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

Progenitors of Low Redshift Gamma-ray Bursts

Bimodal distribution of the observed duration of gamma-ray bursts (GRBs) has led to two distinct progenitors; compact star mergers, either two neutron stars (NSs) or a NS and a black hole (BH), for short GRBs (SGRBs), and so-called collapsars for long GRBs (LGRBs). It is therefore expected that formation rate (FR) of LGRBs should be similar to the cosmic star formation rate (SFR), while that of SGRBs to be delayed relative to the SFR. The localization of some LGRBs in and around the star forming regions of host galaxies and some SGRBs away form such regions support this expectation. Another distinct feature of SGRBs is their association with gravitational wave (GW) sources and kilonovae. However, several independent investigations of the FRs of long and short bursts, using the Efron-Petrosian non-parametric method have shown a LGRB FR that is significantly larger than SFR at low redhift, and similar to the FR of SGRBs. In addition, recent discovery of association of a low redshift long GRB211211A with a kilonova raises doubt about its collapsar origin. In this letter we review these results and show that low redshift LGRBs could also have compact star mergers as progenitor increasing the expected rate of the GW sources and kilonovae significantly.Comment: 6 pages, 4 figures of two panel

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

From the gravitational waves to the exoplanets: the Research Highlights

In this Research Highlights, we summarize 31 contributions provided during the Workshop \textit{Multifrequency Behaviour of High Energy Cosmic Sources - XIV}, held in Palermo (Italy) from the 12th to the 17th of June 2023. We will start with the most recent discoveries in the field of gravitational waves (GWs). We will connect this topic to the contributions of Gamma-Ray Bursts (GRBs) associated with GWs and with the Kilonovae (KNe) hunting and, more in general, on GRBs. Continuing on high-energy astrophysics objects, we will delve into Active Galactic Nuclei (AGNs), neutrino astronomy and the study of the primordial universe, both from the space telescopes' observation and from the very recent proposals in terms of cosmological models. From the faraway universe, we will move to the more local scales and discuss the recent observations in Supernova Remnants (SNRs), massive star binaries, globular cluster dynamics, and exoplanets observed by Kepler.Comment: 23 pages, 8 figures; submitted as a proceeding for the "Multifrequency Behaviour of High Energy Cosmic Sources - XIV", 12-17 June 2023 Mondello, Palermo, Ital

Determination of the Intrinsic Luminosity Time Correlation in the X-Ray Afterglows of Gamma-Ray Bursts

Gamma-ray bursts (GRBs), which have been observed up to redshifts z ≈ 9.5, can be good probes of the early universe and have the potential to test cosmological models. Dainotti’s analysis of GRB Swift afterglow light curves with known redshifts and a 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). Here, we present an update of this correlation with a larger data sample of 101 GRBs with good light curves. 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, the origin of which is still unknown. The present result could help to clarify the debated nature of the plateau emission