The Peculiar Short-duration GRB 200826A and Its Supernova^*

Optical spectroscopy of GRB 200826A was obtained with the Large Binocular Telescope (LBT) on Mount Graham, Arizona, USA, on 2020 August 28, at UT 09:06:45 (MJD 59,089.379692) using both optical Multi-object Double Spectrographs (MODS; Pogge et al. 2010). Each of the MODS was configured using a 1'' wide slit (R ∼ 1500). MODS1 was configured with the red grating (0.58 μm–1.0 μm coverage) due to issues at the time with the blue channel, and MODS2 was configured with the dual grating (0.32 μm–1.05 μm coverage). Observations consisted of 4 × 900 s exposures for the three channels, which yielded an effective exposure time of 7200 s for 0.58 μm−1.0 μm (the red channels). A position angle = 170 was used in order to cover both the reported location of the GRB 200826A afterglow and a nearby galaxy 15'' NE. Observations were obtained at or near transit with an effective airmass of 1.0005. The seeing for the spectroscopic observations ranged from 07 to 10 and the sky conditions were photometric.

The MODS data were reduced first with modsCCDRed version 2.04 to remove bias and flat-field the data. Next, custom IRAF scripts were used to extract along the central slit using a stellar trace, wavelength-calibrate and rectify the tilt in both the X and Y directions using arc-lamp lines, flux-calibrate the data using the spectrophotometric standard star G191-B2B, and remove telluric features from the red channels using the spectrophotometric standard. The data from the three channels were combined into a single spectrum and rebinned to a common scale of Δλ = 0.85 Å pixel⁻¹. The afterglow spectral energy distribution (SED) (Section 3) was then subtracted from the spectrum and the absolute flux calibration was fine-tuned using the late-time g', r', i', and z' MODS and Large Binocular Cameras (LBC; Giallongo et al. 2008) photometry (Table 1), which is free from SN and afterglow emission (see Section 2.4).

Optical follow-up was performed with the LBT in the Sloan filter g' and r' bands using the twin MODS instruments, and in the Sloan r' filter with the Device Optimized for the Low Resolution (DOLoRes) optical imager and spectrograph mounted on Telescopio Nazionale Galileo (TNG) in La Palma (Canary Islands, Spain). We also observed GRB 200826A twice with the AZT-22 1.5 m telescope equipped with a SNUCAM CCD camera (Im et al. 2010) at the Maidanak Astronomical Observatory (MAO; Uzbekistan).

All MODS observations were taken under good seeing conditions (05–10). Due to the binocular capabilities of the LBT, the observations in g' and r' were obtained simultaneously with both MODS instruments. During the summer of 2021, with the aim of expanding our host galaxy photometry data set, we obtained further U, r', i', z', and R_C imaging with the LBC under variable seeing conditions (09–15), followed by further MAO observations in the R_C band. The LBT/MODS and LBC imaging data were reduced using the data reduction pipeline developed at INAF–Osservatorio Astronomico di Roma (Fontana et al. 2014), which includes bias subtraction and flat-fielding, bad-pixel and cosmic-ray masking, astrometric calibration, and coaddition. We also downloaded the Gemini/GMOS data at 28 and 46 days reported in Ahumada et al. (2021), ³³ and afterward reduced them using the dedicated Gemini pipeline DRAGONS (Labrie et al. 2019). Finally, we downloaded the r'-band Gran Telescopio Canarias (GTC) data at 3.99 days reported in Zhang et al. (2021). ³⁴ The TNG/DOLoRes, GTC, and MAO images were reduced in a standard manner using PyRAF/IRAF tasks (Tody 1993).

The first H-band observations were obtained using the LBT Utility Camera in the Infrared (LUCI; Seifert et al. 2003) imager and spectrograph in conjunction with the Single Conjugate Adaptive Optics Upgrade for LBT (SOUL; Pinna et al. 2021), the second-generation adaptive optics (AO) system at the LBT. Data were obtained 37.1 days after the GRB, close to the expected maximum light of the possible associated SN. Compared to the J and K bands, the H band was preferred for three reasons: (1) there is lower sky and thermal background with respect to the K band; (2) the AO performance is better than that in J, allowing the system to achieve an FWHM < 01; and (3) there are no strong emission lines from nonstellar sources or from star formation regions (like Hα in the J band at the redshift of our target), and the host continuum is dominated by light from old, late-type stars. A relatively bright foreground star (R = 15.06 mag) 114 away from the GRB location (and within the FOV) was used as the AO reference star for the AO system. The AO system achieved an average FWHM of the point-spread function (PSF) ∼013. Subsequent observations were able to achieve an FWHM ∼008−013 (see Section 2.3 for more details). The observations were repeated on 2020 December 1 and 2021 February 7, 97 and 158 days, respectively, after the GRB event.

