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γ -ray bursts (GRBs) have recently attracted much attention as a possible way to extend the Hubble diagram to a very high redshift. However, the large scatter in their intrinsic properties prevents directly using them as a distance indicator so that the hunt is open for a relation involving an observable property to standardize GRBs in the same way as the Phillips law makes it possible to use Type Ia supernovae as standardizable candles. We use here the data on the X-ray decay curve and spectral index of a sample of GRBs observed with the Swift satellite. These data are used as input to a Bayesian statistical analysis looking for a correlation between the X-ray luminosity L X (T a ) and the time constant T a of the afterglow curve. We find a linear relation between log [L X (T a )] and log [T a /(1 + z)] with an intrinsic scatter σ int = 0.33 comparable to previously reported relations. Remarkably, both the slope and the intrinsic scatter are almost independent on the matter density M and the constant equation of state w of the dark energy component thus suggesting that the circularity problem is alleviated for the L X -T a relation.

The Large Area Telescope (LAT) aboard the Fermi spacecraft routinely observes high-energy emission from gamma-ray bursts (GRBs). Here we present the second catalog of LAT-detected GRBs, covering the first 10 yr of operations, from 2008 to 2018 August 4. A total of 186 GRBs are found; of these, 91 show emission in the range 30-100 MeV (17 of which are seen only in this band) and 169 are detected above 100 MeV. Most of these sources were discovered by other instruments (Fermi/GBM, Swift/BAT, AGILE, INTEGRAL) or reported by the Interplanetary Network (IPN); the LAT has independently triggered on four GRBs. This catalog presents the results for all 186 GRBs. We study onset, duration, and temporal properties of each GRB, as well as spectral characteristics in the 100 MeV-100 GeV energy range. Particular attention is given to the photons with the highest energy. Compared with the first LAT GRB catalog, our rate of detection is significantly improved. The results generally confirm the main findings of the first catalog: the LAT primarily detects the brightest GBM bursts, and the high-energy emission shows delayed onset as well as longer duration. However, in this work we find delays exceeding 1 ks and several GRBs with durations over 10 ks. Furthermore, the larger number of LAT detections shows that these GRBs not only cover the high-fluence range of GBM-detected GRBs but also sample lower fluences. In addition, the greater number of detected GRBs with redshift estimates allows us to study their properties in both the observer and rest frames. Comparison of the observational results with theoretical predictions reveals that no model is currently able to explain all results, highlighting the role of LAT observations in driving theoretical models.

Gamma Ray Bursts (GRB) observed up to redshifts z > 8 are fascinating objects to study due to their still unexplained relativistic outburst mechanisms and a possible use to test cosmological models. Our analysis of 77 GRB afterglows with known redshifts revealed a physical subsample of long GRBs with canonical plateau breaking to power-law light curves with a significant luminosity L * X -break time T * a correlation in the GRB rest frame. This subsample forms approximately the upper envelope of the studied distribution. We have also found a similar relation for a small sample of GRB afterglows that belong to the intermediate class (IC) between the short and the long ones. It proves that within the full sample of afterglows there exist physical subclasses revealed here by tight correlations of their afterglow properties. The afterglows with regular ('canonical') light curves obey not only a mentioned tight physical scaling, but -for a given T * a -the more regular progenitor explosions lead to preferentially brighter afterglows.

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Context. The discovery by Swift and HETE-2 of an afterglow emission associated possibly with short GRBs opened the new problematic of their nature and classification. This issue has been further enhanced by the observation of GRB 060614 and by a new analysis of the BATSE catalog which led to the identification of a new class of GRBs with "an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission". Aims. We plan a twofold task: a) to fit this new class of "hybrid" sources within our "canonical GRB" scenario, where all GRBs are generated by a "common engine" (i.e. the gravitational collapse to a black hole); b) to propose GRB 970228 as the prototype of the above mentioned class, since it shares the same morphology and observational features. Methods. We analyze BeppoSAX data on GRB 970228 within the "fireshell" model and we determine the parameters describing the source and the CircumBurst Medium (CBM) needed to reproduce its light curves in the 40-700 keV and 2-26 keV energy bands. Results. We find that GRB 970228 is a "canonical GRB", like e.g. GRB 050315, with the main peculiarity of a particularly low average density of the CBM n cbm ∼ 10 −3 particles/cm 3 . We also simulate the light curve corresponding to a rescaled CBM density profile with n cbm = 1 particle/cm 3 . From such a comparison it follows that the total time-integrated luminosity is a faithful indicator of the nature of GRBs, contrary to the peak luminosity which is merely a function of the CBM density. Conclusions. We call attention on discriminating the short GRBs between the "genuine" and the "fake" ones. The "genuine" ones are intrinsically short, with baryon loading B < ∼ 10 −5 , as stated in our original classification. The "fake" ones, characterized by an initial spikelike emission followed by an extended emission lasting tenths of seconds, have a baryon loading 10 −4 < ∼ B ≤ 10 −2 . They are observed as such only due to an underdense CBM consistent with a galactic halo environment which deflates the afterglow intensity.

Gamma-ray bursts (GRBs) have recently attracted much attention as a possible way to extend the Hubble diagram (HD) to very high redshift. To this aim, the luminosity (or isotropic-emitted energy) of a GRB at redshift z must be evaluated from a correlation with a distance-independent quantity so that one can then solve for the luminosity distance d L (z) and hence the distance modulus μ(z). Averaging over five different two-parameter correlations and using a fiducial cosmological model to calibrate them, Schaefer has compiled a sample of 69 GRBs with measured μ(z) which has since then been widely used to constrain cosmological parameters. Here, we update this sample through many aspects. First, we add a recently found correlation to the X-ray afterglow and use a Bayesian-inspired fitting method to calibrate the different GRB correlations known so far by assuming a fiducial cold dark matter model in agreement with the recent Wilkinson Microwave Anisotropy Probe5 data. Averaging over six correlations, we end up with a new GRB HD comprising 83 objects. We also extensively explore the impact of varying the fiducial cosmological model considering how the estimated μ(z) change as a function of the ( M , w 0 , w a ) parameters of the Chevallier-Polarski-Linder phenomenological dark energy equation of state. In order to avoid the need of assuming an a priori cosmological model, we present a new calibration procedure based on a model-independent local regression estimate of μ(z) using the Union Type Ia Supernovae sample to calibrate the GRBs correlations. This finally gives us a GRB HD made out of 69 GRBs whose estimated distance modulus μ(z) is almost independent of the underlying cosmological model.

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) followup with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signatures of Pop III stars, sources and physics of reionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).