High Energy, Multimessenger Astrophysics and Astrobiology
Black hole jets and magnetospheres
My research work related to relativistic jets is principally focused on the physics of their central engine. It has connections with recent observations, like the emission ring around the M87 black hole recently reported by the Event Horizon Telescope, or gamma-ray bursts -- flashes of highly energetic gamma rays lasting from less than a second to several minutes and producing as much energy as the sun will emit during its entire 10-billion-year life! Gamma-ray bursts are associated with the formation of relativistic jets around stellar mass black holes, shortly after the collapse of a massive star or the merger of two neutron stars.
My principal achievements in this field are listed below:
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I studied the jet formation around Kerr black holes during my PhD by constructing self-similar solutions to the GRMHD equations, and also during my first postdoc by calculating GRMHD inflow-outflow solutions where the rotational energy can be extracted from the black hole. There is a critical energy load for the Blandford-Znajek mechanism to operate. Publication
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I applied the model to the central engine of gamma-ray bursts jets using a realistic plasma injection profile derived from neutrino-antineutrino annihilation above the black hole's horizon. Publication.
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I studied the collimation of magnetic jets by disc winds. The M87 jet profile can be reproduced if the jet is collimated by a hydrodynamical disk wind with luminosity ~10% of the jet luminosity. The HST-1 knot can be due to the focusing of the magnetized jet due to the change in the pressure profile around the Bondi radius. Publication.
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Currently, I study the processes at play in the magnetosphere of the spinning black hole in M87 allowing energy transport from the hole to the disk. This leads to different scientific inferences from the results of EHT imaging of M87. Publication.
Multimessenger astroparticle physics
I am investigating the origin of ultra-high energy cosmic-rays and neutrinos. My research work related to cosmic-rays has connections with recent discoveries, like the dipole anisotropy or the high energy hotspots reported by the Telescope Array Collaboration (which I am a member of) and the Pierre Auger Observatory. I also study the multi-messenger signatures (gamma-rays and neutrinos) of cosmic-ray acceleration in high energy sources; and propagation of cosmic rays in the extragalactic medium.
My principal achievements in this field are listed below:
Physics of Cosmic-Ray and Neutrinos Sources
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I co-authored a numerical code during my PhD to calculate the acceleration of cosmic rays at gamma-ray bursts internal shocks, including all the relevant energy loss processes experienced by nuclei. We predicted a softer proton component as a distinctive signature of gamma-ray bursts. Publication.
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I showed the implications of the Fermi-LAT and IceCube data regarding the extragalactic diffuse-ray gamma-ray and neutrino backgrounds on the ultra-high energy cosmic-ray origin. Cosmic ray sources that have a large cosmological evolution are excluded, as they would overproduce the rate of these secondary messengers. Publication.
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I investigated shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and external shocks to provide estimates for neutrino and cosmic-ray signals from self-consistent simulations of gamma-ray bursts jets associated with binary neutron star mergers. Publication.
Phenomenology of Ultra-High Energy Cosmic Rays
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I studied the Galactic-to-extragalactic cosmic-ray transition and showed that a simple generic model with a softer proton component can account for the evolution of the spectrum and composition observed by KASCADE-Grande and Auger. Publication.
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I calculated the lifetime of small scale anisotropies in the interstellar turbulence, and showed that the lifetime of a cosmic-ray hot spot over a 5 years observation period depends on the direction of the source. Publication.
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I investigated the origin of cosmic ray anisotropies, with a focus on interpretation of the first significant large scale anisotropy reported by the Pierre Auger Collaboration. I authored a code to calculate the anisotropy of cosmic rays from the large scale structure, using constrained cosmological simulations. The observed energy dependence of the dipole anisotropy is a natural consequence of the evolution of the size of the cosmic rays observable universe (due to the GZK effect). Publication.
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I mentored a student (Chen Ding) during two years to study the energy dependence of the direction of the dipole. This study shows that the direction and amplitude of the dipole anisotropy is well accounted for when the cosmic-ray source distribution follows the large scale structure, using the Jansson and Farrar state-of-the-art model for the Galactic magnetic field In the press. Publication.
Spin-polarized cosmic radiation and the origin of life
I became fascinated by the problem of biological homochirality after the launch of Rosetta mission. One of the goal of the mission was to search for enantiomeric excesses in a comet. The origin of biological homochirality has been an intense field of research and debate since its discovery by Pasteur in 1848. The weak force, one of the fundamental forces operating in nature, is parity-violating. Cosmic ray muons are caused by a decay involving the weak force and hence, they are spin-polarized. At ground level, nearly 85% of the cosmic radiation dose comes from muons. We proposed that spin-polarized muons could induce persistent structural changes in helical polymers, thereby enabling the emergence of homochirality.
My principal achievements in this field are listed below:
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The difference in the probability of ionization by a spin-polarized cosmic ray is higher for helical polymers of different handedness - such as left and right-handed DNA - than for simple enantiomers such as amino acids or sugars. Publication.
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This ionization difference, translated in a difference in the mutation rate, can influence the evolution of primitive self-replicating polymers, and lead to the emergence of homochirality. In the press, invited news items in Nature and in Scientific American
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We predicted that the biological response depends on the polarization on the radiation and calculated the spin-polarized radiation doses at different environments (Mars, Venus, Titan, small bodies of the solar system). Publication.
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We are developing experiments to study the interaction between polarized radiation and biopolymers (with a focus on DNA) with colleagues from various Departments at the University of California Santa-Cruz: David Deamer, David Kliger, Eefei Chen and Enrico Ramirez-Ruiz. KIPAC Blog post