In 2021 September we also obtained J-band imaging of the host with LBT/LUCI. All near-IR (NIR) data were reduced using the same data reduction pipeline for the LBT/MODS data (Section 2.2), which in the case of LUCI includes dark subtraction, flat-fielding, bad-pixel and cosmic-ray masking, and sky subtraction. Finally, the images were aligned to match the position of the AO reference star and coadded using IRAF tasks. The astrometry was calibrated against field stars in the GAIA DR2 catalog (Gaia Collaboration et al. 2018) and has an astrometric precision of 015.

All data were analyzed by performing aperture photometry using DAOPHOT and APPHOT under PyRAF/IRAF. For the optical data, the photometric calibration was performed using a set of field stars observed during the fifth MODS epoch (which had photometric conditions) and calibrated against the Sloan Digital Sky Survey DR12 catalog magnitudes of brighter nearby stars (Alam et al. 2015). The NIR photometry was calibrated against the AO reference star and a pair of Two Micron All Sky Survey (Skrutskie et al. 2006) stars observed immediately after the GRB field during the first epoch. The photometric calibration has a standard deviation of 0.02 mag. To err on the side of caution, twice this value was used as the photometric uncertainty.

To remove the contribution of the host to the afterglow/SN photometry, a reference frame free of any transient light is required. To better investigate any possible contamination from SN light in these earlier observations, the GRB 200826A data sets were compared with those of GRB 980425/SN 1998bw (Galama et al. 1998), which is the standard template for SNe associated with GRBs (GRB-SNe in the following). Following Zeh et al. (2004) and Klose et al. (2019), the observed SN 1998bw light curves were first corrected for Galactic extinction, moved to the rest frame, interpolated with a polynomial curve at steps of 0.5 days, and finally shifted to the GRB 200826A observed bands, r', i', and H, accounting for filter band differences and cosmological k-corrections.

At the time of the last MODS r' observation (78.9 days after the trigger) any SN 1998bw–like GRB-SNe would have had r' ∼ 27 mag, more than 2 mag fainter than its peak magnitude (r' ∼ 24.7 mag) at 20–40 days, when the first observations had been obtained. Therefore, it is acceptable to consider the late r' MODS images to be free from any contamination by SN light and they can be used as templates for image subtraction. They can also be used to measure the flux of the host galaxy (∼4 mag brighter in the r' band) instead of the later LBC images, which have a lower signal-to-noise ratio. These LBC images were instead used as reference for the image subtraction of the r' imaging at ∼50 days. Similarly, in the H band ³⁵ any SN 1998bw–like GRB-SN contribution at 160 days (H ∼ 26 mag at the time of our last LBT/LUCI observation) would have been ∼2 mag fainter than its peak magnitude (H ∼ 24.3 mag). Differences of ∼2 mag translate to a contribution from the SN to the late-time image of ∼0.2 mag. This contribution does not impact the analysis after the image subtraction, but is included in the photometric errors. On the contrary, any SN 1998bw–like GRB-SN would fade by less than 1 mag between 97 and 160 days; thus the last H-band observation (97 days) may also be affected by oversubtraction, and hence may result in a photometric value for the transient that is too faint at 97 days, though the SN is likely too faint to contribute substantially at this point. For completeness we report the resulting upper limit.

For the i' band we note that at the time of the final Gemini observation (at 75 days; see Ahumada et al. 2020b) any SN 1998bw–like GRB-SN would have had i' ∼ 25.5 mag, just ∼1.5–2 mag fainter than the peak magnitude at ∼30 days (∼24 mag), which could be acceptable. Instead, this difference is certainly smaller in the case of the Gemini observation at 46 days (see Table 1 and Figure 7). Therefore, we caution that using the value at 75 days as pure host can produce an oversubtraction, in particular at 46 days. Therefore, in the image subtraction of the i' band we used the LBC imaging obtained at 287 days.

