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A Clash of Clusters Provides New Clue to Dark Matter
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Why are there so few jet breaks in the Swift era?
A jet break is an achromatic steepening in the flux decay rate of the afterglow emission that occurs when the edges of the jet become visible as it decelerates. Such apparent jet breaks have been frequently detected before the launch of Swift, in a fairly large fraction of well monitored afterglows, and have been used in order to infer the opening angle of the jet and thereby its true energy, in gamma-rays and in the kinetic energy of the afterglow shock, both of which were shown to have a narrower distribution compared to their isotropic equivalent values. After the launch of Swift, however, very few jet breaks were observed. This might be partly due to the larger sensitivity of Swift, which causes it to detect dimmer events on average, which correspond to wider jets with a later jet break time and smaller flux around that time, making it harder to observe the jet break. It is not yet clear whether this is the only reason for the paucity of jet breaks in the Swift era. There is also some initial evidence that the distribution of the true energy (in gamma-rays or kinetic form) is not as narrow as initially thought, and might have a comparable width to that of the isotropic equivalent energy. Furthermore, there are other breaks seen in many afterglows, some of which are chromatic, which are often confused with the jet break. We will discuss these issues as well as possible ways to try and answer some of the related open questions.
- Recommended Reading:
- (Granot 2006, only sub-section 2.6 - not the whole paper!!!): Discusses the cause for the jet break in the afterglow light curve.
- (Frail et al. 2001): Found that when inferring the jet openning angle from the jet break time, the true (corrected for the finite solid angle occupied by the jet) energy in gamma-rays has a much narrower distribution than its isotropic equivalent value, and peaks around 10^{51} erg.
- (O'Brien et al. 2006, only sub-section 3.4 - not the whole paper!!!): Briefly mentions that apparent problem with the small number of jet breaks observed in the Swift era.
- (Sato et al. 2006): Analyses 3 Swift X-ray afterglow light curves that extend out to very late times without showing a jet break, in contrast with the expectations of the Ghirlanda relation.
- (Panaitescu 2005): Compares the light curve breaks expected from a variety of models (jets, structures outflows, and cessation of energy injection) with those measured in the optical emission of 10 GRBs.
- Additional Reading (not required):
- (Rhoads 1997): The original prediction of a jet break in the afterglow light cuve.
- (Sari, Piran, Halpern 1999): Presents a simple and easy to use version of the semi-analytic jet model of Rhoads 1999.
- (Panaitescu & Kumar 2002): Derive the physical parameters for 10 well maonitored afterglows, and find that the true (beaming corrected) kinetic energy in the afterglow shock has a significantly smaller dispersion than its isotropic equivalent value.
- (Bloom et al. 2003): More complete analysis of the original Frail et al. (2001) work.
- (Panaitescu 2005): Numerical modeling of various mechanisms that could cause achromatic breaks in afterglow light curves.
- (Curran et al. 2006): Observations of a peculiar light curve break in GRB 060214.
- (Mundell et al. 2006): Observations of a peculiar light curve break in GRB061007.
- Acknowledgments: I thank Daniel Kocevski for helping me organize this meeting.
MASS: Lance Simms