GRB 210619B: First Gamma-Ray Burst Detection by the Novel Polarimeter MOPTOP

Gamma-ray bursts (GRBs) are short-lived, very energetic cosmological events. When ejected material in GRB jets interacts with the local ambient medium, afterglow emission is observed at various wavelengths (Piran 1999; Zhang & Mészáros 2004). Due to their great distance and their orientation, GRB jets are not spatially resolved via traditional astronomical techniques. Instead, we must rely on photometric variability and any polarimetric signal to decipher the structure and magnetic field properties. However, due to the transient nature of GRBs, traditional polarimeters that make sequential measurements of polarization states can induce an artificial polarization signal. With this in mind, a series of polarimeters RINGO (2006–2009), RINGO2 (2010–2012), and RINGO3 (2013–2020) that used a rapidly rotating polaroid analyzer (Jermak et al. 2016; Arnold 2017) were developed for the Liverpool Telescope (LT). The newest addition to this series is a novel polarimeter MOPTOP (Shrestha et al. 2020) which is designed for rapid optical photometric and polarimetric follow-up of GRBs. MOPTOP utilizes a rapidly rotating half-wave plate and a polarization beam splitter to observe the fading GRB. The instrument started science observation in 2020 October and GRB 210619B was the first GRB detected by the instrument. In this research note, we present the light curve and a polarization upper limit for GRB 210619B and provide a brief interpretation of these observations.

GRB 210619B was triggered by Swift Burst Alert Telescope at 23:59:25 UT (D'Avanzo et al. 2021). It has T₉₀ = 54.7 s (Poolakkil et al. 2021) and was observed at a redshift of 1.937 (de Ugarte Postigo et al. 2021) and Caballero-García et al. (2023) reported E_γ,iso = 4.05 × 10⁵⁴ erg. The burst was followed up by various ground-based telescopes as reported in GCN circulars. ⁴ LT started the follow-up ∼23 minutes after the trigger using MOPTOP in the R band. This was then followed by LT IO:O imager observations ⁵ in the r band. We note an offset in magnitude between the two instruments (∼0.2 mag) due to a slight difference in filter passband.

For both IO:O and MOPTOP, basic CCD reduction with bias subtraction, dark subtraction, flat-fielding and World Coordinate System fitting is done internally. ⁶ The processed images are then used to extract photons counts of the sources using AstroPy Photutils package (Bradley et al. 2019). For MOPTOP and IO:O photometric analysis, we calibrate magnitude using calibration stars from the APASS catalog (Henden et al. 2016). In the case of MOPTOP we follow the "one camera technique" procedure outlined in Shrestha et al. (2020) to get polarization values. ⁷

Figure 1 left panel presents photometric results from MOPTOP R band observations and IO:O observations along with the last three data points are IO:O r observations from GCN by Perley (2021), Blazek et al. (2021). We fit the MOPTOP data both with a single power law and a broken power law (for the case of broken power law we let the break time be a free parameter). We found a broken power law with a break time at 2948 ± 30 s after the trigger gave the best reduced χ² of 1.19 with a degree of freedom 84. From this fit, we get a decay index before the break to be α₁ = 0.84 ± 0.025 and after the break to be α₂ = 0.54 ± 0.018. This shows that we are probably observing the phase where forward shock from the afterglow is dominant (Kobayashi & Sari 2000).

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The right panel of Figure 1 shows how the polarization degree varies with time. A gray solid line represents median %p = 6.95. For the position of GRB, we used the Galactic extinction value from Schlegel et al. (1998) E_B−V  = 0.1715 ± 0.005. We used the relation p^V  ≤ 9E_B−V from Serkowski et al. (1975) to calculate the upper limit in polarization degree in V band and then use relation $p/{p}_{\max }=\exp [-K{\mathrm{ln}}^{2}({\lambda }_{\max }/\lambda )]$ to estimate galactic interstellar polarization (ISP) in the R band where we assume ${\lambda }_{\max }$ to be V band and p_max to be p^V and $K=-0.10+1.86{\lambda }_{\max }$ given by Wilking et al. (1982). Hence we get galactic ISP in the R band to be 1.52% which we do not correct in our results. Though the median value is greater than ISP, the error bars for most of the data crosses 0 which signifies that the observed polarization is upper limit. Thus the non-detection of polarization for the case of GRB 210619B is reasonable as our observation time period is mostly forward shock dominated as seen in the light curve. Mandarakas et al. (2023) also made polarimetric observations of the GRB 210619B with 5σ detection of 1.5%. Their low polarization values are in good agreement with our observations.