Late-time radio observations of the short GRB200522A: constraints on the magnetar model

GRB200522A is a short duration gamma-ray burst (GRB) at redshift $z$=0.554 characterized by a bright infrared counterpart. A possible, although not unambiguous, interpretation of the observed emission is the onset of a luminous kilonova powered by a rapidly rotating and highly-magnetized neutron star, known as magnetar. A bright radio flare, arising from the interaction of the kilonova ejecta with the surrounding medium, is a prediction of this model. Whereas the available dataset remains open to multiple interpretations (e.g. afterglow, r-process kilonova, magnetar-powered kilonova), long-term radio monitoring of this burst may be key to discriminate between models. We present our late-time upper limit on the radio emission of GRB200522A, carried out with the Karl G. Jansky Very Large Array at 288 days after the burst. For kilonova ejecta with energy $E_{rm ej} approx 10^{53} rm erg$, as expected for a long-lived magnetar remnant, we can already rule out ejecta masses $M_{rm ej} lesssim0.03 mathrm{M}_odot$ for the most likely range of circumburst densities $ngtrsim 10^{-3}$ cm$^{-3}$. Observations on timescales of $approx$3-10 yr after the merger will probe larger ejecta masses up to $M_{rm ej} sim 0.1 mathrm{M}_odot$, providing a robust test to the magnetar scenario.

Accurate flux calibration of GW170817: is the X-ray counterpart on the rise?

X-ray emission from the gravitational wave transient GW170817 is well described as non-thermal afterglow radiation produced by a structured relativistic jet viewed off-axis. We show that the X-ray counterpart continues to be detected at 3.3 years after the merger. Such long-lasting signal is not a prediction of the earlier jet models characterized by a narrow jet core and a viewing angle of about 20 deg, and is spurring a renewed interest in the origin of the X-ray emission. We present a comprehensive analysis of the X-ray dataset aimed at clarifying existing discrepancies in the literature, and in particular the presence of an X-ray rebrightening at late times. Our analysis does not find evidence for an increase in the X-ray flux, but confirms a growing tension between the observations and the jet model. Further observations at radio and X-ray wavelengths would be critical to break the degeneracy between models.

A tale of two mergers: constraints on kilonova detection in two short GRBs at z$sim$0.5

We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at $z=0.483$ and GRB 200522A at $z=0.554$. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated to a late-type galaxy with an old stellar population ($approx$3 Gyr) and moderate on-going star formation ($approx$1 $M_{odot}$ yr$^{-1}$). Hubble and Gemini limits on optical/nIR emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavoring large masses of wind ejecta ($lesssim$0.03 $M_odot$). In contrast, observations of GRB 200522A uncovered a luminous ($L_textrm{F125W}approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-Happrox 1.3$ mag) counterpart. The red color can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 $M_odot$ $lesssim$ $M$ $lesssim$ 0.1 $M_odot$) or moderate extinction, $E(B-V)approx0.1-0.2$ mag, along the line of sight. The location of this sGRB in the inner regions of a young ($approx$0.1 Gyr) star-forming ($approx$2-6 $M_{odot}$ yr$^{-1}$) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red color.

Short gamma-ray bursts within 200 Mpc

We present a systematic search for short-duration gamma-ray bursts (GRBs) in the local Universe based on 14 years of observations with the Neil Gehrels Swift Observatory. We cross-correlate the GRB positions with the GLADE catalogue of nearby galaxies, and find no event at a distance $lesssim$100 Mpc and four plausible candidates in the range 100 Mpc$lesssim$$D$$lesssim$200 Mpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. By assuming a local origin, we use these events to constrain the range of properties for X-ray counterparts of neutron star mergers. Optical upper limits place tight constraints on the onset of a blue kilonova, and imply either low masses ($lesssim10^{-3},M_{odot}$) of lanthanide-poor ejecta or unfavorable orientations ($theta_{obs}gtrsim$30 deg). Finally, we derive that the all-sky rate of detectable short GRBs within 200 Mpc is $1.3^{+1.7}_{-0.8}$ yr$^{-1}$ (68% confidence interval), and discuss the implications for the GRB outflow structure. If these candidates are instead of cosmological origin, we set a upper limit of $lesssim$2.0 yr$^{-1}$ (90% confidence interval) to the rate of nearby events detectable with operating gamma-ray observatories, such as Swift and Fermi.