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Alexander, K. D.; Schroeder, G.; Paterson, K.; Fong, W.; Cowperthwaite, P.; Gomez, S.; Margalit, B.; Margutti, R.; Berger, E.; Blanchard, P.; et al(
, The Astrophysical Journal)
Abstract GW190814 was a compact object binary coalescence detected in gravitational waves by Advanced LIGO and Advanced Virgo that garnered exceptional community interest due to its excellent localization and the uncertain nature of the binary’s lighter-mass component (either the heaviest known neutron star, or the lightest known black hole). Despite extensive follow-up observations, no electromagnetic counterpart has been identified. Here, we present new radio observations of 75 galaxies within the localization volume at Δ t ≈ 35–266 days post-merger. Our observations cover ∼32% of the total stellar luminosity in the final localization volume and extend to later timescales than previouslymore »reported searches, allowing us to place the deepest constraints to date on the existence of a radio afterglow from a highly off-axis relativistic jet launched during the merger (assuming that the merger occurred within the observed area). For a viewing angle of ∼46° (the best-fit binary inclination derived from the gravitational wave signal) and assumed electron and magnetic field energy fractions of ϵ e = 0.1 and ϵ B = 0.01, we can rule out a typical short gamma-ray burst-like Gaussian jet with an opening angle of 15° and isotropic-equivalent kinetic energy 2 × 10 51 erg propagating into a constant-density medium n ≳ 0.1 cm −3 . These are the first limits resulting from a galaxy-targeted search for a radio counterpart to a gravitational wave event, and we discuss the challenges—and possible advantages—of applying similar search strategies to future events using current and upcoming radio facilities.« less
Free, publicly-accessible full text available December 1, 2022
Hajela, A.; Margutti, R.; Bright, J. S.; Alexander, K. D.; Metzger, B. D.; Nedora, V.; Kathirgamaraju, A.; Margalit, B.; Radice, D.; Guidorzi, C.; et al(
, The Astrophysical Journal Letters)
Abstract For the first ∼3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n < 0.01 cm −3 . We report on observational evidence for an excess of X-ray emission at δt > 900 days after the merger. With L x ≈ 5 × 10 38 erg s −1 at 1234 days, the recently detected X-ray emission represents a ≥3.2 σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow atmore »earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥3.1 σ , depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7 σ . A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.« less
Free, publicly-accessible full text available March 1, 2023
Rastinejad, J. C.; Fong, W.; Kilpatrick, C. D.; Paterson, K.; Tanvir, N. R.; Levan, A. J.; Metzger, B. D.; Berger, E.; Chornock, R.; Cobb, B. E.; et al(
, The Astrophysical Journal)
Free, publicly-accessible full text available August 1, 2022
Eftekhari, T.; Margalit, B.; Omand, C. M.; Berger, E.; Blanchard, P. K.; Demorest, P.; Metzger, B. D.; Murase, K.; Nicholl, M.; Villar, V. A.; et al(
, The Astrophysical Journal)
Rouco Escorial, A.; Fong, W.; Veres, P.; Laskar, T.; Lien, A.; Paterson, K.; Lally, M.; Blanchard, P. K.; Nugent, A. E.; Tanvir, N. R.; et al(
, The Astrophysical Journal)
Short, P; Nicholl, M; Lawrence, A; Gomez, S; Arcavi, I; Wevers, T; Leloudas, G; Schulze, S; Anderson, J P; Berger, E; et al(
, Monthly Notices of the Royal Astronomical Society)
ABSTRACT We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the All-Sky Automated Survey for Supernova 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 d. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smoothmore »at late times. The Balmer lines are unlike typical active galactic nucleus in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photoionization. The flat Balmer decrement together with the complex profiles suggests that the emission lines originate in a disc 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.« less
Villar, V. A.; Berger, E.; Miller, G.; Chornock, R.; Rest, A.; Jones, D. O.; Drout, M. R.; Foley, R. J.; Kirshner, R.; Lunnan, R.; et al(
, The Astrophysical Journal)