Before applying image subtraction, the input and reference images were aligned using the WCSREMAP package. ³⁶ Image subtraction was then performed using a routine based on HOTPANTS ³⁷ (Becker 2015). This algorithm matches the PSF and count flux of both input images. The PSF is modeled via Gaussian functions in subregions of the original image. The software outputs a noise map of the resulting difference image, which is used to derive the uncertainties in the measured fluxes. The routine creates a grid of input Gaussian widths and subregion sizes and searches for the solution with the lowest noise.

In the case of the H-band imaging, the average FWHM during the first epoch was 013 as measured from the AO reference star in the LUCI-1 FOV ³⁸ and was constant for nearly the entire observation. The small amount of data with larger FWHMs was not used for analysis. A total of 1180 s of exposure time with FWHM ∼ 013 was used for analysis. For the other epochs the FWHMs ranged from 008 to 015. Therefore, only exposures with an FWHM of the AO reference star <013 (to match the first epoch) were selected. This yielded a combined image with a total integration time of 2400 s and 2700 s for the second and last epochs, respectively. In particular, the AO reference star in the last epoch had an FWHM ∼ 010, and this was used as the template for image subtraction. In contrast to the optical images, only two stars (the AO reference star and another star close to it) were useful for mapping the PSF and flux. Unfortunately, this impacts the optimal use of HOTPANTS for scaling the template image to the flux of the science image. Instead, a better result was obtained using the PSF-MATCH task under IRAF and by scaling the images to have the same flux for the two stars. The image subtraction of the two H-band images shows a transient located at R.A. (J2000) = 00^h27^m0855, decl. (J2000) = +34°01'382 with an uncertainty of 015 and calibrated against the MODS images (Figure 1). This is consistent with the position reported by Ahumada et al. (2020a).

All image-subtracted data have been analyzed using aperture photometry as detailed in Section 2.2. Table 1 provides a summary of all the photometry of the transient and the host galaxy including values collected from the literature.

Apparent magnitudes were corrected for Galactic extinction using the Cardelli et al. (1989) interstellar extinction curve, a total-to-selective extinction of R _V = 3.1, and a reddening along the line of sight of E(B − V) = 0.058 mag (Schlafly & Finkbeiner 2011).

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Although the g' and r' light curves are quite sparse, they can be modeled with a single power law with α = 0.94 ± 0.05. However, this model implies that the late afterglow is brighter than the deep r' upper limits obtained at ∼30 days after subtraction of the host galaxy contribution. Therefore, the data were modeled with a smoothly broken power law (Beuermann et al. 1999) (Figure 2). The last detections in the g' and r' bands are at ≈2.2 and ≈4 days (observer-frame). Using SN 1998bw as an SN template (see below for more details), the SN itself would have had g' = 29.8 mag and r' = 26.9 mag at 2.2 days and 4 days, respectively. Therefore, it can safely be assumed to not contribute significantly to the optical transient and can be ignored in the afterglow fitting. This yielded decay slopes α₁ = 0.43 ± 0.55 and α₂ = 1.59 ± 0.20, with a break time t_b = 0.47 ± 0.24 days, and the break smoothness n = 10 fixed. The post-break decay slope easily accommodates the magnitude upper limits at later times, with values beyond r' ∼ 27.8 mag and g' ∼ 28.3 mag at ∼30 days. Therefore, any late emission detected via subtraction is not from the afterglow.

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To investigate whether the optical data are affected by local dust extinction along the line of sight, the SED was modeled spanning the optical to the X-ray band. All Swift/XRT data available were used, which have a mean arrival time of 65,840 s (0.762 days), along with the g' and r' bands obtained from the broken power-law fit (see above), and thus are free from host contamination. The redshift was fixed to match that of the host galaxy and the Galactic hydrogen column density was set to N_H = 6.02 × 10²⁰ cm⁻² (Willingale et al. 2013). The optical-to-X-ray SED is well fit (χ²/dof = 33/47) by a power law with a spectral index β = 0.79 ± 0.03 without the need for additional dust extinction and gas absorption to improve the fit. The negligible dust extinction is well supported by the optical-to-X-ray spectral index β_OX > 0.5 (Jakobsson et al. 2004; Rossi et al. 2012). The optical-to-X-ray spectral slope was then used to extrapolate the late-time brightness in the i' and H bands. For the first i' detection at 28 days, the brightness of the projected afterglow was 27.6 mag, 3.2 mag fainter than the detection. The H-band afterglow is H ∼ 27.4 mag at 37 days, 3.4 mag fainter than the detection.

