Skip to main content

Sub-photospheric shocks in relativistic explosions

Image: In the most common type of gamma-ray burst, illustrated here, a dying massive star forms a black hole (left), which drives a particle jet into space. Light across the spectrum arises from hot gas near the black hole, collisions within the jet, and from the jet's interaction with its surroundings. Credit: NASA's Goddard Space Flight Center


Astrophysical explosions and jets generate shock waves, which produce radiation. Their radiative properties are determined by the dissipation mechanism that sustains the velocity jump in the shock and by its ability to generate nonthermal particles.

A recent paper (Beloborodov 2016) examines the mechanism of internal shocks in gamma-ray bursts (GRBs) that occur before the GRB jets become transparent to radiation. The approach and some of the results may also be of interest for other explosions, e.g. in novae or supernovae.

Sub-photospheric shocks can produce neutrino emission and affect the observed photospheric radiation from the explosion. Shocks develop from internal compressive waves and can be of different types depending on the composition of the flow:
  1. Shocks in 'photon gas' with small plasma inertial mass have a unique structure determined by the 'force-free' condition -- zero radiation flux in the plasma rest frame. Radiation dominance over plasma inertia suppresses formation of collisionless shocks mediated by collective electromagnetic fields. 
  2. Strong collisionless subshocks develop in the opaque flow if it is sufficiently magnetized. The author evaluates the critical magnetization for this to happen. The collisionless subshock is embedded in a thicker radiation-mediated shock structure.
  3. Shocks in outflows carrying a free neutron component involve dissipation through nuclear collisions. At large optical depths, the shock thickness is comparable to the neutron free path, with an embedded radiation-mediated or collisionless subshock.
The author finds that sub-photospheric shocks are capable of generating high-energy particles and boosting the photon number carried by the outflow, and that the shock type changes as the outflow expands toward its photosphere. Strong e± pair creation occurs as the shock wave approaches the photosphere. Then the e±-dressed shock carries the photosphere with it up to two decades in radius, producing a strong observed pulse of nonthermal photospheric radiation.

  • Beloborodov 2016 (preprint) - Sub-photospheric shocks in relativistic explosions - (arXiv)
Etichette:

Comments

Post a Comment

Popular posts from this blog

ORBITAL PERIODS OF THE PLANETS

For orbital period generally we refer to the sidereal period, that is the temporal cycle that it takes an object to make a full orbit, relative to the stars. This is the orbital period in an inertial (non-rotating) frame of reference (365,25 days for the earth).
Post a Comment
Read more

A Sapphire Super-Earth

Twenty-one light years away, in the constellation Cassiopeia, a planet by the name of HD219134 b orbits its star with a year that is just three days long. With a mass almost five times that of Earth, it is what is known as a super-Earth. Unlike our planet, however, these super-Earths were formed at high temperatures close to their host star and contain high quantities of calcium, aluminum and their oxides – including sapphire and ruby. HD219134 b is one of three candidates likely to belong to a new, exotic class of exoplanets. These objects are completely different from the majority of Earth-like planets. They have 10 to 20 percent lower densities than Earth. Researchers looked at different scenarios to explain the observed densities. For example, a thick atmosphere could lead to a lower overall density. But two of the exoplanets studied, 55 Cancri e and WASP-47 e, orbit their star so closely that their surface temperature is almost 3,000 degrees and they would have lost this ...
Post a Comment
Read more

Hubble observes energetic lightshow at Saturn’s north pole

This image is a composite of observations made of Saturn in early 2018 in the optical and of the auroras on Saturn’s north pole region, made in 2017. In contrast to the auroras on Earth the auroras on Saturn are only visible in the ultraviolet — a part of the electromagnetic spectrum blocked by Earth’s atmosphere — and therefore astronomers have to rely on space telescopes like the NASA/ESA Hubble Space Telescope to study them.  Credit:  ESA/Hubble, NASA, A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI), L. Lamy (Observatoire de Paris) In 2017, over a period of seven months, the NASA/ESA Hubble Space Telescope took images of auroras above Saturn’s north pole region using the Space Telescope Imaging Spectrograph. The observations were taken before and after the Saturnian northern summer solstice. These conditions provided the best achievable viewing of the northern auroral region for Hubble. On Earth, auroras are mainly created by particles or...
Post a Comment
Read more