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Title: The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta

Abstract

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component ($$T\approx5500$$ K at 1.5 days) that quickly reddens; spectra later than $$\gtrsim 4.5$$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $$\sim 7900$$ \AA\ at $$t\lesssim 4.5$$ days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $$A\lesssim 140$$. This indicates a sight-line within $$\theta_{\rm obs}\lesssim 45^{\circ}$$ of the orbital axis. Comparison to models suggests $$\sim0.03$$ M$$_\odot$$ of blue ejecta, with a velocity of $$\sim 0.3c$$. The required lanthanide fraction is $$\sim 10^{-4}$$, but this drops to $$<10^{-5}$$ in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $$\lesssim 12$$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.

Authors:
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Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1409351
Report Number(s):
arXiv:1710.05456; FERMILAB-PUB-17-475-AE-CD
Journal ID: ISSN 2041-8213; 1630796
DOE Contract Number:
AC02-07CH11359
Resource Type:
Journal Article
Resource Relation:
Journal Name: The Astrophysical Journal. Letters; Journal Volume: 848; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Nicholl, M., Berger, E., Kasen, D., Metzger, B. D., Elias, J., Briceño, C., Alexander, K. D., Blanchard, P. K., Chornock, R., Cowperthwaite, P. S., Eftekhari, T., Fong, W., Margutti, R., Villar, V. A., Williams, P. K. G., Brown, W., Annis, J., Bahramian, A., Brout, D., Brown, D. A., Chen, H. -Y., Clemens, J. C., Dennihy, E., Dunlap, B., Holz, D. E., Marchesini, E., Massaro, F., Moskowitz, N., Pelisoli, I., Rest, A., Ricci, F., Sako, M., Soares-Santos, M., and Strader, J.. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta. United States: N. p., 2017. Web. doi:10.3847/2041-8213/aa9029.
Nicholl, M., Berger, E., Kasen, D., Metzger, B. D., Elias, J., Briceño, C., Alexander, K. D., Blanchard, P. K., Chornock, R., Cowperthwaite, P. S., Eftekhari, T., Fong, W., Margutti, R., Villar, V. A., Williams, P. K. G., Brown, W., Annis, J., Bahramian, A., Brout, D., Brown, D. A., Chen, H. -Y., Clemens, J. C., Dennihy, E., Dunlap, B., Holz, D. E., Marchesini, E., Massaro, F., Moskowitz, N., Pelisoli, I., Rest, A., Ricci, F., Sako, M., Soares-Santos, M., & Strader, J.. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta. United States. doi:10.3847/2041-8213/aa9029.
Nicholl, M., Berger, E., Kasen, D., Metzger, B. D., Elias, J., Briceño, C., Alexander, K. D., Blanchard, P. K., Chornock, R., Cowperthwaite, P. S., Eftekhari, T., Fong, W., Margutti, R., Villar, V. A., Williams, P. K. G., Brown, W., Annis, J., Bahramian, A., Brout, D., Brown, D. A., Chen, H. -Y., Clemens, J. C., Dennihy, E., Dunlap, B., Holz, D. E., Marchesini, E., Massaro, F., Moskowitz, N., Pelisoli, I., Rest, A., Ricci, F., Sako, M., Soares-Santos, M., and Strader, J.. 2017. "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta". United States. doi:10.3847/2041-8213/aa9029. https://www.osti.gov/servlets/purl/1409351.
@article{osti_1409351,
title = {The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta},
author = {Nicholl, M. and Berger, E. and Kasen, D. and Metzger, B. D. and Elias, J. and Briceño, C. and Alexander, K. D. and Blanchard, P. K. and Chornock, R. and Cowperthwaite, P. S. and Eftekhari, T. and Fong, W. and Margutti, R. and Villar, V. A. and Williams, P. K. G. and Brown, W. and Annis, J. and Bahramian, A. and Brout, D. and Brown, D. A. and Chen, H. -Y. and Clemens, J. C. and Dennihy, E. and Dunlap, B. and Holz, D. E. and Marchesini, E. and Massaro, F. and Moskowitz, N. and Pelisoli, I. and Rest, A. and Ricci, F. and Sako, M. and Soares-Santos, M. and Strader, J.},
