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M. Shrestha

Publications and source records attributed to M. Shrestha.

At least 19 recordsLinked to original sources
arXiv2026

Early Multiwavelength Observations of AT 2026fgk: The Luminous Afterglow to Sub-luminous GRB 260310A, Identified Independently of a Gamma-ray Trigger

The origins of sub-luminous ($L_\mathrm{\gamma,\mathrm{iso}} < 10^{49.5}$\,erg\,s$^{-1}$) gamma-ray bursts (GRBs) associated with broad-lined Type~Ic supernovae (Ic-BL SNe) are poorly understood, in part due to the low discovery rate and faint afterglows. Here we present the identification of the optical afterglow of Fermi-GBM-detected GRB\,260310A (AT\,2026fgk) as a rapidly rising ($>1\,$mag\,d$^{-1}$), red ($g-r=0.4$\,mag) transient using the Gravitational-wave Optical Transient Observatory, Large Array Survey Telescope, and Zwicky Transient Facility (ZTF) data streams. We present multiwavelength follow-up observations from the first 50\,days, which reveal that GRB 260310A/AT\,2026fgk was sub-luminous ($L_\mathrm{\gamma,iso}=10^{48.8}\,$erg\,s$^{-1}$); it was the most nearby ($z=0.153$) afterglow identified blindly by an optical survey; and that it is one of the brightest afterglows ever observed at X-ray, optical, and radio (cm to mm) wavelengths. We spectroscopically confirm an underlying Ic-BL SN with properties typical of GRB-SNe ($M_\mathrm{ej}\approx3\,M_\odot$, $E_{\rm K}\approx 10^{52}\,$erg). With basic modeling of the afterglow, including the long optical rise ($\approx10^{3}\,$s), we infer either a low initial Lorentz factor ($\Gamma_0\approx40$) or a slightly off-axis viewing angle ($\lesssim3^\circ$). The host galaxy's mass and star formation rate are similar to the hosts of other sub-luminous GRBs. ZTF's flux-limited survey gives a volumetric rate of AT\,2026fgk-like events of $0.30^{+1.37}_{-0.29}\,$Gpc\,$^{-3}$\,yr$^{-1}$, which is consistent with the on-axis, high luminosity ($L_{\rm \gamma,iso}>10^{49.5}$\,erg\,s$^{-1}$) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low-$\Gamma_0$ bursts and the beaming of the initial relativistic material in GRBs.

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arXiv2026

Searching for outbursts from Symbiotic Binaries in GOTO and ATLAS data

Symbiotic Binaries contain a white dwarf accreting material from a red giant star through a wind. We present the results of a search for outbursts from Symbiotic Binaries using photometric data obtained using the GOTO all-sky survey taken from 2023 onwards. After identifying ten candidate outbursting systems, we used ATLAS photometry to characterise their photometric behaviour before 2023, leaving five systems which showed photometric behaviour consistent with an outburst. The ATLAS data showed how important the photometric history of an object is in determining whether a photometric feature is a likely outburst event. The outburst from LMC N67 is the first reported Z And-type outburst from a Symbiotic binary in the LMC. OGLE SMC-LPV-4044 and HK Sco show previously unreported outbursts. QW Sge and V4141 Sgr show outbursts starting in 2024, which have already been reported and are ongoing. By better identifying and characterising Z And-type outbursts from many systems, it will be possible to better understand the physics of these events, which are still not fully understood.

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arXiv2026

The Gravitational-wave Optical Transient Observer (GOTO) data pipeline and workflow for transient discovery

Wide-field and high-cadence sky surveys are the first step in the chain of discovery and characterisation of astrophysical transients such as supernovae, kilonovae, and tidal disruption events, each linked to the varied demise of stellar systems. The Gravitational-wave Optical Transient Observer (GOTO) is a telescope array of thirty-two 40 cm unit telescopes split over two almost antipodal sites. It performs a regular time-domain sky-survey in the optical to ~20 mag in addition to immediate scheduling of follow-up observations at the locations of external multi-wavelength and -messenger triggers. To facilitate the timely recovery of optical counterparts to these triggers, as well as the presence of serendipitous discoveries of astrophysical transients in the regular sky-survey, a low-latency data pipeline and workflow was developed. The implementation of this workflow is described herein and the quality of GOTO data delivered by it assessed, alongside its performance for prompt transient recovery. Utilising difference image analysis to identify candidate discoveries, the process is typically complete ~7 minutes after shutter close on the telescope. We further describe later processing of these candidates -- both automated and human-in-the-loop -- including reporting to the wider community and the triggering of more detailed observations, with a focus on immediate, intra-night characterisation. The workflow is meeting the needs of GOTO to promptly discover, report and characterise infant transients. Nevertheless, areas for further development and improvements are also highlighted.

