A study of gamma ray bursts with afterglow plateau phases associated with supernovae

The analysis of 176 gamma ray burst (GRB) afterglow plateaus observed by Swift from GRBs with known redshifts revealed that the subsample of long GRBs associated with supernovae (LONG-SNe) - 19 events - presents a very high correlation coefficient between the luminosity at the end of the plateau phase La and the end time of the plateau T*a, hereafter Dainotti relation. Furthermore, these SNe Ib/c associated with GRBs also obey the peak-magnitude stretch relation, similar to that used to standardize the SNe Ia. We here investigate a category of GRBs with plateau and associated with SNe to compare the Dainotti correlation for this sample with the correlation for long GRBs for which no associated SN has been observed (hereafter LONG-NO-SNe, 128 GRBs) and to check whether there is a difference among these sub-samples. We first adopted a non-parametric statistical method to take redshift evolution into account and to check if and how this effect may steepen the slope for the LONG-NO-SNe sample. Therefore, removing selection bias is the first step before any comparison among samples observed at different redshifts could be properly performed. Then, we applied the T-student test to evaluate a statistical difference between the slopes of the two samples. We demonstrate that there is no evolution for the slope of the LONG-NO-SNe sample and no evolution is expected for the LONG-SNe sample at small redshifts. The difference between the slope of the LONG-NO-SNe and the slope of LONG-SNe with firm spectral detection of SN components, is statistically significant. This possibly suggests that, unlike LONG-NO-SNe, LONG-SNe with firm spectroscopic features of the associated SNe might not require a standard energy reservoir in the plateau phase. Therefore, this analysis may open new perspectives in future theoretical investigations of the GRBs with plateau emission and that are associated with SNe.Comment: 11 pages, 10 figures, 2 Tables, in press on Astronomy and Astrophysics, 8 dicember 201

Supernova spectra below strong circumstellar interaction

We construct spectra of supernovae (SNe) interacting strongly with a circumstellar medium (CSM) by adding SN templates, a blackbody continuum, and an emission-line spectrum. In a Monte Carlo simulation we vary a large number of parameters, such as the SN type, brightness and phase, the strength of the CSM interaction, the extinction, and the signal to noise ratio (S/N) of the observed spectrum. We generate more than 800 spectra, distribute them to ten different human classifiers, and study how the different simulation parameters affect the appearance of the spectra and their classification. The SNe IIn showing some structure over the continuum were characterized as “SNe IInS” to allow for a better quantification. We demonstrate that the flux ratio of the underlying SN to the continuum fV is the single most important parameter determining whether a spectrum can be classified correctly. Other parameters, such as extinction, S/N, and the width and strength of the emission lines, do not play a significant role. Thermonuclear SNe get progressively classified as Ia-CSM, IInS, and IIn as fV decreases. The transition between Ia-CSM and IInS occurs at fV ∼ 0.2−0.3. It is therefore possible to determine that SNe Ia-CSM are found at the (un-extincted) magnitude range −19.5 > M > −21.6, in very good agreement with observations, and that the faintest SN IIn that can hide a SN Ia has M = −20.1. The literature sample of SNe Ia-CSM shows an association with 91T-like SNe Ia. Our experiment does not support that this association can be attributed to a luminosity bias (91T-like being brighter than normal events). We therefore conclude that this association has real physical origins and we propose that 91T-like explosions result from single degenerate progenitors that are responsible for the CSM. Despite the spectroscopic similarities between SNe Ibc and SNe Ia, the number of misclassifications between these types was very small in our simulation and mostly at low S/N. Combined with the SN luminosity function needed to reproduce the observed SN Ia-CSM luminosities, it is unlikely that SNe Ibc constitute an important contaminant within this sample. We show how Type II spectra transition to IIn and how the Hα profiles vary with fV . SNe IIn fainter than M = −17.2 are unable to mask SNe IIP brighter than M = −15. A more advanced simulation, including radiative transfer, shows that our simplified model is a good first order approximation. The spectra obtained are in good agreement with real data

A Chandrasekhar mass progenitor for the type Ia supernova remnant 3C 397 from the enhanced abundances of nickel and manganese

Despite decades of intense efforts, many fundamental aspects of Type Ia supernovae (SNe Ia) remain elusive. One of the major open questions is whether the mass of an exploding white dwarf (WD) is close to the Chandrasekhar limit. Here, we report the detection of strong K-shell emission from stable Fe-peak elements in the Suzaku X-ray spectrum of the Type Ia supernova remnant (SNR) 3C 397. The high Ni/Fe and Mn/Fe mass ratios (0.11–0.24 and 0.018–0.033, respectively) in the hot plasma component that dominates the K-shell emission lines indicate a degree of neutronization in the supernova ejecta that can only be achieved by electron capture in the dense cores of exploding WDs with a near-Chandrasekhar mass. This suggests a single-degenerate origin for 3C 397, since Chandrasekhar mass progenitors are expected naturally if the WD accretes mass slowly from a companion. Together with other results supporting the double-degenerate scenario, our work adds to the mounting evidence that both progenitor channels make a significant contribution to the SN Ia rate in star-forming galaxies.Peer Reviewe

Installation of a Dipole Electromagnet RTAGX(I. Nuclear Physics)

A dipole electromagnet RTAGX has been installed in the GeV-γ experimental hall. It sweeps out charged particles contaminated in the incident γ beam for meson photo-production experiments. It also supplies momentum-analyzed electrons or positrons in a newly constructed test beamline