We present ZTF20aajnksq (AT2020blt), a fast-fading (\(\Delta r=2.4\) mag in \(\Delta t=1.3\) days) red (\(g-r\approx0.6\) mag) and luminous (\(M_{1626}=-25.9\)) optical transient at \(z=2.9\) ...discovered by the Zwicky Transient Facility (ZTF). AT2020blt shares several features in common with afterglows to long-duration gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken power-law with a break at \(t_\mathrm{j}=1\) day (observer-frame); (2) a luminous \((L_X = 10^{46}\) \(\mathrm{erg}\) \(\mathrm{s}^{-1})\) X-ray counterpart; and (3) luminous (\(L_\nu = 4 \times 10^{31}\) \(\mathrm{erg}\) \(\mathrm{sec}^{-1}\) \(\mathrm{Hz}^{-1}\) at 10 GHz) radio emission. However, no GRB was detected in the 0.74d between the last ZTF non-detection (\(r > 20.64\)) and the first ZTF detection (\(r = 19.57\)), with an upper limit on the isotropic-equivalent gamma-ray energy release of \(E_{\gamma,\mathrm{iso}} < 7 \times 10^{52}\) erg. AT2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw) with a redshift measurement. We conclude that the properties of AT2020blt are consistent with a classical (initial Lorentz factor \(\Gamma_0 \gtrsim 100\)) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of AT2020blt in ZTF high-cadence data, we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys.
The accuracy of orbit determination has a strong impact on the scientific output of the Space VLBI mission RadioAstron. The aim of this work is to improve the RadioAstron orbit reconstruction by ...means of sophisticated dynamical modelling of its motion in combination with multi-station Doppler tracking of the RadioAstron spacecraft. The improved orbital solution is demonstrated using Doppler measurements of the RadioAstron downlink signal and by correlating VLBI observations made by RadioAstron with ground-based telescopes using the enhanced orbit determination data. Orbit determination accuracy has been significantly improved from ~ 600 m in 3D position and ~ 2 cm/s in 3D velocity to several tens of metres and mm/s, respectively.
We present time-resolved visible spectrophotometry of minimoon 2020 CD\(_3\), the second asteroid known to become temporarily captured by the Earth-Moon system's gravitational field. The ...spectrophotometry was taken with Keck I/LRIS between wavelengths 434 nm and 912 nm in \(B\), \(g\), \(V\), \(R\), \(I\) and RG850 filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The spectrophotometry of 2020 CD\(_3\) most closely resembles the spectra of V-type asteroids and some Lunar rock samples with a reddish slope of ~18\(\%\)/100 nm between 434 nm and 761 nm corresponding to colors of \(g\)-\(r\) = 0.62\(\pm\)0.08, \(r\)-\(i\) = 0.21 \(\pm\) 0.06 and an absorption band at ~900 nm corresponding to \(i\)-\(z\) = -0.54\(\pm\)0.10. Combining our measured 31.9\(\pm\)0.1 absolute magnitude with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD\(_3\)'s diameter to be ~0.9\(\pm\)0.1 m making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect periodic lightcurve variations with a \(<\)10\(^{-4}\) false alarm probability corresponding to a lightcurve period of ~573 s and a lightcurve amplitude of ~1 mag implying 2020 CD\(_3\) possesses a \(b/a\) axial ratio of ~2.5. In addition, we extend the observational arc of 2020 CD\(_3\) to 37 days between 2020 February 15 UTC and 2020 March 23 UTC. From the improved orbital solution for 2020 CD\(_3\), we estimate its likely duration of its capture to be ~2 y, and we measure the non-gravitation perturbation on its orbit due to radiation pressure with an area-to-mass ratio of 6.9\(\pm\)2.4\(\times\)10\(^{-4}\) m\(^2\)/kg implying a density of 2.3\(\pm\)0.8 g/cm\(^3\), broadly compatible with the densities of other meter-scale asteroids and Lunar rock. We searched for pre-discovery detections of 2020 CD\(_3\) in the ZTF archive as far back as 2018 October, but were unable to locate any positive detections.
The closest ever fly-by of the Martian moon Phobos, performed by the European Space Agency's Mars Express spacecraft, gives a unique opportunity to sharpen and test the Planetary Radio Interferometry ...and Doppler Experiments (PRIDE) technique in the interest of studying planet - satellite systems. The aim of this work is to demonstrate a technique of providing high precision positional and Doppler measurements of planetary spacecraft using the Mars Express spacecraft. The technique will be used in the framework of Planetary Radio Interferometry and Doppler Experiments in various planetary missions, in particular in fly-by mode. We advanced a novel approach to spacecraft data processing using the techniques of Doppler and phase-referenced very long baseline interferometry spacecraft tracking. We achieved, on average, mHz precision (30 {\mu}m/s at a 10 seconds integration time) for radial three-way Doppler estimates and sub-nanoradian precision for lateral position measurements, which in a linear measure (at a distance of 1.4 AU) corresponds to ~50 m.
We present the results of several multi-station Very Long Baseline Interferometry (VLBI) experiments conducted with the ESA spacecraft Venus Express as a target. To determine the true capabilities of ...VLBI tracking for future planetary missions in the solar system, it is necessary to demonstrate the accuracy of the method for existing operational spacecraft. We describe the software pipeline for the processing of phase referencing near-field VLBI observations and present results of the ESA Venus Express spacecraft observing campaign conducted in 2010-2011. We show that a highly accurate determination of spacecraft state-vectors is achievable with our method. The consistency of the positions indicates that an internal rms accuracy of 0.1 mas has been achieved. However, systematic effects produce offsets up to 1 mas, but can be reduced by better modelling of the troposphere and ionosphere and closer target-calibrator configurations.
