article aims to provide an overview of the science prospects for GRBs with CTA, and is organized as follows. We begin by reviewing our current knowledge of GRBs, focusing on their emission in the high-energy ( 100 MeV) and very-high-energy ( 100 GeV) gamma-ray regimes in Sections 2 Current status at GeV energies, 3 Current status at very high energy, respectively. In Section 4, selected science cases for CTA are described in some detail. Section 5 presents demonstrative simulations of GRB spectra and light curve measurements, as a first step toward quantitative assessments of the science goals. In Section 6, predictions for GRB detection rates are given from two different perspectives. Section 7 discusses various issues related to following up GRB alerts with CTA and at other wavelengths, as well as the possibility of discovering GRBs with CTA alone during a wide-field survey mode. We conclude and provide an outlook in Section 8.
Section snippets
From EGRET to Fermi
GeV emission from GRBs was first discovered by EGRET on-board CGRO, active during 1991–2000. While EGRET detected only five GRBs with its spark chambers within 20 MeV–30 GeV and a few more bursts with its Total Absorption Shower Counter within 1–200 MeV, these events already showed diversity [20]. For GRB 940217, GeV emission was seen up to h after the burst trigger, including an 18 GeV photon at h [21]. GRB 941017 displayed a distinct, high-energy spectral component up to MeV with a
Current status at very high energy
Because of their limited effective area, the sensitivity of satellite instruments is often inadequate to measure the decreasing fluxes from gamma-ray sources above few tens of GeV. In this very high energy (VHE) regime, ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) are the most sensitive instruments. GRB follow-up observations are regularly carried out with the latest generation of IACTs including the Major Atmospheric Imaging Cherenkov Telescope (MAGIC),
Physics of GRBs
Many fundamental problems remain unsolved concerning the physical mechanisms behind GRBs. With its large effective area, CTA can detect hundreds or more photons from moderate to bright GRBs (Sections 5 Simulations of GRB observations, 6 Detection rate expectations) and and achieve unprecedented temporal and spectral resolution in the domain above a few tens of GeV. Here we discuss the prospects for studying the physics of GRBs with CTA, focusing on issues related to the bulk Lorentz factor, the
Simulations of GRB observations
In order to quantify the prospects for CTA observations, we now present some simulated spectra and light curves of GRBs. Although our ultimate aim is to assess the different science cases discussed above, in view of the wide range of uncertainties in the current physical models, here we take a purely phenomenological approach as a first step. Choosing as templates some prominent bursts detected by Fermi LAT whose spectra and variability were relatively well characterized up to multi-GeV
Detection rate expectations
We now discuss expectations for the detection rate of GRBs with CTA. Two independent approaches are presented, one by Gilmore et al. (see also [224], [225]) and another by Kakuwa et al. (see also [226]). Although they share some similarities in the assumptions, the main difference lies in the modelling of the GRB population, the former based directly on observed GRB samples, and the latter using a somewhat more theoretical method. The treatment of the CTA performance is also different; Gilmore
Following up alerts and wide-field mode observations
As discussed above, CTA has major scientific potential to advance our understanding of GRBs. It can both follow up GRBs found with other facilities (Sections 6 Detection rate expectations, 7.1 GRB alerts from satellites and other facilities) and also find GRBs using both standard and survey (i.e. wide-field) modes of operation (Section 7.3).
Conclusions and outlook
With high photon statistics measurements of their multi-GeV spectra and temporal variability, the science cases that can be explored by observing GRBs with CTA are varied and far-reaching. In addition to the many mysteries surrounding the physics of GRBs themselves, they include the origin of ultra-high-energy cosmic rays and prospects for high-energy neutrinos via hadronic gamma-ray signatures, the cosmic history of star formation, black hole accretion and intergalactic reionization via
Acknowledgements
We are grateful to Masanori Ohno for providing us with Fermi LAT data on the light curve of GRB 080916C. We thank comments from Felix Aharonian and Elisabetta Bissaldi. This work is supported in part by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, Nos. 22540278 and 24340048 (SI), Nos. 24103006, 24000004, 22244030, 22244019, 21684014 (KI), and No. 21740184 (RY). YI, KM and KT are supported by JSPS Research Fellowships for Young
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