Pulsar Radio Emission Mechanism: Radio Nanoshots as a Low-frequency Afterglow of Relativistic Magnetic Reconnection
Year: 2019
Type: article
Abstract: In this Letter we propose that coherent radio emission of the Crab pulsar, other young energetic pulsars, and millisecond pulsars is produced in the magnetospheric current sheet beyond the light cylinder. We carry out global and local 2D kinetic plasma simulations of reconnection to illustrate the coherent emission mechanism. Reconnection in the current sheet beyond the light cylinder proceeds in the very efficient plasmoid-dominated regime, where the current layer gets fragmented into a dynamic chain of plasmoids that undergo successive coalescence. Mergers of sufficiently large plasmoids produce secondary perpendicular current sheets, which are also plasmoid unstable. Collisions of plasmoids with each other and with the upstream magnetic field eject fast magnetosonic waves, which propagate upstream across the background field and successfully escape from the plasma as electromagnetic waves that fall in the radio band. This model successfully explains many important features of the observed radio emission from the Crab and other pulsars with high magnetic field at the light cylinder: phase coincidence with the high-energy emission, nanosecond duration (nanoshots), and extreme instantaneous brightness of individual pulses.
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Awards: PULSARS ARE ROTATING MAGNETIZED NEUTRON STARS WHICH PRODUCE BROADBAND PULSED EMISSION. OBSERVATIONS WITH FERMI GAMMA-RAY SPACE TELESCOPE HAVE UNCOVERED MORE THAN 100 GAMMA-RAY EMITTING PULSARS DRAMATICALLY EXPANDING THE NUMBER OF SUCH SOURCES KNOWN AND PRODUCING UNPRECEDENTED DATA ON LIGHT CURVES AND SPECTRA IN THE GAMMA-RAY BAND. THE MAGNETOSPHERIC PROCESSES THAT LEAD TO THIS EMISSION ARE VERY INTERESTING AND CURRENTLY NOT UNDERSTOOD IN DETAIL. THE MAIN DIFFICULTY IS THE ABSENCE OF A SELFCONSISTENT MODEL OF THE PULSAR MAGNETOSPHERE THAT CAN PREDICT THE GLOBAL SHAPE OF THE MAGNETIC FIELD AND THE STATE OF THE PLASMA THROUGHOUT THE MAGNETOSPHERE. THE STANDARD PICTURE ENVISIONS THE MAGNETOSPHERE AS FILLED WITH PLASMA DUE TO SPACE-CHARGE-LIMITED FLOW OF CHARGES FROM THE SURFACE OF THE STAR. THESE CHARGES ARE ACCELERATED BY STRONG ELECTRIC FIELDS INDUCED BY RAPID STELLAR ROTATION. AS CHARGES ARE CURVED WHILE TRAVELING ALONG THE MAGNETIC FIELDS THEY EMIT CURVATURE RADIATION WHICH CAN PAIR-PRODUCE AND FILL THE MAGNETOSPHERE WITH PAIR PLASMA. THE CURRENTS AND CHARGES IN THIS PLASMA AFFECT MAGNETIC FIELD STRUCTURE AND PARTICLE ACCELERATION. UNTIL RECENTLY NO SELF-CONSISTENT SOLUTIONS OF THE PULSAR ELECTRODYNAMICS THAT ACCOUNTED FOR PLASMA SUPPLY AND PRODUCTION EXISTED. RECENTLY THE DEVELOPMENT OF MULTIDIMENSIONAL KINETIC SIMULATIONS HAS BROUGHT THE GOAL OF SELF-CONSISTENTLY CALCULATING PLASMA SUPPLY AND MAGNETOSPHERIC SHAPE OF PULSARS CLOSER TO REALITY. IN