We combine electromagnetic (EM) and gravitational wave (GW) information on the binary neutron star (NS) merger GW170817 in order to constrain the radii R ns and maximum mass M max of NSs. GW170817 was followed by a range of EM counterparts, including a weak gamma-ray burst (GRB), kilonova (KN) emission from the radioactive decay of the merger ejecta, and X-ray/radio emission consistent with being the synchrotron afterglow of a more powerful off-axis jet. The type of compact remnant produced in the immediate merger aftermath, and its predicted EM signal, depend sensitively on the high-density NS equation of state (EOS). For a soft EOS which supports a low M max , the merger undergoes a prompt collapse accompanied by a small quantity of shock-heated or disk wind ejecta, inconsistent with the large quantity 10 −2 M of lanthanide-free ejecta inferred from the KN. On the other hand, if M max is sufficiently large, then the merger product is a rapidly-rotating supramassive NS (SMNS), which must spin-down before collapsing into a black hole. A fraction of the enormous rotational energy necessarily released by the SMNS during this process is transferred to the ejecta, either into the GRB jet (energy E GRB ) or the KN ejecta (energy E ej ), also inconsistent with observations. By combining the total binary mass of GW170817 inferred from the GW signal with conservative upper limits on E GRB and E ej from EM observations, we constrain the likelihood probability of a wide-range of previously-allowed EOS. These two constraints delineate an allowed region of the M max − R ns parameter space, which once marginalized over NS radius places an upper limit of M max 2.17M (90%), which is tighter or arguably less model-dependent than other current constraints.

Fast radio bursts (FRB) can arise from synchrotron maser emission at ultra-relativistic magnetized shocks, such as produced by flare ejecta from young magnetars. We combine particle-incell (PIC) simulation results for the maser emission with the dynamics of self-similar shock deceleration, as commonly applied to gamma-ray bursts (GRB), to explore the implications for FRBs. The upstream environment is a mildly relativistic baryon-loaded shell released following a previous flare, motivated by the high electron-ion injection rate M ∼ 10 19 − 10 21 g s −1 needed to power the persistent radio nebula coincident with the repeating burster FRB 121102 and its high rotation measure. The radio fluence peaks once the optical depth ahead of the shock to induced Compton scattering τ c 3. Given intervals between major ion ejection events ∆T ∼ 10 5 s similar to the occurrence rate of the most powerful bursts from FRB 121102, we demonstrate the production of ∼ 0.1 − 10 GHz FRBs with isotropic radiated energies ∼ 10 37 − 10 40 erg and durations ∼ 0.1 − 10 ms for flare energies E ∼ 10 43 − 10 45 erg. Deceleration of the blast wave, and increasing transparency of the upstream medium, generates temporal decay of the peak frequency, similar to the observed downward frequency drift seen in FRB 121102 and FRB 180814.J0422+73. The delay ∆T 10 5 s between major ioninjection events needed to clear sufficiently low densities around the engine for FRB emission could explain prolonged "dark periods" and clustered burst arrival times. Thermal electrons heated at the shock generate a short-lived 1 ms (1 s) synchrotron transient at gamma-ray (X-ray) energies, analogous to a scaled-down GRB afterglow.

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.

Subarcsecond localization of the repeating fast radio burst FRB 121102 revealed its coincidence with a dwarf host galaxy and a steady ("quiescent") nonthermal radio source. We show that the properties of the host galaxy are consistent with those of long-duration gamma-ray bursts (LGRB) and hydrogen-poor superluminous supernovae (SLSNe-I). Both LGRBs and SLSNe-I were previously hypothesized to be powered by the electromagnetic spindown of newly formed, strongly magnetized neutron stars with millisecond birth rotation periods ("millisecond magnetars"). This motivates considering a scenario whereby the repeated bursts from FRB 121102 originate from a young magnetar remnant embedded within a young hydrogen-poor supernova (SN) remnant. Requirements on the gigahertz free-free optical depth through the expanding SN ejecta (accounting for photoionization by the rotationally powered magnetar nebula), energetic constraints on the bursts, and constraints on the size of the quiescent source all point to an age of less than a few decades. The quiescent radio source can be attributed to synchrotron emission from the shock interaction between the fast outer layer of the supernova ejecta with the surrounding wind of the progenitor star, or the radio source can from deeper within the magnetar wind nebula as outlined in Metzger et al. Alternatively, the radio emission could be an orphan afterglow from an initially off-axis LGRB jet, though this might require the source to be too young. The young age of the source can be tested by searching for a time derivative of the dispersion measure and the predicted fading of the quiescent radio source. We propose future tests of the SLSNe-I/LGRB/FRB connection, such as searches for FRBs from nearby SLSNe-I/ LGRBs on timescales of decades after their explosions.

