One unanswered question about the binary neutron star coalescence GW170817 is the nature of its post-merger remnant. A previous search for post-merger gravitational waves targeted high-frequency signals from a possible neutron star remnant with a maximum signal duration of 500 s. Here, we revisit the neutron star remnant scenario and focus on longer signal durations, up until the end of the second Advanced LIGO-Virgo observing run, which was 8.5 days after the coalescence of GW170817. The main physical scenario for this emission is the power-law spindown of a massive magnetar-like remnant. We use four independent search algorithms with varying degrees of restrictiveness on the signal waveform and different ways of dealing with noise artefacts. In agreement with theoretical estimates, we find no significant signal candidates. Through simulated signals, we quantify that with the current detector sensitivity, nowhere in the studied parameter space are we sensitive to a signal from more than 168 Deceased, 2018 February. 169 Deceased, 2017 November.1 Mpc away, compared to the actual distance of 40 Mpc. However, this study serves as a prototype for post-merger analyses in future observing runs with expected higher sensitivity.

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Interstellar neutral atoms, unlike charged particles, freely penetrate the heliosphere, allowing us to sample the physical state of the interstellar matter directly. Most interstellar hydrogen atoms are ionized before reaching the inner heliosphere and become energetic protons picked up by the solar wind and transported away from the Sun. Consequently, observations of interstellar hydrogen atoms by missions operating within a few astronomical units from the Sun are subject to significant systematic uncertainties. We analyze observations from the Solar Wind Around Pluto instrument on New Horizons, the first experiment to provide extensive measurements of the picked-up protons far from the Sun. Analyzing the density of these protons, we find an interstellar neutral hydrogen density at the termination shock of 0.127 ± 0.015 cm−3, i.e., ∼40% higher than previously thought. We show that the Voyager observations of the slowdown of the solar wind further support this value. This result resolves a problem of why energetic neutral atom fluxes, created from pickup ions by charge exchange with hydrogen atoms, are roughly twice that expected from numerical models. Our result also implies higher charge exchange rates at the heliospheric boundaries and, consequently, a less asymmetric shape of the heliosphere. Based on a previous study of the atom filtration in the heliospheric boundaries, we estimate the neutral hydrogen density in the unperturbed local interstellar medium of 0.195 ± 0.033 cm−3. This value agrees with astrophysical observations of the interstellar clouds in the Sun proximity.

The Interstellar Boundary Explorer (IBEX) is a NASA satellite in Earth orbit, dedicated to observing interstellar neutral (ISN) atoms entering the heliosphere and energetic neutral atoms from the heliosheath from 11 eV to 6 keV. This work presents comprehensive maps of ISN hydrogen observed with IBEX at energies between 11 and 41 eV, covering almost an entire solar cycle from 2009 to 2018. ISN hydrogen measurements can provide information on the interstellar medium and on the heliosphere that modifies the incoming ISN flow. Whereas hydrogen is the dominant species in the unperturbed interstellar medium, most ISN hydrogen atoms crossing into the heliosphere do not reach the inner solar system: some are filtered out around the heliopause, while others are held off by solar radiation pressure or may be ionized as they approach the Sun. This paper presents and evaluates several approaches for generating model-free maps of ISN hydrogen from IBEX measurements. We discuss the basic implications of our results for ISN hydrogen inflow and outline the remaining discrepancies between observations and model predictions. Our maps show, during weak solar activity from 2009 to 2011, a clear signal of ISN hydrogen for ecliptic longitudes between 240°and 310°, roughly one month after the signal of ISN helium has peaked. When the solar activity approached its maximum around 2014, the ISN hydrogen signal weakened and dropped below the detection threshold because of increasing solar radiation pressure and ionization. The ISN hydrogen signal then reappeared in 2017.