Radio Linear Polarization of GRB Afterglows: Instrumental Systematics in ALMA Observations of GRB 171205A

Abstract

Polarization measurements of gamma-ray burst (GRB) afterglows are a promising means of probing the structure, geometry, and magnetic composition of relativistic GRB jets. However, a precise treatment of instrumental calibration is vital for a robust physical interpretation of polarization data, requiring tests of and validations against potential instrumental systematics. We illustrate this with Atacama Large Millimeter/Sub-millimeter Array (ALMA) Band 3 (97.5 GHz) observations of GRB 171205A taken ≈5.19 days after the burst, where a detection of linear polarization was recently claimed. We describe a series of tests for evaluating the stability of polarization measurements with ALMA. Using these tests to reanalyze and evaluate the archival ALMA data, we uncover systematics in the polarization calibration at the ≈0.09% level. We derive a 3σ upper limit on the linearly polarized intensity of P < 97.2 μJy, corresponding to an upper limit on the linear fractional polarization of ΠL < 0.30%, in contrast to the previously claimed detection. Our upper limit improves upon existing constraints on the intrinsic polarization of GRB radio afterglows by a factor of 3. We discuss this measurement in the context of constraints on the jet magnetic field geometry. We present a compilation of polarization observations of GRB radio afterglows, and demonstrate that a significant improvement in sensitivity is desirable for eventually detecting signals polarized at the ≈0.1% level from typical radio afterglows.

A search for radio afterglows from gamma-ray bursts with the Australian Square Kilometre Array Pathfinder

“…The optical/UV light curve shows the rise of the SN 2017iuk starting ∼ 3 days post-burst (de Ugarte Postigo et al 2017b); spectroscopic observations confirmed this (Wang et al 2018). The millimetre/submillimetre observations probed for radio linear polarisation from the afterglow; the detection of which is still under debate (Urata et al 2019;Laskar et al 2020). In this subsection, we extended the multiwavelength analysis of this system into radio frequencies, using the data presented in Table 2 to conduct an empirical investigation into the afterglow properties.…”

1000 days of lowest frequency emission from the low-luminosity GRB 171205A

“…This has been explained as Faraday depolarization by non-accelerated, cool electrons in the shocked region. However, one cannot rely on these results due to dispute of the detection claimed by Laskar et al (2020).…”

“…These observations showed a steeply rising spectrum(∝ ν 2 ) at low frequency which indicates a synchrotron self-absorbed spectrum. This is the first GRB for which Urata et al (2019) claimed to have detected polarization at the ALMA 90GHz frequency, though this claim has been disputed by Laskar et al (2020). The upgraded Giant Metrewave Radio Telescope (uGMRT) first detected it on 20th December, 2017 at 1400 MHz (Chandra et al 2017a) after a non detection on 10th and 11th December, 2017 (Chandra et al 2017b).…”

GRB 171205A: Hypernova and Newborn Neutron Star

“…These observations showed that SN 2017iuk is a typical type Ic SN that resembles SN 2006aj, and that the host is an early-type, star-forming galaxy of high mass, low star formation rate, and low solar metallicity. In this source, for the first time, it was observed the polarization in the millimeter and radio bands during the afterglow phase, thanks to the intensive combined use of SMA, ALMA, and VLA, which shows a linear polarization < 1% indicative of Faraday depolarization (Urata et al 2019;Laskar et al 2020). The observation continued for years, the ASKAP, ATCA and µGMRT radio observations lasted till ∼ 1000 d, the radio afterglow decays following a shallow power-law and no jet break was exhibited (Leung et al 2021;Maity & Chandra 2021).…”