GRB spectrum from gradual dissipation in a magnetized outflow

Gamma-ray bursts (GRBs) are the most (electromagnetically) luminous transient phenomenon in the Universe, outshining the entire gamma-ray sky during their brief prompt emission phase.  The latter typically lasts for ~0.03-1 s (in short-hard GRBs, produced by mergers of two neutron stars or a neutron star and a black hole) or ~3-300 s (in long-soft GRBs, produced during the explosion of massive, rapidly rotating stars, accompanied by a powerful supernova explosion). This emission is powered by a relativistic jet but the exact radiation mechanism is still a mystery. Most popular explanations include synchrotron emission from relativistic electrons with a power-law energy distribution, or Compton scattering of thermal photons trapped in the jet by warm electrons. Both of these emission mechanisms often feature in a general class of photospheric emission models that include a prominent quasi-thermal spectral component. In this work, Dr. Ramandeep Gill, Prof. Jonathan Granot and Dr. Paz Beniamini (all from ARCO) studied how the non-thermal smoothly broken power law spectrum of the prompt GRB emission is produced in magnetized jets. Such jets are accelerated by the gradual and continuous dissipation of the magnetic field energy, where particles are accelerated or heated via magnetic reconnection and/or magnetohydrodynamic (MHD) instabilities. This work shows in great detail how the final spectrum is generated by a magnetized jet as it expands (see the figure). It clearly demonstrates that if electrons are accelerated into a relativistic power-law energy distribution, then synchrotron emission is the dominant radiation mechanism (left panel). Alternatively, if electrons are heated into a warm plasma (all particles having the same average energy), then the prompt GRB spectrum forms via Compton scattering of thermal photons (right panel). The shown spectra are obtained from a specially designed numerical code that accurately accounts for all relevant high-energy processes, including production and annihilation of electron-positron pairs, in a relativistically expanding jet. The way the low- and high-energy (below and above the spectral peak) parts of the spectrum are generated has important implications for energy-dependent linear polarization. In the left panel, synchrotron emission can yield strong linear polarization, depending on the magnetic field geometry. In the right panel, however, multiple Compton scatterings wash out any polarization and only produce a negligible global polarization in a uniform (with no local angular structure) jet. Using these results, more sensitive spectro-polarimetric observations from detectors onboard upcoming space missions will help to pin down the exact prompt GRB radiation mechanism.

ARCO TEAM

GRB Polarization: A Unique Probe of GRB Physics

Gamma-ray bursts (GRBs) are  the most powerful explosions in the Universe. They are powered by ultra-relativistic jets (highly collimated outflows moving at ~99.995% of the speed of light). The huge isotropic-equivalent gamma-ray luminosities of L_ɣ,iso~10⁵¹ - 10⁵⁴ erg/s of their brightest prompt emission phase places GRBs as the most (electromagnetically) luminous transient events in nature. This makes them detectable out to the far reaches of the Universe, from down to barely a billion years after the Big Bang. The exact radiation mechanism that produces this prompt emission is still a mystery. The two most popular candidates are synchrotron emission from relativistic electrons with a power-law energy distribution and Compton scattering of thermal radiation by warm electrons. Both mechanisms can explain the majority of the observed prompt GRB spectra equally well, making it hard to distinguish between them. However, they can lead to very different linear polarization, depending on the magnetic field geometry (for synchrotron emission) and the jet’s angular structure (for both). Therefore, high-sensitivity linear polarization measurements can help distinguish between them and reveal the dominant radiation mechanism. Moreover, for synchrotron emission they can also teach us about the magnetic field structure in the emission region. In this work, Dr. Ramandeep Gill (ARCO), Dr. Merlin Kole (University of Geneva) and Prof. Jonathan Granot (ARCO) provide a comprehensive review of the predictions for prompt GRB linear polarization from theoretical models and the current status of polarization measurements. The work gives a concise overview of the fundamental questions in GRB physics, namely what are the jet composition and dynamics, how and where is the energy dissipated, and what are the different candidate radiation mechanisms capable of producing the non-thermal prompt GRB spectrum. It presents relevant formulae and polarization predictions (both time-integrated and time-resolved) for different radiation mechanisms, magnetic field configurations (relevant for synchrotron emission), jet angular structures and dynamics. On the observational front, this work presents the basics of gamma-ray polarimetry and chronicles the GRB polarization measurements made by different detectors over the years. The figure shows example lightcurve (black) and time-resolved polarization (Π) curves for synchrotron emission from different magnetic field configurations in one realization of a uniform (lacking angular structure) jet seen at an angle from the symmetry axis of the jet (for a single pulse in the GRB prompt emission lightcurve). The detailed polarization predictions presented in this work will enable observers to compare high-sensitivity measurements from upcoming gamma-ray polarimeters with theoretical models and constrain fundamental properties of the GRB relativistic jet and prompt emission mechanism.