arXiv subjects

Filip Alamaa

Publications and source records attributed to Filip Alamaa.

7 recordsLinked to original sources

arXiv2026

From Internal Collision to Photon Escape: First-Principles Modeling of Radiation-Mediated Shocks in Gamma-Ray Burst Photospheres

Modeling subphotospheric shocks in a gamma-ray burst (GRB) is challenging due to the various timescales that must be resolved, and the fact that the same radiation dynamically mediates the shocks while forming the observed signal. Here, we present the first self-consistent radiation-hydrodynamic simulation of a subphotospheric internal collision, following the system from formation and propagation of forward and reverse radiation-mediated shocks all the way to photon decoupling and free streaming toward the observer. The simulation evolves the plasma and photon field with full Compton coupling, including the feedback on the hydrodynamic flow. As the ejecta expands and the optical depth decreases, both shocks broaden and the radiation field becomes highly non-thermal. Surprisingly, we find that the reverse shock remains completely radiation-mediated down to upstream optical depths of order a few $\times 10^{-1}$, which indicates that Compton coupling is important even in moderately optically thin regions. The photons undergo last scattering over a broad range of radii rather than at a single photospheric surface. The light curve shows a late, quasi-thermal post-cursor produced by photons that decouple upstream of the reverse shock, which could be searched for in observations. The emitted time-integrated spectrum is GRB-like, with a low-energy photon index $\alpha \sim -1$ and a high-energy photon index $\beta \sim -2.5$. These results show how radiation-mediated shocks evolve close to the photosphere and how they shape the emitted photon field.

astro-ph.HE↗

arXiv2025

Radiation-mediated shocks in gamma-ray bursts: spectral evolution

Radiation-mediated shocks (RMSs) occurring below the photosphere in a gamma-ray burst (GRB) jet could play a crucial role in shaping the prompt emission. In this paper, we study the time-resolved signal expected from such early shocks. We model an internal collision using a 1D special relativistic hydrodynamical simulation, and we follow the photon distributions in the resulting forward and reverse shocks as well as in the common downstream region to well above the photosphere using a designated RMS simulation code. We compute the light curve and time-resolved spectrum of the resulting single pulse taking into account the emission at different optical depths and angles to the line of sight. For the specific case considered, we find a light curve consisting of a short pulse lasting $\sim 0.1~$s for an assumed redshift of $z = 1$, which could constitute a whole short GRB or be a building block within a highly variable longer GRB light curve. The efficiency is large, with $\approx 23$% of the total burst energy being radiated. The spectrum has a complex shape at very early times, after which it settles into a more generic shape with a smooth curvature below the peak energy, $E_p$, and a clear high-energy power law that cuts off at $\sim 5~$MeV in the observer frame. The spectrum becomes narrower and softer at late times with $E_p$ steadily decreasing during the pulse decay from $E_p \approx 250~$keV to $E_p \approx 100~$keV. The low-energy index, $\alpha$, decreases during the bright part of the pulse from $\alpha \approx -0.5$ to $\alpha \approx -1$, although the low-energy part is better fit with a broken power law when the signal-to-noise ratio is high. The high-energy power law is generated by the reverse shock at low optical depths ($\tau < 30$) and has an index that decreases from $\beta \approx -2$ to $\beta \approx -2.4$.

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arXiv2025

Photospheric emission from GRB 211211A altered by a strong radiation-mediated shock

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 analyze 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 ($\Gamma \sim 300$), and a strong RMS (upstream dimensionless specific momentum, $\gamma_u \beta_u \sim 3$) occurring at a moderate optical depth ($\tau \sim 35$) in a relatively cold upstream ($\theta_u = k_{\rm B} T_u / m_e c^2 \sim 10^{-4}$). 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.

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arXiv2025

A generalized method to measure the Lorentz factor from gamma-ray burst photospheric emission

The properties of gamma-ray bursts (GRBs) that are inferred from observations depend on the value of the bulk Lorentz factor, $\Gamma$. Consequently, accurately estimating it is an important aim. In this work, we present a method of measuring $\Gamma$ based on observed photospheric emission, which can also be used for highly dissipative flows that may lead to non-thermal 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 $\Gamma$ with statistical uncertainties of only $\sim 15$\%, while for the other spectrum we only obtain an upper limit.

astro-ph.HE↗

arXiv2024

Intrapulse spectral evolution in photospheric gamma-ray bursts

Photons that decouple from a relativistic jet do so over a range of radii, which leads to a spreading in arrival times at the observer. Therefore, changes to the comoving photon distribution across the decoupling zone are encoded in the emitted signal. In this paper, we study such spectral evolution occurring across a pulse. We track the radiation from the deep subphotospheric regions all the way to the observed time-resolved signal, accounting for emission at various angles and radii. We assume a simple power-law photon spectrum injection over a range of optical depths and let the photons interact with the local plasma. At high optical depths, we find that the radiation exists in one of three characteristic regimes, two of which exhibit a high-energy power law. Depending on the nature of the injection, this power law can persist to low optical depths and manifest itself during the rise time of the pulse with a spectral index $\beta \approx \alpha - 1$, where $\alpha$ is the low-energy spectral index. The results are given in the context of a gamma-ray burst jet but are general to optically thick, relativistic outflows.

astro-ph.HE↗

arXiv2024

Radiation-mediated shocks in GRB prompt emission

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 $\alpha$ 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.

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arXiv2023

Ending the prompt phase in photospheric models of gamma-ray bursts

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper, we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all GRB central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can grant us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.

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