The properties of gamma-ray bursts (GRBs) that are inferred from observations depend on the value of the bulk Lorentz factor, Γ. Consequently, accurately estimating it is an important aim. In this work, we present a method of measuring Γ based on observed photospheric emission, which can also be used for highly dissipative flows that may lead to nonthermal spectral shapes. For the method to be applicable, two conditions need to be met: the photon number should be conserved in the later stages of the jet, and the original photon temperature must be inferred from the data. The case of dissipation via subphotospheric shocks is discussed in detail, and we show that the method is particularly efficient when a low-energy spectral break is identified. We demonstrate the capabilities of the method by applying it to two different GRB spectra. From one of the spectra, we obtain a value for Γ with statistical uncertainties of only ∼15%, while for the other spectrum we only obtain an upper limit.

The prompt phase X-ray and gamma-ray light curves of gamma-ray bursts (GRBs) exhibit erratic and complex behaviors, often with multiple pulses. The temporal shape of the individual pulses is often modeled as “fast rise exponential decay” (FRED). Here, we introduce a novel fitting function to quantify the pulse asymmetry. We conduct a light-curve and a time-resolved spectral analysis on 61 pulses from 22 GRBs detected by the Fermi Gamma-ray Burst Monitor. Contrary to previous claims, we find that only ∼50% of the pulse light curves in our sample show a FRED shape, while about 25% have a symmetric light curve, with the other 25% having a mixed shape. Furthermore, our analysis reveals a clear trend: in multipulse bursts, the initial pulse tends to exhibit the most symmetric light curve, while subsequent pulses become increasingly asymmetric, adopting a more FRED-like shape. Additionally, we correlate the temporal and spectral shapes of the pulses. By fitting the spectra with the classical “Band” function, we find a moderate positive Spearman's correlation index of 0.23 between the pulse asymmetry and the low-energy spectral index α max (the maximum value across all time bins covering an individual pulse). Thus, during GRB light curves, the pulses tend to get more asymmetric and spectrally softer with time. We interpret this as a transition in the dominant emission mechanism, from photospheric (symmetric-like and hard) to nonthermal emission above the photosphere, and show that this interpretation aligns with a GRB jet Lorentz factor of the order of a few tens in many cases.

We present a complete analysis of Fermi Large Area Telescope (LAT) data of GRB 221009A, the brightest gamma-ray burst (GRB) ever detected. The burst emission above 30 MeV detected by the LAT preceded, by 1 s, the low-energy (<10 MeV) pulse that triggered the Fermi Gamma-Ray Burst Monitor (GBM), as has been observed in other GRBs. The prompt phase of GRB 221009A lasted a few hundred seconds. It was so bright that we identify a bad time interval of 64 s caused by the extremely high flux of hard X-rays and soft gamma rays, during which the event reconstruction efficiency was poor and the dead time fraction quite high. The late-time emission decayed as a power law, but the extrapolation of the late-time emission during the first 450 s suggests that the afterglow started during the prompt emission. We also found that high-energy events observed by the LAT are incompatible with synchrotron origin, and, during the prompt emission, are more likely related to an extra component identified as synchrotron self-Compton (SSC). A remarkable 400 GeV photon, detected by the LAT 33 ks after the GBM trigger and directionally consistent with the location of GRB 221009A, is hard to explain as a product of SSC or TeV electromagnetic cascades, and the process responsible for its origin is uncertain. Because of its proximity and energetic nature, GRB 221009A is an extremely rare event.

Gamma-ray burst (GRB) spectra are typically non-thermal, with many including two spectral breaks suggestive of optically thin emission. However, the emitted spectrum from a GRB photosphere, which includes prior dissipation of energy by radiation-mediated shocks (RMSs), can also produce such spectral features. Here, we analyse the bright GRB 211211A using the Kompaneets RMS Approximation (KRA). We find that the KRA can fit the time-resolved spectra well, significantly better than the traditionally used Band function in all studied time bins. The analysis of GRB 211211A reveals a jet with a typical Lorentz factor (Γ∼300), and a strong RMS (upstream dimension-less specific momentum, γuβu∼3) occurring at a moderate optical depth (τ∼35) in a relatively cold upstream (θ_u = k_B T_u /m_e c^2∼10^(−4)). We conclude that broad GRB spectra that exhibit two breaks can also be well explained by photospheric emission. This implies that, in such cases, the spectral shape in the MeV-band alone is not enough to determine the emission mechanism during the prompt phase in GRBs.

