# Dark Energy after GW170817 [CEA]

The observation of GW170817 and its electromagnatic counterpart implies that gravitational waves travel at the speed of light, with deviations smaller than a few parts in $10^{-15}$. We discuss the consequences of this experimental result for models of dark energy and modified gravity characterized by a single scalar degree of freedom. To avoid tuning, the speed of gravitational waves must be unaffected not only for our particular cosmological solution, but also for nearby solutions obtained by slightly changing the matter abundance. For this to happen the coefficients of various operators must satisfy precise relations that we discuss both in the language of the Effective Field Theory of Dark Energy and in the covariant one, for Horndeski, beyond Horndeski and degenerate higher-order theories. The simplification is dramatic: of the three functions describing quartic and quintic beyond Horndeski theories, only one remains and reduces to a standard conformal coupling to the Ricci scalar for Horndeski theories. We show that the deduced relations among operators do not introduce further tuning of the models, since they are stable under quantum corrections.

P. Creminelli and F. Vernizzi
Tue, 17 Oct 17
8/163

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# The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta [HEAP]

We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $\gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $\gtrsim 20^{\circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $\sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $n\sim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $\sim 20^{\circ}-40^{\circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $\sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.

K. Alexander, E. Berger, W. Fong, et. al.
Tue, 17 Oct 17
9/163

Comments: 8 pages, 4 figures, 1 table. ApJL, in press. Keywords: GW170817, LVC

# A cocoon shock breakout as the origin of the $γ$-ray emission in GW170817 [HEAP]

The short Gamma-Ray Burst, GRB170817A, that followed the binary neutron star merger gravitational waves signal, GW170817, is not a usual sGRB. It is weaker by three orders of magnitude than the weakest sGRB seen before and its spectra, showing a hard early signal followed by a softer thermal spectrum, is unique. We show, first, that the $\gamma$-rays must have emerged from at least mildly relativistic outflow, implying that a relativistic jet was launched following the merger. We then show that the observations are consistent with the predictions of a mildly relativistic shock breakout: a minute $\gamma$-ray energy as compared with the total energy and a rather smooth light curve with a hard to soft evolution. We present here a novel analytic study and detailed numerical 2D and 3D relativistic hydrodynamic and radiation simulations that support the picture in which the observed $\gamma$-rays arose from a shock breakout of a cocoon from the merger’s ejecta (Kasliwal 2017). The cocoon can be formed by either a choked jet which does not generate a sGRB (in any direction) or by a successful jet which generates an undetected regular sGRB along the system’s axis pointing away from us. Remarkably, for the choked jet model, the macronova signal produced by the ejecta (which is partially boosted to high velocities by the cocoon’s shock) and the radio that is produced by the interaction of the shocked cocoon material with the surrounding matter, agree with the observed UV/optical/IR emission and with current radio observations. Finally, we discuss the possibility that the jet propagation within the ejecta may photodissociate some of of the heavy elements and may affect the composition of a fraction of ejecta and, in turn, the opacity and the early macronova light.

O. Gottlieb, E. Nakar, T. Piran, et. al.
Tue, 17 Oct 17
10/163

# The unpolarized macronova associated with the gravitational wave event GW170817 [HEAP]

The merger of two dense stellar remnants including at least one neutron star (NS) is predicted to produce gravitational waves (GWs) and short duration gamma ray bursts (GRBs). In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis. The radioactive decay of these heavy elements produces additional transient radiation termed “kilonova” or “macronova”. We report the detection of linear optical polarization P = (0.50 +/- 0.07)% at 1.46 days after detection of the GWs from GW170817, a double neutron star merger associated with an optical macronova counterpart and a short GRB. The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying, component and a red, more slowly evolving, component due to material rich of heavy elements, the lanthanides. The polarization measurement was made when the macronova was still in its blue phase, during which there is an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by Galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45 – 9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.

