arXiv subjects

Hendrik Van Eerten

Publications and source records attributed to Hendrik Van Eerten.

3 recordsLinked to original sources

arXiv2025

The soft X-ray transient EP241021a: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor

X-ray flashes (XRFs) are fast X-ray transients thought to be softer analogs of gamma-ray bursts (GRBs). With its soft X-ray sensitivity, the Einstein Probe (EP) provides a unique opportunity to study these events. We report multiwavelength observations of EP241021a, a soft X-ray transient detected by EP, and interpret its afterglow in the context of leading XRF models. The prompt emission was observed by EP-WXT and Fermi-GBM, followed by a broad campaign across radio (uGMRT, ATCA, e-MERLIN, ALMA), optical (LBT, GTC, CAHA), and X-rays (EP-FXT). Light curves and spectra were analyzed with both empirical and physical models of GRBs and spherical expansions (both nonrelativistic and mildly relativistic cocoons). The afterglow shows multiple components, consistent with a structured jet interacting with a complex environment. The early optical and X-ray decline is explained by wide, low-Lorentz-factor ($γ\sim 40$) wings, while a rebrightening at approximately 7 days arises from the off-axis jet core. Radio data require an additional mildly relativistic cocoon ($γ\sim 2$), and a late (70 days) spectral component peaking at 50 GHz suggests a second, slower cocoon ($γ\sim 1$).

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arXiv2024

Potential biases and prospects for the Hubble constant estimation via electromagnetic and gravitational-wave joint analyses

GW170817 is a binary neutron star merger that exhibited a gravitational wave (GW) and a gamma-ray burst, followed by an afterglow. In this work, we estimate the Hubble constant ($H_0$) using broad-band afterglow emission and relativistic jet motion from the Very Long Baseline Interferometry and Hubble Space Telescope images of GW170817. Compared to previous attempts, we combine these messengers with GW in a simultaneous Bayesian fit. We probe the $H_0$ measurement robustness depending on the data set used, the assumed jet model, the possible presence of a late time flux excess. Using the sole GW leads to a $20\%$ error ($77^{+21}_{-10}$ km/s/Mpc, medians, 16th-84th percentiles), because of the degeneracy between viewing angle ($θ_v$) and luminosity distance ($d_L$). The latter is reduced by the inclusion in the fit of the afterglow light curve, leading to $H_0=96^{+13}_{-10}$ km/s/Mpc, a large value, caused by the fit preference for high viewing angles due to the possible presence of a late-time excess in the afterglow flux. Accounting for the latter by including a constant flux component at late times brings $H_0=78.5^{+7.9}_{-6.4}$ km/s/Mpc. Adding the centroid motion in the analysis efficiently breaks the $d_L-θ_v$ degeneracy and overcome the late-time deviations, giving $H_0 = 69.0^{+4.4}_{-4.3}$ km/s/Mpc (in agreement with Planck and SH0ES measurements) and $θ_v = 18.2^{+1.2}_{-1.5}$ deg. This is valid regardless of the jet structure assumption. Our simulations show that for next GW runs radio observations are expected to provide at most few other similar events.

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arXiv2023

Joint analysis of gravitational-wave and electromagnetic data of mergers: breaking an afterglow model degeneracy in GW170817 and in future events

On August 17, 2017, Advanced LIGO and Virgo observed GW170817, the first gravitational-wave (GW) signal from a binary neutron star merger. It was followed by a short-duration gamma-ray burst, GRB 170817A, and by a non-thermal afterglow emission. In this work, a combined simultaneous fit of the electromagnetic (EM, specifically, afterglow) and GW domains is implemented, both using the posterior distribution of a GW standalone analysis as prior distribution to separately process the EM data, and fitting the EM and GW domains simultaneously. These approaches coincide mathematically, as long as the actual posterior of the GW analysis, and not an approximation, is used as prior for the EM analysis. We treat the viewing angle, $θ_v$, as shared parameter across the two domains. In the afterglow modelling with a Gaussian structured jet this parameter and the jet core angle, $θ_c$, are correlated, leading to high uncertainties on their values. The joint EM+GW analysis relaxes this degeneracy, reducing the uncertainty compared to an EM-only fit. We also apply our methodology to hypothetical GW170817-like events occurring in the next GW observing run at $\sim$140 and 70 Mpc. At 70 Mpc the existing EM degeneracy is broken, thanks to the inclusion of the GW domain in the analysis. At 140 Mpc, the EM-only fit cannot constrain $θ_v$ nor $θ_c$ because of the lack of detections in the afterglow rising phase. Folding the GW data into the analysis leads to tighter constraints on $θ_v$, still leaving $θ_c$ unconstrained, requiring instruments with higher sensitivities, such as Athena.

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