GRB science

Gamma Ray Bursts

Gamma-Ray Bursts (GRBs) are the brightest explosions in the Universe since the Big Bang. A typical GRB emission consists of two main parts: the prompt phase which consists of the immediate γ-rays produced closed to origin of the burst, and the late time afterglow phase which is produced as the outflow interacts with ISM surrounding the burst. The GRBs are classified into two categories depending upon the duration of the prompt emission phase, and they are believed to originate from different progenitors. T90 is the duration over which prompt emission constitutes 5% to 95% of total counts. over which prompt emission constitutes 5% to 95% of total counts. They are long GRBs (LGRBs) with duration T90 > 2 sec and short GRBs (SGRBs) with T90 < 2 sec. Long GRBs are produced by the core collapse SNe of giant star (Bloom et al. 2002; Hjorth et al. 2003; Hjorth & Bloom 2012), while, the short GRBs are produced by the merger of binary compact objects such as binary neutron stars and neutron star - black hole. The observation of GW170817 confirmed that at least a class of short GRBs are produced by binary neutron star (BNS) mergers (Abbott et al. 2017d).

For Astronomers

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The afterglow phase is well studied unlike the prompt phase. The GRB afterglow observations provide the redshift of the the burst which shed light on the constituents of ISM and the underlying physical processes (Greiner, J. et al. 2009; Gendre, B. & Boër, M. 2005; Schady 2015). The broadband observation of the afterglow phase conveys broad picture of the energetics and timeline of the underlying processes.

During the main prompt GRB phase, the source invoke highly relativistic jets with bulk Lorentz factors of a few hundreds emitting highly energetic photons. The exact physical mechanism producing such powerful γ-rays still remains debated (Kumar & Zhang 2015). The composition of GRB jets, the radiative processes giving rise to the prompt γ-rays are some of the open ended questions (Kumar & Zhang 2015). Both in terms of spectral properties and physical mechanisms, prompt emissions are still comparatively poorly explored (Zhang 2011) as opposed to the afterglow phase due to the transient nature of the event and lack of observations in the soft X-ray band (Gehrels & Mészáros 2012; Oganesyan, G. et al. 2018). Daksha mission is expected to improve over both these aspects with its all-sky capability and ability to probe in soft X-ray band and hence improving the population of the GRBs. Below, we highlight on the GRB science we can probe with the Daksha mission especially in the prompt phase.

We use the Swift redshift distributions of long and short GRBs to predict GRB detection rate of Daksha. Using the obtained distributions and the Daksha sensitivity of S = 4 × 10−8 erg cm2 s−1 and spatio-temporal coverage of Ω T∕(4π) = 87%, we estimate that Daksha can detect nearly 500 long GRBs and 50 short GRBs per year (around 7 and 2.5 times greater than currently Swift and Fermi rate respectively).