To consider also the contribution of an emerging SN component, we followed the work of Zeh et al. (2004) and Klose et al. (2019), and modeled the afterglow and SN component in a joint g', r', i', and H fit (rest-frame far-UV, U, B, and z' bands). The analytical light curve of SN 1998bw and the k, s parameterization was used (see Section 2.4). The parameter k is the luminosity factor and it describes the observed luminosity ratio between the GRB-SN at peak time and the SN template in the considered band. The parameter s is a stretch factor with respect to the used template. ³⁹ The observer-frame g' band (rest-frame far-UV) is not covered by observations in the case of SN 1998bw (in contrast to the observer-frame r' band, which is the rest-frame U band), so we assume that the flux density of the SN falls as ν⁻³ to extrapolate and derive the corresponding far-UV SN 1998bw template (e.g., Klose et al. 2019). Note that the g' and r' data points in Figure 2 after 2.2 and 4 days (observer-frame) are upper limits only. In order to constrain the flux density of a possible SN contribution in these bands, we considered these limits as proxy detections in the modeling. We also remind the reader that the i'- and H-band afterglow light curves have been spectrally extrapolated (see above) because there are no observations in these bands during the afterglow phase. The sparse data in i' and H preclude determining the precise temporal evolution of the late emission, but do not preclude determining more information on the luminosity. The stretch factor was fixed at s = 1 for all bands and the luminosity factor k was allowed to vary freely and individually in each band. The results of this joint fit are presented and discussed in Section 3.3.

The duration of GRB 200826A, as reported by the different detectors and in different energy bands, is 1–2 s (see Section 1), placing it right at the divide between SGRBs and LGRBs in the duration distribution commonly used to distinguish between these two classes (but see de Ugarte Postigo et al. 2011; Bromberg et al. 2012).

There are other events that lie between the two populations, but none has an observed T₉₀ shorter than 2 s and an associated SN that clearly points to a collapsar origin. For example, GRB 090426 (z = 2.609) has a duration shorter than 2 s but also has characteristics typical of LGRBs including softer spectra, a bright afterglow, and a blue and low-mass host (Antonelli et al. 2009; Nicuesa Guelbenzu et al. 2011, 2012; Thöne et al. 2011; D. A. Kann et al. 2022, in preparation). It could be argued that one should consider the intrinsic duration in the rest frame (T_90,i). Doing so, the duration of GRB 200826A would be between 0.39 (Konus-Wind) and 0.57 (GBM) s, i.e., well below 2 s. The only other GRB with an associated SN that could be classified as short because of its intrinsic duration (T_90,i = 2.4 s, z = 0.86) is GRB 040924 (Huang et al. 2005; Soderberg et al. 2006; Wiersema et al. 2008). Many other cases exist at high redshift like GRB 080913 (Greiner et al. 2009; Zhang et al. 2009), but the redshift is too high to search for the associated SN (at least with the current facilities).

Bromberg et al. (2012) showed that the 2 s duration commonly used to separate collapsars and noncollapsars is inconsistent with the duration distributions of Swift and Fermi GRBs and only holds for old CGRO/BATSE GRBs. However, analysis of the Konus-Wind data (Svinkin et al. 2020) showed that GRB 200826A has a duration typical of SGRBs. Thus, the short duration of GRB 200826A is instrument-independent. In this case T₉₀ alone is insufficient to determine which population GRB 200826A is best associated with. Another feature typical of LGRBs is a nonnegligible spectral lag (Norris et al. 1996). This parameter has been calculated and the details are provided in the Appendix. We obtain a spectral lag of 96 ± 38 ms, which lies between the values found by Ahumada et al. (2021) and Zhang et al. (2021). In agreement with their analysis, we conclude that the measured spectral lag is more typical of LGRBs.

It is only when both the spectral properties of GRB 200826A and its location in the hardness–duration plane are considered does it appear to be more similar to "long" GRBs. However, if this is indeed the shortest "long" GRB associated with an SN, then additional evidence is needed to demonstrate that it does not belong to the SGRB class. Such a result would challenge the standard paradigm for SGRB and LGRB progenitors.

