GRB classification: short and intermediate GRBs

Though it is clear that the GRB population as a whole is comprised of multiple pro- genitor systems, it is not always clear how best to classify them. Some authors (e.g. Zhang et al. 2009) have suggested a complex classification system based on multi- ple measured and intrinsic properties such as associated supernovae, environment, energy properties and host galaxies in addition to duration and spectral hardness. However, many of these classification schemes require detailed information for each individual GRB which in many cases, especially for the shorter lived fainter GRBs, is simply not available.

The simpler two second divide, more often used to define SGRBs, was based on number statistics measured by the BATSE instrument on the CGRO telescope and not the currently usedSwift telescope with a softer spectral response function. Bromberg et al. (2013) suggest that a division at 0.8 s would include a sample with 20% contamination from collapsar objects as opposed to 40% for a sample defined by the two second divide. In particular, the sample from chapter 4 does contain at least two GRBs which are more likely to be collapsars than merger events: GRBs 090426 and 060121, as evidenced, in particular, by their host galaxy environments and emission properties (Levan et al., 2006a; Antonelli et al., 2009; Levesque et al., 2010a).

However, from both constraints on a bright accompanying supernova and high probabilities of an object being a non-collapsar (fN C >0.5), we find a max-

imum collapsar contamination of ∼ 22% when considering the limited sample of SGRBs with measured redshifts. In addition, when looking at the GRB positions with respect to the brightest regions of the host galaxy (Flight) for the sample of

GRBs between 0.8 < T90 < 2.0 s, the possibility of this sample being drawn from the same underlying population as the long population is strongly rejected. From a rough test of a non-collapsar sample with 40% contamination from randomly drawn collapsar objects we find the same level of rejection only 4% of the time. Consider- ing the prompt emission and afterglow properties, the mild increase in luminosities from the shortest GRBs to this higher duration sample indicates there is some con- tamination from collapsar objects but this is unlikely to be as high as previously suggested. This is encouraging for the creation of clean samples of SGRBs with a larger sample size especially when deriving overall SGRB properties.

The case of GRB 100816A also adds an interesting prospect to the study of SGRBs. This GRB is traditionally long (T90 = 2.9±0.6 s, Swift) but its spectral hardness, afterglow properties and, in particular, host environment all suggest it is more consistent with being a non-collapsar object. In addition, even though the prompt emission properties show this GRB is compatible with the Eiso - Epeak,src

relation, applicable to LGRBs but not to SGRBs, it has a particularly low isotropic equivalent energy for an LGRB. The association of this GRB with the SGRB pop- ulation is actually consistent with the result of Bromberg et al. (2013) if we also consider its spectral hardness. For the hardest GRBs, the duration splits suggested by Bromberg et al. (2013) are much higher than when considering the population as a whole. For instance when examining theFermi duration distribution atT90∼2 s the probability of being a non-collapsar object is as high asfN C ∼0.8, suggesting

GRB 100816A, with a Fermi duration of T90 = 2.04±0.23 s, is most likely a non- collapsar object. This opens up the interesting question of additional objects at this longer duration which may also be non-collapsar objects and should be part of the “short” class of GRBs.

Contemplating these results in the context of future classification schemes, we posit that the host galaxy characteristics, both in terms of galaxy properties and position within the host, and the hardness could be more telling than the duration. Though the population does have a peak at shorter durations, GRBs with extended emission do demonstrate that these objects are capable of producing longer duration GRBs in some form and that considering this population of objects as “short” is perhaps causing some objects to be misclassified. A classification scheme based on hardness, or at least combining duration with hardness, would perhaps prove more fruitful at producing a complete sample while still keeping the sample as clean as possible. The host galaxy properties and, in particular, Flight values are also

particularly telling. The strong preference of SGRBs for lowFlightcould prove as a

coupled with host galaxy properties not consistent with the LGRB population. From the suggestion of a separate population of intermediate GRBs, the sample considered here (defined as GRBs with 2.0 s < T90 ≤ 10.0 s) did not show any significant difference from the LGRB population. This suggests that, if this class is significant, overlap from both the short and long classes are not allowing a clean sample to be determined. However, there are likely to be additional objects other than neutron star mergers and collapsars capable of producing GRB-like objects such as massive white dwarf - neutron star or white dwarf - black hole mergers and it could perhaps be the case that these are represented by this intermediate population.