Year: 2020
Type: article
Abstract: A comprehensive understanding of gamma-ray bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two-dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carlo Radiation Transport (MCRaT) code. MCRaT has been enhanced by the inclusion of polarization; it has been first verified by reproducing a variety of results in the literature and then used to obtain the time-integrated and time-resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees (≲1%) are consistent with the constraints from the POLAR experiment, they are lower than other theoretical studies due to the lack of strong gradients in the model jet profiles that we use. The time-resolved results suggests that GRBs with wide jets observed on axis will have small polarization degrees (≲2%) and constant polarization angles, during the brightest portion of the light curve. GRBs observed off-axis will have larger polarization degrees and polarization angles that change with the temporal structure of radiating shells in the outflow. We then place our results in the context of GRB prompt emission models and future LEAP and POLAR-2 GRB polarimetry detections.
Source: The Astrophysical Journal
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,499
Awards: G4045949753, THE DETECTION OF A GAMMA-RAY TRANSIENT ASSOCIATED WITH GW170817 HERALDED THE NEW ERA OF MULTI-MESSENGER ASTROPHYSICS. HOWEVER ITS ORIGIN HAS YET TO BE FULLY UNDERSTOOD. THE TIMING AND ENERGETICS OF THE GAMMA-RAY TRANSIENT WERE CONSISTENT WITH A SIMPLE COCOON MODEL BUT THE SPECTRUM SHOWED SIGNS OF DISSIPATION AND PAIR SATURATION. A SHOCK BREAKOUT CAN EXPLAIN THE SPECTRUM BUT REQUIRES THE PRESENCE OF AD-HOC EJECTA FROM THE MERGER AND AN UNCOMFORTABLE FINE-TUNING BETWEEN THE COLLAPSE-TO-JET DELAY AND THE JET TRANSIT TIME. WE ARGUE THAT AN UNSTEADY CENTRAL ENGINE DRIVES MULTIPLE COCOON SHELLS WHOSE COLLISIONS CREATE A NATURAL SHOCK BREAKOUT. WE PROPOSE TO DERIVE THE BREAKOUT PROPERTIES ANALYTICALLY AND PERFORM NUMERICAL SIMULATIONS TO CONFIRM THE VALIDITY OF THE MOST INTERESTING CASES., THE DETECTION OF GRAVITATIONAL WAVES (GW) FROM BINARY BLACK HOLE MERGERS HAS BEEN AN HISTORICAL TRANSFORMATIVE EVENT IN PHYSICS AND ASTRONOMY HERALDED BY MOST AS THE BEGINNING OF MULTI-MESSENGER ASTRONOMY. WITH THE INCREASE OF SENSITIVITY OVER THE NEXT FEW YEARS LIGO AND VIRGO ARE PREDICTED TO DETECT MERGERS FROM NEUTRON-STAR (NS) BINARIES. THESE ARE EXPECTED TO BE THE FIRST TRUE MULTI-MESSENGER SOURCES BEING THE PROGENITORS OF SHORT-DURATION GAMMA-RAY BURST (SGRB). THE SIMULTANEOUS DETECTION OF A GRAVITATIONAL ELECTROMAGNETIC AND POSSIBLY NEUTRINO SIGNALS FROM THE SAME SOURCE WOULD DRAMATICALLY ENHANCE THE SCIENTIFIC OUTPUT OF EACH INDIVIDUAL DETECTION. IMPORTANT DETAILS OF THE CONNECTION BETWEEN SGRBS AND NS BINARY MERGERS ARE HOWEVER POORLY KNOWN. THESE INCLUDE THE NATURE OF THE MERGING COMPACT OBJECTS THEIR EQUATION OF STATE THE PHYSICS OF SGRB JETS - SUCH AS THEIR LORENTZ FACTORS AND OPENING ANGLES AND THE POSSIBILITY OF SMALL TEMPORAL DELAYS AMONG THE GW NEUTRINO AND GAMMA-RAY SIGNALS. IN VIEW OF THE EXPECTED INCREASED SENSITIVITY OF LIGO DURING THE UPCOMING OBSERVING PERIOD AND BEYOND THERE IS URGENT NEED OF IMPROVING OUR UNDERSTANDING OF THE PHYSICS OF SGRBS TO SUPPORT THE DETECTION OF GWS (AND POSSIBLY NEUTRINOS) AND TO DEVELOP A CONTEXT IN WHICH THE EXPECTED MULTI-MESSENGER SIGNAL CAN BE PROPERLY INTERPRETED AND ITS POTENTIAL FULLY EXPLOITED. TO ACHIEVE SUCH GOALS WE PROPOSE TO CARRY OUT A COMPREHENSIVE STUDY OF RELATIVISTIC JETS FROM COMPACT BINARY MERGERS EXPLOITING THE MOST RECENT ADVANCES IN NUMERICAL TECHNIQUES DEVELOPED WITHIN THIS RESEARCH GROUP. THE ANSATZ OF THIS STUDY WILL BE THAT WITHIN A SHORT TIME AFTER A COMPACT MERGER A RELATIVISTIC JET IS CREATED. SUBSEQUENTLY THE JET INTERACTS WITH THE MERGER ENVIRONMENT IMPRINTING A SIGNATURE THAT CAN BE DETECTED IN THE TEMPORAL AND SPECTRAL PROPERTIES OF THE PROMPT RADIATION BOTH IN ITS ELECTROMAGNETIC AND NEUTRINO COMPONENTS. ANALOGOUS DYNAMICAL EFFECTS HAVE BEEN OBSERVED AND STUDIED EXTENSIVELY FOR LONG-DURATION GRBS. SINCE DIFFERENT PROGENITORS PRODUCE DIFFERENT ENVIRONMENTS AND PHYSICAL CONDITIONS THE PROPERTIES OF THE GAMMA-RAY AND NEUTRINO SIGNALS WILL BE A PROXY TO THE PHYSICS OF THE MERGER AND ULTIMATELY TO THE EXPECTED GW SIGNAL. WE WILL PERFORM A COMBINATION OF STATE-OF-THEART NUMERICAL SIMULATIONS COVERING ALL DIFFERENT PHASES OF THE EVENT INCLUDING THE COALESCENCE AND MERGER OF THE PROGENITOR COMPACT BINARY SYSTEM THE SMALL TO LARGE SCALE JET DYNAMICS AND THE RADIATION TRANSFER PHYSICS LEADING TO ELECTROMAGNETIC AND NEUTRINO SIGNALS. OUR PRODUCTS WILL INCLUDE MULTI MESSENGER PREDICTIONS NOT ONLY FOR ON-AXIS BURSTS THOSE POINTING DIRECTLY AT EARTH BUT ALSO FOR OFF-AXIS EVENTS THOSE WITH JETS THAT POINT AWAY FROM OUR DETECTORS. OFF-AXIS BURSTS ARE EXPECTED TO HAVE A DIM ELECTROMAGNETIC SIGNATURE BUT THEY CONSTITUTE THE DOMINANT POPULATION OF LIGO DETECTED NS BINARY MERGERS., G6669853532, WoU-MMA: Collaborative Research: Combining Theory with Observations to Unlock the Multi-Messenger Physics of Compact Binary Mergers, G7581042175
OpenAlex ID: W3022849431