In this respect, a very useful tool is the location of a GRB in the E_p,i–E_iso plane, ⁴⁰ where LGRBs follow a strong correlation known as the "Amati relation" (Amati et al. 2002; Amati 2006). In comparison, SGRBs populate a different region in this plane (e.g., D'Avanzo et al. 2014; Minaev & Pozanenko 2020a). Therefore, the fluence and spectral parameters reported by the Konus-Wind and Fermi/GBM teams were used (Mangan et al. 2020; Ridnaia et al. 2020; Svinkin et al. 2020) to compute the values of E_p,i and E_iso, and their uncertainties, for GRB 200826A. As can be seen in Figure 3, this event is fully consistent with the E_p,i–E_iso relation followed by LGRBs. This further supports the hypothesis that it is actually a "long" GRB with a duration in the short tail of the distribution.

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Recently, the energy–hardness (EH) parameter—a combination of E_p,i and E_iso (EH = ${E}_{{\rm{p}},{\rm{i}}}{E}_{\mathrm{iso}}^{-0.4}$)—was introduced in Minaev & Pozanenko (2020a). Although GRB 200826A is placed inside the 95% cluster region of long bursts in the T_90,i–EH diagram (Figure 4), it is evident that this does not permit an unambiguous classification of a burst close to the cluster region intersection (see also Zhang et al. 2021 for a similar analysis and conclusion). Also the position in the E_p,i–E_iso plane may not be sufficient for classification. Indeed, interesting examples such as GRB 201221D ⁴¹ (Agüí Fernández et al. 2021) exist which follow the E_p,i–E_iso relation for LGRBs but are in the SGRB region in the T_90,i–EH diagram.

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A well-known result is that most SGRB afterglows are much fainter than LGRB afterglows (Gehrels et al. 2008; Nysewander et al. 2009; Kann et al. 2010, 2011). Following the method of Kann et al. (2006), using a spectral slope of β = 0.79 and no line-of-sight extinction, in Figure 5 the optical afterglow of GRB 200826A is compared with a large sample of SGRB and LGRB optical afterglows (taken from Kann et al. 2006, 2010, 2011; Agüí Fernández et al. 2021; D. A. Kann et al. 2022, in preparation). Similarly, in Figure 6 the X-ray emission of GRB 200826A is compared with that of ∼500 GRB afterglows (of which 38 are of SGRBs) detected in at least two epochs and with known redshift. The X-ray light curves were obtained from the Swift Burst Analyser ⁴² (Evans et al. 2010) and put in the same frame using the method described in Schulze et al. (2014). The afterglow of GRB 200826A is among the faintest afterglows of LGRBs but also among the brightest ones of SGRBs, in both the X-ray and optical bands. Thus, using the afterglow luminosity is not a good method for classifying, or understanding the nature of, this burst.

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Although the sparse data precludes determining the evolution of the late bump seen in the transient light curve, a comparison can be made with other GRB-SNe to look for similarities. This comparison includes GRB 980425/SN 1998bw (Galama et al. 1998), GRB 060218/SN 2006aj (Ferrero et al. 2006), GRB 100316D/SN 2010bh (Olivares et al. 2012), GRB 130702A/SN 2013dx (D'Elia et al. 2015; Toy et al. 2016; Volnova et al. 2017; Mazzali et al. 2021), and GRB 161219B/SN 2016jca (Cano et al. 2017b; Ashall et al. 2019). In Figure 7 we show only those GRB-SNe with comparable peak luminosities and colors, including SN 1998bw, which has been used to also fit extreme events such as SN 2011kl (Greiner et al. 2015; Kann et al. 2019) and the SN bump of GRB 140506A (Kann et al. 2021). Due to the nonnegligible redshift of GRB 200826A, the observed r'-, i'-, and H-band data sets correspond approximately to the rest-frame U, B, and z' bands, respectively. In the following comparison, we will refer to these rest-frame bands.