The X-ray light curves of gamma-ray bursts (GRBs) display complex features, including plateaus and flares, that challenge theoretical models. Here, we study the properties of flares that are observed in the early afterglow phase (up to a few thousand seconds). We split the sample into two groups: bursts with and without an X-ray plateau. We find that the distributions of flare properties are similar in each group; specifically, the peak time ( t pk ) of the flares and the ratio of the flare width to the flare peak time ( w / t pk ), which is found to be ≈1, regardless of the presence of a plateau. We discuss these results in view of the different theoretical models aimed at explaining the origin of the plateau. These results are difficult to explain by viewing angle effects or late-time energy injection, but do not contradict the idea that GRBs with X-ray plateaus have a low Lorentz factor, on the order of tens. For these GRBs, the dissipation processes that produce the flares naturally occur at smaller radii compared to GRBs with higher Lorentz factors, while the flares maintain a similar behavior. Our results therefore provide independent support for the idea that many GRBs have a Lorentz factor of a few tens rather than a few hundreds.

In many astrophysical systems, photons interact with matter through thermal Comptonization. In these cases, under certain simplifying assumptions, the evolution of the photon spectrum is described by an energy diffusion equation such as the Kompaneets equation, having dependencies on the seed photon temperature, theta(i), the electron temperature, theta e, and the Compton y-parameter. The resulting steady-state spectrum is characterized by the average photon energy and the Compton temperature, which both lack analytical dependencies on the initial parameters. Here, we present empirical relations of these two quantities as functions of theta(i), theta(e), and y, obtained by evaluating the steady-state solution of the Kompaneets equation accounting for energy diffusion and electron recoil. The relations have average fractional errors similar to 1 per cent across a wide range of the initial parameters, which make them useful in numerical applications.

The light curves of the prompt phase of gamma-ray bursts (GRBs) exhibit erratic and diverse behaviour, often with multiple pulses. The temporal shape of individual pulses is often modelled as ‘fast rise exponential decay’ (FRED). Here, we propose a novel fitting function to measure pulse asymmetry. We perform a time-resolved spectrum analysis on a sample of 75 pulses from twenty-seven GRBs that the Fermi Gamma-ray Burst Monitor has identified. When multi-pulse bursts are taken into account, a distinct behaviour becomes evident: the first pulses have the most symmetric-like lightcurve, while subsequent pulses show an increase in the asymmetry parameter, leading to a more FRED-like form. Furthermore, we correlate pulse temporal and spectral shapes after fitting the spectra with the classical “Band" function. A moderate positive Spearman correlation between pulse asymmetry and the low-energy spectral index αmax (where the maximum is taken over all time bins that cover the pulse shape) is identified. An overlapping emission mechanism is indicated by the fact that ∼ 64% of the GRB pulses fall within the limits of the slow-cooling synchrotron and non-dissipative photospheric emission models. Thus, our findings offer a compelling hint towards understanding the origin of GRB pulses.

At 13:16:59.99 UT on October 9th, 2022, the Fermi Gamma-ray Burst Monitor (GBM) triggered on gamma-ray burst (GRB) 221009A. This GRB has the highest fluence value GBM has ever detected. The light curve consists of two distinct emission episodes, a single isolated peak with a thermal spectra followed by a longer, extremely bright, multi-pulsed event with a non-thermal spectra. The two main peaks of the second event, from t0+218 to t0+276 seconds and t0+508 to t0+513\,s, had such high photon rates they caused pulse-pile up effects in the GBM detectors. Afterglow emission is detectable in the GBM energy range out to t0+1467 seconds when the field of view was occulted by Earth. Here we present the key parts of our spectrotemporal analysis for the triggering pulse, prompt emission, and afterglow and the pulse pile-up corrected energetics for the this historically bright event.

Progenitor stars of long gamma-ray bursts (GRBs) could be surrounded by a significant and complex nebula structure lying at a parsec-scale distance. After the initial release of energy from the GRB jet, the jet will interact with this nebula environment. We show here that for a large, plausible parameter space region, the interaction between the jet blast wave and the wind termination (reverse) shock is expected to be weak, and may be associated with a precursor emission. As the jet blast wave encounters the contact discontinuity separating the shocked wind and the shocked interstellar medium, we find that a bright flash of synchrotron emission from the newly formed reverse shock is produced. This flash is expected to be observed at around ∼100 s after the initial explosion and precursor. Such a delayed emission thus constitutes a circumburst medium (CBM) phase in a GRB, having a physically distinct origin from the preceding prompt phase and the succeeding afterglow phase. The CBM phase emission may thus provide a natural explanation for bursts observed to have a precursor followed by an intense, synchrotron-dominated main episode that is found in a substantial minority, ∼10% of GRBs. A correct identification of the emission phase is thus required to infer the properties of the flow and of the immediate environment around GRB progenitors.

The debate regarding the emission mechanism in gamma-ray bursts has been long-standing. Here, we study the spectral signatures of photospheric emission, accounting for subphotospheric dissipation by a radiation-mediated shock. The shocks are modeled using the Kompaneets RMS approximation (KRA). We find that the resulting observed spectra are soft, broad, and exhibit an additional break at lower energies. When fitting a collection of 150 mock data samples generated by the model, we obtain a distribution of the low-energy index α that is similar to the observed one. These results are promising and show that dissipative photospheric models can account for many of the observed properties of prompt gamma-ray burst emission.