Pulsars, transients and relativistic astrophysics

Abstract:

Abstract We present the early radio detection and multiwavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift z = 0.257. The Australia Telescope Compact Array automatically triggered a 9 hr observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hr post-burst. Splitting this observation into 1 hr time bins, the early radio afterglow exhibited flaring, scintillating and plateau phases. The scintillation allowed us to place the earliest upper limit (<10 hr) on the size of a GRB blast wave to date, constraining it to <1 × 10 16 cm. Multiwavelength modeling of the full afterglow required a period of significant energy injection between ∼0.02 and 1 day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of p  = 1.66 ± 0.01. Our model also indicates a jet break at ∼2 days, and a half-opening angle of θ j = 16 . ° 6 ± 1 . ° 1 . Following the period of injection, the total energy is ζ  ∼ 18 times the initial impulsive energy, with a final collimation-corrected energy of E Kf  ∼ 5.7 × 10 49 erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of Γ > 35 to Γ > 5 between ∼0.1 and 1 day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour.

Abstract:

Abstract Fast X-ray Transients (FXTs) are short-lived extra-galactic X-ray sources. Recent progress through multi-wavelength follow-up of Einstein Probe discovered FXTs has shown that several are related to collapsars, which can also produce γ-ray bursts (GRBs). In this paper we investigate the nature of the FXT EP250207b. The VLT/MUSE spectra of a nearby (15.9 kpc in projection) lenticular galaxy reveal no signs of recent star formation. If this galaxy is indeed the host, EP250207b lies at a redshift of z = 0.082, implying a peak observed absolute magnitude for the optical counterpart of $\mathrm{M_{r^\prime }=-14.5}$. At the time when supernovae (SNe) would peak, it is substantially fainter than all SN types. These results are inconsistent with a collapsar origin for EP250207b. The properties favour a binary compact object merger driven origin. The X-ray, optical and radio observations are compared with predictions of several types of extra-galactic transients, including afterglow and kilonova models. The data can be fit with a slightly off-axis viewing angle afterglow. However, the late-time (∼30 day) optical/NIR counterpart is too bright for the afterglow and also for conventional kilonova models. This could be remedied if that late emission is due to a globular cluster or the core of a (tidally disrupted) dwarf galaxy. If confirmed, this would be the first case where the multi-wavelength properties of an FXT are found to be consistent with a compact object merger origin, increasing the parallels between FXTs and GRBs. We finally discuss if the source could originate in a higher redshift host galaxy.

Abstract:

We present the early radio detection and multiwavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift z = 0.257. The Australia Telescope Compact Array automatically triggered a 9 hr observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hr post-burst. Splitting this observation into 1 hr time bins, the early radio afterglow exhibited flaring, scintillating and plateau phases. The scintillation allowed us to place the earliest upper limit (<10 hr) on the size of a GRB blast wave to date, constraining it to <1 × 1016 cm. Multiwavelength modeling of the full afterglow required a period of significant energy injection between ∼0.02 and 1 day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of p = 1.66 ± 0.01. Our model also indicates a jet break at ∼2 days, and a half-opening angle of θj=16.°6±1.°1 . Following the period of injection, the total energy is ζ ∼ 18 times the initial impulsive energy, with a final collimation-corrected energy of EKf ∼ 5.7 × 1049 erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of Γ > 35 to Γ > 5 between ∼0.1 and 1 day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour.

Abstract:

Abstract Fast radio bursts (FRBs) are sufficiently energetic to be detectable from luminosity distances up to at least seven billion parsecs (redshift z > 1). Probing the maximum energies and luminosities of FRBs constrains their emission mechanism and cosmological population. Here we investigate the maximum energetics of a highly active repeater, FRB 20220912A, using 1,500 h of observations. We detect 130 high-energy bursts and find a break in the burst energy distribution, with a flattening of the power-law slope at higher energy – consistent with the behaviour of another highly active repeater, FRB 20201124A. There is a roughly equal split of integrated burst energy between the low- and high-energy regimes. Furthermore, we model the rate of the highest-energy bursts and find a turnover at a characteristic spectral energy density of $E^{\textrm {char}}_{\nu } = 2.09^{+3.78}_{-1.04}\times 10^{32}$ erg Hz−1. This characteristic maximum energy agrees well with observations of apparently one-off FRBs, suggesting a common physical mechanism for their emission. The extreme burst energies push radiation and source models to their limit: at this burst rate a typical magnetar (B = 1015 G) would deplete the energy stored in its magnetosphere in ∼ 2150 h, assuming a radio efficiency εradio = 10−5. We find that the high-energy bursts (Eν > 3 × 1030 erg Hz−1) play an important role in exhausting the energy budget of the source.