ABSTRACT
The correlation between the cosmological rest‐frame νFν spectrum peak energy, Ep,i, and the isotropic‐equivalent radiated energy, Eiso, discovered by Amati et al. in 2002 and ...confirmed/extended by subsequent osbervations, is one of the most intriguing and debated observational evidences in gamma‐ray burst (GRB) astrophysics. In this paper, I provide an update and a re‐analysis of the Ep,i–Eiso correlation basing on an updated sample consisting of 41 long GRBs/X‐ray flashes (XRFs) with firm estimates of z and observed peak energy, Ep,obs, 12 GRBs with uncertain values of z and/or Ep,obs, two short GRBs with firm estimates of z and Ep,obs and the peculiar subenergetic events GRB 980425/SN1998bw and GRB 031203/SN2003lw. In addition to standard correlation analysis and power‐law fitting, the data analysis here reported includes modelling that accounts for sample variance. All 53 classical long GRBs and XRFs, including 11 Swift events with published spectral parameters and fluences, have Ep,i and Eiso values, or upper/lower limits, consistent with the correlation, which shows a chance probability as low as ∼7 × 10−15, a slope of ∼0.57 (∼0.5 when fitting by accounting for sample variance) and an extra‐Poissonian logarithmic dispersion of ∼0.15, it extends over ∼5 orders of magnitude in Eiso and ∼3 orders of magnitude in Ep,i and holds from the closer to the higher z GRBs. Subenergetic GRBs (980425 and possibly 031203) and short GRBs are found to be inconsistent with the Ep,i–Eiso correlation, showing that it can be a powerful tool for discriminating different classes of GRBs and understanding their nature and differences. I also discuss the main implications of the updated Ep,i–Eiso correlation for the models of the physics and geometry of GRB emission, its use as a pseudo‐redshift estimator and the tests of possible selection effects with GRBs of unknown redshift.
The prompt emission mechanism of gamma-ray bursts (GRB) even after several decades remains a mystery. However, it is believed that correlations between observable GRB properties, given their huge ...luminosity/radiated energy and redshift distribution extending up to at least z 9, are promising possible cosmological tools. They also may help to discriminate among the most plausible theoretical models. Nowadays, the objective is to make GRBs standard candles, similar to supernovae (SNe) Ia, through well-established and robust correlations. However, differently from SNe Ia, GRBs span over several order of magnitude in their energetics, hence they cannot yet be considered standard candles. Additionally, being observed at very large distances, their physical properties are affected by selection biases, the so-called Malmquist bias or Eddington effect. We describe the state of the art on how GRB prompt correlations are corrected for these selection biases to employ them as redshift estimators and cosmological tools. We stress that only after an appropriate evaluation and correction for these effects, GRB correlations can be used to discriminate among the theoretical models of prompt emission, to estimate the cosmological parameters and to serve as distance indicators via redshift estimation.
In the current framework, the standard parametrization of our Universe is the so-called Lambda cold dark matter (ΛCDM) model. Recently, a ∼4σ tension with the ΛCDM model was shown to exist via a ...model-independent parametrization of a Hubble diagram of type Ia supernovae (SNe Ia) from the JLA survey and quasars. Model-independent approaches and independent samples over a wide redshift range are key to testing this tension and any possible systematic errors. Here we present an analysis of a combined Hubble diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement of the quasar and GRB cosmological parameters at high redshifts (z > 2) and to test the concordance flat ΛCDM model with improved statistical accuracy. We build a Hubble diagram with SNe Ia, quasars, and GRBs, where quasars are standardised through the observed non-linear relation between their ultraviolet and X-ray emission and GRBs through the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called Amati relation). We fit the data with cosmographic models consisting of a fourth-order logarithmic polynomial and a fifth-order linear polynomial, and compare the results with the expectations from a flat ΛCDM model. We confirm the tension between the best-fit cosmographic parameters and the ΛCDM model at ∼4σ with SNe Ia and quasars, at ∼2σ with SNe Ia and GRBs, and at > 4σ with the whole SNe Ia+quasars+GRB data set. The completely independent high-redshift Hubble diagrams of quasars and GRBs are fully consistent with each other, strongly suggesting that the deviation from the standard model is not due to unknown systematic effects but to new physics.
