The Optical Afterglow of GW170817:

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Sep 9, 2019.

  1. paddoboy Valued Senior Member

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    27,543
    https://phys.org/news/2019-09-afterglow-nature-neutron-star-collisions.html

    Afterglow sheds light on the nature, origin of neutron star collisions:

    The final chapter of the historic detection of the powerful merger of two neutron stars in 2017 officially has been written. After the extremely bright burst finally faded to black, an international team led by Northwestern University painstakingly constructed its afterglow—the last bit of the famed event's life cycle.

    Not only is the resulting image the deepest picture of the neutron star collision's afterglow to date, it also reveals secrets about the origins of the merger, the jet it created and the nature of shorter gamma ray bursts.

    "This is the deepest exposure we have ever taken of this event in visible light," said Northwestern's Wen-fai Fong, who led the research. "The deeper the image, the more information we can obtain."

    more at link.....

    the paper:

    https://arxiv.org/pdf/1908.08046.pdf

    The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin:

    ABSTRACT:

    We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at ≈584 days post-merger, which provide a robust optical template. The light curve spans ≈ 110 − 362 days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of hβi = −0.583 ± 0.013, demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster of L . 6.7×103 L, or M . 1.3 × 104 M, at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy, NGC4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster. As highlighted here, HST (and soon JWST) enables critical observations of the optical emission from neutron star merger jets and outflows.

    5. CONCLUSIONS & FUTURE OUTLOOK:
    We present the first observation following GW170817 in which an optical source is not detected to deep limits, allowing us to determine the complete F606W light curve of its optical afterglow from ≈ 110 − 584 days. The afterglow evolution is fully consistent with the optical emission emanating from a relativistic structured jet at an observer angle of ≈ 30◦ , as indicated by radio and X-ray observations. This study highlights the importance of template observations in determining accurate light curves, especially for the late and faintest stages of evolution. This is especially important for local events detected by gravitational wave facilities which are embedded in their host galaxies, for which galactic low surface brightness features are more prominent and cannot be easily modeled. We also compare GW170817 to on-axis cosmological short GRBs. Extrapolating the optical post-peak temporal evolution of GW170817 to the luminosities of short GRBs, the predicted jet break times for short GRBs are consistent with their observed breaks. Thus, we find that the two populations can be easily connected if their explosion properties (e.g., energetics, circum-merger densities and jet opening angles) are similar, and that the factor which primarily dictates their different evolution is the observer angle. Continued studies of short GRBs to & 5 days, as well as similarly in-depth studies of local binary neutron star mergers, will continue to shed light on any intrinsic differences in these populations. We provide a deep and direct constraint on the presence of an underlying globular cluster to M . 1.3×104 M, providing direct evidence that GW170817 did not form and merge in a cluster in situ at the 4σ level. However, we cannot place meaningful constraints on the possibility that the progenitor system was dynamically formed and ejected from its parent cluster. Future simulations which calculate accurate rates of such systems taking into account the full cluster evolution, coupled with further observational constraints on mergers at . 200 Mpc, will help to elucidate this formation channel. Finally, we remark that HST had a singular role in the optical afterglow of the relatively nearby GW170817. As gravitational wave facilities increase in sensitivity, most binary neutron star mergers will be detected farther away. If the optical luminosity of the GW170817 afterglow is representative, the advent of extremely large telescopes and future space-based initiatives, such as JWST will play an incredibly important role in the detection and characterization of off-axis afterglows from binary neutron star (and neutron star-black hole) mergers.


     
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  3. paddoboy Valued Senior Member

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    The above article and paper was of course inspired by the first GW detection event of two colliding Neutron stars, which also resulted in detection via electromagnetic radiation...or light.
    More incredible science.
     
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