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arXiv2026

GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B

Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while ALMA provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $E_{K,iso} \sim 4\times10^{52}$ erg, $n_0 \sim 3.6$ cm$^{-3}$, $\theta_j \simeq 0.10$ rad ($\sim 5.7$ deg), and $p \simeq 1.64$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($r_{AB} \sim 24.7$) is offset by $\sim 4$ arcsec ($\sim 34$ kpc) with chance-alignment probability $P_{cc} \sim 0.2$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.

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arXiv2025

Limits on the Ejecta Mass During the Search for Kilonovae Associated with Neutron Star-Black Hole Mergers: A case study of S230518h, GW230529, S230627c and the Low-Significance Candidate S240422ed

Neutron star-black hole (NSBH) mergers, detectable via their gravitational-wave (GW) emission, are expected to produce kilonovae (KNe). Four NSBH candidates have been identified and followed-up by more than fifty instruments since the start of the fourth GW Observing Run (O4), in May 2023, up to July 2024; however, no confirmed associated KN has been detected. This study evaluates ejecta properties from multi-messenger observations to understand the absence of detectable KN: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF). First, our analysis on follow-up observation strategies shows that, on average, more than 50% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the $g,r,i$- bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate. In particular, we can only confirm the kilonova was not missed for 1% of the GW230529 and S230627c sky localization region, given the large sky localization error of GW230529 and the large distance for S230627c and, their respective KN faint luminosities. More constraining, for S230518h, we infer the dynamical ejecta and post-merger disk wind ejecta $m_{dyn}, m_{wind}$ $<$ $0.03$ $M_\odot$ and the viewing angle $\theta>25^\circ$. Similarly, the non-astrophysical origin of S240422ed is likely further confirmed by the fact that we would have detected even a faint KN at the time and presumed distance of the S240422ed event candidate, within a minimum 45% credible region of the sky area, that can be larger depending on the KN scenario.

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arXiv2024

JWST NIRSpec+MIRI Observations of the nearby Type IIP supernova 2022acko

We present JWST spectral and photometric observations of the Type IIP supernova (SN) 2022acko at ~50 days past explosion. These data are the first JWST spectral observations of a core-collapse SN. We identify ~30 different H I features, other features associated with products produced from the CNO cycle, and s-process elements such as Sc II and Ba II. By combining the JWST spectra with ground-based optical and NIR spectra, we construct a full Spectral Energy Distribution from 0.4 to 25 microns and find that the JWST spectra are fully consistent with the simultaneous JWST photometry. The data lack signatures of CO formation and we estimate a limit on the CO mass of < 10^{-8} solar mass. We demonstrate how the CO fundamental band limits can be used to probe underlying physics during stellar evolution, explosion, and the environment. The observations indicate little mixing between the H envelope and C/O core in the ejecta and show no evidence of dust. The data presented here set a critical baseline for future JWST observations, where possible molecular and dust formation may be seen.

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arXiv2023

JWST MIRI/MRS Observations and Spectral Models of the Under-luminous Type Ia Supernova 2022xkq

We present a JWST mid-infrared spectrum of the under-luminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) $\sim130$ days post-explosion. We identify the first MIR lines beyond 14 $\mu$m in SN Ia observations. We find features unique to under-luminous SNe Ia, including: isolated emission of stable Ni, strong blends of [Ti II], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Co III] 11.888 $\mu$m feature and the SN light curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements we constrain the mass of the exploding white dwarf. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD (M$_{\rm ej}$ $\approx 1.37$ M$_{\odot}$) of high-central density ($\rho_c \geq 2.0\times10^{9}$ g cm$^{-3}$) seen equator on, which produced M($^{56}$Ni) $= 0.324$ M$_{\odot}$ and M($^{58}$Ni) $\geq 0.06$ M$_{\odot}$. The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of sub-sonic carbon burning followed by an off-center DDT beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.

