Varying linear polarisation in the dust-free GRB 210610B

Long gamma ray bursts (GRBs) are produced by the collapse of some very massive stars, which emit ultra-relativistic jets. When the jets collide with the interstellar medium they decelerate and generate the so-called afterglow emission, which has been observed to be polarised. In this work we study the polarimetric evolution of GRB 210610B afterglow, at z=1.1341. This allows to evaluate the role of geometric and/or magnetic mechanisms in the GRB afterglow polarisation. We observed GRB 210610B using imaging polarimetry with CAFOS on the 2.2 m Calar Alto Telescope and FORS2 on the 4 × 8.1 m Very Large Telescope. Complementary optical spectroscopy was obtained with OSIRIS on the 10.4 m Gran Telescopio Canarias. We study the GRB light-curve from X-rays to optical bands and the Spectral Energy Distribution (SED). This allows us to strongly constrain the line-of-sight extinction. Finally, we study the GRB host galaxy using optical/NIR data to fit the SED and derive its integrated properties. GRB 210610B had a bright afterglow with a negligible line-of-sight extinction. Polarimetry was obtained at three epochs: during an early plateau phase, at the time when the light curve breaks, and after the light curve steepened. We observe an initial polarisation of ∼4% that goes to zero at the time of the break, and then increases again to ∼2% with a change of the position angle of 54±9 deg. The spectrum show features with very low equivalent widths, indicating a small amount of material in the line-of-sight within the host. The lack of dust and the low amount of material on the line-of-sight to GRB 210610B allow us to study the intrinsic polarisation of the GRB optical afterglow. We find the GRB polarisation signals are consistent with ordered magnetic fields in refreshed shock or/and hydrodynamics-scale turbulent fields in the forward shock.</p

The nature of the X-ray flash of August 24 2005 Photometric evidence for an on-axis z = 0.83 burst with continuous energy injection and an associated supernova?

Aims.Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005. Methods.We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 min to 104 days after the burst. In addition we present observations taken during the first day in the $\it BRIK$ bands and two epochs of spectroscopy. We also analyse available X-ray data. Results.The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope of $\alpha=0.6$ ( $F_{\nu} \propto t^{-\alpha}$). Our late R-band images reveal the host galaxy. The rest-frame B-band luminosity is ~0.5 L*. The star-formation rate as determined from the [O II] emission line is ~ $1.8~M_{\odot}$ yr-1. When accounting for the host contribution, the slope is $\alpha=0.65$ $\pm$ 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the lightcurve in the form of wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle ( \theta_j \ga {10}^\circ). If the energy injections are episodic this could potentially help explain the bumps and wiggles. Spectroscopy of the afterglow gives a redshift of z=0.828 $\pm$ 0.005 from both absorption and emission lines. The spectral energy distribution (SED) of the afterglow has a power-law ( $F_{\nu} \propto \nu ^{-\beta}$) shape with slope ${\beta}=0.56$ $\pm$ 0.04. This can be compared to the X-ray spectral index which is ${\beta_{\rm X}}=1.0$ $\pm$ 0.1. The curvature of the SED constrains the dust reddening towards the burst to $A_{\rm v}<0.5$ mag

Evidence for the magnetar nature of 1E 161348-5055 in RCW 103

We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/Burst Alert Telescope (BAT) monitor, and on the follow-up campaign made with Swift/X-ray Telescope, Swift/UV/Optical Telescope, and the optical/near-infrared (NIR) Gamma-Ray burst Optical and Near-infrared Detector. The characteristics of this flash, such as duration and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E 161348−5055. Its nature has been long debated due to a periodicity of 6.67 h in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E 161348−5055 was a factor of ∼100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the 2 yr before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 h pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E 161348−5055. Based on these findings, we associate the BAT burst with 1E 161348−5055, we classify it as a magnetar, and pinpoint the 6.67 h periodicity as the magnetar spin period

Spectroscopy and multiband photometry of the afterglow of intermediate duration γ-ray burst GRB 040924 and its host galaxy

Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB 040924. This GRB had a rather short duration of T90 ~2.4 s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor. Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data. Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ~0.03 days. We determine the redshift z = 0.858 $\pm$ 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the [Ne III] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne III] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter. Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star

ALMA and GMRT Constraints on the Off-axis Gamma-Ray Burst 170817A from the Binary Neutron Star Merger GW170817

Binary neutron-star mergers (BNSMs) are among the most readily detectable gravitational-wave (GW) sources with the Laser Interferometer Gravitational-wave Observatory (LIGO). They are also thought to produce short γ-ray bursts (SGRBs) and kilonovae that are powered by r-process nuclei. Detecting these phenomena simultaneously would provide an unprecedented view of the physics during and after the merger of two compact objects. Such a Rosetta Stone event was detected by LIGO/Virgo on 2017 August 17 at a distance of ∼44 Mpc. We monitored the position of the BNSM with Atacama Large Millimeter/submillimeter Array (ALMA) at 338.5 GHz and the Giant Metrewave Radio Telescope (GMRT) at 1.4 GHz, from 1.4 to 44 days after the merger. Our observations rule out any afterglow more luminous than ´ - - 3 10 erg s Hz 26 1 1 in these bands, probing >2–4 dex fainter than previous SGRB limits. We match these limits, in conjunction with public data announcing the appearance of X-ray and radio emission in the weeks after the GW event, to templates of off-axis afterglows. Our broadband modeling suggests that GW170817 was accompanied by an SGRB and that the γ-ray burst (GRB) jet, powered by EAG,iso ~ 1050 erg, had a half-opening angle of ~20, and was misaligned by ~41 from our line of sight. The data are also consistent with a more collimated jet: EAG,iso ~ 1051 erg, q1 2,jet obs ~ ~ 5 , 17 q . This is the most conclusive detection of an off-axis GRB afterglow and the first associated with a BNSM-GW event to date. We use the viewing angle estimates to infer the initial bulk Lorentz factor and true energy release of the burst