GRB 210217A: a short or a long GRB?

Abstract

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{90}$ value (the time interval during which an instrument observes 5% to 95% of gamma-ray/hard X-ray fluence). However, $T_{90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{90}$ for a burst. GRB 210217A is detected with different duration by Swift and Fermi. It is classified as a long/soft GRB by Swift-BAT with a $T_{90}$ value of 3.76 s. On the other hand, the sub-threshold detection by Fermi-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 s. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{90}$-hardness ratio, $T_{90}-E_{p}$ and $T_{90}-t_\mathrm{mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and placed the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

Appendix

1.1 Afterglow properties

The afterglow of the GRB can be well explained by the synchrotron fireball model (Piran 1999). The spectra, as well as the light curves, consist of a combination of power-law and broken power-law characterized by electron distribution index p (Sari et al. 1998; Piran 2004). We used the spectral and temporal indices to constrain p and the break frequencies using well-known closure relations (Sari et al. 1998). For this purpose, we fitted the X-ray and optical light curves/spectra at different epochs with single and broken power-law models. Both X-ray and optical light curves are well explained with a single power with indices values of $1.10 \pm 0.1$ and $0.65\pm 0.02$, respectively. The values of spectral indices at different epochs are given in Table 4. At around 0.3 ks, the spectral index is almost the same for optical and X-ray within the errorbar, suggesting no cooling break between X-ray to optical data. However, we found that the X-ray spectral index is almost twice the optical index at later epochs, indicating some break. So, we further created the spectral energy distributions using optical and X-ray data at two epochs centered at ${\sim }0.27$ and ${\sim }$125 ks. We fitted the SEDs with power-law and broken power-law models. The SED at the early epoch is best fitted with a single power-law with an index of $0.607\pm 0.02$. However, the SED at the later epoch is best fitted with a broken power-law with indices $0.65^{+0.08}_{-0.07}$ (pre-break) and $1.32^{+0.28}_{-0.21}$ (post-break) and a break at $1.7\pm 0.3 \times 10^{17}$ Hz, which we identify as a cooling break. The best-fitted SEDs are shown in Figure 7.

Table 4 Optical and X-ray spectral indices were obtained from fitting SEDs at different epochs and their best described spectral regime. p is the mean value of the electron distribution indices calculated using temporal and spectral indices in that spectral regime.

Figure 7

The optical/X-ray SEDs at two different epochs ($\sim$270 s and at $\sim$1.45 days after the trigger). The SED at the earlier epoch is well fitted with a simple power-law with a spectral index of 0.61$^{+0.03}_{-0.02}$. However, a spectral break can be seen at the later epoch, which is identified as the synchrotron cooling break.