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arXiv2023

SN 2022joj: A Potential Double Detonation with a Thin Helium shell

We present photometric and spectroscopic data for SN 2022joj, a nearby peculiar Type Ia supernova (SN Ia) with a fast decline rate ($\rm{\Delta m_{15,B}=1.4}$ mag). SN 2022joj shows exceedingly red colors, with a value of approximately ${B-V \approx 1.1}$ mag during its initial stages, beginning from $11$ days before maximum brightness. As it evolves the flux shifts towards the blue end of the spectrum, approaching ${B-V \approx 0}$ mag around maximum light. Furthermore, at maximum light and beyond, the photometry is consistent with that of typical SNe Ia. This unusual behavior extends to its spectral characteristics, which initially displayed a red spectrum and later evolved to exhibit greater consistency with typical SNe Ia. We consider two potential explanations for this behavior: double detonation from a helium shell on a sub-Chandrasekhar-mass white dwarf and Chandrasekhar-mass models with a shallow distribution of $\rm{^{56}Ni}$. The shallow nickel models could not reproduce the red colors in the early light curves. Spectroscopically, we find strong agreement between SN 2022joj and double-detonation models with white dwarf masses around 1 $\rm{M_{\odot}}$ and thin He-shell between 0.01 and 0.02 $\rm{M_{\odot}}$. Moreover, the early red colors are explained by line-blanketing absorption from iron-peak elements created by the double detonation scenario in similar mass ranges. However, the nebular spectra composition in SN 2022joj deviates from expectations for double detonation, as we observe strong [Fe III] emission instead of [Ca II] lines as anticipated from double detonation models. More detailed modeling, e.g., including viewing angle effects, is required to test if double detonation models can explain the nebular spectra.

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arXiv2023

Optimising the observation of optical kilonovae with medium size telescopes

We consider the optimisation of the observing strategy (cadence, exposure time and filter choice) using medium size (2-m class) optical telescopes in the follow-up of kilonovae localised with arcminute accuracy to be able to distinguish among various kilonova models and viewing angles. To develop an efficient observation plan, we made use of the synthetic light curves obtained with the Monte Carlo radiative transfer code POSSIS for different kilonova models and as a function of different viewing angles and distances. By adding the appropriate photon counting noise to the synthetic light curves, we analysed four alternative sequences having the same total time exposure of 8 hours, with different time windows (0.5, 1, 2, 4 h), each with $i$, $r$, and $u$ filters, to determine the observing sequence that maximises the chance of a correct identification of the model parameters. We suggest to avoid $u$ filter and to avoid the use of colour curves. We also found that, if the error on distance is $\le$ 2%, 0.5, 1, 2-hour time window sequences are equivalent, so we suggest to use 2-hour one, because it has 1 day cadence, so it can be easily realised. When the distance of the source is unknown, 0.5 h time window sequence is preferable.

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arXiv2023

GRB 210619B: first gamma-ray burst detection by the novel polarimeter MOPTOP

GRB~210619B was a bright long gamma-ray burst (GRB) which was optically followed up by the novel polarimeter MOPTOP on the Liverpool Telescope (LT). This was the first GRB detection by the instrument since it began science observations. MOPTOP started observing the GRB 1388 seconds after the Swift Burst Alert Telescope (BAT) trigger. The $R$ band light-curve decays following a broken power law with a break time of 2948 s after the trigger. The decay index values are $\alpha_1 = 0.84 \pm 0.03$ (pre-break) and $\alpha_2 = 0.54 \pm 0.02$ (post-break), indicating that the observation was most probably during the forward shock-dominated phase. We find a polarization upper limit of $\sim7$\%. In the forward shock we expect the polarization to mostly come from dust in the local ambient medium which only produces low degrees of polarization. Hence our non-detection of polarization is as expected for this particular burst.

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arXiv2023

Shock shaping? Nebular Spectroscopy of Nova V906 Carinae

V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution UVES spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line SHAPE model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionised environment. Temperature, density and abundance analyses of the evolving nova shell are presented.

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arXiv2023

GRB minimum variability timescale with Insight-HXMT and Swift: implications for progenitor models, dissipation physics and GRB classifications