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The SN associated with GRB 200826A appears fainter than SN 1998bw in the rest-frame B band, but is of comparable luminosity to or slightly brighter (≲0.5 mag) than SN 1998bw in the rest-frame z band. The most striking feature is the suppression in the rest-frame U band, by at least 1 mag compared to SN 1998bw at 28 days. This prompted a further investigation into GRB-SNe with redder colors, and produced possible agreement with GRB 130702A/SN 2013dx. This GRB-SN (like GRB 161219B/SN 2016jca and possibly GRB 050525A/SN 2005nc; Della Valle et al. 2006) is characterized by a rise-to-maximum time Δt ≲ 15 days in the respective rest frame (e.g., Belkin et al. 2020b), which is a parameter related to properties of progenitor stars (e.g., González-Gaitán et al. 2015).

In Section 2.4 we have modeled the late light curve considering the analytical light curve of SN 1998bw and the k, s parameterization following the work of Zeh et al. (2004) and Klose et al. (2019), which allows us to place quantitative constraints on the above analysis. We remind the reader that there are no detections in g' and r' after 2.2 and 4 days (observer-frame), and thus with our modeling we can only place deep upper limits on the flux density of the SN contribution in these bands (see Figure 2). In terms of luminosity, a potential SN contribution in the rest-frame far-UV (observed g' band) is found as k_g' < 1.5, an unremarkable limit to its luminosity. Therefore, although the spectral slope of the extrapolation mentioned above is insecure, this does not have any significant impact on our modeling. However, there is clear UV suppression in the rest-frame U band: we find ${k}_{{r}^{{\prime} }}\lt 0.43$, implying that the SN associated with GRB 200826A is less than half as luminous as SN 1998bw in the same rest-frame band. In the rest-frame B band the suppression is also strong compared to SN 1998bw with ${k}_{{i}^{{\prime} }}=0.56\pm 0.10$. The rest-frame z'-band detection (observed H-band) is, however, marginally brighter than SN 1998bw (k_H = 1.18 ± 0.22). Moreover, it lies close to the SN peak for s = 1, and thus a significantly faster or slower SN would be even brighter. When compared with the list of k, s values of Klose et al. (2019) and Cano et al. (2017a) the k factor ranges between 0.5 and 2. This provides plausible support for an SN.

In Figure 8, the late-time photometry of the transient is compared with the light curve of AT 2017gfo, the KN associated with the binary NS merger GW170817 and the SGRB 170817A (Abbott et al. 2017c). The KN light curves are obtained from observed spectra shifted and scaled to the same redshift of GRB 200826A, following the method described in Rossi et al. (2020b). The comparison shows that the luminosity of the transient was at all times at least 3 mag brighter than the peak luminosity of AT 2017gfo. This is not extraordinary, because KNe ≳10 times more luminous in the optical have been claimed to exist (e.g., Gao et al. 2017; Fong et al. 2021). However, what makes this event very different from all KNe observed so far is its slower temporal evolution: assuming that the rest-frame B-band detection (i'-band detection in the observer frame) is close to maximum light, i.e., ∼16 days in the rest frame, the peak would be more than ∼15 days later than that of AT 2017gfo. The analysis below investigates whether such behavior is theoretically possible.

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Modeling of KNe powered by radioactive decay of r-process nuclei shows that KNe produced by nearly equal-mass binary NS mergers are expected to have peak times and magnitudes similar (within a few days and one or two magnitudes, respectively) to the ones observed in AT 2017gfo, especially in the case of face-on mergers characterized by lanthanide-free high-latitude ejecta (e.g., Radice et al. 2018b; Kawaguchi et al. 2020). Viewing angle and composition effects possibly decrease the KN luminosity as compared with face-on configurations (Korobkin et al. 2021). In the case of very unequal-mass binary NS or black hole–NS mergers, the peak evolution can be slower, especially in the rest-frame NIR bands, but with similar peak magnitudes (e.g., Barbieri et al. 2019; Bernuzzi et al. 2020; Kawaguchi et al. 2020; Zhu et al. 2020). These theoretical results seem to also disfavor the interpretation of the observed transient as a KN powered by radioactive decay.

