We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient (FBOT), AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 10 44 erg s −1 exceeding those of superluminous supernovae (SNe), before declining as L ∝ t −2 . Initial spectra at δt 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1 c and temperatures arXiv:1810.10720v1 [astro-ph.HE] 25 Oct 2018 2 MARGUTTI ET AL. reaching T ∼ 3 × 10 4 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s −1 with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blastwave with v sh ∼ 0.1 c with a dense environment (Ṁ ∼ 10 −3 − 10 −4 M yr −1 for v w = 1000 km s −1 ). While these properties exclude 56 Ni-powered transients, our multi-wavelength analysis instead indicates that AT 2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼ 10 50 − 10 51.5 erg over ∼ 10 3 − 10 5 s and resides within lowmass fast-moving material with equatorial-polar density asymmetry (M ej,fast 0.3 M ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

We present X-ray and radio observations of the Fast Blue Optical Transient CRTS-CSS161010 J045834 −081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray (L x ∼5× 10 39 erg s −1 ) and radio (L ν ∼10 29 erg s −1 Hz −1 ) emission. The radio emission peaked at ∼100 days posttransient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blast wave. CSS161010 launched a mildly relativistic outflow with velocity Γβc0.55c at ∼100 days. This is faster than the non-relativistic AT 2018cow (Γβc∼0.1c) and closer to ZTF18abvkwla (Γβc0.3c at 63 days). The inferred initial kinetic energy of CSS161010 (E k 10 51 erg) is comparable to that of long gamma-ray bursts, but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ( -). This is consistent with the lack of observed γ-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ∼150 Mpc in a dwarf galaxy with stellar mass M * ∼10 7 M e and specific star formation rate sSFR∼0.3 Gyr −1 . This mass is among the lowest inferred for host galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally located intermediate-mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare

We present Chandra and VLA observations of GW 170817 at ∼ 521−743 days post merger, and a homogeneous analysis of the entire Chandra dataset. We find that the late-time non-thermal emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay F ν ∝ t −1.95±0.15 and power-law spectrum F ν ∝ ν −0.575±0.007 . We present a new method to constrain the merger environment density based on diffuse Xray emission from hot plasma in the host galaxy and find n ≤ 9.6×10 −3 cm −3 . This measurement is independent from inferences based on jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy E 0 = 1.5 +3.6 −1.1 × 10 49 erg (E iso = 2.1 +6.4 −1.5 × 10 52 erg), jet opening angle θ 0 = 5.9 +1.0 −0.7 deg with characteristic Lorentz factor Γ j = 163 +23 −43 , expanding in a low-density medium with n 0 = 2.5 +4.1 −1.9 × 10 −3 cm −3 and viewed θ obs = 30.4 +4.0 −3.4 deg offaxis. The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15 +0.01 −0.02 . The shock microphysics parameters are constrained to e = 0.18 +0.30 −0.13 and B = 2.3 +16.0 −2.2 × 10 −3 . Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥ 2.5σ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy E KN k ∝ (Γβ) −α into the environment, finding that shallow stratification indexes α ≤ 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties. arXiv:1909.06393v3 [astro-ph.HE]

Radio emission from non-magnetic cataclysmic variables (CVs, accreting white dwarfs) could allow detailed studies of outflows and possibly accretion flows in these nearby, numerous and non-relativistic compact accretors. Up to now, however, very few CVs have been detected in the radio.We have conducted a VLA pilot survey of four close and optically-bright novalike CVs at 6 GHz, detecting three, and thereby doubling the number of radio detections of these systems. RW Sex, V603 Aql and the old nova TT Ari were detected in both of the epochs, while V1084 Her was not detected (to a 3σ upper-limit of 7.8 µJy beam −1 ). These observations clearly show that the sensitivity of previous surveys was typically too low to detect these objects and that non-magnetic CVs can indeed be significant radio emitters.The three detected sources show a range of properties, including flaring and variability on both short (∼200 s) and longer-term (days) time-scales, as well as circular polarization levels of up to 100%. The spectral indices range from steep to inverted; TT Ari shows a spectral turnover at ∼6.5 GHz, while the spectral index of V603 Aql flattened from α = 0.54 ± 0.05 to 0.16 ± 0.08 (F ν ∝ ν α ) in the week between observations. This range of properties suggests that more than one emission process can be responsible for the radio emission in non-magnetic CVs. In this sample we find that individual systems are consistent with optically thick synchrotron emission, gyrosynchrotron emission or cyclotron maser emission.

The luminosity distance measurement of GW170817 derived from GW analysis in Abbott et al. (2017) (A17:H0) is highly correlated with the measured inclination of the NS-NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broad-band X-ray-toradio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at t < 40 days since merger. We find that the afterglow emission from GW170817 is consistent with an off-axis relativistic jet with energy 10 48 erg < E k ≤ 3×10 50 erg propagating into an environment with density n ∼ 10 −2 −10 −4 cm −3 , with preference for wider jets (opening angle θ j = 15• ). For these jets, our modeling indicates an off-axis angle θ obs ∼ 25• − 50• . We combine our constraints on θ obs with the joint distance-inclination constraint from LIGO. Using the same ∼ 170 km/sec peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of H 0 =74.0 ± 11.5 7.5 km/s/Mpc, which is higher than the value of H 0 =70.0 ± 12.0 8.0 km/s/Mpc found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km/sec derived from previous work, we find H 0 =75.5± 11.6 9.6 km/s/Mpc for H0 from this system. We note that this is in modestly better agreement with the local distance ladder than the Planck CMB, though a significant such discrimination will require ∼ 50 such events. Future measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.