The dissipation process of GRB prompt emission is still unknown. Study of temporal variability may provide a unique way to discriminate the imprint of the inner engine activity from geometry and propagation related effects. We define the minimum variability timescale (MVT) as the shortest duration of individual pulses that shape a light curve for a sample of GRBs and test correlations with peak luminosity, Lorentz factor, and jet opening angle. We compare these correlations with predictions from recent numerical simulations for a relativistic structured -- possibly wobbling -- jet and assess the value of MTV as probe of prompt-emission physics. We used the peak detection algorithm mepsa to identify the shortest pulse within a GRB time history and estimate its full width half maximum (FWHM). We applied this framework to two sets of GRBs: Swift (from 2005 to July 2022) and Insight-HXMT (from June 2017 to July 2021, including 221009A). We then selected 401 GRBs with measured z to test for correlations. On average short GRBs have significantly shorter MVT than long GRBs. The MVT distribution of short GRBs with extended emission such as 060614 and 211211A is compatible only with that of short GRBs. This provides a new clue on the progenitor's nature. The MVT for long GRBs anticorrelates with peak luminosity. We confirm the anticorrelation with the Lorentz factor and find a correlation with the jet opening angle as estimated from the afterglow, along with an inverse correlation with the number of pulses. The MVT can identify the emerging putative new class of long GRBs that are suggested to be produced by compact binary mergers. For otherwise typical long GRBs, the different correlations between MVT and peak luminosity, Lorentz factor, jet opening angle, and number of pulses can be explained within the context of structured, possibly wobbling, weakly magnetised relativistic jets. (summarised)

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arXiv2022

The role of the magnetic fields in GRB outflows

Gamma-ray bursts (GRBs) are bright extragalactic flashes of gamma-ray radiation and briefly the most energetic explosions in the Universe. Their catastrophic origin (the merger of compact objects or the collapse of massive stars) drives the formation of a newborn compact remnant (black hole or magnetar) that powers two highly relativistic jets. To distinguish between magnetized and baryonic jet models and ultimately determine the power source for these energetic explosions, our team studies the polarization of the light during the first minutes after the explosion (using novel instruments on fully autonomous telescopes around the globe) to directly probe the magnetic field properties in these extragalactic jets. This technology allowed the detection of highly polarized optical light in GRB 120308A and confirmed the presence of mildly magnetized jets with large-scale primordial magnetic fields in a reduced sample of GRBs (e.g. GRB 090102, GRB 110205A, GRB 101112A, GRB 160625B). Here we discuss the observations of the most energetic and first GRB detected at very high TeV energies, GRB 190114C, which opens a new frontier in GRB magnetic field studies suggesting that some jets can be launched highly magnetized and that the collapse and destruction of these magnetic fields at very early times may have powered the explosion itself. Additionally, our most recent polarimetric observations of the jet of GRB 141220A indicate that, when the jetted ejected material is decelerated by the surrounding environment, the magnetic field amplification mechanisms at the front shock (needed to generate the observed synchrotron emission) produce small magnetic domains. These measurements validate theoretical expectations and contrast with previous observations that suggest large magnetic domains in collisionless shocks (i.e. GRB 091208B).

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arXiv2022

A short gamma-ray burst from a proto-magnetar remnant

The contemporaneous detection of gravitational waves and gamma rays from the GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron-star mergers as progenitors of short-duration gamma-ray bursts (GRBs), and cosmic sources of heavy r-process nuclei. However, the nature (and lifespan) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with the short GRB 180618A with extended gamma-ray emission, with ultraviolet and optical multicolour observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission at 15 minutes post-burst, which fades abruptly and chromatically (flux density $F_ν \propto t^{-α}$, $α=4.6 \pm 0.3$) just 35 minutes after the GRB. Our observations from gamma rays to optical wavelengths are consistent with a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly-spinning and highly magnetized neutron star (i.e. a millisecond magnetar), whose rotational energy is released at a rate $L_{\rm th} \propto t^{-(2.22\pm 0.14)}$ to reheat the unbound merger-remnant material. These results suggest such neutron stars can survive the collapse to a black hole on timescales much larger than a few hundred milliseconds after the merger, and power the GRB itself through accretion. Bright thermal optical counterparts to binary merger gravitational wave sources may be common in future wide-field fast-cadence sky surveys.

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arXiv2022

Polarimetry and Photometry of Gamma-Ray Bursts Afterglows with RINGO3

We present photometric and polarimetric measurements of gamma-ray burst (GRB) optical afterglows observed by the RINGO3 imaging polarimeter over its $\sim$7 year lifetime mounted on the Liverpool Telescope. During this time, RINGO3 responded to 67 GRB alerts. Of these, 28 had optical afterglows and a further ten were sufficiently bright for photometric and polarimetric analysis ($R\lessapprox{17}$). We present high quality multicolour light curves of ten sources: GRB 130606A, GRB 130610A, GRB 130612A, GRB 140430A, GRB 141220A, GRB 151215A, GRB 180325A, GRB 180618A, GRB 190114C, and GRB 191016A and polarimetry for seven of these (excluding GRB 130606A, GRB 130610A, and GRB 130612A, which were observed before the polarimetry mode was fully commissioned). Eight of these ten GRBs are classical long GRBs, one sits at the short-long duration interface with a $T_{90}$ $\sim$ 4 seconds and one is a classical short, hard burst with extended emission. We detect polarization for GRB 190114C and GRB 191016A. While detailed analyses of several of these GRBs have been published previously, here we present a uniform re-reduction and analysis of the whole sample and investigation of the population in a broad context relative to the current literature. We use survival analysis to fully include the polarization upper limits in the comparison with other GRB properties, such as temporal decay rate, isotropic energy and redshift. We find no clear correlation between polarization properties and wider sample properties and conclude that larger samples of early time polarimetry of GRB afterglows are required to fully understand GRB magnetic fields.

