The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on ‘Black holes, Gravitational waves and Fundamental Physics’.

GRB 980425 and GRB 031203 are apparently two outliers with respect to the correlation between the isotropic equivalent energy E_iso emitted in the prompt radiation phase and the peak frequency E_peak of the spectrum in a vF(v) representation (the so-called Amati relation). We discuss if these two bursts are really different from the others or if their location in the E_iso-E_peak plane is the result of other effects, such as viewing them off-axis, or through a scattering screen, or a misinterpretation of their spectral properties. The latter case seems particularly interesting after GRB 060218, that, unlike GRB 031203 and GRB 980425, had a prompt emission detected both in hard and soft X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and total emitted energy. Although it shares with GRB 031203 the total energetics, it is not an outlier with respect to the Amati correlation. We then investigate if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent with all the observed properties, can give rise to a time integrated spectrum with an E_peak consistent with the Amati relation.Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in MNRA

We derive the correlation between the peak luminosity Liso and the peak energy of the νFν spectrum Epeak using 25 long gamma‐ray bursts (GRBs) with firm redshift measurements. We find that its slope is similar to that of the correlation between the time‐integrated isotropic emitted energy Eiso and Epeak. For the 16 GRBs in our sample with estimated jet opening angle, we compute the collimation‐corrected peak luminosity Lγ, and find that it correlates with Epeak. This correlation has, however, a scatter larger than that of the correlation between Epeak and Eγ (the time‐integrated emitted energy, corrected for collimation), which we ascribe to the fact that the opening angle is estimated through the global energetics. We have then selected a large sample of 442 GRBs with pseudo‐redshifts, derived through the lag–luminosity relation, to test the existence of the Liso–Epeak correlation. With this sample we also explore the possibility of a correlation between time‐resolved quantities, namely Lpiso and the peak energy at the peak of emission Eppeak.

For a sample of long γ‐ray bursts (GRBs) with known redshift, we study the distribution of the evolutionary tracks on the rest‐frame luminosity‐peak energy Liso−E′p diagram. We are interested in exploring the extension of the ‘Yonetoku’ correlation to any phase of the prompt light curve, and in verifying how the high‐signal prompt duration time, T′f, in the rest frame correlates with the residuals of such correlation. For our purpose, we separately analyse two samples of time‐resolved spectra corresponding to 32 GRBs with peak fluxes Fp > 1.8 phot cm−2 s−1 from the Swift‐BAT detector, and seven bright GRBs from the Compton Gamma‐ray Observatory (CGRO)‐BATSE detector previously processed by Kaneko et al. After constructing the Liso−E′p diagram, we discuss the relevance of selection effects, finding that they could significantly affect the correlation. However, we find that these effects are much less significant in the Liso T′f−E′p diagram, where the intrinsic scatter reduces significantly. We apply further corrections in order to reduce the intrinsic scatter even more. For the subsamples of GRBs (seven from Swift and five from CGRO) with measured jet break time, tj, we analyse the effects of correcting Liso by jet collimation. We find that (i) the scatter around the correlation is reduced, and (ii) this scatter is dominated by the internal scatter of the individual evolutionary tracks. These results suggest that the time‐integrated ‘Amati’ and ‘Ghirlanda’ correlations are consequences of the time‐resolved features, not of selection effects, and therefore call for a physical origin. We finally remark the relevance of looking inside the nature of the evolutionary tracks.

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