THIS PROPOSAL WE PLAN TO PERFORM THREE-DIMENSIONAL KINETIC SIMULATIONS OF PULSAR MAGNETOSPHERES USING PARTICLE-IN-CELL (PIC) METHOD. PIC IS AN ESTABLISHED TECHNIQUE FOR PLASMA SIMULATIONS THAT REPRESENTS THE PLASMA AS CHARGED MACROPARTICLES THAT ARE ADVANCED IN ELECTROMAGNETIC FIELDS. THE FIELDS ARE EVOLVED WITH PARTICLE CURRENTS AS SOURCES COMPLETING THE CYCLE. WE RECENTLY DEMONSTRATED THAT SELF-CONSISTENT PIC SIMULATIONS WITH PARTICLE EXTRACTION FROM THE STAR AND PAIR PRODUCTION IN THE MAGNETOSPHERE LEAD TO SOLUTIONS WHICH RESEMBLE THE FORCE-FREE STATE BUT CARRY INFORMATION ABOUT PARTICLE ACCELERATION AND BEAMING. WE WILL PERFORM A NUMERICAL SURVEY OF PULSAR MAGNETOSPHERES WITH A REALISTIC PAIR PRODUCTION PRESCRIPTION IN ORDER TO ASCERTAIN THE REGIMES OF PULSAR PARAMETER SPACE WHERE MAGNETOSPHERES WITH ABUNDANT PLASMA OCCUR. WE WILL ALSO ADDRESS THE FATE OF PULSARS NEAR THE DEATH LINE AND A SPECIAL CLASS OF WEAK PULSARS WITH LIMITED PAIR FORMATION. WE WILL SIMULATE THE PHYSICS OF RELATIVISTIC RECONNECTION WITH PAIR FORMATION IN ORDER TO PREDICT THE GAMMA-RAY LUMINOSITY AS A FUNCTION OF SPINDOWN ENERGY LOSS RATE FOR A RANGE OF PULSARS. WE WILL ALSO ANALYZE THE PLASMA DISTRIBUTION FUNCTION FOR EVIDENCE OF PLASMA INSTABILITIES THAT CAN LEAD TO COHERENT RADIO EMISSION. FINALLY WE WILL STUDY THE ENERGY AND PARTICLE DISTRIBUTION IN THE PULSAR WIND AND THE WAY THESE CAN TRANSFORM WITH WIND PROPAGATION. WE ALSO WILL SIMULATE THE INTERACTION OF PULSAR WINDS WITH OBSTACLES AND MAGNETOSPHERIC INTERACTIONS DURING NEUTRON STAR MERGERS. 50 YEARS AFTER DISCOVERY OF PULSARS THE STUDY OF PULSAR MAGNETOSPHERES IS NOW UNDERGOING AN EXCITING TRANSFORMATION WHERE A CALCULABLE STANDARD MODEL IS BEING FORMULATED. THE ADVENT OF GLOBAL KINETIC SIMULATIONS THAT WE WILL UTILIZE IN THIS RESEARCH WILL ENABLE US TO EXPLORE COMPLETE AB-INITIO MODELS OF PULSAR MAGNETOSPHERES AND THEIR EMISSION SOLVING ONE OF THE OLDEST UNSOLVED PROBLEMS IN ASTROPHYSICS., Why Blazar Jets Shine: Radiative Relativistic Reconnection and the Minijet Model, KINETIC MODELING OF RADIATIVE TURBULENCE IN RELATIVISTIC ASTROPHYSICAL PLASMAS: PARTICLE ACCELERATION AND HIGH-ENERGY FLARES PI: DMITRI UZDENSKY (UNIV. COLORADO) CO-IS: MITCH BEGELMAN AND GREG WERNER RELATIVISTIC ASTROPHYSICAL PLASMA ENVIRONMENTS ROUTINELY PRODUCE INTENSE HIGH-ENERGY EMISSION WHICH IS OFTEN OBSERVED TO BE NONTHERMAL AND RAPIDLY FLARING. THE RECENTLY DISCOVERED GAMMA-RAY (>100 MEV) FLARES IN CRAB PULSAR WIND NEBULA (PWN) PROVIDE A QUINTESSENTIAL ILLUSTRATION OF THIS BUT OTHER NOTABLE EXAMPLES INCLUDE RELATIVISTIC ACTIVE GALACTIC NUCLEI (AGN) JETS INCLUDING BLAZARS AND GAMMA-RAY BURSTS (GRBS). UNDERSTANDING THE PROCESSES RESPONSIBLE FOR THE VERY EFFICIENT AND RAPID RELATIVISTIC PARTICLE ACCELERATION AND SUBSEQUENT EMISSION THAT OCCURS IN THESE SOURCES POSES A STRONG CHALLENGE TO MODERN HIGH-ENERGY ASTROPHYSICS ESPECIALLY IN LIGHT OF THE NECESSITY TO OVERCOME