However, it is known from several studies (e.g., Gompertz et al. 2018; Ascenzi et al. 2019; Rossi et al. 2020b; Rastinejad et al. 2021) that the distribution of luminosities covers a large range, and some KNe can be 10 times more luminous than AT 2017gfo, such as those associated with GRBs 060614 and 050724 (Gao et al. 2017). Even more extreme cases that are ∼100 times more luminous have been claimed to exist in association with GRB 070714B (Gao et al. 2017) and more recently with GRB 200522A (Fong et al. 2021; O'Connor et al. 2021), although the evidence is not very strong. To achieve this high blue luminosity, the KN can be boosted by energy deposition from a magnetar remnant. Indeed, long-lived merger remnants could have a significant impact on KN light curves. While larger amounts of ejecta, expelled due to angular momentum excess, can increase peak magnitudes, times, and widths only marginally (e.g., Radice et al. 2018a), the presence of a fast-rotating magnetar (in addition to its being a possible GRB central engine) can boost the KN luminosity via spin-down luminosity (Yu et al. 2013; Metzger & Piro 2014; Siegel & Ciolfi 2016).

To explore this scenario, the KN model presented in Perego et al. (2017) was used and extended to consider spin-down energy injection, as well as sources at cosmological distances for which the fluxes, the (thermal) spectra, and the observer time were corrected for the cosmological redshift. The computed KN light curves are shown in Figure 8. For simplicity, a spherically symmetric ejecta model was considered for which 0.02 M_⊙ of matter expands homologously at 0.2c. Given that the adopted model foresees the presence of a long-lived magnetar, an effective gray opacity κ = 1 cm² g⁻¹, typical of lanthanide-poor ejecta, was assumed. P₀ = 0.7 ms was fixed as a typical initial rotation period (Radice et al. 2018a), while the strength of the magnetic field B and the energy deposition efficiency for the spin-down luminosity _th were varied to obtain plausible agreement with the observations. A reasonable match for B ∼ 10¹⁴ G and _th = 0.03 is shown in Figure 8. For the considered bands, larger magnetic field strengths (B = 10¹⁵ G) result in lower, more rapidly declining peaks, and a plausible match with the data would require _th ≳ 1. Weaker field strengths (B = 10¹³ G), on the other hand, produce more luminous, more slowly evolving peaks, which plausibly match the data for ≲ 0.001. The ejecta properties also influence the light curves, but less significantly.

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The calculations suggest that the observed transient could be compatible with the late emission of a spin-down-powered KN for plausible values of the magnetic field and thermalization efficiency. Unfortunately, all of the magnetically boosted KN models fail to explain the red color of the transient, and in particular the strong UV suppression observed in the observer-frame r' band, although modeling greatly simplifies the ejecta and the emission properties, particularly the opacity. But most importantly, during the first 10 days the theoretical KN is comparable to or brighter than the observed data without accounting for the afterglow. Therefore, the magnetically boosted KN models are excluded.

As first reported in Rothberg et al. (2020a, 2020b), the continuum of the host galaxy was clearly detected between 0.38 μm and 0.98 μm. A redshift of z = 0.748577 ± 0.000065 was determined from the simultaneous detection of the following emission lines: [O ii], [O iii], and [Ne iii] doublets; He i λ3889; and Balmer Hδ λ4101, Hγ λ4340, and Hβ λ4861 (Figure 9). The fluxes of the lines were measured using the slinefit code (Schreiber et al. 2018), which also allows correction of the Balmer-line fluxes for the underlying stellar absorption. All the measured fluxes are reported in Table 2. The absorption lines of the Mg ii λλ2796, 2803 doublet were also detected at a lower redshift (z = 0.7462 ± 0.0002), which may possibly imply that they are due to an intervening Mg ii absorber along the line of sight (Vergani et al. 2009; Christensen et al. 2017). The rest-frame equivalent width for both lines in the doublet is W_rest = 2.4 ± 0.6 Å.

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Using the derived redshift as a fixed input, the optical/NIR SED of the host galaxy was modeled (Table 1) with the code Le PHARE (Arnouts et al. 1999; Ilbert et al. 2006). The best fit (χ²/dof = 2.2/9) points to a galaxy dominated by a young population (${0.04}_{-0.02}^{+0.08}$ Gyr) with global dust reddening E(B – V) ∼0.2 mag using the Small Magellanic Cloud (SMC) curves from Pei (1992) and low stellar mass $\mathrm{log}{M}_{* }/{M}_{\odot }=8.6\pm 0.2$ (Figure 10). The B-band dust-corrected absolute magnitude is M_B = −20.01 mag and the star formation rate is ${\rm{SFR}}\,={13.0}_{-6.0}^{+10.9}\,{M}_{\odot }$ yr⁻¹. With this large uncertainty, this SFR is consistent with what is measured from the emission lines (see below). The dust extinction is in agreement with the broad range of values found for LGRB hosts, also in comparison with the SFR (Hunt et al. 2014; Japelj et al. 2016). It is different from the afterglow-derived negligible extinction, indicating that the GRB sight line might be crossing a relatively dust-free region (e.g., by being placed at the front side of the host galaxy). Compared to Ahumada et al. (2021), we find a mass that is a factor 10 lower. The computed lower mass is most likely due to the fact that the NIR observations were obtained sufficiently late to be free of any contributions from the transient.

