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Register a new userGamma--Ray Bursts associated with Supernovae: A systematic analysis of BATSE GRB candidates
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Added by
Giancarlo Ghirlanda
Publication date
2005
fields
Physics
and research's language is
English
Authors
Z. Bosnjak
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We examined the properties of a sample of BATSE Gamma--Ray Bursts (GRBs) comprising events which have indications of association with a supernova (SN), some on the basis of indications of re--brightening in the optical afterglow light curve, but in most cases based only on the `loose temporal and directional coincidence inferred from the cross correlation of catalogs. Despite of the large uncertainties in the latter selection method, the temporal and spectral analysis reveal three interesting statistical results when the sample is compared with that of all the BATSE GRBs: the GRBs tentatively associated with SNe are found to predominantly (in $sim$ 80% of the cases) have single-peaked light curves, a softer spectrum (i.e. low energy power law index $alpha sim$ --1.5) and tend not to follow the Lag-Luminosity and Isotropic Energy--Peak Energy correlations. These three independent statistical properties point toward the existence of a significant number of under-luminous,GRB 980425-like events constituting -- at least from an observational point of view -- a tail or a separate class with respect to the whole of the BATSE GRB events. The unusually high percentage of SN Ibc among those identified by the catalog cross--correlation (factor $sim 4$ higher than expected from SN catalog statistics) reinforces the non-randomness of (some of) the selected events.
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The connection between long GRBs and supernovae is now well established. I briefly review the evidence in favor of this connection and summarise where we are observationally. I also use a few events to exemplify what should be done and what type of data are needed. I also look at what we can learn from looking at SNe not associated with GRBs and see how GRBs fit into the broad picture of stellar explosions.
It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard collapsar model predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge this simple view. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these long-duration bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in star-forming regions. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death. From the supplementary material: Now we have observed SN-less GRBs in star-forming regions, suggesting that a non-detection of a SN does not preclude a massive progenitor. The position of the GRB, i.e. in a star-forming region or in an older component, may be the only way to discriminate between merging compact objects and massive stars as progenitors. In fact, several host galaxies for short GRBs have been found to be as actively star-forming as some host galaxies of long-duration GRBs. The GRB labels long and short have become synonymous with massive stars and other progenitors. These distinctions may need to be relaxed.
GRB 111209A, one of the longest Gamma-Ray Bursts (GRBs) ever observed, is linked to SN 2011kl, the most luminous GRB-Supernova (SN) detected so far, which shows evidence for being powered by a magnetar central engine. We place SN 2011kl into the context of large samples of SNe, addressing in more detail the question of whether it could be radioactively powered, and whether it represents an extreme version of a GRB-SN or an underluminous Superluminous SN (SLSN). We model SN 2011kl using SN 1998bw as a template and derive a bolometric light curve including near-infrared data. We compare the properties of SN 2011kl to literature results on stripped-envelope and superluminous supernovae. Comparison in the k,s context, i.e., comparing it to SN 1998bw templates in terms of luminosity and light-curve stretch, clearly shows SN 2011kl is the most luminous GRB-SN to date, and it is spectrally very dissimilar to other events, being significantly bluer/hotter. Although SN 2011kl does not reach the classical luminosity threshold of SLSNe and evolves faster than any of them, it resembles SLSNe more than the classical GRB-associated broad-lined Type Ic SNe in several aspects. GRB 111209A was a very energetic event, both at early (prompt emission) and at very late (SN) times. We have shown in a further publication that with the exception of the extreme duration, the GRB and afterglow parameters are in agreement with the known distributions for these parameters. SN 2011kl, on the other hand, is exceptional both in luminosity and spectral characteristics, indicating that GRB 111209A was likely not powered by a standard-model collapsar central engine, further supporting our earlier conclusions. Instead, it reveals the possibility of a direct link between GRBs and SLSNe.
Six intensity groups with ~150 BATSE gamma-ray bursts each are compared using average emissivity curves. Time-stretch factors for each of the dimmer groups are estimated with respect to the brightest group, which serves as the reference, taking into account the systematics of counts-produced noise effects and choice statistics. A stretching/intensity anti-correlation is found with good statistical significance during the average back slopes of bursts. A stretch factor ~2 is found between the 150 dimmest bursts, with peak flux <0.45ph cm^{-2} s^{-1}, and the 147 brightest bursts, with peak flux >4.1 ph cm^{-2} s^{-1}. On the other hand, while a trend of increasing stretching factor may exist for rise fronts for burst with decreasing peak flux from >4.1 ph cm^{-2} s^{-1} down to 0.7 ph cm^{-2} s^{-1}, the magnitude of the stretching factor is less than ~ 1.4 and is therefore inconsistent with stretching factor of back slope.
We have recently completed a search of 6 years of archival BATSE data for gamma-ray bursts (GRBs) that were too faint to activate the real-time burst detection system running onboard the spacecraft. These non-triggered bursts can be combined with the triggered bursts detected onboard to produce a GRB intensity distribution that reaches peak fluxes a factor of 2 lower than could be studied previously. The value of the V/Vmax statistic (in Euclidean space) for the bursts we detect is 0.177 +/- 0.006. This surprisingly low value is obtained because we detected very few bursts on the 4.096 s and 8.192 s time scales (where most bursts have their highest signal-to-noise ratio) that were not already detected on the 1.024 s time scale. If allowance is made for a power-law distribution of intrinsic peak luminosities, the extended peak flux distribution is consistent with models in which the redshift distribution of the gamma-ray burst rate approximately traces the star formation history of the Universe. We argue that this class of models is preferred over those in which the burst rate is independent of redshift. We use the peak flux distribution to derive a limit of 10% (99% confidence) on the fraction of the total burst rate that could be contributed by a spatially homogeneous (in Euclidean space) subpopulation of burst sources, such as type Ib/c supernovae. These results lend support to the conclusions of previous studies predicting that relatively few faint classical GRBs will be found below the BATSE onboard detection threshold.
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