GN-z11-flash in the Context of Gamma-Ray Burst Afterglows

Gamma-Ray Bursts (GRBs) are among the most powerful explosions in the universe, and their optical emission at very early times can be dominated by extremely luminous ultraviolet (UV) flashes (Akerlof et al. 1999; Racusin et al. 2008; Bloom et al. 2009; Vestrand et al. 2014), which may be directly linked to the higher-energy prompt emission (e.g., Blake et al. 2005; Vestrand et al. 2005). Such prompt flashes are the most luminous UV/optical sources known (Kann et al. 2007; Perley et al. 2011), implying GRBs are detectable across a broad wavelength range up to the highest studied redshifts (Salvaterra et al. 2009; Tanvir et al. 2009; Cucchiara et al. 2011; Tanvir et al. 2018). This makes them highly interesting probes of cosmology, and the main target of mission concepts such as the Gamov Explorer (White 2020) and THESEUS (Amati et al. 2018).

Recently, Jiang et al. (2020a) reported a spectroscopic redshift for the ultra-high-z candidate galaxy GN-z11 (Oesch et al. 2016), finding it to lie at z = 10.957, confirming it to be the most distant object known at the time. Serendipitously, as (Jiang et al. 2020b, J20 hereafter) report, a single (2D spectrum) image of their 5.3 hr K-band spectrum contains a strong signal that is not seen in the previous or the subsequent integration. They exclude atmospheric, terrestrial, and solar system sources and present evidence that the transient is very likely associated with GN-z11 and therefore at the same redshift. This implies extreme luminosity, and they therefore argue that GN-z11-flash is most likely the prompt flash of a GRB exploding while they were observing the galaxy. It is extremely unlikely that such an incidence would occur, however, all other explanations J20 present are even more unlikely, mostly significantly so.

Assuming GN-z11-flash is indeed associated with the z ≈ 11 galaxy, we here compare its luminosity with a large sample of GRB afterglows (Kann et al. 2006, 2010, 2011, D. A. Kann et al. 2021a, 2021b, in preparation). J20 report the redshift (z = 10.957), a lack of dust extinction, and the spectral slope they measure from their calibrated spectrum (β = −1.2 ± 0.4 assuming F_ν ∝ ν^−β). Following J20, we here assume the emission is synchrotron radiation and this spectral slope can be extrapolated. We neglect Galactic foreground extinction, as this galaxy is in the GOODS/Hubble Deep Field North, and it is negligible (a few millimag). We use a λCDM concordance cosmology with H₀ = 72 km s⁻¹ Mpc⁻¹, Ω_M = 0.27, and Ω_λ = 0.73. J20 report a slit-loss corrected flux density in the K band of 0.1 − 0.2 mJy, we assume 0.15 ± 0.05 mJy. This corresponds to $K={18.46}_{-0.31}^{+0.44}$ mag (AB system, K = 16.59 mag in Vega). From the spectral slope, we find K − R_C = 1.54 mag (AB; K − R_C = − 0.15 mag Vega). Using the redshift and spectral slope, we find a magnitude correction to z = 1 of ${dRc}=-{1.98}_{-0.76}^{+0.78}$ mag (the blue spectrum and the large distance partially cancel each other out) following Kann et al. (2006). In total, this yields ${R}_{C}={14.46}_{-0.82}^{+0.90}$ mag at z = 1, Vega system. Following J20, the non-detection in the following spectrum implies a flux density at least ten times fainter, giving us a corresponding R_C > 17.0 mag. J20 did not find a coincident GRB detection. We assume the GRB began (t = T₀) with the end of the previous spectral integration, this yields logarithmic central points of 14.0 s for the detection and 53.9 s for the upper limit, in the z = 1 frame. For these times, the decay slope is α ≳ 1.7 (F(t) ∝ t^−α), a reasonable value for early GRB flashes.

We plot the final result (red) in Figure 1, highlighting a sample of z > 6 GRB afterglows (D. A. Kann et al. 2021a, in preparation). It can clearly be seen that GN-z11-flash is in full accordance with the luminosity distribution of early GRB optical emission, supporting the interpretation of J20. Considering no further emission was detected in subsequent spectra, we even infer this was a quite faint afterglow. In the sample of D. A. Kann et al. (2021a, in preparation), we find 70 GRB afterglows with detections at the time of the upper limit from J20 (assuming our T₀; note some are extrapolations), 63/70 (90%) are brighter than the upper limit of R_C > 17 mag.

Zoom In Zoom Out Reset image size