Using the Balmer decrement, and assuming the theoretical values for the Hγ/Hβ and Hδ/Hβ ratios, in the absence of dust, for case B recombination at a gas temperature of 10⁴ K (Osterbrock 1989), and a Milky Way (MW) extinction law ⁴³ (Pei 1992) with R _V = 3.1, we derive an E(B – V) of ∼0.16 mag and an A_V ∼ 0.5 mag. Using extinction-corrected Hβ and [O ii] fluxes and the prescriptions provided by Kennicutt (1998) and Savaglio et al. (2009), the resulting SFR of the host galaxy is SFR ∼ 4.0 M_⊙ yr⁻¹. The SFR determined from the line fluxes is within 2σ of that derived from SED fitting. The high SFR obtained from the two separate methods implies a specific SFR (sSFR; SFR per unit stellar mass) of 1–3 × 10⁻⁸ yr⁻¹, among the highest measured so far in the LGRB host population (e.g., Savaglio et al. 2009; Perley et al. 2013; Hunt et al. 2014; Vergani et al. 2015; Japelj et al. 2016; Schulze et al. 2018; Modjaz et al. 2020). For comparison, this sSFR is about two orders of magnitude higher than that for the Large Magellanic Cloud (LMC). Moreover, if the derived SFR were constant, this would imply that the time required for the host to build the measured stellar mass would be about 0.03–0.1 Gyr, which is consistent with the upper limit obtained from the SED best fit. Using the appropriate diagnostic tools in Lamareille (2010), Trouille et al. (2011), and Marocco et al. (2011) the presence of an active galactic nucleus, which may mimic a high SFR, can be confidently excluded.

The diagnostic ratios of the [O ii], [O iii], and Hβ emission lines were used to derive the gas metallicity in the host galaxy. Many calibrators have been proposed in the past (for a review, see Maiolino & Mannucci 2019); the method proposed by Curti et al. (2017) was adopted, which computes the best-fit metallicity by minimizing χ² in the space defined by the different possible diagnostics given the available emission lines. This yielded $12+\mathrm{log}({\rm{O/H}})=8.31\pm 0.02$. This is in agreement with the value of $12+\mathrm{log}({\rm{O/H}})\simeq 8.3$ obtained from using the [Ne iii] λ3869/[O ii] line ratio (e.g., Bian et al. 2018), a method that has more uncertainties. The resulting subsolar metallicity (Z/Z_⊙ ≃ 0.4) is consistent with the values derived for LGRB hosts at similar redshift (Japelj et al. 2016). However, we note that the redshift evolution of the mass–metallicity relation in normal star-forming galaxies derived by Bellstedt et al. (2021) shows that galaxies at z = 0.62 or 0.8 and stellar masses similar to that of the GRB 200826A host have a typical metallicity of about $12+\mathrm{log}({\rm{O/H}})=7.85$ or 7.80, respectively, significantly below our measured value. Taken together, these results indicate that the host galaxy of GRB 200826A is characterized by a typical metallicity but a higher than usual SFR and sSFR for an LGRB host.

For the adopted cosmological parameters and using the last deep H-band observation, the half-light radius of the host galaxy is R_h = 016 or 1.2 kpc at the redshift of the GRB. This size is consistent with LGRB hosts of similar masses (Kelly et al. 2014). The high angular resolution from the LUCI-AO observations also makes it possible to measure the GRB-SN position within the host. The transient has an offset of R = 01 from the center of the host, which corresponds to 0.75 kpc. This corresponds to a normalized offset R/R_h = 0.62, which is also consistent with the vast majority of LGRBs (e.g., Bloom et al. 2002; Blanchard et al. 2016).