Radio data challenge the broadband modelling of GRB160131A afterglow

Context. Gamma-ray burst (GRB) afterglows originate from the interaction between the relativistic ejecta and the surrounding medium. Consequently, their properties depend on several aspects: radiation mechanisms, relativistic shock micro-physics, circumburst environment, and the structure and geometry of the relativistic jet. While the standard afterglow model accounts for the overall spectral and temporal evolution for a number of GRBs, its validity limits emerge when the data set is particularly rich and constraining, especially in the radio band. Aims. We aimed to model the afterglow of the long GRB160131A (redshift $z = 0.972$), for which we collected a rich, broadband, and accurate data set, spanning from $6\times10^{8}$ to $7\times10^{17}$ Hz in frequency, and from 330 s to 160 days post burst in time. Methods. We modelled the spectral and temporal evolution of this GRB afterglow through two approaches: the adoption of empirical functions to model optical/X-rays data set, later assessing their compatibility with the radio domain; the inclusion of the entire multi-frequency data set simultaneously through the Python package named sAGa (Software for AfterGlow Analysis), to come up with an exhaustive and self-consistent description of the micro-physics, geometry, and dynamics of the afterglow. Results. From deep broadband analysis (from radio to X-ray frequencies) of the afterglow light curves, GRB160131A outflow shows evidence of jetted emission. Moreover, we observe dust extinction in the optical spectra, and energy injection in the optical/X-ray data. Radio spectra are characterised by several peaks, that could be due to either interstellar scintillation (ISS) effects or a multi-component structure. Conclusions. The inclusion of radio data in the broadband set of GRB160131A makes a self-consistent modelling hardly attainable within the standard model of GRB afterglows.Comment: 36 pages, 16 figures, 8 tables, accepted by A&A; v2: updated Acknowledgement

A flash of polarized optical light points to an aspherical "cow"

The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3 observations of AT2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached ~7% at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient, and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT~2018cow at early times. A second ``bump" in the polarization is observed at blue wavelengths at ~12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 day), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be ~10%.Comment: MNRAS Accepted, 10 pages, 8 figure

Lowly Polarized Light from a Highly Magnetized Jet of GRB 190114C

We report multicolor optical imaging and polarimetry observations of the afterglow of the first TeV-detected gamma-ray burst (GRB), GRB 190114C, using the RINGO3 and MASTER II polarimeters. Observations begin 31 s after the onset of the GRB and continue until ~7000 s postburst. The light curves reveal a chromatic break at ~400–500 s, with initial temporal decay α = 1.669 ± 0.013 flattening to α ~ 1 postbreak, which we model as a combination of reverse and forward shock components with magnetization parameter R B ~ 70. The observed polarization degree decreases from 7.7% ± 1.1% to 2%–4% 52–109 s postburst and remains steady at this level for the subsequent ~2000 s at a constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms that GRB 190114C is highly obscured (A v,HG = 1.49 ± 0.12 mag; ${N}_{{\rm{H}},\mathrm{HG}}=(9.0\pm 0.03)\,\times {10}^{22}\,$ cm−2). We interpret the measured afterglow polarization as intrinsically low and dominated by dust —in contrast to the P > 10% measured previously for other GRB reverse shocks—with a small contribution from polarized prompt photons in the first minute. We test whether first- and higher-order inverse Compton scattering in a magnetized reverse shock can explain the low optical polarization and subteraelectronvolt emission but conclude that 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

Methods for detection and analysis of weak radio sources with single-dish radio telescopes

The detection of mJy/sub-mJy point sources is a significant challenge for single-dish radio telescopes. Detection or upper limits on the faint afterglow from GRBs or other sources at cosmological distances are important means of constraining the source modeling. Using the Sardinia Radio Telescope (SRT), we compare the sensitivity and robustness of three methods applied to the detection of faint radio sources from raster maps around a known source position: the smart quick-look method, the source extraction method (typical of high-energy astronomy), and the fit with a 2-D Gaussian. We developed a Python code specific for the analysis of point-like radio sources applied to the SRT C-band (6.9 GHz) observations of both undetected sources (GRB afterglows of 181201A and 190114C) and the detected Galactic X-ray binary GRS 1915+105. Our comparative analysis of the different detection methods made extensive use of simulations as a useful complement to actual radio observations. The best method for the SRT data analysis is the fit with a 2-D Gaussian, as it pushes down the sensitivity limits of single-dish observations -- with respect to more traditional techniques -- to ~ 1.8 mJy, improving by ~ 40 % compared with the initial value. This analysis shows that -- especially for faint sources -- good maps of the scanned region pre- or post-outburst are essential.Comment: 22 pages, 11 figures, 3 tables, pre-print of an article published in Experimental Astronomy; v2: updated abstract and reference

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

Long gamma-ray burst GRB 191016A was a bright and slow rising burst that was detected by the Swift satellite and followed up by ground based Liverpool Telescope (LT). LT follow up started 2411 s after the 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 ± 7.2 per cent) 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 magnetized 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 Burst Observer and Optical Transient Exploring System (BOOTES) (reported via GCN) and IO:O we see evidence of a jet break with jet opening angle 2◦

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 ∼7 yr 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 ⪅ 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 T90 ∼ 4 s 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 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

A flash of polarized optical light points to an aspherical ‘cow’

The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached ∼7% at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient, and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second ‘bump’ in the polarization is observed at blue wavelengths at ∼12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period (<1 day), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be ∼10%⁠

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)Comment: 18 pages, 15 figures, accepted by A&

A Short Gamma-Ray Burst from a Protomagnetar Remnant

The contemporaneous detection of gravitational waves and gamma rays from 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 life span) 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 short GRB 180618A with extended gamma-ray emission—with ultraviolet and optical multicolor observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission 15 minutes post-burst, which fades abruptly and chromatically (flux density Fν ∝ t−α, α = 4.6 ± 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 Lth ∝ t−(2.22±0.14) to reheat the unbound merger-remnant material. These results suggest that 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

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. As these jets continue to travel outwards, they collide with the external material surrounding the dying star, producing a long-lasting afterglow that can be seen across the entire electromagnetic spectrum, from the most energetic gamma-ray emission to radio wavelengths. But how can such material be accelerated and focused into narrow beams? The internal shock model proposes that repeated collisions between material blasted out during the explosion can produce the gamma-ray flash. The competing magnetic model credits primordial large-scale ordered magnetic fields that collimate and accelerate the relativistic outflows. To distinguish between these 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