Featured Research
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, we 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.
See my latest article: Gill, R., Granot, J., & Beniamini, P. 2020, MNRAS, 499, 1356
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 1e51 to 1e54 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, we 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.
See my latest article: Gill, R., Kole, M., and Granot, J. 2021, Galaxies, 9, 82
Constraining the magnetic field structure in collisionless relativistic shocks with a radio afterglow polarization upper limit in GW 170817
Constraining the magnetic field structure in collisionless relativistic shocks with a radio afterglow polarization upper limit in GW 170817 Shocks (a.k.a blast waves) are a ubiquitous phenomenon in the Universe. In many astrophysical objects, emission all the way from radio through X-rays to very high energy gamma-rays is powered by collisionless shocks (which are mediated by collective electromagnetic interactions rather than by particle collisions). These include relativistic (traveling at very close to the speed of light) blast waves powered by stellar-mass black holes in gamma-ray bursts (GRBs) and supermassive black holes in active galactic nuclei, or non-relativistic blast waves in supernovae.
It is well established that the broad-band (radio to gamma-rays) afterglow emission in GRBs is produced by relativistic electrons that are accelerated at collisionless shock fronts. In particular, these electrons emit synchrotron radiation as they gyrate in the shock-generated magnetic fields behind the shock front. Several theoretical works and numerical (particle in cell - PIC) simulations are devoted to understanding the structure of this magnetic field, which is posited to be tangled and anisotropic - constrained entirely to the plane of the shock - perpendicular to the shocked fluid’s local velocity. Since synchrotron emission is generally partially linearly polarized, this has important implications for the measured degree of polarization (Π). GRB afterglows show polarization of up to a few percent, which already suggests that the shock-generated field is closer to isotropic (a fully isotropic field gives Π = 0) than anisotropic. Better understanding of the magnetic field structure in GRBs, the majority of which have been observed at distances of billions of light-years, is limited by our incomplete knowledge of the angular structure of the relativistic jet and our viewing angle relative to its symmetry axis, which also affects the degree of polarization Π, thereby creating a degeneracy.
A golden opportunity was provided by the afterglow of GW170817 / GRB 170817A that originated in the aftermath of a merger between two neutron stars only 130 million lightyears away from us. We used the afterglow from this source to study the properties of the relativistic jet, including its angular structure, in several joint works. In the present work, they used their knowledge of the jet’s angular structure (either a gaussian jet (GJ) or power-law jet (PLJ) and our viewing angle (about 25 to 30 degrees or 5 to 6 times the jet’s core angle), to isolate the effects of the shock-generated magnetic field structure on the afterglow linear polarization. The figure shows several polarization curves marked by ξf, the parameter characterizing the anisotropy of the magnetic field (becoming more anisotropic away from unity) just behind the shock front. The two arrows show the upper limit on the afterglow polarization obtained from radio observations. This work improved the constraints on the magnetic field anisotropy (0.57 < ξf < 0.89) and clearly showed that the magnetic field just behind the shock front is only mildly anisotropic, contrary to several earlier works. The implications of this study are of fundamental importance for collisionless shock physics. In particular, it suggests that the magnetic field may be produced not only by small-scale plasma instabilities, but is instead also strongly affected by macroscopic turbulence in the region behind the shock front, which generally yields more isotropic magnetic fields.
See my latest article: Gill, R. and Granot, J. 2020, MNRAS, 491, 5815
2D Relativistic MHD Simulations of Kruskal-Schwarzschild Instability (KSI) in a Strongly Magnetized Striped Wind
Figure: Development of the instability from the linear to the non-linear phase and dripping of hot plasma from the current layer. The unmagnetized hot current layer is surrounded by cold strongly magnetized fluid with anti-aligned magnetic field lines going into and out of the page in, respectively, the top and bottom layers.
We study the linear and non-linear development of the Kruskal-Schwarzchild Instability in a relativisitically expanding striped wind. This instability is the generalization of Rayleigh-Taylor instability in the presence of a magnetic field. It has been suggested to produce a self-sustained acceleration mechanism in strongly magnetized outflows found in active galactic nuclei, gamma-ray bursts, and micro-quasars. The instability leads to magnetic reconnection, but in contrast with
steady-state Sweet-Parker reconnection, the dissipation rate is not limited by the current layer`s small aspect ratio. We performed two-dimensional (2D) relativistic magneto-hydrodynamic (RMHD) simulations featuring two cold and highly magnetized plasma layers with an anti-parallel magnetic field separated by a thin layer of relativistically hot plasma with a local effective gravity induced by the outflow`s acceleration. Our simulations show how the heavier relativistically hot plasma in the reconnecting layer drips out and allows oppositely oriented magnetic field lines to reconnect. The instability`s growth rate in the linear regime matches the predictions of linear stability analysis. We find turbulence rather than an ordered bulk flow near the reconnection region, with turbulent velocities up to 0.1c, largely independent of model parameters. However, the magnetic energy dissipation rate is found to be much slower. This occurs due to the slow evacuation of hot plasma from the current layer, largely because of the Kelvin-Helmholtz instability experienced by the dripping plasma.