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Strongly-magnetized, rapidly-rotating neutron stars are contenders for the central engines of both long-duration gamma-ray bursts (LGRBs) and hydrogen-poor super-luminous supernovae (SLSNe-I). Models for typical (∼ minute long) LGRBs invoke magnetars with high dipole magnetic fields (B d ∼ > 10 15 G) and short spin-down times, while models for SLSNe-I invoke neutron stars with weaker fields and longer spin-down times of weeks. Here we identify a transition region in the space of B d and birth period for which a magnetar can power both a long GRB and a luminous SN. In particular, we show that a 2 ms period magnetar with a spin-down time of ∼ 10 4 s can explain the observations of both the ultra-long GRB 111209 and its associated luminous SN2011kl. For magnetars with longer spin down times, we predict even longer duration (∼ 10 6 s) GRBs and brighter supernovae, a correlation that extends to Swift J2058+05 (commonly interpreted as a tidal disruption event). We further show that previous estimates of the maximum rotational energy of a proto-magnetar were too conservative and energies up to E max ∼ 1 − 2 × 10 53 ergs are possible. The magnetar model can therefore comfortably accommodate the extreme energy requirements recently posed by the most luminous supernova ASASSN-15lh. The high ionization flux from a pulsar wind nebula powering ASASSN-15lh may lead to an ionization break-out X-ray burst over the coming months, which would be accompanied by an abrupt change in the optical spectrum. We conclude by briefly contrasting millisecond magnetar and black hole models for SLSNe and ultra-long GRBs.

The combined detection of a binary neutron-star merger in both gravitational waves (GWs) and electromagnetic (EM) radiation spanning the entire spectrum -GW170817 / AT2017gfo / GRB170817A -marks a breakthrough in the field of multi-messenger astronomy. Between the plethora of modeling and observations, the rich synergy that exists among the available data sets creates a unique opportunity to constrain the binary parameters, the equation of state of supranuclear density matter, and the physical processes at work during the kilonova and gamma-ray burst. We report, for the first time, Bayesian parameter estimation combining information from GW170817, AT2017gfo, GRB170817 to obtain truly multi-messenger constraints on the tidal deformabilityΛ ∈ [302, 860], total binary mass M ∈ [2.722, 2.751]M , the radius of a 1.4 solar mass neutron star R ∈ [11.3, 13.5]km (with additional 0.2 km systematic uncertainty), and an upper bound on the mass ratio of q ≤ 1.27, all at 90% confidence. Our joint novel analysis makes use of new phenomenological descriptions of the dynamical ejecta, debris disk mass, and remnant black hole properties, all derived from a large suite of numerical relativity simulations. arXiv:1812.04803v2 [astro-ph.HE]

The fast radio burst FRB 121102 has repeated multiple times, enabling the identification of its host galaxy and of a spatially-coincident, compact, steady ('persistent') radio synchrotron source. It was proposed that FRB 121102 is powered by a young flaring magnetar, embedded within a decadesold supernova remnant. Using a time-dependent one-zone model, we show that a single expanding magnetized electron-ion nebula (created by the same outbursts likely responsible for the FRBs) can explain all the basic properties of the persistent source (size, flux, self-absorption constraints) and the large but decreasing rotation measure (RM) of the bursts. The persistent emission is powered by relativistic thermal electrons heated at the termination shock of the magnetar wind, while the RM originates from non-relativistic electrons injected earlier in the nebula's evolution and cooled through expansion and radiative losses. The model contains few free parameters, which are tightly constrained by observations: the total energy injected into the nebula over its history, ∼ 10 50 −10 51 erg, agrees with the magnetic energy of a millisecond magnetar; the baryon loading of the magnetar outflow (driven by intermittent flares) is close to the neutron star escape speed; the predicted source age ∼ 10 − 40 years is consistent with other constraints on the nebula size. For an energy input rateĖ ∝ t −α following the onset of magnetar activity, we predict secular decay of the RM and persistent source flux, which approximately follow RM ∝ t −(6+α)/2 and F ν ∝ t −(α 2 +7α−2)/4 , respectively.