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]

We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I λ7774 line and higher-ionisation states of oxygen. The iron-dominated region around 5000Å is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows that 5 'eigenspectra' capture 70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN sub-classes. Line velocities are 5000-8000 km s −1 , and show stratification of the ejecta. O I λ7774 likely arises in a dense inner region that also produces calcium emission, while [O I] λ6300 comes from further out until 300-400 days. The luminosities of O I λ7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II] λ7300/[O I] λ6300 favour progenitors with relatively massive helium cores, likely 6 M , though more modelling is required here. SLSNe with broad light curves show the strongest [O I] λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density and ionisation structure point to a central power source.

On 2019 August 14 the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo gravitational wave interferometer announced the detection of a binary merger, S190814bv, with a low false alarm rate (FAR) of about 1 in 1.6 × 10 25 years, a distance of 267 ± 52 Mpc, a 90% (50%) localization region of about 23 (5) deg 2 , and a probability of being a neutron star-black hole (NS-BH) merger of > 99%. The LIGO/Virgo Collaboration (LVC) defines NS-BH such that the lighter binary member has a mass of < 3 M and the more massive one has > 5 M , and this classification is in principle consistent with a BH-BH merger depending on the actual upper mass cut-off for neutron stars. Additionally, the LVC designated a probability that the merger led to matter outside the final BH remnant of < 1%, suggesting that an electromagnetic (EM) counterpart is unlikely. Here we report our optical follow-up observations of S190814bv using the Magellan Baade 6.5 m telescope to target all 96 galaxies in the GLADE catalog within the 50% localization volume (representing about 70% of the integrated luminosity within this region). No counterpart was identified to a median 3σ limiting magnitude of i = 22.2 (M i ≈ −14.9 mag), comparable to the brightness of the optical counterpart of the binary neutron star merger GW170817 at the distance of S190814bv; similarly, we can rule out an on-axis jet typical of short GRBs. However, we cannot rule out other realistic models, such as a kilonova with only ∼ 0.01 M of lanthanide-rich material, or an off-axis jet with a viewing angle of θ obs 15 • .

On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory and the Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at d L = 156 ± 41 Mpc, while S190426c is possibly the first NS-black hole (BH) merger ever detected, at d L = 377 ± 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field γ-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the γ-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg 2 are likely essential.

We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the ASAS-SN survey at an absolute magnitude of MV ∼ −20.2 mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He I in early spectra, with He II making an appearance after ∼70 − 100 days. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disk emission, and back to smooth at late times. The Balmer lines are unlike typical AGN in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photo-ionisation. The flat Balmer decrement together with the complex profiles suggest that the emission lines originate in a disk chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He II may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.