Final Report Summary - GRBS (Gamma Ray Bursts as a Focal Point of High Energy Astrophysics)
The research, under the advanced ERC project GRBs dealt as its name suggests with Gamma-Ray Bursts, the strongest explosion in the Universe and related issues. Our main achievements include:
According to the Collapsar model a long GRB arises when a central engine ejects a relativistic jet. The jet penetrates the stellar envelope and the burst is produces once the jet has emerged from the star. We have explored baryonic and magnetic dominated jet propagation within stellar envelope. We have found that this propagation is consistent for baryonic flux but inconsistent for Poynting flux dominated flux. These results have a few implications. First a new classification scheme of GRBs demonstrating that there are three groups of GRBs. Long, short and low-luminosity. We have demonstrated that the third group must arise from a different physical mechanism than the first two. Second, we have also obtained a statistical model predicting the probability that a given GRB that is shorter than 2 sec is indeed a genuine ``short” one (i.e. arising from a merger) rather than simply a short Collapsar. Third, following this analysis, our finding of a plateau in the distribution of time duration of long GRBs provides a new proof for the validity of the Collapsar model. Finally, the observed properties of long GRBs and concluded that if GRBs’ central engines produce magnetic dominated jets, then the magnetic jets dissipate their magnetic energy before emerging from the star. This results that sheds new light on the inner structure of GRBs are consistent with our independent findings that show that the emitting regions in which the prompt emission of the GRBs is produced are not dominated by magnetic fields.
We have predicted that neutron star binary mergers will be followed by long lasting radio flares. These ratio flares provide us with another tool with which we can identify the location of these mergers. This is particularly important, as these flares will follow gravitational radiation events detected by the upcoming advanced gravitational radiation detectors.
An unusual near Infra red flare was observed by the Hubble space telescope following the short GRB 130603B. Indeed what was observed was only a single measurement at a single epoch. Still if the interpretation of this observation as a Macronova (a signal arising from radioactive decay of debris of a neutron star merger) is correct than it has a few spectacular implications: It confirms our long standing (from 1989) prediction that short GRBs arise from neutron star mergers and that these mergers are the source of heavy rare metals like gold in the Universe. We have suggested that a radio flare should follow this event and we have begun searching using the VLA these radio signals. We have also identified a similar event following GRB 060614, providing further support for this interpretation and we begun similar radio searches.
We have explored the likelihood that a strong nearby GRB disturbed life of Earth. We have found that there is a 50:50 chance that such an event indeed took place within the last Billion years. It is possible and even likely that a nearby GRB led to the Ordovician-Silurian extinction about 430 Millions years ago. We have extended the study to the whole Milky Way galaxy and have shown that GRBs are much more dangerous for life in the inner region of the galaxy, providing an possible explanation why life developed on Earth, a planet located in the outer region of the Milky Way, and not at the center. We have also shown that life is unlikely in small Galaxies and one needs a large Galaxy like ours for life to develop.
In a project related to GRBs we have suggested novel ideas concerning the tidal disruption of stars by massive black holes. These ideas revolutionize the standard paradigm concerning these tidal disruption events and suggest a model, whose predictions agree with all present observations, in which the observed radiation arise from shocks within tidal streams interacting with each other and doesn’t arise from regular accretion disks.
According to the Collapsar model a long GRB arises when a central engine ejects a relativistic jet. The jet penetrates the stellar envelope and the burst is produces once the jet has emerged from the star. We have explored baryonic and magnetic dominated jet propagation within stellar envelope. We have found that this propagation is consistent for baryonic flux but inconsistent for Poynting flux dominated flux. These results have a few implications. First a new classification scheme of GRBs demonstrating that there are three groups of GRBs. Long, short and low-luminosity. We have demonstrated that the third group must arise from a different physical mechanism than the first two. Second, we have also obtained a statistical model predicting the probability that a given GRB that is shorter than 2 sec is indeed a genuine ``short” one (i.e. arising from a merger) rather than simply a short Collapsar. Third, following this analysis, our finding of a plateau in the distribution of time duration of long GRBs provides a new proof for the validity of the Collapsar model. Finally, the observed properties of long GRBs and concluded that if GRBs’ central engines produce magnetic dominated jets, then the magnetic jets dissipate their magnetic energy before emerging from the star. This results that sheds new light on the inner structure of GRBs are consistent with our independent findings that show that the emitting regions in which the prompt emission of the GRBs is produced are not dominated by magnetic fields.
We have predicted that neutron star binary mergers will be followed by long lasting radio flares. These ratio flares provide us with another tool with which we can identify the location of these mergers. This is particularly important, as these flares will follow gravitational radiation events detected by the upcoming advanced gravitational radiation detectors.
An unusual near Infra red flare was observed by the Hubble space telescope following the short GRB 130603B. Indeed what was observed was only a single measurement at a single epoch. Still if the interpretation of this observation as a Macronova (a signal arising from radioactive decay of debris of a neutron star merger) is correct than it has a few spectacular implications: It confirms our long standing (from 1989) prediction that short GRBs arise from neutron star mergers and that these mergers are the source of heavy rare metals like gold in the Universe. We have suggested that a radio flare should follow this event and we have begun searching using the VLA these radio signals. We have also identified a similar event following GRB 060614, providing further support for this interpretation and we begun similar radio searches.
We have explored the likelihood that a strong nearby GRB disturbed life of Earth. We have found that there is a 50:50 chance that such an event indeed took place within the last Billion years. It is possible and even likely that a nearby GRB led to the Ordovician-Silurian extinction about 430 Millions years ago. We have extended the study to the whole Milky Way galaxy and have shown that GRBs are much more dangerous for life in the inner region of the galaxy, providing an possible explanation why life developed on Earth, a planet located in the outer region of the Milky Way, and not at the center. We have also shown that life is unlikely in small Galaxies and one needs a large Galaxy like ours for life to develop.
In a project related to GRBs we have suggested novel ideas concerning the tidal disruption of stars by massive black holes. These ideas revolutionize the standard paradigm concerning these tidal disruption events and suggest a model, whose predictions agree with all present observations, in which the observed radiation arise from shocks within tidal streams interacting with each other and doesn’t arise from regular accretion disks.