Gamma Ray Burst Prompt correlations

The mechanism responsible for the prompt emission of gamma-ray bursts (GRBs) is still a debated issue. The prompt phase-related GRB correlations can allow to discriminate among the most plausible theoretical models explaining this emission. We present an overview of the observational two-parameter correlations, their physical interpretations, their use as redshift estimators and possibly as cosmological tools. The nowadays challenge is to make GRBs, the farthest stellar-scaled objects observed (up to redshift $z=9.4$), standard candles through well established and robust correlations. However, GRBs spanning several orders of magnitude in their energetics are far from being standard candles. We describe the advances in the prompt correlation research in the past decades, with particular focus paid to the discoveries in the last 20 years

Study of GRB light curve decay indices in the afterglow phase

In this work we study the distribution of temporal power-law decay indices, $\alpha$, in the Gamma Ray Burst (GRB) afterglow phase, fitted for $176$ GRBs (139 long GRBs, 12 short GRBs {\it with extended emission} and 25 X-Ray Flashes (XRFs)) with known redshifts. These indices are compared with the temporal decay index, $\alpha_W$, derived with the light curve fitting using the \cite{willingale07} model. This model fitting yields similar distributions of $\alpha_W$ to the fitted $\alpha$, but for individual bursts a difference can be significant. Analysis of ($\alpha$, $L_a$) distribution, where $L_a$ is the characteristic luminosity at the end of the plateau, reveals only a weak correlation of these quantities. However, we discovered a significant regular trend when studying GRB $\alpha$ values along the \cite{dainotti2008} correlation between $L_a$ and the end time of the plateau emission in the rest frame, $T_a^*$, hereafter LT correlation. We note a systematic variation of the $\alpha$ parameter distribution with luminosity for any selected $T_a^*$. We analyze this systematics with respect to the fitted LT correlation line, expecting that the presented trend may allow to constrain the GRB physical models. We also attempted to use the derived correlation of $\alpha(T_a)$ versus $L_a(T_a)$ to diminish the luminosity scatter related to the variations of $\alpha$ along the LT distribution, a step forward in the effort of standardizing GRBs. A proposed toy model accounting for this systematics applied to the analyzed GRB distribution results in a slight increase of the LT correlation coefficient.Comment: Accepted for publication in Ap

Determination of the intrinsic Luminosity Time Correlation in the X-ray Afterglows of GRBs

Gamma-ray bursts (GRBs), which have been observed up to redshifts z approx 9.5 can be good probes of the early universe and have the potential of testing cosmological models. The analysis by Dainotti of GRB Swift afterglow lightcurves with known redshifts and definite X-ray plateau shows an anti-correlation between the rest frame time when the plateau ends (the plateau end time) and the calculated luminosity at that time (or approximately an anti-correlation between plateau duration and luminosity). We present here an update of this correlation with a larger data sample of 101 GRBs with good lightcurves. Since some of this correlation could result from the redshift dependences of these intrinsic parameters, namely their cosmological evolution we use the Efron-Petrosian method to reveal the intrinsic nature of this correlation. We find that a substantial part of the correlation is intrinsic and describe how we recover it and how this can be used to constrain physical models of the plateau emission, whose origin is still unknown. The present result could help clarifing the debated issue about the nature of the plateau emission.Comment: Astrophysical Journal accepte

Slope evolution of GRB correlations and cosmology

Gamma -ray bursts (GRBs) observed up to redshifts $z>9.4$ can be used as possible probes to test cosmological models. Here we show how changes of the slope of the {\it luminosity $L^*_X$ -break time $T^*_a$} correlation in GRB afterglows, hereafter the LT correlation, affect the determination of the cosmological parameters. With a simulated data set of 101 GRBs with a central value of the correlation slope that differs on the intrinsic one by a $5\sigma$ factor, we find an overstimated value of the matter density parameter, $\Omega_M$, compared to the value obtained with SNe Ia, while the Hubble constant, $H_0$, best fit value is still compatible in 1$\sigma$ compared to other probes. We show that this compatibility of $H_0$ is due to the large intrinsic scatter associated with the simulated sample. Instead, if we consider a subsample of high luminous GRBs ($HighL$), we find that both the evaluation of $H_0$ and $\Omega_M$ are not more compatible in 1$\sigma$ and $\Omega_M$ is underestimated by the $13\%$. However, the $HighL$ sample choice reduces dramatically the intrinsic scatter of the correlation, thus possibly identifying this sample as the standard canonical `GRBs' confirming previous results presented in Dainotti et al. (2010,2011). Here, we consider the LT correlation as an example, but this reasoning can be extended also for all other GRB correlations. In literature so far GRB correlations are not corrected for redshift evolution and selection biases, therefore we are not aware of their intrinsic slopes and consequently how far the use of the observed correlations can influence the derived `best' cosmological settings. Therefore, we conclude that any approach that involves cosmology should take into consideration only intrinsic correlations not the observed ones.Comment: 8 pages, 4 figures, accepted to MNRAS Main Journa

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

Toward a standard Gamma Ray Burst: tight correlations between the prompt and the afterglow plateau phase emission

To reveal and understand astrophysical processes responsible for the Gamma Ray Burst (GRB) phenomenon, it is crucial to discover and understand relations between their observational properties. The presented study is performed in the GRB rest frames and it uses a sample of 62 long GRBs from our sample of 77 Swift GRBs with known redshifts. Following the earlier analysis of the afterglow {\it characteristic luminosity $L^*_a$ -- break time $T^*_a$} correlation for a sample of long GRBs \citep{Dainotti2010} we extend it to correlations between the afterglow and the prompt emission GRB physical parameters. We reveal a tight physical scaling between the mentioned afterglow luminosity $L^*_a$ and the prompt emission {\it mean} luminosity $_{45} \equiv E_{iso}/T^*_{45}$. The distribution, with the Spearman correlation coefficient reaching 0.95 for the data subsample with most regular light curves, can be fitted with approximately $L^*_a \propto {_{45}}^{0.7}$. We also analyzed correlations of $L^*_a$ with several other prompt emission parameters, including the isotropic energy $E_{iso}$, the peak energy in the $\nu F_{\nu}$ spectrum, $E_{peak}$, and the variability parameter, $V$, defined by \cite{N000}. As a result, we reveal significant correlations also between these quantities, with an exception of the variability parameter. The main result of the present study is the discovery that the highest correlated GRB subsample in the \citet{Dainotti2010} afterglow analysis, for the GRBs with canonical X\,-\,ray light curves, leads also to the highest {\it prompt-afterglow} correlations and such events can be considered to form a sample of standard GRBs for astrophysics and cosmology.Comment: The Data Table will appear after the paper will be accepte