RADIATION REACTION DURING THE ACCELERATION PROCESS. MAGNETIC RECONNECTION AND COLLISIONLESS SHOCKS HAVE BEEN INVOKED AS POSSIBLE MECHANISMS. HOWEVER THE INFERRED EXTREME PARTICLE ACCELERATION REQUIRES THE PRESENCE OF COHERENT ELECTRIC-FIELD STRUCTURES. HOW SUCH LARGE-SCALE ACCELERATING STRUCTURES (SUCH AS RECONNECTING CURRENT SHEETS) CAN SPONTANEOUSLY ARISE IN TURBULENT ASTROPHYSICAL ENVIRONMENTS STILL REMAINS A MYSTERY. THE PROPOSED PROJECT WILL CONDUCT A FIRST-PRINCIPLES COMPUTATIONAL AND THEORETICAL STUDY OF KINETIC TURBULENCE IN RELATIVISTIC COLLISIONLESS PLASMAS WITH A SPECIAL FOCUS ON NONTHERMAL PARTICLE ACCELERATION AND RADIATION EMISSION. THE MAIN COMPUTATIONAL TOOL EMPLOYED IN THIS STUDY WILL BE THE RELATIVISTIC RADIATIVE PARTICLE-IN-CELL (PIC) CODE ZELTRON DEVELOPED BY THE TEAM MEMBERS AT THE UNIV. OF COLORADO. THIS CODE HAS A UNIQUE CAPABILITY TO SELF-CONSISTENTLY INCLUDE THE SYNCHROTRON AND INVERSE-COMPTON RADIATION REACTION FORCE ON THE RELATIVISTIC PARTICLES WHILE SIMULTANEOUSLY COMPUTING THE RESULTING OBSERVABLE RADIATIVE SIGNATURES. THIS PROPOSAL ENVISIONS PERFORMING MASSIVELY PARALLEL LARGE-SCALE THREE-DIMENSIONAL SIMULATIONS OF DRIVEN AND DECAYING KINETIC TURBULENCE IN PHYSICAL REGIMES RELEVANT TO REAL ASTROPHYSICAL SYSTEMS (SUCH AS THE CRAB PWN) INCLUDING THE RADIATION REACTION EFFECTS. IN ADDITION TO MEASURING THE GENERAL FLUID-LEVEL STATISTICAL PROPERTIES OF KINETIC TURBULENCE (E.G. THE TURBULENT SPECTRUM IN THE INERTIAL AND SUB-INERTIAL RANGE) AS WELL AS THE OVERALL ENERGY DISSIPATION AND PARTICLE ACCELERATION THE PROPOSED STUDY WILL ALSO INVESTIGATE THEIR INTERMITTENCY AND TIME VARIABILITY RESULTING IN DIRECTION- AND TIME-RESOLVED EMITTED PHOTON SPECTRA AND DIRECTION- AND ENERGY-RESOLVED LIGHT CURVES WHICH CAN THEN BE COMPARED WITH OBSERVATIONS. TO GAIN DEEPER PHYSICAL INSIGHT INTO THE INTERMITTENT PARTICLE ACCELERATION PROCESSES IN TURBULENT ASTROPHYSICAL ENVIRONMENTS THE PROJECT WILL ALSO IDENTIFY AND ANALYZE STATISTICALLY THE CURRENT SHEETS SHOCKS AND OTHER RELEVANT LOCALIZED PARTICLE-ACCELERATION STRUCTURES FOUND IN THE SIMULATIONS. IN PARTICULAR IT WILL ASSESS WHETHER RELATIVISTIC KINETIC TURBULENCE IN PWN CAN SELF-CONSISTENTLY GENERATE SUCH STRUCTURES THAT ARE LONG AND STRONG ENOUGH TO ACCELERATE LARGE NUMBERS OF PARTICLES TO THE PEV ENERGIES REQUIRED TO EXPLAIN THE CRAB GAMMA-RAY FLARES AND WHERE AND UNDER WHAT CONDITIONS SUCH ACCELERATION CAN OCCUR. THE RESULTS OF THIS RESEARCH WILL ALSO ADVANCE OUR UNDERSTANDING THE ORIGIN OF ULTRA-RAPID TEV FLARES IN BLAZAR JETS AND WILL HAVE IMPORTANT IMPLICATIONS FOR GRB PROMPT EMISSION AS WELL AS AGN RADIO-LOBES AND RADIATIVELY-INEFFICIENT ACCRETION FLOWS SUCH AS THE FLOW ONTO THE SUPERMASSIVE BLACK HOLE AT OUR GALACTIC CENTER., RADIATIVE SIGNATURES OF RECONNECTION IN X-RAY BINARY SPECTRAL STATES ACCRETING BLACK HOLES (BHS) IN GALACTIC X-RAY BINARY (XRB) SYSTEMS REPRESENT SOM
OpenAlex ID: W2915367205