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The first neutron star merger

A new era, of multi-messenger astronomy (combining gravitational waves, particles, photons), has been born today. LIGO/VIRGO plus 70 teams around the world announced the first detection of colliding neutron stars, which means that a gravitational wave detection (GW170817) triggered the observations using almost everything available (from gamma rays to radio wavelengths) that identify and follow its electromagnetic counterpart.

I feel very fortunate that I participated in the above exciting event, along with Thodoris Bitsakis and Carlson Adams, by obtaining spectroscopic and photometric observations from the Clay telescope (at Las Campanas Observatory), a few days after the event. These are included in the following papers published in Science:

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis

M. R. Drout, A. L. Piro, B. J. Shappee, C. D. Kilpatrick, J. D. Simon, C. Contreras, D. A. Coulter, R. J. Foley, M. R. Siebert, N. Morrell, K. Boutsia, F. Di Mille, T. W.-S. Holoien, D. Kasen, J. A. Kollmeier, B. F. Madore, A. J. Monson, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, T. C. Beers, R. A. Bernstein, T. Bitsakis, A. Campillay, T. T. Hansen, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, C. Moni Bidin, J. L. Prieto, K. C. Rasmussen, C. Rojas-Bravo, A. L. Strom, N. Ulloa, J. Vargas-González, Z. Wan, D. D. Whitten

Abstract:
On 17 August 2017, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the universe.

2017, Sci, 358, 1570 / NASA/ADS / Science 16 Oct 2017, eaaq0049

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger

B. J. Shappee, J. D. Simon, M. R. Drout, A. L. Piro, N. Morrell, J. L. Prieto, D. Kasen, T. W.-S. Holoien, J. A. Kollmeier, D. D. Kelson, D. A. Coulter, R. J. Foley, C. D. Kilpatrick, M. R. Siebert, B. F. Madore, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, R. A. Bernstein, T. Bitsakis, K. Boutsia, J. R. Bravo, F. Di Mille, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, V. M. Placco, G. Prieto, Z. Wan

Abstract:
On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from 11000(+3400,-900) Kto 9300(+300,-300) K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.

2017, Sci, 358, 1574 / NASA/ADS / Science 16 Oct 2017, eaaq0186

[1] About GW170817https://www.ligo.caltech.edu/page/press-release-gw170817

Gravitational waves detected

Today (February 11, 2016), Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first-ever detection of gravitational waves [1], opening a new era in Astronomy.

The signal recorded by LIGO on September 14, 2015, is consistent with a black-hole merger of 36 and 29 solar masses – resulting in a black hole of 62 solar masses. The remaining 3 solar masses “was converted into gravitational waves in a fraction of a second — with a peak power output about 50 times that of the whole visible universe” [2].

ligo20160211-gravitational wave announcement

Plots of the signals of gravitational waves detected by the twin LIGO observatories.
(Credit: LIGO).


[1] LIGO detection announcement (https://www.ligo.caltech.edu/detection)
[2] LIGO press-release (http://ligo.org/news/detection-press-release.pdf)