S. Covino, K. Wiersema, Y. Fan, et. al.
Tue, 17 Oct 17
11/163

Comments: 18 pages, 1 figure, 2 tables, Nature Astronomy, in press

# GRB170817A associated with GW170817: multifrequency observations and modeling of prompt gamma-ray emission [HEAP]

We present details of our observational campaigns of electromagnetic transients associated with GW170817/GRB170817A using optical telescopes of Chilescope observatory and Big Scanning Antenna (BSA) of Pushchino Radio Astronomy Observatory at 110~MHz. The Chilescope observatory detected an optical transient of $\sim19^m$ on the third day in the outskirts of the galaxy NGC 4993; we continued observations following its rapid decrease. We put an upper limit of $1.5\times10^4$ Jy on any radio source of duration 10–60 s which may be associated with GW170817/GRB170817A. The prompt gamma-ray emission consists of two distinctive components – a hard short pulse delayed by $\sim 2$ seconds with respect to the LIGO signal and softer thermal pulse with $T\sim 10$ keV lasting for another $\sim2$ seconds. The appearance of the thermal component at the end of the burst is uncharacteristic for GBRs. Both the hard and the soft components do not satisfy the Amati relation, making GRB170817A distinctively different from other short GRBs. Based on gamma-ray and optical observations we develop a model of prompt high-energy emission associated with GRB170817A. The merger of two neutron stars create an accretion torus of $\sim10^{-2} M_\odot$, which supplies the black hole with magnetic flux and confines the Blandford-Znajek-powered jet. We associate the hard prompt spike with the quasi-spherical break-out of the jet from the disk wind. As the jet plows through the wind with sub-relativistic velocity, it creates a radiation dominated shock that heats the wind material to tens of keV, producing the soft thermal component. After the break out the continuing jet regains collimation so that its emission, as well as early afterglows, is beamed away from the observer. The model explains both the off-axis viewing geometry and observations of the prompt emission.

A. Pozanenko, M. Barkov, P. Minaev, et. al.
Tue, 17 Oct 17
12/163

Comments: 19 pages, 11 figures, submitted to ApJL

# Gamma-ray emission from internal shocks in novae [HEAP]

Gamma-ray emission at energies >100MeV has been detected from nine novae using the Fermi-LAT, and it can be explained by particle acceleration at shocks in these systems. Eight out of these nine objects are classical novae in which interaction of the ejecta with a tenuous circumbinary material is not expected to generate detectable gamma-ray emission. We examine whether particle acceleration at internal shocks can account for the gamma-ray emission from these novae. The shocks result from the interaction of a fast wind radiatively-driven by nuclear burning on the white dwarf with material ejected in the initial runaway stage of the nova outburst. We present a one-dimensional model for the dynamics of a forward and reverse shock system in a nova ejecta, and for the associated time-dependent particle acceleration and high-energy gamma-ray emission. Non-thermal proton and electron spectra are calculated by solving a time-dependent transport equation for particle injection, acceleration, losses, and escape from the shock region. The predicted emission is compared to LAT observations of V407 Cyg, V1324 Sco, V959 Mon, V339 Del, V1369 Cen, and V5668 Sgr. The 100MeV gamma-ray emission arises predominantly from particles accelerated up to ~100GeV at the reverse shock and undergoing hadronic interactions in the dense cooling layer downstream of the shock. The internal shock model can account for the gamma-ray emission of the novae detected by Fermi-LAT, including the main features in the observations of the recent gamma-ray nova ASASSN-16ma. Gamma-ray observations hold potential for probing the mechanism of mass ejection in novae, but should be combined to diagnostics of the thermal emission at lower energies to be more constraining. (abridged version)

P. Martin, G. Dubus, P. Jean, et. al.
Tue, 17 Oct 17
13/163

Comments: Submitted to Astronomy and Astrophysics. This is the revised version after a first round of referee comments

# Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories [CEA]

The LIGO/VIRGO collaboration has recently announced the detection of gravitational waves from a neutron star-neutron star merger (GW170817) and the simultaneous measurement of an optical counterpart (the gamma-ray burst GRB 170817A). The close arrival time of the gravitational and electromagnetic waves limits the difference in speed of photons and gravitons to be less than about one part in $10^{15}$. This has three important implications for cosmological scalar-tensor gravity theories that are often touted as dark energy candidates and alternatives to $\Lambda$CDM. First, for the most general scalar-tensor theories—beyond Horndeski models—three of the five parameters appearing in the effective theory of dark energy can now be severely constrained on astrophysical scales; we present the results of combining the new gravity wave results with galaxy cluster observations. Second, the combination with the lack of strong equivalence principle violations exhibited by the supermassive black hole in M87, constrains the quartic galileon model to be cosmologically irrelevant. Finally, we derive a new bound on the disformal coupling to photons that implies that such couplings are irrelevant for the cosmic evolution of the field.

J. Sakstein and B. Jain
Tue, 17 Oct 17
14/163