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arXiv2021

GRB 191016A: A highly collimated gamma-ray burst jet with magnetised energy injection

Long gamma-ray burst GRB 191016A was a bright and slow rising burst that was detected by the \textit{Swift} satellite and followed up by ground based Liverpool Telescope (LT). LT follow-up started $2411$-s after the \textit{Swift} Burst Alert Telescope (BAT) trigger using imager IO:O around the time of the late optical peak. From $3987-7687$-s, we used the LT polarimeter RINGO3 to make polarimetric and photometric observations of the GRB simultaneously in the $V,R$ and $I$ bands. The combined optical light curve shows an initial late peak followed by a decline until 6147-s, 6087-s, and 5247-s for $I,R$ and $V$ filters respectively followed by a flattening phase. There is evidence of polarization at all phases including polarization ($P = 14.6 \pm 7.2 \%$) which is coincident with the start of the flattening phase. The combination of the light curve morphology and polarization measurement favours an energy injection scenario where slower magnetised ejecta from the central engine catches up with the decelerating blast wave. We calculate the minimum energy injection to be $ΔE / E>0.36$. At a later time combining the optical light curve from BOOTES (reported via GCN) and IO:O we see evidence of a jet break with jet opening angle 2 \degree.

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arXiv2021

Coherence scale of magnetic fields generated in early-time forward shocks of GRBs

We report the earliest-ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured $129.5-204.3\,$s after the burst using the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from $86\,$s to $\sim 2200\,$s post-burst is typical of classical forward shocks with $α= 1.091 \pm 0.008$. The low optical polarization $P_{BV} = 2.8_{- 1.6} ^{+ 2.0} \, \%$ (2$σ$) at mean time $\sim 168\,$s post-burst is compatible with being induced by the host galaxy dust ($A_{V, {\rm HG}}= 0.71 \pm 0.15 \,$mag), leaving low polarization intrinsic to the GRB emission itself -- as theoretically predicted for forward shocks and consistent with previous detections of low degrees of optical polarization in GRB afterglows observed hours to days after the burst. The current sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P \sim 1\%-3\%$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P \leq 2\%$), which depends on the magnetic field coherence length scale and the size of the observable emitting region (burst energetics, circumburst density).

astro-ph.HE↗
arXiv2020

Lowly polarized light from a highly magnetized jet of GRB 190114C

We report multi-color optical imaging and polarimetry observations of the afterglow of the first TeV- detected gamma-ray burst, GRB 190114C, using RINGO3 and MASTER II polarimeters. Observations begin 31 s after the onset of the GRB and continue until $\sim 7000\,$s post-burst. The light curves reveal a chromatic break at $\sim 400- 500\,$s, with initial temporal decay $α= 1.669 \pm 0.013$ flattening to $α\sim 1$ post-break, which we model as a combination of reverse and forward-shock components, with magnetization parameter $R_{\rm B} \sim 70$. The observed polarization degree decreases from $7.7 \pm 1.1\%$ to $2-4\%$ during $52-109\,$s post-burst and remains steady at this level for the subsequent $\sim 2000$-s, at constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms GRB 190114C is highly obscured (A$_{\rm v, HG} = 1.49 \pm 0.12 \,$mag; N$_{\rm H, HG}= (9.0 \pm 0.3) \times 10^{22}\,$cm$^{-2}$). We interpret the measured afterglow polarization as intrinsically low and dominated by dust, in contrast to ${\rm P} >10\%$ measured previously for other GRB reverse shocks, with a small contribution from polarized prompt photons in the first minute. We test whether 1st and higher-order inverse Compton scattering in a magnetized reverse shock can explain the low optical polarization and the sub-TeV emission but conclude neither is explained in the reverse shock Inverse Compton model. Instead, the unexpectedly low intrinsic polarization degree in GRB 190114C can be explained if large-scale jet magnetic fields are distorted on timescales prior to reverse shock emission.

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