ABSTRACT
Multimessenger astronomy received a great boost following the discovery of kilonova (KN) AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the ...short gamma-ray burst GRB 170817A. AT2017gfo was the first KN that could be extensively monitored in time using both photometry and spectroscopy. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like KN emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue KN can be more than 10 times brighter. Finally, we exclude a KN as luminous as AT2017gfo in GRBs 050509B and 061201.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source ...powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
The origin, reliability, and dispersion of the Ep,i – Eiso and other spectral energy correlations is a highly debated topic in GRB astrophysics. GRB 080916C, with its enormous radiated energy ...(${E_{\rm iso}}$ ~ 1055 erg in the 1 keV-10 GeV cosmological rest-frame energy band) and its intense GeV emission measured by Fermi, provides a unique opportunity to investigate this issue. In our analysis, we also study another extremely energetic event, GRB 090323, more recently detected and localized by Fermi/LAT, whose radiated energy is comparable to that of GRB 080916C in the 1 keV-10 MeV energy range. Based on Konus/WIND and Fermi spectral measurements, we find that both events are fully consistent with the Ep,i – Eiso correlation (updated to include 95 GRBs with the data available as of April 2009), thus further confirming and extending it, and providing evidence against a possible flattening or increased dispersion at very high energies. This also suggests that the physics behind the emission of peculiarly bright and hard GRBs is the same as for medium-bright and soft-weak long events (XRFs), which all follow the correlation. In addition, we find that the normalization of the correlation obtained by considering these two GRBs and the other long ones for which ${E_{\rm p,i}}$ was measured to high accuracy by the Fermi/GBM are fully consistent with those obtained by other instruments (e.g., BeppoSAX, Swift, Konus/WIND), thus indicating that the correlation is not affected significantly by “data truncation” because of detector thresholds and limited energy bands. A Fermi/GBM accurate estimate of the peak energy of a very bright and hard short GRB with a measured redshift, GRB 090510, provides robust evidence that short GRBs do not follow the Ep,i – Eiso correlation and that the Ep,i – Eiso plane can be used to discriminate between, and understand, the two classes of events. Prompted by the extension of the spectrum of GRB 080916C to several GeV (in the cosmological rest-frame) without any excess or cut-off, we also investigated whether the evaluation of ${E_{\rm iso}}$ in the commonly adopted 1 keV-10 MeV energy band may bias the Ep,i – Eiso correlation and/or contribute to its scatter. By computing ${E_{\rm iso}}$ from 1 keV to 10 GeV, the slope of the correlation becomes slightly flatter, while its dispersion does not change significantly. Finally, we find that GRB 080916C is also consistent with most of the other spectral energy correlations derived from it, with the possible exception of the Ep,i – Eiso – tb correlation.
Context. Timing analysis can be a powerful tool with which to shed light on the still obscure emission physics and geometry of the prompt emission of gamma-ray bursts (GRBs). Fourier power density ...spectra (PDS) characterise time series as stochastic processes and can be used to search for coherent pulsations and, more in general, to investigate the dominant variability timescales in astrophysical sources. Because of the limited duration and of the statistical properties involved, modelling the PDS of individual GRBs is challenging, and only average PDS of large samples have been discussed in the literature thus far. Aims. We aim at characterising the individual PDS of GRBs to describe their variability in terms of a stochastic process, to explore their variety, and to carry out for the first time a systematic search for periodic signals and for a link between PDS properties and other GRB observables. Methods. We present a Bayesian procedure that uses a Markov chain Monte Carlo technique and apply it to study the individual PDS of 215 bright long GRBs detected with the Swift Burst Alert Telescope in the 15−150 keV band from January 2005 to May 2015. The PDS are modelled with a power-law either with or without a break. Results. Two classes of GRBs emerge: with or without a unique dominant timescale. A comparison with active galactic nuclei (AGNs) reveals similar distributions of PDS slopes. Unexpectedly, GRBs with subsecond-dominant timescales and duration longer than a few tens of seconds in the source frame appear to be either very rare or altogether absent. Three GRBs are found with possible evidence for a periodic signal at 3.0–3.2σ (Gaussian) significance, corresponding to a multi-trial chance probability of ~1%. Thus, we found no compelling evidence for periodic signal in GRBs. Conclusions. The analogy between the PDS of GRBs and of AGNs could tentatively indicate similar stochastic processes that rule BH accretion across different BH mass scales and objects. In addition, we find evidence that short dominant timescales and duration are not completely independent of each other, in contrast with commonly accepted paradigms.
Observations of gamma-ray bursts up to z ∼ 9 are best suited to study the possible evolution of the universe equation of state at intermediate redshifts. We apply the Combo relation to a sample of ...174 gamma-ray bursts to investigate possible evidence of evolving dark energy parameter w(z). We first build a gamma-ray burst Hubble's diagram and then we estimate the set ( m, Λ) in the framework of flat and non-flat ΛCDM paradigm. We then get bounds over the wCDM model, where w is thought to evolve with redshift, adopting two priors over the Hubble constant in tension at 4.4 , i.e., H0 = (67.4 0.5) km s−1 Mpc−1 and H0 = (74.03 1.42) km s−1 Mpc−1. We show our new sample provides tighter constraints on m since at z ≤ 1.2 we see that w(z) agrees within 1 with the standard value w = −1. The situation is the opposite at larger z, where gamma-ray bursts better fix w(z) that seems to deviate from w = −1 at 2 and 4 level, depending on the redshift bins. In particular, we investigate the w(z) evolution through a piecewise formulation over seven redshift intervals. From our fitting procedure we show that at z ≥ 1.2 the case w < −1 cannot be fully excluded, indicating that dark energy's influence is not negligible at larger z. We confirm the Combo relation as a powerful tool to investigate cosmological evolution of dark energy. Future space missions will significantly enrich the gamma-ray burst database even at smaller redshifts, improving de facto the results discussed in this paper.
Abstract
We study the high-energy properties of GRB 181123B, a short gamma-ray burst (sGRB) at redshift
z
≈ 1.75. We show that, despite its nominal short duration with
T
90
< 2 s, this burst displays ...evidence of a temporally extended emission (EE) at high energies and that the same trend is observed in the majority of sGRBs at
z
≳ 1. We discuss the impact of instrumental selection effects on the GRB classification, stressing that the measured
T
90
is not an unambiguous indicator of the burst physical origin. By examining their environment (e.g., stellar mass, star formation, offset distribution), we find that these high-
z
sGRBs share many properties of long GRBs at a similar distance and are consistent with a short-lived progenitor system. If produced by compact binary mergers, these sGRBs with EE may be easier to localize at large distances and herald a larger population of sGRBs in the early universe.
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