abstractNote = {We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component ($T\approx5500$ K at 1.5 days) that quickly reddens; spectra later than $\gtrsim 4.5$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $\sim 7900$ \AA\ at $t\lesssim 4.5$ days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $A\lesssim 140$. This indicates a sight-line within $\theta_{\rm obs}\lesssim 45^{\circ}$ of the orbital axis. Comparison to models suggests $\sim0.03$ M$_\odot$ of blue ejecta, with a velocity of $\sim 0.3c$. The required lanthanide fraction is $\sim 10^{-4}$, but this drops to $<10^{-5}$ in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $\lesssim 12$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.},
doi = {10.3847/2041-8213/aa9029},
journal = {The Astrophysical Journal. Letters},
number = 2,
volume = 848,
place = {United States},
year = 2017,
month =
}
  • 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 (more » $13.7$ 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.« less
  • We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart atmore » $0.47$ days to $18.5$$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy distribution (SED) inferred from this photometry at $$0.6$ days is well described by a blackbody model with $$T\approx 8300$$ K, a radius of $$R\approx 4.5\times 10^{14}$$ cm (corresponding to an expansion velocity of $$v\approx 0.3c$$), and a bolometric luminosity of $$L_{\rm bol}\approx 5\times10^{41}$$ erg s$$^{-1}$$. At $1.5$ days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set we find that models with heating from radioactive decay of $$^{56}$$Ni, or those with only a single component of opacity from $r$-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data, the resulting "blue" component has $$M_\mathrm{ej}^\mathrm{blue}\approx 0.01$$ M$$_\odot$$ and $$v_\mathrm{ej}^\mathrm{blue}\approx 0.3$$c, and the "red" component has $$M_\mathrm{ej}^\mathrm{red}\approx 0.04$$ M$$_\odot$$ and $$v_\mathrm{ej}^\mathrm{red}\approx 0.1$$c. These ejecta masses are broadly consistent with the estimated $r$-process production rate required to explain the Milky Way $r$-process abundances, providing the first evidence that BNS mergers can be a dominant site of $r$-process enrichment.« less
  • Here, we present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational-wave emission, GW170817. Our observations commenced 10.5 hr post-merger, as soon as the localization region became accessible from Chile. We imaged 70 deg 2 in the i and z bands, covering 93% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hr post-merger we detected a bright optical transient locatedmore » $$10\buildrel{\prime\prime}\over{.} 6$$ from the nucleus of NGC 4993 at redshift z = 0.0098, consistent (for $${H}_{0}=70$$ km s –1 Mpc –1) with the distance of 40 ± 8 Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes of $i=17.3$ and $z=17.4$, and thus an absolute magnitude of $${M}_{i}=-15.7$$, in the luminosity range expected for a kilonova. We identified 1500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC 4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5% confidence level. We therefore conclude that the optical counterpart we have identified near NGC 4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.« less
    Cited by 6
  • We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational wave emission, GW170817. Our observations commenced 10.5 hours post-merger, as soon as the localization region became accessible from Chile. We imaged 70 degmore » $^2$ in the $i$ and $z$$ bands, covering 93\% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detected a bright optical transient located $$10.6''$$ from the nucleus of NGC\,4993 at redshift $$z=0.0098$, consistent (for $$H_0 = 70$$\, km s$$^{-1}$$ Mpc$$^{-1}$$) with the distance of $$40 \pm 8$$\, Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes $$i\approx 17.30$$ and $$z\approx 17.45$$, and thus an absolute magnitude of $$M_i = -15.7$$, in the luminosity range expected for a kilonova. We identified 1,500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC\,4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5\% confidence level. We therefore conclude that the optical counterpart we have identified near NGC\,4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves, and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.« less
    Cited by 6