See my latest article on the KSI: Gill, R., Granot, J., & Lyubarsky, Y. 2018, MNRAS, 474, 3535
Gamma-Ray Bursts
Figure: Formation of GRB spectrum using my one-zone time-dependent kinetic code. Here, I start by injecting a quasi-thermal soft seed spectrum and let it evolve in time while accounting for Compton scattering, pair production/annihilation, Coulomb interaction and volumetric heating of pairs.
My recent study with Chris Thompson at CITA and two other works co-authored by me present a complete description of the physics that gives rise to the GRB spectrum. We show that the low energy spectrum forms pre-breakout when the jet, powered by a central black hole in our model, is mildly relativistic and working its way out of the confining medium provided by the Wolf-Rayet progenitor. The inertia in the jet is dominated by the magnetic field and the low-energy spectrum is formed by quasi-thermal Comptonization in a photon-rich electron-positron-pair fireball, with no baryon contamination. The high-energy spectrum forms post-breakout when the jet is relativistic and optically thin. The dissipation of magnetic energy is provided by the interaction of the relativistic magnetofluid with baryons entrained in the jet from the confining medium. In both the optically thick and thin phases, the pairs are heated volumetrically due to the damping of hydromagnetic turbulence in the flow. To simulate the high energy spectrum, I developed a one-zone kinetic code with all radiative processes treated in an exact manner. We have developed a cogent explanation for the prompt emission mechanism in a series of articles:
Magnetar Giant Flares
The soft gamma-ray repeaters (SGRs) dissipate large amounts of magnetic energy in the form of highly luminous gamma-ray bursts called giant flares. How the flares are triggered and what controls their duration and energy is still an open question. The energetics of the bursts, however, do point towards the extremely strong fields that these objects are endowed with. According to the Thompson & Duncan (1995) model of magnetar bursts, the giant flares involve major restructuring of the global magnetic field of the star on timescales longer than the Alfven crossing time of the magnetosphere. This takes place in concert with a magnetic reconnection event in the magnetosphere that ultimately triggers the hyperflare.
Figure: It displays the setup of the different reconnecting current layers. The macroscopic Sweet-Parker layer with length L∼1E5 cm and width δ∼0.01 cm is the largest of the three. This layer is then thinned down vertically as strong magnetic flux is convected into the dissipation region. The Hall reconnection layer, represented by the dark gray region, develops when δ becomes comparable to the ion-inertial length. The system makes a transition from the slow to the impulsive reconnection and powers the main flare. The tiny region embedded inside the Sweet-Parker layer is the super-hot turbulent current layer, which aids in creating sufficient anomalous resistivity to facilitate the formation of the Sweet-Parker layer. The strongly accelerated plasma downstream of the reconnection layer is trapped inside magnetic flux lines and forms a plasmoid moving at some speed V. This plasmoid is then finally ejected during the initial spike when the external field undergoes a sudden relaxation.
One interesting observation these models do not resolve is the connection between the precursor burst and the main flare. In two out of the total three giant flares that have been observed, a precursor burst of very short duration was detected a few seconds before the main flare. In my study, I show that the precursor bursts are not only causally connected to the main flares but also triggers them. I examine two reconnection models that explain how the wound up flux tubes inside the NS and the sheared field lines in the magnetosphere can form tangential discontinuities and dissipate stored magnetic energy. In this work, I introduced a novel idea of Hall reconnection operating in the inner magnetosphere which ultimately leads to an explosive energy release.
Work on magnetars:
Axions and Axion-like Particles
Unlike weak interactions, that are responsible for the radioactive decay of subatomic particles, strong interactions have thus far been experimentally verified to preserve CP-symmetry. However, this observation is in conflict with QCD which necessarily violates CP. This can be remedied in many ways but the one most favored solution to the Strong CP problem has been the introduction of a new particle into the Standard Model, dubbed the axion. In several extensions of the Standard Model axion-like particles (ALPs) are produced naturally. Such particles remain elusive due to their very small mass and extremely weak coupling to matter and radiation. The limit on its coupling to radiation, derived from astrophysical arguments, in the given mass range is a few orders of magnitude larger than that predicted by theoretical axion models. Several laboratory experiments have been conducted and many are currently underway to exclude as much of the parameter space as possible.
In my work on ALPs, I take a novel approach to constrain the properties of ALPs. I use the polarized light of magnetic white dwarfs (mWDs) to determine the absolute upper limit on the ALPs' coupling to radiation as a function of its mass. My treatment is based on the observation that light is polarized as it traverses the magnetosphere encompassing the tenuous plasma permeated by the ∼ 100 MG magnetic field of some mWDs. Since the photon-axion interaction in a strongly magnetized plasma leads to additional polarization, the observed degree of polarization can be used to constrain axion properties if the plasma contribution is correctly modeled. The derived constraints in my work are hitherto the best estimates of ALP parameters to come from compact objects.
Work on Axions: