Martin Topinka

Observing early optical emission from gamma-ray bursts (GRBs) contemporaneous with the MeV prompt emission phase remains rare. Such observations require rapid-response robotic facilities. The Ondřejov D50 telescope detected the optical counterpart of GRB 250702F at z = 1.520 only 27.8 s after trigger, enabling high-cadence monitoring during the brightest prompt emission pulses. The optical light curve revealed two distinct flares. The first (30–100 s) is spectrally consistent with the MeV prompt emission. The second flare (100–1400 s) exhibits an unusual morphology ( F _ν ∝ t ^{− α} ): a rapid rise turning gradually into a steep decay ( α ∼ 1.6) before transitioning to a standard power-law afterglow ( α = 0.79). This steep decay phase cannot be explained by non-thermal electrons accelerated at the forward shock, and a reverse-shock scenario is disfavoured due to the long duration of the flare and the temporal offset from the underlying deceleration time. We interpret the steep decay as the synchrotron frequency of a thermal (Maxwellian) electron population sweeping through the optical band. Modelling yields a non-thermal energy fraction of δ ≈ 0.8, with the remaining energy heating electrons at a characteristic Lorentz factor of γ _th ∼ 900. These observations provide evidence of thermal electron signatures in GRB afterglows consistent with predictions from particle-in-cell simulations of ultra-relativistic collisionless shocks.

One of the distinctive properties of magnetars, young neutron stars powered mainly by magnetic energy, is the emission of short (≲1 s) bursts of hard X-rays. Such bursts have been observed in nearly all the known magnetars, although at different and time-variable rates of occurrence. In the past two decades, the INTEGRAL satellite has extensively covered the Galactic plane with good imaging capabilities, where most magnetars reside. We present the results of a comprehensive search for magnetar bursts in more than 20 yr of archival data of the INTEGRAL IBIS instrument (15 keV–1 MeV). This led to the detection of 1349 bursts with 30–150 keV fluence in the ∼2 × 10 ⁻⁹ to 3 × 10 ⁻⁶ erg cm ⁻² range from 21 of the 34 examined magnetars and candidate magnetars with well-known positions. The durations of the bursts, in terms of T ₉₀ , follow a lognormal distribution centered at ∼0.1 s. Most of the detected bursts originated from three particularly active sources: 1E 1547–5408, SGR 1806–20, and SGR 1935+2154. The integral distributions of their burst fluences follow power laws with slopes β = 0.76±0.04, 0.95±0.06, and 0.92±0.10, respectively. The burst spectra are generally well fit with an exponentially cutoff power law with peak energy E _peak in the range ∼20−60 keV for SGR 1806–20 and SGR 1935+2154, while the bursts of 1E 1547–5408 are slightly harder ( E _peak ∼ 35−100 keV). A significant anticorrelation between E _peak and fluence is found for SGR 1806–20, which provided the largest number of bursts among the sources of our sample.

Swift J1818.0−1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B ∼ 3 × 10¹⁴ G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in 2020 March until 2021 October, as well as INTEGRAL upper limits on its hard X-ray emission. The 1–10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT _BB ∼ 1.1 keV and apparent reduction in the radius of the emitting region from ∼0.6 to ∼0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ∼50″ and ∼110″. A timing analysis revealed large torque variability, with an average spin-down rate ν ̇ ∼ −2.3 × 10⁻¹¹ Hz² that appears to decrease in magnitude over time. We also observed Swift J1818.0−1607 with the Karl G. Jansky Very Large Array on 2021 March 22. We detected the radio counterpart to Swift J1818 measuring a flux density of S _v = 4.38 ± 0.05 mJy at 3 GHz and a half-ringlike structure of bright diffuse radio emission located at ∼90″ to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust-scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology.